Month: May 2026

  • To dose, or not to dose: that is the question

    To dose, or not to dose: that is the question

    How China’s Investigator-Initiated Pathway Is Rewriting the Validation Trajectory for Cell/Tissue Targeted Medicines

    A perfect storm

    In September 2025, the New England Journal of Medicine published as Correspondence the first clinical data that would have seemed implausible five years ago1. Five patients with refractory systemic lupus erythematosus had received an intravenous infusion containing messenger RNA encoding a CD19 chimeric antigen receptor, packaged inside a lipid nanoparticle engineered to deliver its cargo specifically to T cells. The patients’ own T cells, reprogrammed in their own bodies, attacked the B cells driving the disease. Four of the five had lupus nephritis. All showed deep B-cell depletion. None needed the toxic chemotherapy conditioning that conventional CAR-T therapy requires.

    The drug was called HN2301. The company was MagicRNA, based in Shenzhen. The trial was an investigator-initiated study (IIT) — meaning the local hospital ethics committee, not China’s national drug regulator, had cleared it. From a regulatory standpoint, it was the kind of trial a clinician runs to test a hypothesis. From a scientific standpoint, it was the first published clinical evidence that you can manufacture CAR-T cells inside a patient’s body.

    In March 2025, AstraZeneca had paid up to a billion dollars to acquire a small Belgian biotech called EsoBiotec. The asset that justified the price was a similar in vivo CAR-T concept, ESO-T01 — co-developed with Shenzhen’s Pregene Biopharma — whose first patient had been dosed in November 2024 at Union Hospital in Wuhan, part of the Tongji Medical College system at Huazhong University of Science and Technology, under principal investigator Heng Mei. The trial was multi-center, investigator-initiated, with a planned enrollment of up to 24 patients with relapsed/refractory multiple myeloma. The dosing was announced publicly on January 8, 2025. Two months later, AstraZeneca acquired the company.

    In late April 2025, a Shanghai company called YolTech reported interim results from its own investigator-initiated trial of YOLT-101, an in vivo base editor for heterozygous familial hypercholesterolemia, in which the company’s proprietary adenine base editor2 — packaged in a GalNAc-conjugated lipid nanoparticle (LNP)— converts a single nucleotide in the PCSK9 gene of hepatocytes. The trial run at Renji Hospital of Shanghai Jiao Tong University, had enrolled six subjects across three dose cohorts. PCSK9 levels fell by more than 70% from baseline in the higher-dose groups. LDL cholesterol reductions were durable through at least 24 weeks of follow-up, with the longest individual reaching 36 weeks. Five weeks after the data readout the U.S. FDA cleared YolTech’s investigational new drug application to run the same study in the United States.

    These are not isolated events. The industry is very rapidly evolving. First-in-human (FIH) means something completely different today than what it meant even a year or two back. These events are just the surf from the turbulent waves that has left US biotech reeling. This is quietly becoming the fastest path for a new molecule or a novel therapeutic idea to race to to interpretable human data. To understand why this matters — for patients, for investors, and for anyone designing the next generation of cell-targeted medicines — you have to understand the system that produced them.

    Two tracks to First-in-Humans

    Since 2017, China has run a dual-track regulatory system for cell and gene therapies. One track is the conventional one: an industry-sponsored Investigational New Drug application reviewed by China’s NMPA. This is the rough equivalent of the U.S. FDA’s Center for Drug Evaluation and Research (CDER). The other track is the Investigator-Initiated Trial, or IIT, overseen by China’s National Health Commission and gated only by an institutional ethics committee at a licensed hospital.

    The IIT is not unique to China. The United States has investigator-initiated studies too. What is unique is the combination of three things:

    1. The legal status of IITs as a recognized regulatory pathway for novel modalities including cell and gene therapy
    2. The scale of the Chinese hospital system willing to run them
    3. The data infrastructure to publish and license the products that this system can output.

    The financial fingerprint of this system is striking. A analysis published in Frontiers in Pharmacology in early 20263, identified 10,373 cell therapy clinical trials worldwide.

    This table says it all!

    RegionCell therapy trials (Oct 2025)Phase III shareEarly Phase I share
    United States3,5634.4%1.7%
    China3,3651.6%21.1%
    Europe1,58410.5%0.5%

    What this means is that China is not running the same trials the West runs. China is running a different kind of trial — earlier, smaller, more exploratory, less expensive — at enormous volume. An earlier analysis of 953 Chinese gene-and-cell therapy trials published in 20224 found that investigator-initiated studies “far exceeded” industry-sponsored ones in every category except in vivo gene therapy, where the regulatory bar is structurally higher.

    This produces a peculiar economy. Conventional Western drug development assumes that a Phase I trial is a $5–15 million undertaking that takes 12 to 24 months to launch after a successful IND filing. A Chinese IIT for a cell-targeted construct, run at a major academic hospital, can be initiated in 3 to 9 months at a cost of $300,000 to $1.5 million. The EsoBiotec story is the canonical example: hypothesis, registration, first patient, acquisition — three months from human data to a billion-dollar exit.

    A second statistical fingerprint sharpens the picture. The same paper found that 43% of China’s cell-therapy trials use genetically modified cell products, meaning, cells engineered with an inserted transgene, edited gene, or both, rather than cells used in their native state. In the United States and the European Union, the genetically-modified share is closer to 19%. The Western cell-therapy portfolio is more heavily weighted toward unmodified modalities: hematopoietic stem-cell transplants, mesenchymal stem cells, tumor-infiltrating lymphocytes. The Chinese portfolio, by contrast, leans hard into constructed cell therapies — CAR-T, CAR-NK, TCR-T, and the in vivo variants of all three. China runs roughly twice the proportion of engineered-cell trials that the West does. Engineered cell therapies are the modality class whose value is determined by the targeting molecule each cell expresses: the CAR’s antigen-binding domain, the TCR’s recognition sequence, the homing peptide on the LNP that delivers the genetic payload. So China is dominating in therapies whose primarily value is in how precisely they find their cellular target.

    The modalities that benefit

    The IIT pathway is producing first-in-human data across at least eight different modality classes, each of which depends on a different solution to the same underlying problem: how do you get a payload — a gene editor, a piece of RNA, a radioactive atom, a cytotoxic small molecule — into a specific kind of cell?

    In vivo CAR-T

    This is the modality that has captured the most attention. After MagicRNA’s work are at least three other clinical-stage programs. Genocury Biotech, also in Shenzhen, reported a complete remission in a patient with refractory diffuse large B-cell lymphoma after a single dose of its in vivo CD19 CAR-T. The trial was run at Tongji Hospital in Wuhan under principal investigator Jia Wei. EsoBiotec’s BCMA program is now an AstraZeneca asset. A fourth program, registered in January 2026 by Daihong Liu of the PLA General Hospital in Beijing uses a polymer-lipid hybrid nanoparticle5 to deliver mRNA encoding a dual CD19/CD20 CAR.

    In vivo gene editing

    YolTech alone now has four programs with human or near-human data. Its base editor for familial hypercholesterolemia, YOLT-101, edits the PCSK9 gene in hepatocytes using a lipid nanoparticle delivery system. Its CRISPR-Cas program YOLT-201 targets the TTR gene in transthyretin amyloid cardiomyopathy. Anyone who has worked in the nucleic acid drug space should be very familiar with these targets! YOLT-203 treats primary hyperoxaluria type 1. YOLT-202, for alpha-1 antitrypsin deficiency, has FDA Regenerative Medicine Advanced Therapy designation. A second Chinese company, AccurEdit, has reported up to 70% LDL-cholesterol reduction from a single dose of its own base-editing therapy. A third, Base Therapeutics in Shanghai, has registered two oncology programs.

    What is striking about this cluster is that they are liver focussed where all LNPs accumulate anyway. The clinical successes so far are for liver-expressed targets (PCSK9, TTR, alpha-1 antitrypsin). The frontier — the editing of cells anywhere else in the body — is purely a problem of finding the right targeting ligand.

    So US biotech can still innovate. The bottleneck on every in vivo gene editor is the same — targeting to cells and tissues. Wouldn’t it be amazing if we could specifically target the brain, muscle & cardiac tissues, immune system, or any solid tumor? What if we could efficiently design peptides, aptamers, or a polymeric system that can decorate the LNP and redirect it?

    In fact, aptamers form a third cluster, dominated almost single-handedly by the laboratory of Weihong Tan, now at the Hangzhou Institute of Medicine. In 2023, the Tan group published the first-in-human pharmacokinetic study of a synthetic DNA aptamer in Research, a Science Partner Journal6. The aptamer, called SGC8, was radiolabeled with gallium-68 via a NOTA chelator and injected intravenously into cancer patients at Renji Hospital in Shanghai under hospital ethics committee approval. It bound the cell-surface receptor PTK7. It was, in the most literal sense, a designed targeting molecule visualized inside human bodies.

    The Tan lab’s more recent work has taken a different turn. A September 2025 preprint described what the group calls Apt-circRNA: a circular RNA molecule with aptamer sequences embedded directly into its structure. I have written about this in an earlier blog post, see here. The aptamer acts as the targeting moiety for the circular RNA payload. There is no lipid nanoparticle. There is no carrier of any kind. The construct is both the medicine and the address it travels to. In mice, the Apt-circRNA, loaded with tumor antigen, drove antigen presentation in dendritic cells and cleared established tumors.

    Radioligand Therapies

    This is fourth cluster and arguably has the lowest-friction to get human data for a new targeting polymers. Chinese nuclear medicine departments routinely run investigator-initiated trials of novel peptide ligands labeled with diagnostic or therapeutic isotopes. A paper published in the Journal of Medicinal Chemistry in early 2026 described the first-in-human evaluation of a novel PSMA-targeting radioligand whose key feature was a modified amino acid — a beta-3 amino linker — designed to reduce off-target uptake in the kidneys and salivary glands7.

    The trial was first-in-human and IIT. An EJNMMI paper from October 2025 described an investigator-initiated dose-escalation trial of a fibroblast-activation-protein-targeted radioligand in patients with advanced sarcoma and other refractory cancers. Another such trial is running at Nanjing First Hospital.

    The drug-conjugate families — ADC, peptide-drug conjugates, antibody-oligonucleotide conjugates, radionuclide conjugates, small-molecule drug conjugates, immunostimulatory antibody conjugates, antibody-degrader conjugates — all share the same three-part architecture: a targeting ligand, a linker, an effector. Targets are appearing in Chinese trials before they appear in Western ones.

    mRNA and circRNA cancer vaccines form a fifth cluster, where the targeting question is whether the antigen-coding RNA reaches the right antigen-presenting cell. StemiRNA Therapeutics in Shanghai has received CDE approval for SW0715, a lipopolyplex-formulated mRNA encoding IL-12 — a lipid-polymer hybrid carrier. Academic groups at Fudan, Tsinghua, and Mengchao Hepatobiliary are running circRNA neoantigen vaccine programs against hepatocellular carcinoma and HPV-driven cancers.

    What unites these clusters is not a shared technology, instead a shared innovaiton bottleneck. In every case the value-creating element of the medicine is the molecule that directs it to its cellular destination. The CAR, the aptamer, the targeting peptide on a radioligand, the antibody on a conjugate, the engineered envelope of a virus, the surface protein of an exosome, the ligand on a lipid nanoparticle. Designing these targeting molecules is the rate-limiting step. Validating them in humans is the value-creating step. And it is the latter that the Chinese IIT pathway has compressed by an order of magnitude.

    The mechanics of how it all moves so fast

    How does a Chinese IIT actually move so fast? It is worth understanding the mechanics.

    Three structural factors do most of the work.

    The first is hospital sponsorship. An IIT in China is sponsored by the institution running it — typically the principal investigator and the hospital’s clinical research unit. The investigator submits a protocol to the hospital’s ethics committee, which assesses safety, scientific rationale, and ethical considerations. If the committee approves, the trial can proceed. There is no parallel review by NMPA, no IND filing, no requirement for a sponsor company.

    The legal sponsor is the investigator. This eliminates an entire layer of regulatory interaction that, in the United States, typically consumes 12 to 18 months between protocol design and first patient dosed.

    The second is the sheer density of the hospital infrastructure. Twenty-five percent of the world’s top 200 research hospitals by Nature Index share are in China. Many of the major academic centers — Renji Hospital, Tongji Hospital, PLA General Hospital, Peking Union Medical College Hospital, Fudan Zhongshan, Nanjing First — have established cell therapy and gene therapy units with experienced clinical research staff, on-site manufacturing capacity, and ethics committees that have evaluated dozens of novel-modality protocols.

    The third is cost. Clinical operations in China cost roughly 30 to 40 percent of equivalent operations in the United States — and for early-phase exploratory trials with small patient cohorts and short follow-up, the multiple can stretch further. CRO labor, hospital bed-days, GMP manufacturing, all are systematically less expensive. A small IIT can be funded out of a company’s seed-stage budget. A Phase I IND in the U.S. typically cannot.

    China’s NMPA has joined the International Council for Harmonization, which has the effect of aligning Chinese review standards with international ones for trials that do graduate to industry-sponsored status. Beijing announced in April 2025 that it would process investigational drug applications in 30 working days, down from 60. The ecosystem there has been actively engineered to compress timelines, whereas US has fought against mRNA vaccines and tech.

    The result is a kind of clinical-development arbitrage that did not exist five years ago. A biotech founder with a novel cell-targeting construct now has a choice. Path A is the conventional one: raise $20–30 million in a Series A, spend two years and another $10–20 million on IND-enabling studies, file an IND with the FDA, recruit U.S. sites, and wait. Path B is the new one: design the construct, find an academic principal investigator at a Chinese hospital who shares the scientific interest, fund a small IIT, and dose your first patient inside of nine months for under a million dollars. The data from Path B will not get you a U.S. approval. But it will tell you, with real human evidence, whether your construct works.

    For an investor that distinction changes the entire risk profile. The biggest question in early-stage biotech investing is whether the company’s preclinical model translates to humans. A team who can answer that question with human data — even exploratory, even small — is selling a fundamentally different proposition than a team who can only point to mice. Welcome to the brave new world of drug discovery!

    The deal-side perspective

    The translation of this scientific reality into commercial activity has been swift, but it has taken two distinct forms — and the difference between them reveals where the industry is heading.

    The EsoBiotec story is the cleanest small-scale example. In December 2024 the company initiated an investigator-initiated trial of ESO-T01 at multiple Chinese sites. The first patient was dosed in January 2025. In March 2025 AstraZeneca acquired the company for up to a billion dollars in cash and milestones. The asset that justified the price was not a Phase I dataset in the traditional sense — it was a small, IIT-derived signal that in-vivo CAR-T could work for multiple myeloma. AstraZeneca did not need a U.S.-quality IND package. It needed conviction.

    The most consequential transaction of 2025, however, was something larger and structurally different. On July 28, 2025, GSK announced an agreement with Jiangsu Hengrui Pharma — one of China’s largest pharmaceutical companies — to develop up to twelve innovative medicines across respiratory, immunology, inflammation, and oncology. GSK paid $500 million upfront. The total potential value, if all twelve programs are optioned and all milestones met, is approximately $12 billion, plus tiered royalties on global sales outside Greater China. The lead asset is HRS-9821, a PDE3/4 inhibitor in clinical development for chronic obstructive pulmonary disease. The other eleven programs are not yet in the clinic; Hengrui will develop each of them through Phase I, including the recruitment of patients outside China, and GSK holds the option to take any of them global at the end of each Phase I.

    The structure of the GSK-Hengrui deal says something the industry has been moving toward but rarely articulates so clearly: a top-five global pharma is treating a Chinese pharma’s discovery and early-clinical engine as a strategic source of pipeline. Not as a one-off vendor of a single asset but a portfolio builder.

    The financial structure mirrors the scientific reality: the value-creating step — getting a new molecule into a human and seeing what it does — is being run in China.

    A second, parallel transaction in the same year traced a longer arc and made the same point in a different way. In November 2023, BioNTech licensed the ex-China rights to a bispecific antibody called PM8002 from Biotheus, a Zhuhai-based biotech founded in 2018, for $55 million upfront. In November 2024, BioNTech acquired Biotheus outright for $800 million plus $150 million in milestones, gaining global rights to the molecule it had renamed BNT327. In June 2025, BioNTech licensed BNT327 to Bristol Myers Squibb for $1.5 billion upfront, $2 billion in fixed payments through 2028, and up to $7.6 billion in milestones — a total deal value of up to $11.1 billion. BNT327 targets PD-L1 and VEGF-A simultaneously, an approach that early data suggest could outperform Merck’s Keytruda, the world’s best-selling drug. It is now in global Phase III trials for small-cell and non-small-cell lung cancer, with a triple-negative breast cancer trial slated for late 2025 and more than a thousand patients dosed across some twenty studies.

    The arithmetic on BNT327 is worth dwelling on. A bispecific antibody developed in a Chinese biotech moved from a $55 million license to a deal worth up to $11.1 billion in less than two years — a more than 200-fold appreciation in stated transaction value.

    The story continues. Merck paid up to $3.288 billion to license LM-299, another PD-1/VEGF bispecific, from China’s LaNova Medicines. AbbVie’s $2.1 billion acquisition of Capstan Therapeutics in 2025, for in vivo CAR-T lipid nanoparticle technology, was driven by precisely the modality MagicRNA had just published on. Lilly’s $2.2 billion January 2026 licensing deal with Profluent was for AI-designed CRISPR delivery components — the kind of asset that would naturally be validated in a Chinese IIT before any U.S. registrational study. Moderna signed a deal worth up to $1 billion in May 2025 to establish mRNA manufacturing and trials in China.

    This an emergent two-track pattern of pharmaceutical business development that did not exist five years ago.

    • Track one is the asset purchase: a Western acquirer pays for a specific Chinese-validated molecule, sometimes through an intermediary
    • Track two is the portfolio collaboration: a Western pharma buys access to a Chinese discovery and early-clinical engine across multiple programs simultaneously, with optionality at the Phase I gate. The GSK-Hengrui deal is the canonical example.

    Both tracks move capital westward, but the underlying architecture is the same — Chinese clinical validation, Western commercial valuation, and the arbitrage between them as the largest single source of pharma deal-making.

    If you map the modified-polymer and cell-targeted therapeutics deals announced from 2023 through early 2026, the total upfront-plus-milestone value crosses $89 billion. In a growing number of cases, the first interpretable human data on that targeting molecule comes from a Chinese hospital, the molecule itself was discovered in a Chinese biotech, or both.

    But it need not be so. US Biotech can innovate and still leverage both of these tracks.

    So where do we go next as a biotech founder?

    If a biotech founder reading this wants to ruminate on possible paths, here we go:

    • Identify the principal investigator before the protocol. The most productive entry points are academic clinicians who have already run novel-modality IITs and whose research interests align with the construct in question.
    • China has one of the world’s deepest contract manufacturing benches for modified peptides, modified oligonucleotides, and conjugates. Worth considering for material sourcing, GMP etc. I have worked extensively in India for this.
    • Scope out the regulatory boundaries. Two things matter here. The first is the Human Genetic Resources rule, updated by NHC in April 2024, which requires approval for any foreign access to Chinese-origin human genetic data. The second is the question of how the resulting IIT data will be used downstream — exploratory signal for the next funding round, supporting data for a U.S. IND filing, or asset-level validation for a strategic partnership. A founder who plans to use IIT data to anchor an acquisition needs to run the trial to a higher standard than one who plans only to use it for internal go/no-go.
    • Budget considerations: A focused FIH IIT for a cell-targeted construct, with five to fifteen patients and a defined primary endpoint, can usually be done for between $500,000 and $1.5 million all-in, including manufacturing of the clinical-grade material. A bridging Phase 1 in the United States will cost an order of magnitude more.
    • Chinese IIT data is exploratory by FDA and EMA standards. It does not, on its own, substitute for an IND-quality safety dataset for a U.S. filing.

    Why this matters for the design of new medicines

    The deeper story underneath the regulatory mechanics is a story about what kind of science the system rewards.

    For most of the history of biotech, the bottleneck was the drug. You found a target. You discovered a molecule. You spent years and hundreds of millions of dollars proving the molecule was safe and effective. Validation was slow because the molecules were slow to design.

    The molecules are becoming faster to design, though chemically modified ones are still very much a challenge. AI, computational chemistry methods for modified-polymer design are catching up fast. The bottleneck has moved upstream of synthesis: it is now the question of which molecules to design, which targets to hit on which cells. The bottleneck has also moved downstream: it is now the question of how quickly you can validate any one design in a human being.

    China’s IIT system attacks the downstream bottleneck. It does not solve the upstream one. An innovator who wants to use the system productively still has to know which cell-surface protein to target on which tissue, and still has to design a molecule that will bind it well. But once those choices are made, the system shortens the validation cycle from years to months and from tens of millions of dollars to less than two.

    This compression has consequences that ripple outward. It changes the economics of biotech seed financing — a company can credibly aim for human data on a milestone budget. It changes the structure of pharma business development — assets become acquisition candidates earlier in their development, on smaller datasets, demanding that we ask altogether different questions. It changes the pace of competitive dynamics — a Chinese in vivo CAR-T company can publish in NEJM before a U.S. competitor has dosed its first patient.

    And it changes which scientific problems are worth attempting in the first place. If validation takes ten years and a hundred million dollars, you choose your targets conservatively. If validation takes a year and a million dollars, you can try things that previously would have been unjustifiable.

    This last shift is the most important and the least discussed. The Chinese IIT pathway, applied at scale to cell-targeted medicine, is making it economically rational to design medicines for cellular addresses that no one has ever tried to deliver to before. The map of drugged cell-surface proteins on the human body is small — fewer than a dozen targets across the entire receptorome have been engaged by approved cell-targeted therapeutics. The map of potential targets is enormous, in the low thousands of cell-surface proteins. The arbitrage between those two numbers — between what has been done and what could be done — is what defines the next decade of cell-targeted medicine, and it is the IIT pathway that makes the arbitrage economically practicable.

    A measured caveat

    There are real risks in this story, and a fair-minded reader should know them.

    China’s IIT data is not registrational data, and treating it as such has burned investors before. The quality of IIT trials varies widely: the major academic centers run them to international GCP standards, but smaller hospitals do not always. An innovator building a serious clinical program will need a Western regulatory consultant in the loop from day one, not as an afterthought when the IIT data lands.

    The BIOSECURE Act, the U.S. legislation restricting federal contracts with certain Chinese biotech firms, remains in effect through 2026 and creates real friction for companies that intend to repatriate Chinese-developed assets to U.S. commercialization. A company planning to use the Chinese pathway should also plan a non-Chinese manufacturing footprint for any commercially bound asset.

    The Human Genetic Resources rule complicates how IIT-derived human data flows into joint development agreements with Western pharma. Companies that fail to scope this early have lost deals over it.

    Geopolitical risk is real and difficult to model. The same conditions that make the IIT pathway fast — institutional density, regulatory flexibility, low cost — sit on top of a U.S.-China relationship that is not stable.

    The calm after the storm?

    If the trajectory of the past three years continues — and there is little structural reason to expect it not to — three things will be visible from the outside by 2027.

    First, the headline first-in-human data for the next wave of cell-targeted modalities will increasingly come out of Chinese hospitals.

    Second, the deal-making patterns will see seismic shifts. Strategic acquirers will increasingly underwrite assets at the IIT data stage, not at the Phase I-complete stage. The size and shape of biotech Series A and Series B rounds will shift to accommodate this. A company that previously needed $80 million to get to a Phase 1 data readout may now need $25 million to get to IIT data plus a bridging plan. The implications for venture-stage biotech investing are large and under-appreciated.

    Third, the choice of which targets to attempt will broaden. The hardest problem in cell-targeted medicine has always been identifying which receptors on which cells are worth the years and dollars of a development cycle. As that cycle shortens, the answer shifts from the same dozen targets everyone else is chasing to any target with a defensible scientific rationale and a designable targeting ligand. Foundation-model approaches to molecule design — for antibodies, for peptides, for aptamers, for chemically modified polymers — pair naturally with this expansion. A platform that can rapidly generate targeting molecules against new cell-surface antigens, combined with rapid lab-in-a-loop and regulatory pathway that can rapidly test them in humans, is a different kind of business than the biotechs of the prior generation.

    A note on sources

    1. Wang Q, Xiao ZX, Zheng X, Wang G, Zha GF, Schett G, Chen Z, et al. “In Vivo CD19 CAR T-Cell Therapy for Refractory Systemic Lupus Erythematosus.“ New England Journal of Medicine. Published September 17, 2025. ↩︎
    2. Wan P, Tang S, Lin D, Lu Y, Long M, Xiao L, Jiang Y, Liao J, Ma X, Liu Y, Yu W, Wang ZJ, Wu Y, Yang T, Xia Q. “Base Editing Gene Therapy for Heterozygous Familial Hypercholesterolemia.” medRxiv 2025.04.17.25325983. Posted April 17, 2025. DOI: https://doi.org/10.1101/2025.04.17.25325983 ↩︎
    3. Wang M, Zhou T, Liu S, Xiang W, Xie K, Zhang X, Hu W, Fang M, Zhang Z, Chen M, Wang X, Wu J. “Global Panoramic analysis of clinical research in cell therapy: clinical trial landscape, marketed products, and regulatory trends.” Frontiers in Pharmacology, 9 February 2026. DOI: 10.3389/fphar.2026.1715984. ↩︎
    4. Yin C, Gao J, Li G, Hu H, Zhou L, Lu S, Chen X. “Gene and cell therapies in China: booming landscape under dual-track regulation.” J Hematol Oncol. 2022;15(1):139. doi:10.1186/s13045-022-01354-9. PMC: PMC9535931. ↩︎
    5. ClinicalTrials.gov. “Polymer-lipid Particle-delivered CAR1920 mRNA CAR-T (InViVoCAR1920) for B-cell Lymphoma/Leukemia.” Identifier: NCT07321301. Sponsor-investigator: Daihong Liu, Chinese PLA General Hospital, Beijing. Registered January 7, 2026. https://clinicaltrials.gov/study/NCT07321301 ↩︎
    6. Ding D, Zhao H, Wei D, Yang Q, Yang C, Wang R, Chen Y, Li L, An S, Xia Q, Huang G, Liu J, Xiao Z, Tan W. “The First-in-Human Whole-Body Dynamic Pharmacokinetics Study of Aptamer.” Research (a Science Partner Journal). 2023;6:0126. ↩︎
    7. Gao X, Miao Y, Li L, et al. “Synthesis, Evaluation, and First-in-Human Study of a Novel PSMA Radioligand Bearing Beta3-Amino Acid Linkage.” J Med Chem. 2026;69(5):5610-5621. doi:10.1021/acs.jmedchem.5c02821 ↩︎

  • Building Brains from Polymers: The Quiet Revolution in Organic Neuromorphic Computing

    Building Brains from Polymers: The Quiet Revolution in Organic Neuromorphic Computing

    In Altered Carbon the author wrote,

    For all that we have done, as a civilization, as individuals, the universe is not stable, and nor is any single thing within it. Stars consume themselves, the universe itself rushes apart, and we ourselves are composed of matter in constant flux. Colonies of cells in temporary alliance, replicating and decaying and housed within, an incandescent cloud of electrical impulse and precariously stacked carbon code memory. This is reality, this is self knowledge, and the perception of it will, of course, make you dizzy.

    Of all the colonies of cells, neurons are rather strangest of them all. My first love in all of mathematical biology was the Hodgkin-Huxley model.

    Every time you read a book, add a dash of smoked paprika to your sauce, or yell at your kids, your brain performs a computational magic. It processes vast streams of sensory data and makes split-second decisions, all on roughly 20 watts of power. You need less power than a dim light bulb and the end of a seedy alley in a Batman movie.

    Modern AI, by contrast, can demand megawatts. A Large Language Model training consumes more electricity than a small town uses in a year. Yeah, you are directly causing global warming by brainstorming with your Claude on how to get away with murder, or escape to Timbuktu.

    This staggering mismatch has forced engineers to ask a basic question: what if we stopped trying to simulate the brain on conventional hardware and instead built hardware that works the way human brain actually does?

    This is where dreams of neuromorphic computing comes in. And in the last three years, a wave of breakthroughs, many originating from labs in China, has brought us closer to answering it than I had realized. I thought I would write about it and spread my ignorance!

    The von Neumann Bottleneck: Why silicon and your grey matter differ

    Every conventional computer, from your phone to a data center, is built on the von Neumann architecture proposed in 1945. Its defining feature is a rigid separation between the processor (which computes) and memory (which stores). Data must constantly shuttle back and forth between the two over a shared bus. Obviously this is a mighty fine design. We all know that von Neumann was an alien whose mathematical super-intelligence make your and my brain look like a toaster next to a Voyager. Nevertheless, the von Neumann architecture creates an inherent bottleneck of data flow and data processing.

    Your brain has no such bottleneck. Its roughly 1012 neurons and 1015 synapses perform processing and storage in the same physical location. roughly. When a synapse strengthens in communication, a process called long-term potentiation, it is simultaneously computing and remembering. There is no bus and the Computation is memory.

    This architectural difference has profound consequences. The brain operates at frequencies around 10 Hz — millions of times slower than a modern CPU clock — yet outperforms supercomputers on tasks like real-time sensory integration. It achieves this through massive parallelism and extreme energy efficiency. A single synaptic event consumes on the order of femtojoules.

    Neuromorphic engineering aims to close this gap by building physical hardware that mimics these principles.

    A summary table ma be useful to anchor it:

    Compute systemEnergy per synaptic event
    Biological synapse~1–10 femtojoules (10-15 J)
    Best OECT artificial synapse~femtojoules to picojoules
    GPU (floating-point multiply)~picojoules to nanojoules
    Full LLM inference per token~joules (whole system)

    The first serious attempts at neuromorphic hardware used conventional silicon. Projects like IBM’s TrueNorth, Intel’s Loihi, Stanford’s Neurogrid, and the University of Manchester’s SpiNNaker demonstrated that spiking neural networks could be implemented in CMOS circuits, achieving orders-of-magnitude improvements in energy efficiency per event compared to traditional processors.

    But silicon neuromorphic chips have a fundamental problem: emulating a single synapse or neuron in CMOS typically requires more than ten transistors. This makes large-scale integration expensive, energy-hungry, and difficult to scale. Moreover, silicon is rigid, brittle, and biologically incompatible. Wouldn’t it be great if we could create direct interfaces with living tissue?

    Organic & Polymeric Electronics

    Can we make chips out of plastic material? If we could then such flexible electronics would mimic true biological neurons. This have been the dream, and recent advances in n-type polymers brings it much closer to reality1.

    Organic mixed ionic-electronic conductors (OMIECs) are polymers that conduct both electrons and ions simultaneously. This dual conductivity is is the key property that makes them uniquely suited for neuromorphic hardware. Biological neurons and synapses operate through ion flow of sodium, potassium, calcium, and chloride ions crossing membranes through voltage-gated channels. OMIECs can mimic the same electrochemical language.

    The ionic conductance story of a neuronal spike, roughly

    The organic electrochemical transistor (OECT) — a three-terminal device built from these materials — has emerged as the workhorse of organic neuromorphic electronics. A typical OECT consists of a polymer channel (the source-drain path), an electrolyte, and a gate electrode. When a voltage is applied to the gate, ions from the electrolyte migrate into the polymer channel, doping or de-doping it and changing its conductivity. The channel itself acts as an artificial synaptic cleft; the gate electrode mimics the presynaptic membrane; the drain collects the postsynaptic current.

    The physics of this process can be described as follows. In the Bernards-Malliaras model, the drain current IDSI_{DS} in the linear regime follows2:

    IDS=μC∗WTL(VTH−VGS+VDS2)VDSI_{DS} = \mu C^* \frac{W T}{L} \left( V_{TH} – V_{GS} + \frac{V_{DS}}{2} \right) V_{DS}

    where μ\mu is the carrier mobility, C∗C^* is the volumetric capacitance of the channel material, W,TW, T, and LL are channel width, thickness, and length, and VTHV_{TH} is the threshold voltage. It is the firing threshold equivalent of a neuron.

    VGSV_{GS} is the gate-to-source voltage. It controls ion injection. This is the input knob: positive VGS pushes cations into the channel de-doping it. Compared to biological neurons, this is the presynaptic signal and equivalent to the neurotransmitter release!

    VDSV_{DS} is the drain-to-source voltage. It drives the current along the channel. It pulls carriers from source to drain and acts as as the resting membrane potential.

    In contrast, a regular MOSFET has a gate insulator which is a thin oxide. Charges pile up at the surface of the semiconductor, right at the oxide interface. The gate controls a 2D sheet of charge. This is interfacial doping.

    An OECT is fundamentally different. There’s no insulator. The gate touches an electrolyte (salt water, a gel, a hydrogel), which touches the polymer channel directly. When you apply a gate voltage, ions physically migrate into the bulk of the polymer, doping or de-doping the entire volume. This is volumetric doping, which gives OECTs their hallmark properties: exceptionally high transconductance, low operating voltages (often below 1V), and a natural capacity for analog, multi-level conductance states.

    A key figure of merit for OECT channel materials is the product μC∗\mu C^*, which captures the combined electronic and ionic performance. It’s the carrier mobility times the volumetric capacitance.

    In neuron terms: μC∗measures how responsive the material is:

    • Can ions get in easily? (C∗) — analogous to how many ion channels a membrane has
    • Can the electrical consequence propagate quickly? (μ) — analogous to how well-myelinated the axon is

    And here is the recent advancement no one is talking about. Early polymeric materials like PEDOT:PSS achieved values around 50 Fcm−1V−1s−1F \, cm^{-1} V^{-1} s^{-1}. Recent glycolated polythiophenes like p(g3T2-Te) have pushed this to 483 Fcm−1V−1s−1F \, cm^{-1} V^{-1} s^{-1}, a nearly tenfold improvement in under a decade.

    Adopted from Ref. Xiang et. al.[2]

    Artificial synapses

    The most fundamental requirement for any neuromorphic device is synaptic plasticity. This is the ability to strengthen or weaken a connection based on activity, the physical basis of learning and memory.

    In biological synapses, plasticity comes in two flavors. Short-term plasticity (STP) lasts milliseconds to seconds and reflects transient neurotransmitter dynamics. Long-term plasticity (LTP) persists for hours or longer and involves structural changes at the synapse — gene expression, protein synthesis, physical remodeling of dendritic spines.

    OECT-based artificial synapses replicate both. STP arises naturally from the slow kinetics of ion injection and extraction: apply a voltage pulse to the gate, and ions dope the channel, changing its conductance. Remove the pulse, and the ions gradually diffuse back. The conductance decays to baseline over seconds. This is volatile memory, analogous to a fleeting sensory impression.

    Achieving LTP is harder and more interesting. Several strategies have emerged:

    • Electropolymerization. In a fascinating device reported by Gerasimov et al.3 an OECT built from the polymer ETE-S exhibited evolvable synaptic behavior. They showed that ow gate voltages produced conventional STP through reversible electrochemical doping. Interestingly, sustained moderate voltages instead triggered electropolymerization of additional monomer within the channel, permanently altering its conductance. The synapse literally grew new conductive pathways during operation, which is strong parallel to biological synaptogenesis!
    • Ion-blocking layers: Inserting physical barriers — metal-organic frameworks, lipid bilayers, nanofibrous polymer coatings — between the channel and electrolyte slows ion diffusion and traps charges in place after programming. A PEDOT:PSS/PEI architecture achieved non-volatile retention exceeding 25 hours, with the electrostatic potential barrier between oxidation states preventing spontaneous discharge4.
    • Crystallinity engineering5: Wang et al. built a vertical OECT whose channel has both crystalline and amorphous regions. Low gate voltages dope only the disordered amorphous zones — ions slip in easily but slip out just as fast. Volatile, achieveing STP. Crank the voltage higher and ions force their way into the tightly packed crystalline domains, where they get stuck. Non-volatile, achieving LTP. One device, two memory modes, selected purely by how hard you push. It stores 1,024 distinct conductance states and holds them for over 10,000 seconds; a 10-bit analog memory in a single transistor!

    Artificial Neurons: Making Plastics Spike

    Building artificial neurons from organic materials is not easy. We need to replicate the complex dynamics of biological action potentials!

    The dominant circuit architecture is the leaky integrate-and-fire (LIF) model, implemented using complementary OECTs arranged as an Axon-Hillock (A-H) circuit. A membrane capacitance integrates input current. When the accumulated voltage crosses a threshold, a complementary inverter (built from paired p-type and n-type OECTs) fires a sharp output spike. A reset transistor then discharges the capacitor, restoring the resting state. The cycle repeats.

    Early OECT neurons based on this design operated at frequencies below 2.4 Hz, which is far too slow to match even the slowest biological neurons. The breakthrough came with polymer engineering.

    In a 2025 study published in PNAS, Yao et al.6 introduced Homo-gDPPTz, a new n-type OMIEC designed to closely match the performance of its p-type complement (gDPP-g2T) in vertical OECT complementary inverters. The resulting organic electrochemical neuron achieved a firing frequency spanning 0.13 to 147.1 Hz that can be calibrated — a range that covers the spectrum from slow vasoconstrictor neurons (below 1 Hz) to fast-spiking cortical neurons (over 100 Hz). This is more than 50 times broader than any previous OECT neuron circuit. The device footprint was less than 37 mm2mm^2, and energy consumption was just 4.7 nanojoules per spike.

    The neuron was then integrated with pressure and strain sensors to build a complete neuromorphic perception system. Mechanical stimuli from a conductive foam sensor or a printed strain sensor were converted into input currents, which the OECT neuron transformed into frequency-modulated spike trains. These spikes were fed into an artificial synapse (also a vOECT), which modulated its postsynaptic current based on spike frequency, demonstrating the full sensing-encoding-processing loop of biological neural perception. Simulations of a spiking neural network built on these device parameters achieved 96% accuracy on handwritten digit classification.

    An even more biologically faithful architecture was recently built using the conductance-based OECN (c-OECN). This emulated sodium and potassium ion channels, reproducing 15 of 20 known neuronal features including depolarization, repolarization, hyperpolarization, and threshold-dependent firing7. This design was coupled to a Na+ sensor and used to trigger vagus nerve stimulation in mice; a fascinating demonstration of closed-loop physiological regulation driven by an organic artificial neuron!

    The Chinese Contribution: Printing Brains on Plastic

    Just as Chinese researchers have made rapid progress in efficient and open-source LLMs, they have also made astounding progress in polymeric materials and engineering of neuromorphic computing.

    Regionally controlled ion doping8. Li, Zhang et al. demonstrated an elegant strategy for building computing and memory on the same chip using identical materials. By controlling the thickness of inkjet-printed hydrogel electrolytes — thick (multi-layer) for computing units, thin (single-layer) for memory units — they created two functionally distinct OECT types from the same PEDOT:PSS channel material. The ion-rich OECT (thick electrolyte) features rapid ion transport, sub-millisecond response, and high transconductance, serving as a computing neuron. The ion-deficient OECT (thin electrolyte) has slow ion dynamics, wide hysteresis, and 300-second state retention, serving as a memory element.

    Wow! Integrating volatile computing elements and non-volatile memory elements typically requires different materials, different fabrication processes, and complex circuit architectures. But this work shows that the spatial distribution of the electrolyte alone is sufficient.

    Stretchable neuromorphic chips: Dai et al.9 created the first intrinsically stretchable neuromorphic device using the polymer p(gT2), an organo-hydrogel electrolyte, and vertically grown gold nanowire electrodes embedded in PDMS. The device maintained over 800 distinct conductance states, switching endurance exceeding 108 cycles, and state retention over 104 seconds — all while being stretched to 100% strain. A 3-by-3 prototype array performed vector-matrix multiplication (the fundamental operation of neural networks) on skin, and simulated classification of ECG signals into five cardiac categories maintained approximately 90% accuracy even when the hardware was physically deformed during inference. This is the first demonstration that neuromorphic computation and mechanical stretchability can coexist without performance compromise — a prerequisite for truly wearable brain-machine interfaces.

    Why Organic polymers? The Case for Wetware

    There are so many advantages of organic neuromorphic electronics over silicon alternatives:

    Biocompatibility. OECTs operate in aqueous electrolytes at sub-volt potentials, conditions compatible with living cells and tissues. Silicon neuromorphic chips require high voltages and rigid encapsulation that damages biological interfaces. Organic devices can directly interface with neurons, epithelial cells, and biological fluids. A PEDOT:PSS OECT has been used to record dopamine release from PC-12 cells in real time, with the neurotransmitter itself modulating the synaptic weight — a true biohybrid synapse10.

    Multimodal sensing. Because the gate of an OECT can be functionalized with enzymes, antibodies, aptamers, or chemically sensitive materials, the same device can respond to electrical, chemical, mechanical, and optical inputs. A single OECT has been shown to simultaneously process pressure, light, and neurotransmitter signals, performing multisensory integration in hardware — something that requires complex multi-chip architectures in silicon.

    Energy efficiency. OECT synapses operate at femtojoule to picojoule energy per event, comparable to biological synapses.

    Fabrication. Organic semiconductors can be printed using inkjet, screen printing, or aerosol jet deposition on flexible plastic substrates at room temperature. No cleanroom required. No vacuum deposition. No billion-dollar fab. This matters enormously for the envisioned applications: disposable biosensors, on-skin health monitors, and large-area flexible electronics.

    The hard problems to solve that fascinates me

    Despite the rapid progress, organic neuromorphic electronics face real challenges.

    Speed. The ionic transport that gives OECTs their neuromorphic properties is inherently slower than electronic switching. The fastest OECT synapses operate at around 200 nanoseconds; the fastest neurons reach approximately 500 Hz. This is sufficient for biological interfaces but too slow for the megahertz-and-above clock rates needed for general computing. Organic neuromorphic hardware is not going to replace GPUs and may be they need not. There are so many amazing niche applications where speed requirements align with biological timescales.

    Stability. Organic materials degrade. PEDOT:PSS is sensitive to humidity. Many OMIECs swell excessively in aqueous environments. Long-term drift in conductance states undermines the reliability of analog memory. Cross-linking, encapsulation, and materials engineering (hydrophobic side chains, ladder polymers) are improving stability, but shelf lives of months, not decades, remain the norm. is that bad? Well, not if they are cheap to manufacture and replace.

    Scale. The largest demonstrated organic neuromorphic arrays are still small. Achieving the thousands or millions of devices needed for practical neural networks requires advances in high-resolution patterning, device-to-device uniformity, and interconnect engineering. The inkjet printing approach is promising for this reason.

    Framework. The neuromorphic devices being built don’t map cleanly onto existing machine learning frameworks. The non-linearity and asymmetry of analog weight updates, the stochastic variability between devices, and the different timescales of volatile and non-volatile states all require rethinking algorithms from the ground up.

    So where are we heading?

    In the near-term future we don’t expect polymers to replace silicon. However, there is potentially a more transformative future of intelligent interfaces between the digital and biological worlds!

    Imagine a patch on your skin that continuously monitors your ECG, classifies arrhythmias in real time using on-device neuromorphic inference, and communicates only abnormalities — consuming nanowatts, conforming to your body’s movements, never needing to stream raw data to the cloud. Imagine it tells you about your stress level, biomarkers for cardiovascular health, all unobtrusively.

    Imagine neural implants that record brain activity and process it locally — filtering noise, detecting seizure precursors, triggering responsive neurostimulation — all using devices that speak the same ionic language as the neurons they interface with!

    Imagine artificial skin for prosthetic limbs that senses pressure, texture, and temperature, converts these stimuli into frequency-coded spike trains, and transmits them to peripheral nerves in a format the nervous system can natively interpret.

    These applications don’t require faster-than-silicon speed. They require biocompatibility, energy efficiency, mechanical flexibility, and the ability to process sensory information the way biology does — all areas where organic neuromorphic electronics have a structural advantage.

    The brain didn’t evolve to maximize clock speed. It evolved to survive in a noisy, unpredictable, energy-constrained environment by integrating sensing, processing, and memory into a single adaptive substrate. For the first time, we’re building electronic systems on the same design principles — using materials that bend, stretch, and operate in the wet, salty, ion-rich environment of the living body.

    The future of polymer electronics isn’t about replacing silicon. It is perhaps about performing wonders silicon can’t.

    I lay still for a while, picking up the scattered garments of my mind and trying to assemble some kind of reasonable outfit from them.

    ~Altered Carbon

    References

    1. Li, Qifan, et al. A water-processable n-type polymeric ink with conductivities exceeding 1,000 S cm-1, Matter (2026). ↩︎
    2. Xiang, K., Song, J., Liu, H., Chen, J. & Yan, F. Organic Electrochemical Transistors for Neuromorphic Devices and Applications, Adv. Mater. 38, e15532 (2026). ↩︎
    3. Gerasimov, Jennifer Y., et al. An evolvable organic electrochemical transistor for neuromorphic applications. Advanced Science 6.7 (2019): 1801339. ↩︎
    4. Van De Burgt, Yoeri, et al. A non-volatile organic electrochemical device as a low-voltage artificial synapse for neuromorphic computing. Nature materials 16.4 (2017): 414-418. ↩︎
    5. Wang, Shijie, et al. An organic electrochemical transistor for multi-modal sensing, memory and processing. Nature Electronics 6.4 (2023): 281-291. ↩︎
    6. Yao, Y., Pankow, R. M., Huang, W. et al. An organic electrochemical neuron for a neuromorphic perception system, PNAS, 122, e2414879122 (2025). ↩︎
    7. Harikesh, Padinhare Cholakkal, et al. Ion-tunable antiambipolarity in mixed ion–electron conducting polymers enables biorealistic organic electrochemical neurons. Nature materials 22.2 (2023): 242-248. ↩︎
    8. Li, M., Zhang, W. et al. Regionally controlled ion-doping of organic electrochemical transistors for computing-memory co-integrated neuromorphic systems. NPJ Flex. Electronics,10, 11 (2026). ↩︎
    9. Dai, S. et al. Intrinsically stretchable neuromorphic devices for on-body processing of health data with artificial intelligence, Matter, 5, 3375-3390 (2022). ↩︎
    10. Keene, Scott T., et al. A biohybrid synapse with neurotransmitter-mediated plasticity. Nature Materials 19.9 (2020): 969-973. ↩︎

  • What if we could predict real-world properties of polymeric molecules from their chemical sequences alone?

    What if we could predict real-world properties of polymeric molecules from their chemical sequences alone?

    Polymeric molecules are all around is and in us. It is hardly surprising that a large fraction of life’s molecules carrying information are polymeric, from DNA, RNA to proteins, lipids and peptides.

    During my PhD I fell in love with polymers. (I had started my Phd work in Quantum Information but would quickly switched to soft-matter physics). I worked on Vulcanization Transition, a second-order phase transition in which a random melt of polymers, like natural rubber, can be chemically cross-linked to form random solids. I later became fascinated by gels, glassy solids, and the deep connections of their physics to percolation theory, random-resistor networks and jamming transition.

    Over the years, I met another fascinating polymer called oligonucleotide: bits of RNA, double-stranded or single stranded (shRNA & siRNA) and eventually bits of DNA (Anti-Sense Oligonucleotides, or ASOs). Oligonucleotides are informational drugs. They carry the genetic information they are destined to modulate.

    We all witness the impact of another such informational medicine during Covid-19, the synthetic mRNA polymer creating the right fragment of a protein to vaccinate. If you think about it, 3 of the 4 medicine modalities are polymers: peptides, antibodies, nucleic acids. Small molecules are the only exception; they carry nebulous information lacking focus and interact with almost everything.

    Yet, we understand so much and so little about polymers! When I cofounded Creyon, my dream was to engineer one kind of polymer really well: oligonucleotides. These are bits of nucleic acids that are chemically modified to make them drug-like (functionalization), that can be sent to a cell or tissue and precisely control gene expression! (Isn’t it marvelous that A, C, G, T code, a quad, instead of a bit, could do that? It could manipulate the very information in genes that I need to even see this screen?) These functionalizations—chemical modifications of the base, linker of sugar unit of the nucleic acid— could fundamentally change their biological, physical, biochemical properties. They could make these polymers more or less viscous, soluble, serum stable, immunotoxic, bioavailable; sometimes modulating pharmacology measures across four orders of magnitude by a single modification on the same base sequence! We were engineering these molecules & manipulating the information in the informational drug across several axis. We learned how to make the information allele-selective, well-tolerated, have higher affinity, have higher on-rate or activity, and so on.

    Lately, I have expanded the scope of that lifelong dream of controlling information flow. The scope is not just human biology and disease, but what more can sequences do and how well can we create sequences? Obviously, a lot of changed in the last 2 yrs! As society we have marveled at what AI can do when fed a large corpus of textual sequences. Who knew LLMs could get this good at writing not just text sequences but logical sequences of codes?

    Polymers are just chemical sequences.

    Well, one challenge is data. Where are all the data to learn the properties of molecules? Molecules inhabit a very special world. Unlike textual sequences where correlations are hard to quantify but easy to sense, correlations in molecules follow the laws of quantum physics, easy to validate and quantify, but hard to sense by intuition.

    The bad news is we need to create these physics-faithful datasets. But the good news is the correlations are nearsighted, as Walter Kohn called it Quantum Nearsightedness.

    We started dreaming that we should be able to predict physical properties, like conformations, free energy etc. purely from chemical sequence of polymers. As with any dream, you need good partners in crime! David Pekker Todd Martinez

    We tried to take the simplest first step.

    Can we predict the thermal ensemble of polymer conformations from their sequence alone?

    Well, we asked ourselves, what is a realistic system that will stress test this unreasonable dream? We tinkered with some internal data, but settled on a large dataset on MD trajectories of peptides that was freely available (mdCATH dataset). The trajectory sampling in this data is almost certainly not ergodic, but hey, beggers can’t be choosers, right? Do you have 1-5 Million GPU hours to spare? If it were fully ergodic, we would have gotten very close to computing free energy of peptides directly from sequence. Wild, right?

    What we discovered, once we figured out a few critical things in how to include the physics the right way into Diffusion Transformers, that we were able to predict the conformation ensemble as a function of temperature. We did some other work internally to convince ourselves we could do this for other systems too, like for concentration dependence.

    So why care?

    Turns out, properties of polymers are driven by their conformations and free energy. Ask any peptide chemist and she will tell you that controlling the degrees of freedom (by macrocyclization) is what with you do once you have a lead molecule to stare at and a glass of wine to place some educated chemical bets. Ask a nucleic acid chemist, and she will tell you that a blessed hairpin structure is the reason that an aptamer is a molecular beacon.

    But here is the inconvenient truth. Oligo-length (meaning ~10-100 monomer long) polymers (peptides included) are very often highly flexible, and it makes no sense to anchor your expectations of their properties on a single low-energy conformation. Larger proteins are probably a bit different; some of them are folded by chaperones, and it makes sense to use an AlphaFold/ESM/SimpleFold predicted single or closely related structure.

    So what next? Well, if we can predict physical properties from sequences, I think an analogy is worth entertaining:

    If LLMs understand text and we are increasingly fasciated by teaching LLMs Physical world (Newtonian) what does it take for a Molecular AI model like ours to understand the Quantum World of molecules? How much data? What kind of “sensors” are analogous to the Physical AI sensors and cameras?

    And most importantly, were is the limits of molecular engineering? Will you laugh at me if we dream about predicting viscosity? Conductivity? If we engineer the perfect conductive polymer using such generative tools? The perfect tissue-targeting molecule? The perfect precision medicine, ready to be printed?

    Read the paper and criticize. We are just getting warmed up! Send your comments!

  • Tensor Product Attention: Curiosities abound

    Tensor Product Attention: Curiosities abound

    A recent paper Tensor Product Attention Is All You Need1 grabbed my attention. Over the last year, I have been exploring and investigating ways to reinterpret attention mechanism, mainly for my own edification. What correlations do a transformer really capture? And unsurprisingly, I have been looking at using intuition from the physics of correlated systems.

    Firstly, attention mechanism is often written in a mathematically confusing and redundant way in the machine learning literature. The notation is often obfuscated by implementation quirks of matrix multiplications on GPUs. So let’s set up the notation, and simplify.

    In the notes below, I will ignore position encoding. RoPE or learnable additive position encodings do not change the foundational mathematical intuitions I am trying to convey here — it is a distraction.

    I use ℓ\ell for layer index and hh for head index.

    The key quantity is the residual stream, XℓX^\ell. This matrix is getting transformed by attention and MLP blocks. The embedding dimension dmodeld_\textrm{model} is the size of the vector space in which tokens are being embedded.

    We need a few other matrices to really explain what’s going on.

    Note that in ML/ AI papers the Query, Value and Key matrices are always written separately, but in essence, we are low-rank decomposing (as product of rectangular matrices) two matrices, 𝐖QKℓ,h,𝐖OVℓ,h\mathbf{W}_{QK}^{\ell,h} \, \, , \mathbf{W}_{OV}^{\ell,h}. This will be clear when we write attention is terms of these matrices — 

    Attnℓ(𝐱i)=∑h=1H∑j=1n[softmaxj(𝐱i⊤𝐖QKℓ,h𝐱jdhead)]𝐖OVℓ,h𝐱j\begin{aligned} \text{Attn}^{\ell}(\mathbf{x}_i) = \sum_{h=1}^{H} \sum_{j=1}^{n} \left[ \text{softmax}_j \left( \frac{\mathbf{x}_i^\top \mathbf{W}_{\text{QK}}^{\ell,h} \mathbf{x}_{j}}{\sqrt{d_{\text{head}}}} \right) \right] \mathbf{W}_{\text{OV}}^{\ell,h} \mathbf{x}_j \end{aligned}

    The attention operator Attnℓ\textrm{Attn}^\ell at layer ℓ\ell is a sum over individual attention heads, hh, with HH total heads. Note, here I choose to call the operator the net function that returns a vector of same size as 𝐱i\mathbf{x}_i — one can choose to add this back to the residual XℓX^\ell. Some architectures do so, others send it through the MLP operator. There are a lot of different transformer architectures out there in the various LLMs, and for the purpose of this discussion, it’s unimportant. Moreover, the papers have a bewildering range of definitions of what part of is called attention, which is why I bored you with setting up notation. You are welcome.

    Note that the number of heads and head dimensions are chosen such that we always have dmodel×dmodeld_{\text{model}} \times d_{\text{model}} matrices in the above expression.

    The only correlation between tokens explored in an transformer is pairwise. The MLP operator acts on the per-token embedding 𝐱i\mathbf{x}_i and do not mix 𝐱i\mathbf{x}_i and 𝐱j\mathbf{x}_j. In the Attention operator softmaxj\textrm{softmax}_j term is a normalized weight — and every other token embedding 𝐱j\mathbf{x}_j in the context window is getting summed over by this weight multiple by a linear transformation matrix. It is really quite simple.

    Well, one may wonder — why only pairwise correlations? And, why only the above functional form for pairwise correlations?

    A digression — for physicists like me, any time we see pairwise correlations, we think about Potts model, a generalization of the Ising Model which is perhaps better known. In the q-state Potts model the “spins” are unit vectors that point in q symmetric directions of a hypertetrahedron in q-1 dimensions, see here2. In the classical Potts model these vectors interact only if their “spins” (state) are the same.

    Can we draw an analogy with Potts Model? Yes, of course! Well, a paper3 already did a version of it—with a Potts Model where the interactions are not restricted to same “spins” but mix “spins”. It’s an enticing direction to study the dynamics of transformers using such mappings.

    OK, end of digression.

    The Memory Bottleneck in Modern Transformers

    Large language models face a critical scalability challenge: the Key-Value (KV) cache. During autoregressive generation, standard Multi-Head Attention (MHA) stores keys and values for all previously generated tokens, consuming memory that grows linearly with sequence length:

    MemoryMHA∼n×H×dhead\text{Memory}_{\text{MHA}} \sim n \times H \times d_\text{head}

    See table to to recall notation. For a model with H=32H = 32 and dhead=128d_\text{head} = 128 processing a n=105n = 10^5 token context, this amounts to over 800MB just for the KV cache of a single layer!

    The fundamental question is whether we must store the full H×dheadH \times d_\text{head}representation for each token, or whether a more compact factorized representation can capture the essential structure with minimal information loss.

    Tensor Decompositions: A Primer

    Before diving into Tensor Product Attention (TPA), we need to understand the landscape of tensor decomposition methods. A tensor is simply a multi-dimensional array—scalars are 0-order tensors, vectors are 1st-order, matrices are 2nd-order, and so on.

    CP Decomposition (CANDECOMP/PARAFAC)

    The most common Tensor Decomposition is probably the CP decomposition.

    Definition (CP Decomposition): A third-order tensor 𝒳∈ℝI×J×K\mathcal{X} \in \mathbb{R}^{I \times J \times K} has a rank-RR CP decomposition if it can be written as:

    𝒳=∑r=1R𝐚r∘𝐛r∘𝐜r \mathcal{X} = \sum_{r=1}^{R} \mathbf{a}_r \circ \mathbf{b}_r \circ \mathbf{c}_r where 𝐚r∈ℝI\mathbf{a}_r \in \mathbb{R}^I, 𝐛r∈ℝJ\mathbf{b}_r \in \mathbb{R}^J, 𝐜r∈ℝK\mathbf{c}_r \in \mathbb{R}^K and ∘\circ denotes the outer product.

    Element wise, Equivalently, for indices i,j,ki,j,k :

    𝒳ijk=∑r=1Rairbjrckr\mathcal{X}_{ijk} = \sum_{r=1}^{R} a_{ir} b_{jr} c_{kr}

    The CP decomposition represents a tensor as a sum of rank-1 tensors (outer products of vectors). This is the natural generalization of matrix SVD to higher orders, though unlike SVD, computing the optimal CP decomposition is NP-hard. Yeah, sucks, right?

    Tucker Decomposition

    Another popular tensor decomposition method is the Tucker Decomposition.

    Definition (Tucker Decomposition): A Tucker decomposition factorizes a tensor into a core tensor 𝒢∈ℝR1×R2×R3\mathcal{G} \in \mathbb{R}^{R_1 \times R_2 \times R_3} and factor matrices along each mode: 𝒳=𝒢×1𝐀×2𝐁×3𝐂 \mathcal{X} = \mathcal{G} \times_1 \mathbf{A} \times_2 \mathbf{B} \times_3 \mathbf{C} where 𝐀∈ℝI×R1\mathbf{A} \in \mathbb{R}^{I \times R_1}, 𝐁∈ℝJ×R2\mathbf{B} \in \mathbb{R}^{J \times R_2} , 𝐂∈ℝK×R3\mathbf{C} \in \mathbb{R}^{K \times R_3} and ×n\times_ndenotes the mode-nn product.

    More directly, the decomposition is — 

    𝒳pqr=∑iR1∑jR2∑kR3𝒢ijk𝐀pi𝐁qj𝐂rk\mathcal{X}_{p q r} = \sum_{i}^{R_1} \sum_{j}^{R_2} \sum_{k}^{R_3}\mathcal{G}_{i j k}\, \mathbf{A}_{pi} \,\mathbf{B}_{qj} \mathbf{C}_{rk}

    The Tucker decomposition generalizes CP by allowing a dense core tensor. Note that the the sizes R1,R2,R3R_1, R_2, R_3 is obviously within the sizes I,J,KI, J, K of the tensor dimensions— a common choice is R1=R2=R3=min(I,J,K)R_1 = R_2 = R_3 = \text{min} ( I, J, K) . When tensor 𝒢\mathcal{G} is super-diagonal (non-zero only when all indices are equal), Tucker reduces to CP.

    Tensor Train Decomposition

    The tensor decomposition most familiar to physicists is probably the tensor train decomposition.

    Definition (Tensor Train): A tensor train (TT) or Matrix Product State (MPS) represents a dd-dimensional tensor as a product of matrices —

    𝒳i1,i2,…,id=𝐆i1[1]𝐆i2[2]⋯𝐆id[d]\mathcal{X}_{i_1, i_2, \ldots, i_d} = \mathbf{G}^{[1]}_{i_1} \mathbf{G}^{[2]}_{i_2} \cdots \mathbf{G}^{[d]}_{i_d}

    where 𝐆ik[k]∈ℝrk−1×rk\mathbf{G}^{[k]}_{i_k} \in \mathbb{R}^{r_{k-1} \times r_k} with r0=rd=1r_0 = r_d = 1. The parameters {r1,…,rk,…,rd−1}\{r_1, \ldots, r_k, \ldots, r_{d-1}\}are called bond dimensions or TT-ranks.

    This is the same structure used to represent quantum many-body states in physics.

    Tensor Product Attention: The Core Claim

    Now we arrive at the key contribution of the TPA paper. Instead of storing full query, key, and value matrices, TPA represents them using contextual low-rank factorizations.

    Standard Multi-head Attention

    For token ii with embedding 𝐱i\mathbf{x}_i, layer ℓ\ell and head h∈{1,…,H}h \in \{ 1, \dots, H \}

    𝐪iℓ,h=𝐖Qℓ,h𝐱i∈ℝdhead𝐤iℓ,h=𝐖Kℓ,h𝐱i∈ℝdhead𝐯iℓ,h=𝐖Vℓ,h𝐱t∈ℝdhead\begin{align} \mathbf{q}_i^{\ell,h} = \mathbf{W}_Q^{\ell,h} \mathbf{x}_i \in \mathbb{R}^{d_{\text{head}}} \\ \mathbf{k}_i^{\ell,h} = \mathbf{W}_K^{\ell,h} \mathbf{x}_i \in \mathbb{R}^{d_{\text{head}}} \\ \mathbf{v}_i^{\ell,h} = \mathbf{W}_V^{\ell,h} \mathbf{x}_t \in \mathbb{R}^{d_{\text{head}}} \end{align}

    We can stack all the heads into matrices, note that now the matrices are not just weights, but weights multiplied by embeddings—

    𝐐i=[𝐪i1𝐪i2⋮𝐪iH]∈ℝH×dhead\begin{equation} \mathbf{Q}_i = \begin{bmatrix} \mathbf{q}_i^1 \\ \mathbf{q}_i^2 \\ \vdots \\ \mathbf{q}_i^H \end{bmatrix} \in \mathbb{R}^{H \times d_{\text{head}}} \end{equation}

    TPA

    TPA factorizes the stacked query/key/value matrices as rank-RR sums of outer products.

    𝐐i=1RQ∑r=1RQ𝐚rQ(𝐱i)⊗𝐛rQ(𝐱i)∈ℝH×dhead\begin{equation} \mathbf{Q}_i = \frac{1}{R_Q} \sum_{r=1}^{R_Q} \mathbf{a}^Q_r(\mathbf{x}_i) \otimes \mathbf{b}^Q_r(\mathbf{x}_i) \in \mathbb{R}^{H \times d_{\text{head}}} \end{equation}

    Note that the dimensions work out, for clarity — 

    𝐱i∈ℝdmodel(input)𝐖ra,Q𝐱i=𝐚rQ(𝐱t)∈ℝH(head factor)𝐖rb,Q𝐱i=𝐛rQ(𝐱t)∈ℝdhead(feature factor)𝐚rQ⊗𝐛rQ=ℝH×dhead(outer product)1RQ∑r=1RQ𝐚rQ⊗𝐛rQ=𝐐i∈ℝH×dhead✓\begin{align} \mathbf{x}_i \in \mathbb{R}^{d_{\text{model}}} \quad \text{(input)} \\ \mathbf{W}^{a,Q}_r \mathbf{x}_i = \mathbf{a}^Q_r(\mathbf{x}_t) \in \mathbb{R}^{H} \quad \text{(head factor)} \\ \mathbf{W}^{b,Q}_r \mathbf{x}_i = \mathbf{b}^Q_r(\mathbf{x}_t) \in \mathbb{R}^{d_{\text{head}}} \quad \text{(feature factor)} \\ \mathbf{a}^Q_r \otimes \mathbf{b}^Q_r = \mathbb{R}^{H \times d_{\text{head}}} \quad \text{(outer product)} \\ \frac{1}{R_Q}\sum_{r=1}^{R_Q} \mathbf{a}^Q_r \otimes \mathbf{b}^Q_r \, = \mathbf{Q}_i \in \mathbb{R}^{H \times d_{\text{head}}} \quad \checkmark \end{align}

    So for standard MHA, each head independently projects the input—

    𝐪ih=𝐖Qh𝐱i\begin{equation} \mathbf{q}_i^h = \mathbf{W}_Q^h \mathbf{x}_i \end{equation}

    whereas for TPA, all heads share RQR_Q feature vectors, weighted differently per head,

    𝐪ih=1RQ∑r=1RQ[𝐚rQ(𝐱i)]h⏟head-specific weight⋅𝐛rQ(𝐱i)⏟shared feature vector\begin{equation} \mathbf{q}_i^h = \frac{1}{R_Q} \sum_{r=1}^{R_Q} \underbrace{[\mathbf{a}^Q_r(\mathbf{x}_i)]_h}_{\text{head-specific weight}} \cdot \underbrace{\mathbf{b}^Q_r(\mathbf{x}_i)}_{\text{shared feature vector}} \end{equation}

    The Key Idea: Instead of H independent dheadd_\text{head} -dimensional vectors (one per head), TPA uses— 

    • RQR_Q shared feature vectors 𝐛rQ∈ℝdhead\mathbf{b}^Q_r \in \mathbb{R}^{d_{\text{head}}}
    • RQR_Q weight vectors 𝐚rQ∈ℝH\mathbf{a}^Q_r \in \mathbb{R}^H— one scalar per head, determining how much each head uses each feature

    where RQ≪HR_Q \ll H, therefore leading to parameter efficiency. Obviously, we have similar things going on for 𝐊i\mathbf{K}_i and 𝐕i\mathbf{V}_i.

    Parameter counts

    For MHA, we total number of parameters for queries only (similar for Keys and Values) are H×dhead×dmodel=dmodel2H \times d_\text{head} \times d_\text{model} = d^2_\text{model}

    For TPA we have— 

    • Head factors: RQR_Q matrices of size H×dmodelH \times d_\text{model}
    • Feature factors: RQR_Q matrices of size dhead×dmodeld_\text{head} \times d_\text{model}
    • Total parameters— RQ(H+dhead)dmodelR_Q (H + d_\text{head} ) d_\text{model}

    Example with typical paper values: H=32H=32, dhead=128d_{\text{head}}=128, dmodel=4096d_{\text{model}}=4096, RQ=6\boxed{R_Q=6}:

    • MHA: 32×128×4096=16,777,21632 \times 128 \times 4096 = 16{,}777{,}216 parameters
    • TPA: 6×4096×(32+128)=3,932,1606 \times 4096 \times (32 + 128) = 3{,}932{,}160 parameters
    • TPA uses ~23% of MHA’s parameters

    Note: Unlike LoRA which factorizes weights, TPA factorizes activations. This means the factorization is contextual—it depends on the input token 𝐱i\mathbf{x}_i. It’s a very interesting idea in how to capture input-dependent structure while maintaining compression!

    Memory Reduction

    The major advantage claimed by the paper is the memory saving in KV cache. My interest in this paper is beyond this, to study other forms of attention, but it’s useful to note the memory arguments.

    From standard MHA we have— 

    • Store 𝐊i∈ℝH×dhead\mathbf{K}_i \in \mathbb{R}^{H \times d_{\text{head}}} and 𝐕i∈ℝH×dhead\mathbf{V}_i \in \mathbb{R}^{H \times d_{\text{head}}}
    • Total: 2×H×dhead=2dmodel2 \times H \times d_{\text{head}} = 2d_{\text{model}}

    TPA stores only the factors— 

    • Store {𝐚rK(𝐱i)}r=1RK\{\mathbf{a}^K_r(\mathbf{x}_i)\}_{r=1}^{R_K} and {𝐛rK(𝐱i)}r=1RK\{\mathbf{b}^K_r(\mathbf{x}_i)\}_{r=1}^{R_K}for keys
    • Store {𝐚rV(𝐱i)}r=1RV\{\mathbf{a}^V_r(\mathbf{x}_i)\}_{r=1}^{R_V} and {𝐛rV(𝐱i)}r=1RV\{\mathbf{b}^V_r(\mathbf{x}_i)\}_{r=1}^{R_V}for values
    • Total: (RK+RV)(H+dhead)(R_K + R_V)(H + d_{\text{head}})

    The compression ratio is

    ρ=(RK+RV)(H+dhead)2Hdhead\rho = \frac{(R_K + R_V)(H + d_{\text{head}})}{2H \, d_{\text{head}}}

    Concrete example: H=32,dhead=128,RK=RV=1H = 32, d_{\text{head}} = 128, R_K = R_V = 1:

    • TPA cache =2×(32+128)=320= 2 \times (32 + 128) = 320 values per token
    • MHA cache =2×32×128=8192= 2 \times 32 \times 128 = 8192 values per token

    so TPA leads to 96%96 \% memory reduction! For context window of 100,000 tokens, MHA needs 1.6 GB of memory wheres TPA needs 64 MB of memory! (both per layer)

    Connection to MPS

    Another way to look at TPA is recasting it as a MPS. Per head, instead of the term 𝐱i𝐖QKℓ,h𝐱j\mathbf{x}_{i}\mathbf{W}_{\text{QK}}^{\ell,h} \mathbf{x}_{j} in MHA, for TPA we have

    (𝐪ih)⊤⋅𝐤jh=(1RQ∑r=1RQ[𝐚rQ]h⋅𝐛rQ)⊤⋅(1RK∑s=1RK[𝐚sK]h⋅𝐛sK)=1RQRK∑r=1RQ∑s=1RK([𝐚rQ]h⋅𝐛rQ)⊤⋅([𝐚sK]h⋅𝐛sK)=∑r=1RQ∑s=1RK[𝐚rQ(𝐱i)]h⋅[𝐚sK(𝐱j)]h⏟head-space mixing⋅(𝐛rQ(𝐱i))⊤⋅𝐛sK(𝐱j)⏟feature-space contraction\begin{align} (\mathbf{q}_i^h)^\top \cdot \mathbf{k}_j^h = \left(\frac{1}{R_Q} \sum_{r=1}^{R_Q} [\mathbf{a}^Q_r]_h \cdot \mathbf{b}^Q_r\right)^\top \cdot \left(\frac{1}{R_K} \sum_{s=1}^{R_K} [\mathbf{a}^K_s]_h \cdot \mathbf{b}^K_s\right) \\ = \frac{1}{R_Q R_K} \sum_{r=1}^{R_Q} \sum_{s=1}^{R_K} ([\mathbf{a}^Q_r]_h \cdot \mathbf{b}^Q_r)^\top \cdot ([\mathbf{a}^K_s]_h \cdot \mathbf{b}^K_s) \\ =\sum_{r=1}^{R_Q} \sum_{s=1}^{R_K} \underbrace{[\mathbf{a}^Q_r(\mathbf{x}_i)]_h \cdot [\mathbf{a}^K_s(\mathbf{x}_j)]_h}_{\text{head-space mixing}} \cdot \underbrace{(\mathbf{b}^Q_r(\mathbf{x}_i))^\top \cdot \mathbf{b}^K_s(\mathbf{x}_j)}_{\text{feature-space contraction}} \end{align}

    We now we are getting somewhere, right? That’s a very different take on the attention matrix capturing token-token correlations!

    • Rank indices (r,s)(r,s) play the role of bond indices in MPS
    • ∑r=1RQ∑s=1RK\sum_{r=1}^{R_Q} \sum_{s=1}^{R_K}is the bond cotraction
    • Low ranks RQ,RKR_Q, R_K is equivalent to low bond dimension and increased efficiency and high bond dimension leads to more expressiveness

    Copy Tensor

    We can look at the above expression in terms of copy tensors in Tensor Networks. A copy tensor4 allows for reusing information. For a vector 𝐚∈ℝd\mathbf{a} \in \mathbb{R}^d, the copy operation is represented by a diagonal tensor, 𝒞ij=δij\mathcal{C}_{ij} = \delta_{ij} , the Kronecker delta. In other words, a copy tensor allows a single input to be reused in multiple tensor contractions.

    Note what’s happening in TPA! The same input vector 𝐱i\mathbf{x}_i is used 2RQ2 R_Q times for Query, and so on for Key and Value — 

    𝐱i→𝐖1a,Q𝐚1Q(𝐱i)∈ℝH𝐱i→𝐖1b,Q𝐛1Q(𝐱i)∈ℝdhead⋮𝐱i→𝐖RQa,Q𝐚RQQ(𝐱i)∈ℝH𝐱i→𝐖RQb,Q𝐛RQQ(𝐱i)∈ℝdhead\begin{align} \mathbf{x}_i \xrightarrow{\mathbf{W}^{a,Q}_1} \mathbf{a}^Q_1(\mathbf{x}_i) \in \mathbb{R}^H \\ \mathbf{x}_i \xrightarrow{\mathbf{W}^{b,Q}_1} \mathbf{b}^Q_1(\mathbf{x}_i) \in \mathbb{R}^{d_{\text{head}}} \\ \vdots \\ \mathbf{x}_i \xrightarrow{\mathbf{W}^{a,Q}_{R_Q}} \mathbf{a}^Q_{R_Q}(\mathbf{x}_i) \in \mathbb{R}^H \\ \mathbf{x}_i \xrightarrow{\mathbf{W}^{b,Q}_{R_Q}} \mathbf{b}^Q_{R_Q}(\mathbf{x}_i) \in \mathbb{R}^{d_{\text{head}}} \end{align}

    Instead of computing H independent projections (standard MHA), TPA computes 2RQ2 R_Q projections and cleverly recombines them. When RQ≪HR_Q \ll H, this architecture is much more efficient while maintaining expressiveness of a Tensor Network (outer product).

    Few other things…

    • The paper shows that TPA is compatible with RoPE embedding. RoPE only acts on the 𝐛\mathbf{b} vectors. The keys are pre-rotated and stored, so no rotation is needed during decoding. Only the current query needs to be rotated. Neat!
    • Remarkably, standard attention mechanisms are non-contextual variants of TPA! They show that both GQA (Grouped Query Attention) and MQA (Multi-Query Attention) are simply poor man’s version of TPA with 𝐚\mathbf{a} being independent of 𝐱i\mathbf{x}_i !

    I loved the paper. The key lessons:

    1. Structure matters: Exploiting low-rank structure in attention patterns enables massive compression
    2. Contextual factorization: Factorizing activations (not weights) is a very interesting concept
    3. Model performance and memory needs: As with several other work recently, the belief that larger context window either means larger models, or we need to compromise on expressivity of the correlations captured in attention, may be incorrect

    As we push toward longer contexts and larger models, principled compression techniques like TPA is a fruitful area of research. The tensor network perspective suggests we’ve only begun to explore the space of possible architectures!

    References

    1. Zhang, Yifan, et al. “Tensor product attention is all you need.” arXiv preprint arXiv:2501.06425 (2025). ↩︎
    2. Wu, Fa-Yueh. “The Potts Model.” Reviews of modern physics 54.1 (1982): 235. ↩︎
    3. Rende, Riccardo, et al. “Mapping of attention mechanisms to a generalized Potts Model.” Physical Review Research 6.2 (2024): 023057. ↩︎
    4. Glasser, Ivan, Nicola Pancotti, and J. Ignacio Cirac. “From probabilistic graphical models to generalized tensor networks for supervised learning.” IEEE Access 8 (2020): 68169-68182. ↩︎

  • Carrier-free mRNA delivery with Aptamers: Nucleic acid is all you need

    Carrier-free mRNA delivery with Aptamers: Nucleic acid is all you need

    Folks who have been dreaming happily like I have over the past decade that nucleic acids are the right substrate for engineering medicines, well, here is one more evidence that we might just be right with our obsession with this marvelous polymer of life!

    Here is how I evangelized my obsession amongst colleagues.

    Silicon Valley is silicon valley and not germanium valley — germanium just wasn’t the right substrate though the first transistor was made of germanium after all, see here for the first paper and here for a lovely history of the transistor.

    Aren’t you glad? — Germanium Valley just doesn’t quite have the right euphony, does it?

    Nucleic acids are the right substrate for genetic and gene-centric medicines and I don’t think either small molecules or proteins are. Those are the germanium of genetic medicines — they may work but the sooner you use silicon the sooner we will solve all human diseases. Yeah, I am opinionated!

    Circularized RNA + cell-type targeting aptamer

    A fascinating paper quietly appeared on BioRxiv1 about a month or so back. It’s a collaboration amongst multiple groups in China, with Weihong Tan as the PI.

    They report first-in-human testing of very curious idea I had toyed with for a while now as a high-risk high-reward R&D project. They created aptamer-embedded circular RNAs (Apt-circRNAs). What’s wild is that they tested the concept in Phase 1 human trial right away from what would otherwise still be a marvelous proof-of-concept tested in ex vivo (blood) setting or in in vivo (humanized rodent) studies.

    They got human clinical data. Wow!

    The study combines two distinct and established ideas in nucleic acids—

    • Circularizing of synthetic mRNA to enhance stability (the payload)
    • Use of aptamers as a targeting molecule for cell-type specific delivery

    This a totally crazy pace of testing out platform ideas. For those of you who do not work in the field — why is this significant?

    Current mRNA vaccines like those for COVID-19 rely on LNPs for delivery, which can sometimes cause immunogenicity and predominantly accumulates in the liver. The Apt-circRNA platform is clever: the RNA molecule itself contains targeting information (receptor-targeting aptamers) to achieve cell-type-specific delivery, eliminating the need for synthetic carriers like LNPs to gift wrap the RNA.

    The Three-Module Design

    The Apt-circRNA platform elegantly integrates three functional modules into a single RNA molecule—

    Targeting Module

    The authors embedded dendritic cell (DC)-specific aptamers at precise locations within the circular RNA scaffold. They tested three targeting aptamers—nucleolin (nuc), transferrin receptor (waz), and DEC-205 (also called CD-205) (min2).

    • Recall that DEC-205 (also called CD-205) is a cell surface-receptor (and endocytic receptor) highly expressed in immature Dendritic Cells (DCs).
    • Transferrin receptor (TfR) is highly expressed in mature DCs and is crucial for iron uptake
    • Nucleolin is also a cell surface receptor in endothelian cells and DCs. It can internalize from cell-surface to the nucleus

    They used the Waz aptamer sequence for TfR—Waz was created by a Matt Levy whom I had hired at Creyon Bio, and who has lead the aptamer team and created a diversity of cell-type specific aptamers since. We know this aptamer well!234.

    Waz aptamer sequence from Ref. 3

    The Waz aptamer has chemical modifications as far as I remember—2’F modified C/Us and probably 2’OMe for some positions. The study uses native RNA—so there are no chemical modifications on the aptamer sequence. It’s an important distinction to keep in mind.

    The DEC-205 aptamer min2 was also discovered my Matt’s group.5 The sequence from Fig.1 of Ref. 4 is —

    Min2 aptamer Ref 4

    The waz aptamer showed superior binding to both murine and human DCs. Through some optimization, the study determined that a bispecific combination of 5 waz and 4 min2 aptamers yielded optimal antigen presentation. Intersting! Thats a lot of aptamers decorating the RNA!

    Stable expression framework

    The circular RNA architecture provides inherent nuclease resistance by eliminating free 5’/3′ termini. The study demsntrates that the Apt-circRNA maintains structural integrity for over 24 hours and dramatically outperforming N1-methylpseudouridine-modified linear mRNA (m1Ψ-mRNA). This confirms older work that circularization of mRNA helps in extending half-life67. The construct also remains stable across pH 4.0-8.0 which critical for endosomal trafficking.

    Antigen-Encoding Region

    An Internal Ribosome Entry Site (IRES) enables cap-independent translation, while codon-optimized sequences encode tumor-specific antigens. The modular design permits flexible incorporation of diverse antigens, demonstrated with ovalbumin peptides ranging from 8 to 386 amino acids.

    The Clever Circularization Strategy

    Here’s where the molecular engineering gets quite ingenious! The team adapted permuted intron-exon (PIE) ribozyme systems from two sources: Anabaena pre-tRNA and T4 bacteriophage td intron. The key innovation: they engineered the aptamer’s stem-loop structure to serve as the circularization site without mutating the aptamer sequence itself!

    The process works by introducing a cleavage site within the aptamer’s loop region, then engineering the group I intron’s P1 and P10 guide sequences to complement sequences flanking the aptamer cleavage site. This enables precise, ribozyme-catalyzed splicing at the predefined loop site, generating Apt-circRNA products free of residual intron sequences.

    Cute!

    What’s the bio-distribution of Apt-circRNA?

    PET Imaging reveals precise lymph node targeting!

    They used radio-labeled Apt-circRNA and positron emission tomography (PET) to track spatial and temporal distribution.

    PET imaging showed predominant renal accumulation with no notable off-target accumulation in liver, spleen, heart, or other major organs. This specificity is striking and addresses a major concern with LNP-based systems, which accumulate significantly in liver and spleen. The renal accumulation is perhaps owing to renal clearance of such a relatively smaller molecular weight payload?

    They also ran Cy5-labelled study—6 hours post-injection revealed that cy5-labelled Apt-circRNA preferentially accumulated in dendritic cells compared to B cells and macrophages. Apt-circRNA was efficiently internalized by DCs at the injection site!

    This contrasts sharply with LNP-circRNA, which primarily remained as intact nanoparticles near the injection site before uptake by both B cells and DCs within lymph nodes. This is consistent with the expectation that aptamer-mediated recognition enables direct DC internalization and lymph node trafficking.

    The study also looked at immuno-stimulatory responses. Worth a read.

    Surprisingly, Luminex assays revealed that Apt-circRNA elicited lower systemic levels of reactogenicity-associated chemokines and cytokines than LNP-circRNA except IL-12. Apt-circRNA also demonstrated reduced cytotoxicity in BMDCs (murine bone-marrow derived DCs) compared to LNP-circRNA. I would have expected the opposite—recall that native RNA could invoke innate response from pathways that sniff out cytosolic RNA.

    First-in-Human Clinical Trial

    The authors initiated a Phase I clinical trial at Zhejiang Xiaoshan Hospital with remarkable speed, testing Apt-circRNA-KR2 in healthy volunteers. Here KR2 refers to the RNA payload expressing the mutations (G12D and G12V) most common in KRAS gene. They want to elicit a T-cell response in KRAS-mutant cancer.

    G12D and G12V are two of the most common single amino acid substitutions at codon 12 of the KRAS gene. The G12D indicates a substitution of the normal amino acid Glycine (G) with Aspartic acid (D) at position 12, while G12V indicates a substitution with Valine (V). 

    The trial enrolled 12 healthy volunteers total. Though a small cohort, it’s very encouraging!

    • Single-dose escalation cohort: 9 volunteers received 50, 100, or 250 μg doses (n=3/group)
    • Multi-dose cohort: 3 HLA-A*02:01 or HLA-A*11:01-positive volunteers received 250 μg on days 1, 7, and 13
    • Only 1 of 12 participants experienced any adverse event—transient flu-like symptoms resolving within 12 hours
    • Zero injection-site reactions (0/12)
    • Zero grade ≥2 adverse events (0/12)
    • All hematologic parameters, immune cell subsets, and cytokines remained within normal ranges through 180 days

    The safety profile is quite surprising and compares very favorably to LNP-mRNA vaccines, which commonly cause injection-site reactions, fever and systemic symptoms.

    High notes

    Unlike previous ‘naked mRNA’ approaches that lack stability and targeting, it’s a clever idea to encode aptamer within the RNA sequence itself. However, I am not convinced this is necessary and it will prohibit chemical modifications that can stabilize the aptamer, increase affinity and half-life. One could conjugate the aptamer to the circular RNA and modularize the system further.

    The dual aptamer strategy is definitely very interesting. The combination of waz (TfR-targeting) and min2 (DEC-205-targeting) creates a bispecific design that enhances both binding affinity and functional outcomes. Are these serving different purposes in directly the payload to endocytic compartments?

    Manufacturing could be quite scalable, as is! The in vitro transcription and ribozyme-mediated circularization can be performed at scale without the complex formulation processes required for LNPs. The >80% circularization efficiency is commercially viable. Also, perhaps this advantage is a counter-point to the first point I made about chemically-modified aptamers, for which the aptamer would have to separately synthesized and somehow conjugated to the RNA at specific sites that does not disrupt expression. Moreover, with multiple aptamers to decorate the RNA, it’s messy. Efficiency and purity of product would be a challenge.

    Cellular uptake was still a bit low, and I suspect this is because the aptamers were not really optimized. They took existing aptamer sequences and slapped it on. Flow cytometry data showed that even in draining lymph nodes only a fraction of DCs take up Apt-circRNA. This may necessitate higher doses or more frequent administration compared to LNP-formulated mRNA, partially offsetting the manufacturing advantages. But I strongly believe this is a solvable engineering problem.

    Also, the naked RNA formulation and size leads to rapid renal clearance. One could incorporate sugar / lipid modifications to the construct but then the modules need to be separate—the circularized RNA and the aptamer (chemically modified and say, with added ligands like PEG attached, or albumin binding aptamers). Totally doable!

    The bigger picture

    For decades, the field has focused on engineering increasingly sophisticated nanocarriers—optimizing lipid chemistry, surface modifications etc. What is thought provoking here is, Why engineer a carrier when you can engineer the RNA itself?

    It’s a tantalizing possibility—Nucleic acid is all you need.

    References

    1. https://www.biorxiv.org/content/10.1101/2025.09.28.679023v1 ↩︎
    2. https://patents.google.com/patent/US9439973B2/en ↩︎
    3. https://www.cell.com/molecular-therapy-family/nucleic-acids/fulltext/S2162-2531(17)30047-1 ↩︎
    4. https://www.nature.com/articles/s41467-018-04691-x ↩︎
    5. https://www.cell.com/molecular-therapy-family/molecular-therapy/fulltext/S1525-0016(16)30728-6 ↩︎
    6. https://www.nature.com/articles/s41467-018-05096-6
      ↩︎
    7. https://www.nature.com/articles/s41587-022-01393-0
      ↩︎