RIPTACs Explained: How Regulated Induced Proximity Targeting Chimeras Are Redefining Precision Oncology
Executive Overview
Targeted protein degradation taught the field of drug discovery an important lesson: a small molecule doesn’t need to occupy an active site to be therapeutic. It can instead force two proteins into proximity and let the biological consequences of that proximity do the work. Proteolysis-targeting chimeras (PROTACs) proved this principle by recruiting E3 ubiquitin ligases to catalytically destroy disease-driving proteins, and have since matured into a validated clinical modality with degraders such as vepdegestrant reaching regulatory submission [1].
Regulated Induced Proximity Targeting Chimeras — RIPTACs — apply the same induced-proximity logic to a different problem. Rather than degrading a disease target, a RIPTAC uses a tumor-restricted protein as a tether to selectively disable a separate protein that is essential for cell survival, and it does so only inside the cells that express that tether [2]. The concept was formalized by a Yale-associated team working with Halda Therapeutics, and it moved unusually fast from mechanistic proof-of-concept to the clinic: the first RIPTAC, HLD-0915, entered a Phase 1/2 trial in metastatic castration-resistant prostate cancer within roughly two years of the founding publication [3]. Validation continued to compound from there — in November 2025, Johnson & Johnson agreed to acquire Halda Therapeutics for $3.05 billion in cash, described in the trade press as the largest deal yet paid for an induced-proximity platform [4].
That commercial validation has, in turn, exposed the field’s central open problem. Making the linker chemistry that connects two ligands is now a comparatively solved problem; finding two ligands, for a tumor-restricted target protein and an essential effector protein, that actually cooperate to form a functional ternary complex is not. This article walks through how RIPTACs work, how they compare mechanistically to PROTACs, molecular glues, and TCIPs, what the clinical and deal-making record shows so far, and why high-throughput, cell-based screening approaches — including DNA-encoded library (DEL) selection — are increasingly positioned as part of the answer to the field’s pairing bottleneck.
What Is a RIPTAC?
A RIPTAC is a heterobifunctional small molecule: structurally, two ligands joined by a chemical linker, much like a PROTAC. Functionally, though, it is built for a different job. One ligand binds a target protein (TP) — a protein whose expression is restricted to, or strongly enriched in, the diseased cell. In the founding published example, the target protein is the androgen receptor, a well-characterized driver of prostate cancer with expression heavily skewed toward prostate tissue [2]. The second ligand binds an effector protein (EP) — a protein that is essential for cell viability but, critically, is expressed broadly across both healthy and diseased tissue.
On its own, a ligand for a broadly expressed, essential protein would be a poor drug candidate: inhibiting something the cell cannot live without, everywhere it is expressed, is a recipe for toxicity rather than therapy. The RIPTAC’s design solves this by making the two ligands functionally interdependent. The linker is built so that the compound preferentially engages both proteins simultaneously, forcing a stable three-protein (“ternary”) complex between the RIPTAC, the TP, and the EP [2,5]. That complex — and the loss of EP function that follows from it — can only form efficiently in cells that also express the TP. In TP-negative healthy cells, the EP-binding ligand has no tether to anchor a productive complex, and normal EP activity is left largely undisturbed.
Halda’s own publications describe the resulting cellular phenotype as a “hold-and-kill” mechanism: the target protein effectively “holds” the RIPTAC in place long enough for it to disable the effector protein’s function, and that disabled function then drives cell death — selectively, and only in cells expressing the tether [2,3]. A commentary accompanying the founding mechanistic paper in Cell Chemical Biology framed this as “opening the therapeutic window” for essential-protein targets that would otherwise be undruggable on their own [5].
Figure 1. In a target-protein-positive tumor cell, the RIPTAC bridges TP and EP into a ternary complex that disables EP and kills the cell. In a target-protein-negative healthy cell, no productive complex forms, EP function is unaffected, and the cell survives.
RIPTACs vs. PROTACs, Molecular Glues, and TCIPs
RIPTACs sit within a broader and fast-growing family of induced-proximity modalities. Each family member forces two proteins together, but the purpose of that proximity, and the source of the resulting selectivity, differ substantially.
PROTACs recruit an E3 ubiquitin ligase to a disease target, tagging it for destruction via the ubiquitin-proteasome system. Because the ligase acts catalytically, a single PROTAC molecule can support degradation of many target protein copies, and selectivity is driven largely by target expression and “degradability” — how amenable the target’s conformation and cellular trafficking are to productive ternary complex formation with the ligase [1].
Molecular glues are typically monovalent or near-monovalent small molecules that stabilize a novel interface between two proteins that would not otherwise interact — classically an E3 ligase and a neosubstrate, as with the immunomodulatory drugs that redirect cereblon toward new targets. Because molecular glues often lack an obvious bifunctional architecture, their discovery has historically depended more on serendipity than the modular, linker-driven design used for PROTACs and RIPTACs, though systematic chemical-profiling approaches are now closing that gap [6].
TCIPs (transcriptional/chaperone-based inducers of proximity) redirect a disease-associated regulatory machine — for example, a transcription factor or chromatin reader bound to an oncogenic locus — toward an effector that converts that binding event into a therapeutic outcome, such as forced transcription of a pro-apoptotic gene. A 2023 study published in Nature demonstrated this by rewiring bromodomain-containing protein 4 (BRD4) at super-enhancers driving oncogene expression to instead drive expression of the pro-apoptotic gene BCL2L11, killing cancer cells that depend on that oncogenic transcriptional program [7].
RIPTACs, distinct from all three, are explicitly non-degradative. Nothing is destroyed; the essential effector protein is instead sequestered into an inactive ternary complex. Selectivity is not a function of target degradability or of a spontaneously novel interface — it is a function of differential expression of the target protein between diseased and healthy tissue [2]. That reframing matters commercially: it means an essential, otherwise “undruggable” housekeeping protein can become a viable effector, as long as a sufficiently tissue- or tumor-restricted target protein exists to pair it with.
| Modality | What the ligands bind | Mechanism | Basis for selectivity | Key reference |
|---|---|---|---|---|
| PROTAC | Disease target + E3 ligase | Catalytic degradation via ubiquitin-proteasome system | Target expression / degradability | Békés et al., Nat Rev Drug Discov 2022 [1] |
| Molecular glue | Novel protein–protein interface (often E3 ligase + neosubstrate) | Stabilizes a new interaction, often triggering degradation | Novelty/selectivity of the induced interface | Mayor-Ruiz et al., Nat Chem Biol 2020 [6] |
| TCIP | Disease-locus-bound factor + transcriptional/chaperone machinery | Redirects regulatory activity toward a therapeutic transcriptional outcome | Localization of the disease-associated anchor | Gourisankar et al., Nature 2023 [7] |
| RIPTAC | Tumor-restricted target protein + broadly expressed essential protein | Non-degradative, forces a ternary complex that disables the effector’s function | Differential expression of the target protein | Raina et al., Cell Chem Biol 2024 [2] |
From Proof-of-Mechanism to the Clinic
The founding RIPTAC publication, in Cell Chemical Biology, reported selective killing of prostate cancer cells in preclinical models — including models resistant to enzalutamide, a standard-of-care androgen receptor antagonist — while sparing androgen-receptor-negative cells [2]. That result was the proof-of-mechanism the field needed: it showed that a non-degradative, ternary-complex-dependent mechanism could achieve tumor selectivity even against a target (an essential transcriptional/proliferative regulator) that would never be druggable as a standalone inhibitor.
That preclinical work matured into HLD-0915, an oral RIPTAC pairing the androgen receptor with an essential effector protein, which entered a Phase 1/2 clinical trial in metastatic castration-resistant prostate cancer (mCRPC) — the first RIPTAC to reach human testing. A 2025 news feature in Nature Reviews Drug Discovery covering that milestone described the approach as extending induced-proximity pharmacology “beyond protein degraders,” toward tissue-selective target inhibition rather than destruction [3]. A companion perspective in the Journal of Medicinal Chemistry, published the same year, used HLD-0915 as the motivating example for a broader discussion of RIPTAC medicinal chemistry and design principles, underscoring how quickly the modality had moved from a single mechanistic paper to a recognized class worth reviewing in its own right [8].
The clearest signal of validation, though, came from the deal market rather than the clinic. In November 2025, Johnson & Johnson agreed to acquire Halda Therapeutics for $3.05 billion in cash — coverage in Nature Reviews Drug Discovery called it the biggest induced-proximity deal to date, for a company that, at the time, had a single asset in Phase 1/2 [4]. Halda’s president and CEO, Christian Schade, characterized the acquisition as “a tribute to the years of scientific effort to develop this novel, first-in-class modality” [9]. For a platform built on one clinical-stage molecule, that price tag is a strong market signal that large pharmaceutical companies view RIPTACs as a durable new pharmacological category rather than a one-off curiosity.
A Fast-Moving Competitive Field
Halda’s exit accelerated interest across the broader induced-proximity space. In August 2025, Halda and the AI drug-discovery company VantAI announced a computational discovery alliance potentially worth more than $1 billion in payments, explicitly aimed at using structure-prediction and generative modeling to identify new target–effector protein pairs beyond the androgen receptor axis that anchors HLD-0915 [10]. The scale of that alliance is itself informative: a company that has just closed a multi-billion-dollar acquisition is still investing heavily and immediately in better ways to find RIPTAC pairs, which tells you where the field believes the remaining risk sits.
Elsewhere, Flare Therapeutics — a company focused on drugging transcription factors — disclosed that it is advancing an androgen-receptor-directed RIPTAC program through preclinical development, positioned as a companion effort alongside its lead AR(ON) degrader program, backed by an $85 million insider-led Series C financing round closed in mid-2026 [11]. Multiple other oncology-focused biotech and pharma groups are reportedly building early induced-proximity pipelines, with initial efforts still concentrated on nuclear hormone receptors and other transcriptional targets, where tissue-restricted expression patterns are best characterized and easiest to exploit for a target protein.
The Real Bottleneck: Pairing, Not Chemistry
Once you look across these programs, a pattern emerges: nearly every disclosed RIPTAC candidate still pairs the androgen receptor with an effector protein, and the number of validated, functional target–effector pairs reported publicly remains small relative to the number of plausible candidate combinations that could, in principle, be tried. Linker chemistry connecting two known-good ligands is now a well-trodden medicinal chemistry problem, informed by two decades of PROTAC linker optimization work [1]. Identifying which essential effector protein, paired with which tissue-restricted target protein, will actually cooperate to form a stable, functional ternary complex is not — and unlike ligand potency or linker geometry, that compatibility can’t be fully predicted from structure alone, because it also depends on subcellular localization, relative abundance, and cellular context.
That is precisely the kind of combinatorial search problem that favors empirical, high-throughput approaches over one-pair-at-a-time rational design — and it’s why two very different discovery strategies (VantAI’s computational, structure-based approach and DEL-based empirical screening) are both being pursued as complementary answers to the same underlying constraint. DNA-encoded library selection, in particular, was purpose-built for combinatorial search problems of exactly this shape: screening formats that can interrogate hundreds of millions of candidate binders against a protein in a single experiment, rather than testing designed candidates one at a time, are a natural fit for a field whose main bottleneck is combinatorial pair discovery rather than potency optimization [12].
Where DEL Screening Fits Into RIPTAC Discovery
DNA-encoded libraries work by attaching a unique DNA “barcode” to each member of a combinatorial small-molecule library, so that after an affinity-based selection against a protein target, sequencing the surviving barcodes reveals which chemical structures bound. This lets a single experiment screen library sizes — often hundreds of millions of compounds — that would be entirely impractical with conventional one-compound-at-a-time high-throughput screening, and the approach has matured into a well-established hit-finding technology across the pharmaceutical industry [12].
Critically, this combinatorial capability has already been demonstrated directly against the proximity-inducing molecule problem, not just against conventional single-target binders. A 2024 study in Nature Chemical Biology used a DEL screening strategy to directly discover functional PROTAC-type degraders — bifunctional compounds recruiting the VHL E3 ligase — identifying a potent bromodomain-containing protein 4 (BRD4) degrader, including a novel BRD4-binding ligand, directly from the selection itself, with minimal downstream optimization required [13]. That result is a proof-of-concept for a broader idea: DEL-based selection doesn’t just find binders to a single protein — it can be configured to find and characterize the chemical matter needed for a functional bifunctional molecule, ternary complex formation included.
Applied to RIPTAC discovery specifically, that combinatorial capability addresses two distinct needs simultaneously. First, DEL selection can generate high-quality, novel ligands against each side of a candidate pair independently — screening directly against a tumor-restricted target protein candidate, and separately against an essential effector protein candidate, without requiring either to already have a known, drug-like starting ligand. Second — and this is the piece that matters most for a modality whose selectivity is fundamentally a cellular phenomenon — because RIPTAC selectivity depends on which proteins are actually co-expressed and able to physically form a complex inside a living cell, not merely on binary in vitro affinity, screening formats that operate under physiologically relevant, cellular conditions are especially well matched to this modality’s discovery needs. Assessing differential engagement and complex formation directly in a cellular context, from the earliest stages of hit-finding, means that a candidate pair’s cell-selectivity profile is characterized early, rather than discovered as a costly surprise deep into a medicinal chemistry campaign.
Outlook
RIPTACs remain a young modality — one validated clinical candidate, one landmark acquisition, and a small but growing set of preclinical follow-on programs. But the underlying design principle — exploit differential expression rather than differential degradability — generalizes well beyond androgen receptor biology in prostate and breast cancer, and the field’s own diagnosis of its limiting factor is unambiguous: target–effector pairing, not linker chemistry, is what is currently constraining how far and how fast the platform concept can expand [4,10]. That diagnosis points directly toward the kind of empirical, cell-based, high-throughput discovery approaches that DNA-encoded library screening platforms were built to deliver. As more validated effector-protein ligands and tissue-restricted target-protein ligands are identified and cataloged across a wider range of cancers and essential cellular machinery, the combinatorial space of viable RIPTAC pairs — and the modality’s reach beyond prostate and breast cancer — should expand accordingly.
Frequently Asked Questions
References
- Békés M, Langley DR, Crews CM. PROTAC targeted protein degraders: the past is prologue. Nat Rev Drug Discov (2022), 21(3), 181-200. DOI: https://doi.org/10.1038/s41573-021-00371-6
- Raina K, Forbes CD, Stronk R, et al. Regulated induced proximity targeting chimeras—RIPTACs—a heterobifunctional small molecule strategy for cancer selective therapies. Cell Chem Biol (2024), 31(8), 1490-1502. DOI: https://doi.org/10.1016/j.chembiol.2024.07.005
- Mullard A. Induced proximity pushes beyond protein degraders, as first RIPTAC moves into the clinic. Nat Rev Drug Discov (2025), 24(4), 235-237. DOI: https://doi.org/10.1038/d41573-025-00037-7
- Mullard A. Halda secures biggest induced-proximity deal yet, with J&J’s US$3 billion buy out. Nat Rev Drug Discov (2025), 25(1), 9. DOI: https://doi.org/10.1038/d41573-025-00190-z
- Mangano K, Potts PR. Feel the breeze: Opening the therapeutic window with RIPTACs and induced proximity. Cell Chem Biol (2024), 31(8), 1391-1393. DOI: https://doi.org/10.1016/j.chembiol.2024.07.013
- Mayor-Ruiz C, Bauer S, Brand M, et al. Rational discovery of molecular glue degraders via scalable chemical profiling. Nat Chem Biol (2020), 16(11), 1199-1207. DOI: https://doi.org/10.1038/s41589-020-0594-x
- Gourisankar S, Krokhotin A, Ji W, et al. Rewiring cancer drivers to activate apoptosis. Nature (2023), 620(7973), 417-425. DOI: https://doi.org/10.1038/s41586-023-06348-2
- Ma Z, Zhang C, Shen Q, Zhou J. RIPTACs for Precision Cancer Therapy: A Novel Modality with the Inspiration of HLD-0915 as the First Candidate in Clinical Trials. J Med Chem (2025), 68(11), 10503-10506. DOI: https://doi.org/10.1021/acs.jmedchem.5c01250
- DDW Staff. Oncology company Halda acquired by Johnson & Johnson. Drug Discovery World (2025). www.ddw-online.com/oncology-company-halda-acquired-by-johnson-johnson-39055-202511
- BusinessWire. VantAI and Halda Therapeutics Forge Alliance to Discover Next-Generation RIPTAC Medicines. BusinessWire (Aug 19, 2025). www.businesswire.com/news/home/20250819757409/en
- Flare Therapeutics. Flare Therapeutics Secures $85M in Insider-Led Series C Financing and Appoints Anna Protopapas as Chief Executive Officer. Flare Therapeutics press release (2026). www.flaretx.com/flare-therapeutics-secures-85m-in-insider-led-series-c-financing-and-appoints-anna-protopapas-as-chief-executive-officer
- Peterson AA, Liu DR. Small-molecule discovery through DNA-encoded libraries. Nat Rev Drug Discov (2023), 22(9), 699-722. DOI: https://doi.org/10.1038/s41573-023-00713-6
- Mason JW, Chow YT, Hudson L, et al. DNA-encoded library-enabled discovery of proximity-inducing small molecules. Nat Chem Biol (2024), 20(2), 170-179. DOI: https://doi.org/10.1038/s41589-023-01458-4