Inobrodib

Inobrodib (CCS1477) is CellCentric's oral small-molecule that blocks the bromodomains of p300 and CBP - two enzymes that switch on the gene programs multiple myeloma depends on to survive. The Phase 2 DOMMINO-1 trial (NCT07096778) tests inobrodib on top of pomalidomide and dexamethasone in patients whose myeloma has come back after standard treatments. It matters because p300/CBP inhibition shuts down IRF4 and MYC, the same wiring that immunomodulatory drugs like pomalidomide attack from a different angle - so the biological logic for combining them is unusually tight [1][2]. ASH 2025 dose-optimization data already showed combination ORR of 60-75% in heavily pretreated patients, roughly 2x historical pom/dex benchmarks [8].

Inobrodib is a novel compound, never approved anywhere. The lead program is the Phase 2 DOMMINO-1 combination study with pomalidomide and dexamethasone in relapsed/refractory multiple myeloma, recruiting toward n=100 at the selected 20 mg dose [1][8]. A parallel Phase 1 dose-finding study in hematologic malignancies (NCT04068597) is ongoing, with a target enrollment of 250 - that trial produced the monotherapy signal that justified moving into combinations [2]. CellCentric has reported FDA Fast Track designation and FDA Orphan Drug Designation for inobrodib in multiple myeloma based on the Phase 1 monotherapy responses presented at ASH 2023 [3]. No breakthrough therapy designation has been disclosed. At ASH 2025 (December 2025), CellCentric presented Phase 2 dose-optimization data from DOMMINO-1 showing 60-75% ORR across the 20 mg and 30 mg cohorts in heavily pretreated patients (median ~5 prior lines, including bispecific- and anti-BCMA-exposed populations) - at least a 2-fold improvement over historical real-world response rates for pom-refractory patients [8]. The 20 mg dose was selected for the key portion of DOMMINO-1 in line with FDA Project Optimus dose-optimization expectations. A global Phase 3 trial (DOMMINO-2) is planned to initiate in H2 2026 [9]. Funding profile is strong: CellCentric closed an oversubscribed $220M Series D in May 2026, led by Venrock Healthcare Capital Partners with Fidelity, Sofinnova, HBM, RA Capital, Forbion, and Pfizer participating [9]. Pfizer has held a strategic stake since a $25M loan note (issued July 2024) converted to equity [10] - meaning CellCentric is no longer a typical capital-constrained private biotech, and has a named pharma partner already on the cap table. In parallel, inobrodib has an active Phase 1/2a clinical program in castration-resistant prostate cancer (CRPC) led by Prof. Johann de Bono at the Royal Marsden / Institute of Cancer Research, with monotherapy and combination arms with abiraterone and enzalutamide [11]. The CRPC rationale: AR, AR-splice variants, and c-Myc all depend on p300/CBP transcriptional co-activator activity, so inhibition is mechanistically positioned to delay or reverse resistance to standard hormonal therapies. This is a separate registration track, not a sub-indication of the MM program - it materially expands the asset's licensing and pipeline optionality.

p300 and CBP are nearly identical enzymes that act like the body's gene-activation amplifiers. They paste small chemical tags (acetyl groups) onto histones, the proteins DNA wraps around, and acetylated histones tell the cell 'read this gene' [4]. The bromodomain is the part of p300/CBP that recognizes already-acetylated histones and recruits more activity to the same spot - a self-reinforcing loop. Inobrodib jams that loop by binding the bromodomain. In myeloma, this loop is hijacked to maintain high expression of two transcription factors the cancer can't live without: IRF4 and MYC. IRF4 is the master switch of the plasma cell program - myeloma cells are essentially addicted to it [5]. MYC drives proliferation. Knock down p300/CBP bromodomain function, and IRF4 and MYC drop within hours in myeloma cell lines. The genetic case is also strong: CREBBP and EP300 are recurrently mutated in B-cell lymphomas, and loss-of-function in one paralog creates synthetic vulnerability (when losing one gene makes a cell hyperdependent on its close cousin) to inhibition of the other. Validation tier: target biology is well-established, but no approved drug yet hits this mechanism - so clinical proof is still being built, not assumed.

NCT07096778 (DOMMINO-1) is a Phase 2 single-arm study of inobrodib plus pomalidomide and dexamethasone in relapsed/refractory multiple myeloma, n=100, sponsored by CellCentric, with 20 mg as the selected dose following randomized dose-optimization [1][8]. Primary endpoint is Objective Response Rate (≥PR by IMWG criteria - the international oncology body that defines what counts as a drug response in myeloma) adjudicated by an Independent Review Committee, a reasonable choice for a single-arm registrational-track study in heavily pretreated MM. No randomized comparator arm, which is standard at this stage and indication but means any future approval would lean on cross-trial benchmarking against pomalidomide/dexamethasone alone (historical ORR ~30%) [6]. The combination logic is the strongest part of the design: pomalidomide degrades IKZF1/IKZF3 (proteins that normally sustain plasma cell identity, whose loss collapses IRF4 transcription), which lowers IRF4 transcriptionally; inobrodib lowers IRF4 epigenetically. Two mechanisms converging on the same node should give additive or synergistic effects if the biology holds - and the 60-75% ASH 2025 combination ORR is consistent with that prediction [8]. Risks in the design: single-arm ORR readouts can flatter combinations because they don't isolate inobrodib's contribution, but the ~2x lift over pom/dex historical performance is large enough that mechanism-specific contribution is plausible. Recruitment status is RECRUITING; pace is not publicly disclosed. The parallel Phase 1 (NCT04068597) continues to generate monotherapy data that helps interpret the combination signal.

The model estimates a 12% chance this drug is eventually approved. It starts from the historical approval rate for Phase 2 drugs in this area - about 21% - then adjusts based on ten facts about the trial and its sponsor. The estimate is nudged up by a non-randomized trial design and open-label blinding, and nudged down by the sponsor's thin approval record and weak earlier-phase results. The remaining factors fall close to average for this stage, so they leave the estimate roughly where it started.

Efficacy risk: the ASH 2025 combination signal is encouraging but limited to early dose-optimization cohorts; durability (median duration of response) is the next critical readout. Biomarker selection remains unaddressed; we don't yet know which patients respond best, and a target this central to normal hematopoiesis tends to have a narrow therapeutic window. Safety risk: p300/CBP regulate megakaryocyte differentiation, so thrombocytopenia is the predictable on-target toxicity, and it has been the dose-limiting issue across the BET inhibitor field - multiple BET programs in heme malignancies have been shelved over this exact problem. Inobrodib's bromodomain selectivity may help, but the risk isn't theoretical. GI toxicity and fatigue have also been reported in early data [3]. Execution risk: CellCentric closed an oversubscribed $220M Series D in May 2026 to advance DOMMINO-1 and initiate the global Phase 3 DOMMINO-2 trial in H2 2026 [9], and Pfizer holds a $25M strategic equity stake since 2024 [10] - so capital and a potential commercial partner are already in place. The execution question is no longer 'can they fund the next study,' it's 'can a UK-based clinical-stage biotech operationally run a global Phase 3 on the timeline implied by the financing.' Enrollment competition in relapsed MM is brutal - bispecifics (teclistamab, talquetamab, elranatamab) and CAR-T (cilta-cel, ide-cel) are now standard later-line options and may pull patients away from this trial. Commercial risk: if approved, an oral combination drug needs to differentiate against bispecifics that are showing 60%+ ORRs as monotherapy - but the ASH 2025 combination data is already in that range, and convenience plus outpatient delivery is a real moat for community oncology.

**Near-term catalyst (imminent): EHA 2026, June 11-14, 2026 in Milan - ~9 days from this writeup.** Given ASH 2025 already produced positive combination data, an EHA follow-up update on DOMMINO-1 (durability, expanded cohort, safety) is highly likely and would materially move the read on this program either direction. Worth watching, leaning constructive. The mechanism is real, the combination logic is tight, the ASH 2025 ORR of 60-75% lands meaningfully above the ~30% pom/dex historical benchmark [8], and CellCentric is the only company with a clinical-stage p300/CBP inhibitor in heme - a clean first-in-class read that doesn't get muddied by competitor data. The signal to wait for from EHA 2026 and ASH 2026: confirmation that the ORR holds in a larger, more diverse population, and median duration of response ≥6 months. Watch the platelet curves more than the response rate - thrombocytopenia is what will kill this program if it kills it. Capital and partnership posture have shifted materially: with $220M Series D in hand and Pfizer already on the cap table [9][10], CellCentric does not need a partnership to fund Phase 3 - meaning any partnership announcement would be on better terms, not survival terms. The parallel CRPC program led by Johann de Bono [11] adds a second, fully independent registration track that is mostly absent from market consensus on this asset. Next check-ins: EHA 2026 (June 11-14) for any DOMMINO-1 update; ASH 2026 (December 2026) for expanded combination data and DOMMINO-2 launch update. If the company goes quiet through both, that's a signal in itself.