Investigating the Therapeutic Potential of the Novel CBP/p300 Protein Degrader CBPD409 in Multiple Myeloma

Introduction: Multiple Myeloma (MM) remains incurable with 10-year survival rates <30%. Although treatment options available to newly diagnosed and RRMM patients have vastly expanded in the past decade, disease progression still occurs within the majority of patients (Malard et al. Nature Review Disease Primers 2024). Thus, there is an unmet need for the introduction of therapies for patients who have experienced disease progression and relapse. It has been shown, via inhibition of the histone acetyltransferases and transcription co-activators CREB-binding protein (CBP)/enhancer (p300), that MM cell survival is reliant on the activity of CBP/P300. CCS-1477, a specific CBP/P300 inhibitor, has demonstrated single treatment efficacy in an early Phase 1 trial (NCT04068597) in RRMM patients (Nicosia et al. Cancer Cell 2023). Proteolysis Targeting Chimeras (PROTACs) represent an emergent class of drugs achieving targeted protein degradation through their unique structural features, and has demonstrated superior efficacy across multiple cancers as compared to traditional inhibitors. CBPD-409 is a novel, highly selective and potent PROTAC, that targets the bromodomain of CBP/P300. CBPD-409 demonstrates superior preclinical and clinical efficacy versus CCS-1477 in CBP/p300-driven cancers, such as prostate cancer (Chen et al. Journal of Medicinal Chemistry 2024). However, whether CBPD-409 exhibits better anti-myeloma efficacy as compared to CCS-1477 has still to be elucidated.

Aim: To determine the in vitro and in vivo impact of CBPD-409 monotherapy, and to evaluate its potential synergy with the standard of care treatment dexamethasone (Dex) in MM cells.

Methods: MM cell lines U266 and MM1.S, and peripheral blood mononuclear cells (PBMCs) obtained from healthy donors, were exposed to CBPD-409 or CCS-1477 for 72h, as to establish the therapeutic window of CBPD-409. MM1.S cells were treated with CBPD-409 or CCS-1477 as single agents or in combination with dexamethasone (Dex) (MM1.S) for 72h. CBP and c-MYC (CBP target) protein levels were assessed via immunoblotting after 24h treatment with CBPD-409 or CCS-1477. Apoptosis was evaluated via the Annexin V-FITC/eFluor 450 viability assay and flow cytometry. Myeloma disease was established in the MM1.S-GFP-Luc xenograft mouse model, with mice then treated with 3 mg/kg of CBPD-409 or vehicle control, via daily oral gavage. Tumour burden was assessed, via bioluminescent imaging (BLI) at day 7,14 and 21 post-treatment.

Results: CBPD-409 (10 nM) led to a significant reduction in CBP levels (95±5% SEM) in U266 and MM1.S cells after 24h. Furthermore, 10 nM of CBPD-409 induced a reduction in c-MYC levels (95±5% SEM) in MM1.S cells after 24h. CBPD-409 induced a greater reduction in CBP and c-MYC protein as compared to a higher (10x) concentration of CCS-1477 (100 nM). At low nanomolar concentration (10 nM) CBPD-409 reduced survival of MM1.S (97±3% SEM) and U266 (64±5% SEM) cells after 72h. CBPD-409 did not exhibit a dose-dependent impact on the viability of PBMCs with CBPD-409 impacting PBMC viability similarly to CCS-1477 (28±6% SEM) across all concentrations of CBPD-409 tested (1-100 nM). Next, we tested the effect of the combination of 0.5 nM (IC50) CBPD-409 with 5 nM Dex (IC50) in MM1.S cells, which resulted in synergistic kill (96±3% SEM), as compared to Dex (10±6% SEM) or CBPD-409 (<55±12% SEM) used as monotherapies after 72h of treatment. Daily oral dosing of 3 mg/kg of CBPD-409 as a monotherapy led to a 7-fold, 54-fold and 908-fold reduction in MM tumour burden as compared to the control group at day 7,14 and 21 respectively.

Conclusion: Our findings are the first to demonstrate that the novel CBP/P300 degrader CBPD-409 exhibits preclinical efficacy in MM, with CBPD-409 leading to significantly greater pro-apoptotic effects in MM cells, both as a monotherapy and in combination with Dex, as compared to CCS-1477. CBPD-409 significantly reduced tumour burden in the MM1.S xenograft model of myeloma, demonstrating in vivo efficacy. Future studies will examine the impact of CBPD-409 on proliferation and CBP/p300 downstream signalling pathways in primary MM cells.

Guzman:SeqRX LLC: Current equity holder in private company; BridgeMedicines: Research Funding.