Elucidating molecularly stratified single agent, and combination, therapeutic strategies targeting MCL1 for lethal prostate cancer.

Metastatic castration-resistant prostate cancer (mCRPC) is a lethal disease requiring additional therapeutic strategies. MCL1, an anti-apoptotic BCL2 family member, promotes cancer-cell survival, but its role in mCRPC remains poorly understood. Here, we characterise MCL1 in multiple mCRPC biopsy cohorts and patient-derived models, assessing responses to MCL1 inhibition. MCL1 copy number gain (14%-34%) correlates with increased MCL1 expression and worse outcomes. MCL1 inhibition exhibits anti-tumour effects in MCL1-gained mCRPC models. Co-inhibition of MCL1 and AKT induces cancer-specific cell death in PTEN-loss/PI3K-activated models in vitro and in vivo, modulating BAD-BCLXL and BIM-MCL1 interactions, with durable anti-tumour activity in models with AKT inhibitor acquired resistance. Finally, CDK9-mediated MCL1 downregulation combined with AKT inhibition recapitulates these findings, providing further opportunities for clinical translation. These data support early phase clinical trials targeting MCL1, both as monotherapy for MCL1-gained mCRPC, and in combination with AKT inhibition for PTEN-loss/PI3K-activated mCRPC.