TulmiSTAR-02: A Phase I/II Open-label Study of Tulmimetostat in Combination With Darolutamide vs. Darolutamide, and Tulmimetostat With Abiraterone in Patients With Metastatic Hormone-sensitive Prostate Cancer (mHSPC)

This is a Phase I/II, open-label, global, multicenter study evaluating tulmimetostat in combination with androgen receptor pathway inhibitors in participants with de novo or recurrent metastatic hormone-sensitive prostate cancer (mHSPC). The study consists of two phases: Phase I (Dose Escalation) Phase I includes two treatment groups: 1. Group A (Part 1): tulmimetostat in combination with darolutamide. 2. Group B (Part 2): tulmimetostat in combination with abiraterone. The primary objective of Phase I is to determine the recommended dose(s) for expansion (RDE) of tulmimetostat in each combination regimen through a parallel dose-escalation design guided by a Bayesian Logistic Regression Model (BLRM). Enrollment will follow a staggered approach between the two groups. Participants may have received prior taxane-based chemotherapy and/or prior androgen receptor pathway inhibitor (ARPI) therapy and must not have received prior radioligand therapy. Participants will continue androgen deprivation therapy (ADT) to maintain castrate testosterone levels (\<50 ng/dL or \<1.7 nmol/L) according to investigator choice and local prescribing information. In Group B, abiraterone will be administered with prednisone or prednisolone according to local prescribing information. Phase II (Dose Expansion) Phase II is a randomized, open-label, multicenter dose-expansion study designed to further characterize the recommended dose(s) of tulmimetostat in combination with darolutamide and to provide proof-of-concept for the efficacy and safety of tulmimetostat plus darolutamide compared with darolutamide alone. Eligible participants are adult men with de novo or recurrent mHSPC who have not received prior radioligand therapy and may have received prior taxane-based chemotherapy and/or prior ARPI therapy (excluding darolutamide), with a maximum prior ARPI exposure of 4 months in the mHSPC setting. Participants will continue ADT throughout the study according to investigator choice and local prescribing information. Based on Phase I results, one or two tulmimetostat dose levels in combination with darolutamide may be evaluated. Study Duration and Follow-up For each participant, the study consists of a screening period, a treatment period, and post-treatment follow-up. Post-treatment follow-up includes: 1. A 30-day safety follow-up visit after permanent discontinuation of study treatment. 2. An efficacy follow-up period for participants who discontinue study treatment without documented disease progression or initiation of new anti-neoplastic therapy, including scheduled imaging assessments. 3. Long-term follow-up to collect survival status, new anti-neoplastic therapies, suspected treatment-related serious adverse events, and second primary malignancies. Participants will be contacted approximately every 12 weeks during the first 3 years and every 24 weeks thereafter until death, loss to follow-up, or withdrawal of consent. The study is designed to evaluate whether inhibition of EZH1/2 with tulmimetostat in combination with androgen receptor pathway inhibition may improve clinical outcomes in men with metastatic hormone-sensitive prostate cancer.