A Study of Mevrometostat for Treatment of Relapsed/Refractory SCLC, Castration Resistant Prostate Cancer, and Follicular Lymphoma

The purpose of this study is to learn about the safety and effects of the study medicine (called Mevrometostat) for the possible treatment of Relapsed/ Refractory Small Cell Lung Cancer (SCLC), Castration Resistant Prostate Cancer (CRPC) and Follicular Lymphoma (FL). The study consists of 3 parts; Part 1 and 2 enrolled participants with SCLC, metastatic CRPC, and FL are closed for enrollment. Part 3, which is open for enrollment is seeking men who: * have Castration Resistant Prostate Cancer (CRPC) and * have previously received treatment for CRPC and have progressed from the last treatment All participants in Part 3 of this study will receive mevrometostat and/ or enzalutamide. Part 3 consists of 2 sub studies each has an assessment phase and a maintenance phase. The Part 3 DDI substudy consist of 2 cohorts, Cohort 1 (monotherapy cohort) and Cohort 2 (Combination cohort). In the assessment phase: * participants in the BE substudy will take 3 single doses of mevrometostat by mouth over 3 periods. * participants in the DDI substudy Cohort 1 (monotherapy cohort) will take mevrometostat 2 times a day and/or itraconazole 1 time a day based on a present schedule. * participants in the DDI substudy Cohort 2 (combination cohort) will take mevrometostat 2 times a day, enzalutamide 1 time a day, and/or itraconazole 1 time a day based on a present schedule. After completion of the assessment phase, participants will enter the maintenance phase where they will receive mevrometostat 2 times a day and enzalutamide 1 time a day by mouth until their cancer is no longer responding. The study will look at the experiences of participanrs receiving the study medicine. This will help see if the study medicine is safe and effective.