A Study of Tazemetostat (Tazverik) Before Prostatectomy in Men With Prostate Cancer

Inclusion Criteria: * Men electing to undergo radical prostatectomy with preoperative tissue available * Able to take oral medications * Age ≥18 years * Have an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1 * Adequate organ function * Men with partners of childbearing potential, must have had a successful vasectomy (with medically confirmed lack of sperm that are alive) OR must either practice complete abstinence or agree to use adequate contraception * Inclusion criteria for remote monitoring component of study with Fitbit only (failure to meet inclusion criterion for Fitbit-monitoring portion of study should not preclude participating in treatment trial): Access to a device (e.g., smart phone, tablet) that has the capability to sync to the Fitbit. Note: access can be via family member, friend, caregiver, or study-designated device provided by study member Exclusion Criteria: * Active intercurrent illness or malignancy requiring therapy outside of prostate cancer * History of bleeding disorders * Concurrent use of anti-platelet agents or therapeutic anticoagulants with the exception of low-dose aspirin (81 mg daily) or NSAIDs for pain/inflammation. Concurrent use of strong and moderate CYP3A inhibitors and inducers * Subjects who have previously received tazemetostat * Currently participating in a study using an investigational, medicinal anti-cancer agent within 4 weeks prior to first dose of tazemetostat * Exclusion criteria for remote monitoring component of study with Fitbit only (failure to meet exclusion criterion for Fitbit-monitoring portion of study should not preclude participating in treatment trial): Using a pacemaker, implantable cardiac defibrillator, neurostimulator, cochlear implants (removable hearing aids permitted), or other electronic medical equipment, per PI's discretion.