A Study to Learn About the Investigational Drug Rinzimetostat (ORIC-944) in Patients With mCRPC Who Were Previously Treated With Abiraterone Acetate (Himalayas-1)

Radiographic Progression Free Survival is defined as the time from the date of randomization to first evidence of radiographic progression as assessed in soft tissue by RECIST 1.1 or in bone per PCWG3 guideline by BICR, or death, whichever occurs first

OS is defined as time from date of randomization to date of death due to any cause

ORR is defined as proportion of patients with measurable soft tissue disease at baseline who have a confirmed objective response of complete response (CR) or partial response (PR)

DOR is defined as time of first response to first documentation of radiographic progression or death, whichever occurs first

Defined as the proportion of patients with a confirmed ≥50% and ≥90% decline in PSA from baseline as per the PCWG3 criteria, along with the maximum decline in PSA occurring at any point on study

Measured from the start of treatment until criteria for PSA progression are met as per PCWG3

Time from randomization to first symptomatic skeletal event (symptomatic bone fractures, spinal cord compression, surgery or radiation to the bone whichever is first)

Time from randomization to first symptomatic skeletal event (symptomatic bone fractures, spinal cord compression, surgery or radiation to the bone whichever is first) Compare patient reported outcomes (PROs) between the investigational treatment and control arms

Analysis of Brief Pain Inventory-Short Form (BPI-SF) will be based on the pain severity score (mean of individual BPI-SF items 3, 4, 5 and 6), the pain interference score (the mean of items 9A-9G), and the single BPI-SF Item 3 which asks the patient to rate pain at its worst in the last 24 hours

Change from baseline in HRQoL (FACT-P total score) will be presented. The FACT-P total score will be calculated based on the participant responses to the 39 items in the FACT-P questionnaire. Each item is rated on a 0 to 4 Likert-type scale and then combined to produce the FACT-P total score (0-156), with higher scores representing better health-related quality of life

Change from baseline in social/family well-being score will be presented. The social/family well-being score will be calculated based on 7 items in the FACT-P questionnaire; score range 0-28. Each item is rated on a 0 to 4 Likert-type scale

Change from baseline in functioning well-being score will be presented. The functioning well-being score will be calculated based on 7 items in the FACT-P questionnaire; score range 0-28. Each item is rated on a 0 to 4 Likert-type scale

Change from baseline in physical well-being score will be presented. The physical well-being score will be calculated based on 7 items in the FACT-P questionnaire; score range 0-28. Each item is rated on a 0 to 4 Likert-type scale

Change from baseline prostate cancer symptoms (PCS) score will be presented. The PCS score will be calculated based on 12 items in the FACT-P questionnaire; score range 0-48. Each item is rated on a 0 to 4 Likert-type scale

Participants will self-rate their current state of mobility, self-care, usual activities, pain/discomfort, and anxiety/depression by choosing 1 of 5 possible responses that record the level of severity (no problems, slight problems, moderate problems, severe problems, or extreme problems) within each dimension. The questionnaire also includes a visual analog scale to self-rate general health state on a scale from "the worst health you can imagine" to "the best health you can imagine."

Each selected PRO-CTCAE items will be assessed related to one or more attributes that include counts for the frequency, severity, and/or interference with usual or daily activities

The FACT-GP5 question assesses overall side effect burden with the following response options "I am bothered by side effects of treatment," is rated on a 5-point scale ranging from "not at all" (0) to "very much" (4) and counts will be presented

Defined as the time from randomization to onset of pain progression, which is defined as > 2-point increase from baseline in the score from the BPI-SF Question 3 that is confirmed at the next consecutive assessment > 4 weeks apart or an initial deterioration followed by death before the next assessment

Defined as the time from randomization to onset of definitive deterioration in FACT-P total score, which is defined as >10 point decrease from baseline and no subsequent observations with a <10 point decrease from baseline FACT-P total score

Time to definitive deterioration in physical well-being (PWB) per FACT-P is defined as the time from randomization to onset of definitive deterioration in physical well-being, which is defined as ≥3-point

Type, incidence, relationship to study treatments, severity, and seriousness of AEs [as graded by National Cancer Institute (NCI) common terminology criteria for adverse events (CTCAE) v6.0] and other clinical assessments

Rinzimetostat characterized by predose and postdose plasma concentrations at selected time points