A Clinical Study of Ifinatamab Deruxtecan Based Treatment Combinations or as Monotherapy to Treat Metastatic Castrate Resistant Prostate Cancer (mCRPC) (MK-2400-01A/IDeate-Prostate02)

The purpose of this substudy is to assess the efficacy and safety of ifinatamab deruxtecan (I-DXd), given alone or with other treatments in participants with metastatic castration-resistant prostate cancer (mCRPC). The goals of this study are to learn about: * The safety of the study treatment and if people tolerate it. * A safe dose level of I-DXd that can be used with other treatments. * Participant levels of prostate specific antigen (PSA) during treatment. Lead Sponsor: Merck Sharp & Dohme LLC Start Date: 2025-07-03 Primary Completion: 2031-04-01

• : The main inclusion criteria include but are not limited to the following: * Has histologically- or cytologically-confirmed adenocarcinoma of the prostate without small cell histology * Has prostate cancer progression while on androgen deprivation therapy (ADT) (or post bilateral orchiectomy) within 6 months before Screening * Has current evidence of metastatic disease * Has received prior treatment with 1 or 2 androgen receptor pathway inhibitors (ARPIs) and progressed during or after treatment * Participants receiving bone resorptive therapy (including, but not limited to bisphosphonate or denosumab) must have been on stable doses for ≥4 weeks before allocation/randomization * An Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 assessed within 10 days before allocation/randomization * Has prior treatment with poly-ADP-ribose polymerase inhibitors (PARPi) if indicated by local approved regimen or were deemed ineligible to receive PARPi by the investigator

Use the source registry for the full inclusion and exclusion criteria before discussing referral or enrolment.