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 following events if considered drug related by the Investigator, will be considered a DLT: Grade 4 nonhematologic toxicity (not based on laboratory value); Grade 4 hematologic toxicity lasting ≥7 days, except thrombocytopenia; Grade 4 thrombocytopenia of any duration; Grade 3 thrombocytopenia associated with clinically significant bleeding; Nonhematologic AEs ≥Grade 3 with exceptions; Grade 3 or Grade 4 non-hematologic laboratory values if requires medical intervention, leads to hospitalization, persists for >1 week, or results in a Drug-induced Liver Injury with exceptions; Grade 3 or 4 febrile neutropenia; Study intervention - related toxicities that lead to discontinuation of study treatment during Cycle 1; Prolonged delay (>2 weeks) in initiating Cycles 2 due to treatment-related toxicity; Missing >25% of study intervention doses as a result of treatment-related AE during Cycle 1; Grade 5 toxicity.

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention.

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention.

PSA response is defined per prostate cancer working group (PCWG) criteria as a reduction in the PSA level of 50% or more from baseline measured at consecutive assessments at least 3 weeks apart.

The following events if considered drug related by the Investigator, will be considered a DLT: Grade 4 nonhematologic toxicity (not based on laboratory value); Grade 4 hematologic toxicity lasting ≥7 days, except thrombocytopenia; Grade 4 thrombocytopenia of any duration; Grade 3 thrombocytopenia associated with clinically significant bleeding; Nonhematologic AEs ≥Grade 3 with exceptions; Grade 3 or Grade 4 non-hematologic laboratory values if requires medical intervention, leads to hospitalization, persists for >1 week, or results in a Drug-induced Liver Injury with exceptions; Grade 3 or 4 febrile neutropenia; Study intervention - related toxicities that lead to discontinuation of study treatment during Cycle 1; Prolonged delay (>2 weeks) in initiating Cycles 2 due to treatment-related toxicity; Missing >25% of study intervention doses as a result of treatment-related AE during Cycle 1; Grade 5 toxicity.

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention.

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention.

ORR is defined as the percentage of participants with Complete Response (CR: disappearance of all target lesions) or Partial Response (PR: at least a 30% decrease in the sum of diameters of target lesions) per PCWG-Modified Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1). The percentage of participants who experience CR or PR as assessed by Blinded Independent Central Review (BICR) will be presented.

rPFS is defined as the time from randomization to the first documented progressive disease (PD) per PCWG-modified RECIST 1.1 based on BICR or death due to any cause, whichever occurred first. Per PCWG-modified RECIST 1.1, PD is defined as ≥20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also have demonstrated an absolute increase of ≥5 mm. The appearance of one or more new lesions or ≥2 new bone lesions was also considered PD. PCWG-modified RECIST is similar to RECIST 1.1 with the exception that a confirmation assessment of PD (>4 weeks after the initial PD) is required for participants who remain on treatment following a documented PD per RECIST 1.1 and PCWG rules include new bone lesions. The rPFS per PCWG-modified RECIST as assessed by BICR for all participants is presented. The nonparametric Kaplan-Meier method will be used to estimate the rPFS curve in each treatment arm

Overall survival (OS) is defined as the time from randomization to death due to any cause. The nonparametric Kaplan-Meier method will be used to estimate the survival curves.

DOR is defined as the time from first documented evidence of CR or PR until progressive disease (PD) or death. Per PCWG-modified RECIST 1.1, PD is defined as at least a 20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of at least 5 mm. The appearance of ≥ 2 new bone lesions is also considered PD. DOR as assessed by BICR is presented. The nonparametric Kaplan-Meier method will be used to estimate the DOR in each treatment arm.

TFST is defined as the time from randomization to initiation of the first subsequent anticancer therapy or death, whichever occurs first. The nonparametric Kaplan-Meier method will be used to estimate the TFST in each treatment arm.

Time to PSA progression is defined as the time from randomization to PSA progression. Participants without PSA progression will be censored at the last PSA assessment date. The PSA progression date is defined as the date of:

• ≥25% increase and ≥2 ng/mL above the nadir, confirmed by a second value ≥3 weeks later if there is PSA decline from baseline, OR 2) ≥25% increase and ≥2 ng/mL increase from baseline beyond 12 weeks if there is no PSA decline from baseline.

The time from allocation/randomization to pain progression based on the Brief Pain Inventory-Short Form (BPI-SF) Item 3 "worst pain in 24 hours" and by the Analgesic Quantification Algorithm (AQA) score.