Efficacy and Safety of Pembrolizumab (MK-3475) Plus Enzalutamide Plus Androgen Deprivation Therapy (ADT) Versus Placebo Plus Enzalutamide Plus ADT in Participants With Metastatic Hormone-Sensitive Prostate Cancer (mHSPC) (MK-3475-991/KEYNOTE-991)

rPFS was defined as the time from randomization to occurrence of: radiological tumor progression using RECIST 1.1 as assessed by BICR; progression of bone lesions using PCWG criteria; or death due to any cause. Progression as per modified RECIST 1.1 was ≥20% increase in sum of diameters of target lesions and progression of existing non-target lesions. Progression of bone lesions by PCWG criteria was the appearance of ≥2 new bone lesions on bone scan, that have been confirmed to not represent tumor flare, and was persistent for ≥6 weeks. The rPFS was calculated using the product-limit (Kaplan-Meier) method for censored data. Participants without a rPFS event were censored at the date of last disease assessment.

OS was defined as the time from randomization to death due to any cause. The OS was calculated using the product-limit (Kaplan-Meier) method for censored data. Participants without documented death at the time of the analysis were censored at the date of the last follow-up.

TFST was defined as the time from randomization to initiation of the first subsequent anti-cancer therapy or death; whichever occurred first. The TFST was calculated using the product limit (Kaplan-Meier) method for censored data. Participants without documented event at time of analysis will be censored at the date of last known time to have not received subsequent new anti-cancer therapy.

TTSSRE was the time from randomization to the first symptomatic skeletal-related event defined as: use of external-beam radiation therapy (EBRT) to prevent or relieve skeletal symptoms, occurrence of new symptomatic pathologic bone fracture (vertebral or nonvertebral), occurrence of spinal cord compression, or tumor-related orthopedic surgical intervention, whichever occurs first. The TTSSRE was calculated using the Kaplan-Meier method for censored data. Participants without symptomatic skeletal-related events were censored at the last evaluable assessment.

Time to PSA progression was the time from randomization to PSA progression. The PSA progression date was defined as the date of 1) ≥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. Time to PSA was calculated using Kaplan-Meier method for censored data. Participants without PSA progression were censored at the last PSA date.

The time to radiographic soft tissue progression was defined as the time from randomization to radiographic soft tissue progression per soft tissue rules of PCWG-modified RECIST 1.1 as assessed by BICR. Progression was 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 was also considered progression. Time to radiographic soft tissue progression was calculated using the product-limit (Kaplan-Meier) method for censored data. Participants without radiographic soft tissue progression were censored at the last evaluable assessment.

TTPP was defined as the time from randomization to pain progression as determined by Item 3 of the BPI-SF. Pain progression was defined as: 1) For participants asymptomatic at baseline: a >2-point change from baseline in the average BPI-SF item 3 score at 2 consecutive visits or initiation of opioid use for pain 2) For participants symptomatic at baseline (average BPI-SF Item 3 score >0 and/or currently taking opioids; a >2-point change from baseline in the average BPI-SF Item 3 score and the average worst pain score >4 and no decrease in average opioid use. TTPP was calculated using the Kaplan-Meier method for censored data. Participants who had > 2 consecutive visits that were not evaluable for pain progression were censored at the last evaluable assessment.

PFS2 was defined as the time from randomization to disease progression as determined by investigator assessment of radiological or clinical progression after next-line of therapy or death from any cause, whichever occurs first.

PSA response rate was the percentage of participants who had PSA response defined as a reduction in the PSA level from baseline by >50%. The reduction in PSA level was confirmed by an additional PSA evaluation performed >3 weeks from the original response.

PSA undetectable rate was defined as the percentage of participants with detectable PSA (> 0.2 ng/mL) at baseline, which becomes undetectable (< 0.2 ng/mL) during study treatment.

ORR was defined as the percentage of participants with complete response (CR: disappearance of all target lesions per RECIST 1.1; and no evidence of disease (NED) on base scan per PCWG) or partial response (PR: at least a 30% decrease in the sum of diameters of target lesions per RECIST 1.1; and non-progressive disease, non-evaluable [NE], or NED on bone scan or CR with non-progressive disease or NE bone scan per PCWG).

DOR was defined as the time from first documented evidence of complete response (CR) or partial response (PR) per PCWG and RECIST 1.1 criteria until progressive disease (PD) or death. PD per RECIST 1.1 was 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 a least 5 mm. PD per PCWG was the appearance of >2 new bone lesions on bone scan, that have been confirmed to not represent tumor flare, and were persistent for >6 weeks. The DOR was calculated using the product-limit (Kaplan-Meier) method for censored data.

An AE was defined as 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 could therefore have been any unfavourable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. Participants who experienced an AE will be reported for each arm.

An AE was 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 unfavourable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The number of participants who discontinue study treatment due to an AE will be reported for each arm.