A Study of Atezolizumab (Anti-PD-L1 Antibody) in Combination With Enzalutamide in Participants With Metastatic Castration-Resistant Prostrate Cancer (mCRPC) After Failure of an Androgen Synthesis Inhibitor And Failure of, Ineligibility For, or Refusal of a Taxane Regimen

Overall Survival is defined as the time from randomization to death from any cause.

Time frame: Baseline until death from any cause (up to approximately 42 months)

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

Safety visit-Discontinued participants had these visits within approximately 120 days after the last dosing; Study Completion Pre-dose visit-Participants had these visits at the last treatment dosing, samples were taken before the last dosing; Early Discontinuation Pre-dose visit-Participants who discontinued the study had these visits within the 30 days after their last dosing but samples were takes before their last dosing; Study Completion visit-Participants had these visits at their last treatment dosing but samples were taken after their last dosing; Early Discontinuation visit-Participants who discontinued the study had the visits within the 30 days after their last dosing but samples were taken after their last dosing; Unscheduled and Unscheduled Pre-dose Visits- At these visits, participants' samples were collected without dosing event and before dosing dosing at a visit unscheduled per protocol respectively. Enzalutamide Arm had no Atezolizumab dosing.

Time frame: Day Cycle 1 Day 1 0.5 hr post-infusion (infusion duration: 60 minutes [min])

Population: PK evaluable population. This population is defined as all randomized participants regardless of whether the assigned study treatment was received. For PK analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

Safety visit-Discontinued participants had these visits within approximately 120 days after the last dosing; Study Completion Pre-dose visit-Participants had these visits at the last treatment dosing, samples were taken before the last dosing; Early Discontinuation Pre-dose visit-Participants who discontinued the study had these visits within the 30 days after their last dosing but samples were takes before their last dosing; Study Completion visit-Participants had these visits at their last treatment dosing but samples were taken after their last dosing; Early Discontinuation visit-Participants who discontinued the study had the visits within the 30 days after their last dosing but samples were taken after their last dosing; Unscheduled and Unscheduled Pre-dose Visits- At these visits, participants' samples were collected without dosing event and before dosing dosing at a visit unscheduled per protocol respectively. Enzalutamide Arm had no Atezolizumab dosing.

Time frame: Pre-infusion (0 hour[hr]) on Day 1 Cycles 1, 2, 3, 4, 8, 12, 16 (Cycle length: 21 days); treatment discontinuation visit, 120 days after last dose (up to approximately 42 months)

Population: PK evaluable population. This population is defined as all randomized participants regardless of whether the assigned study treatment was received. For PK analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

The numbers and proportions of ADA-positive participants and ADA-negative participants at baseline (baseline prevalence) and after baseline (post-baseline incidence) will be summarized by treatment group. Enzalutamide Arm has no Atezolizumab dosing therefore no participants to include here for Atezolizumab ADA.

Time frame: Predose (0 hr) on Day 1 Cycles 1, 2, 3, 4, 8, 12, 16 (Cycle length: 21 days); at atezolizumab discontinuation visit (30 days after last dose); 120 days after last dose of atezolizumab; up to 42 months

Population: The safety population is defined as participants who received any amount of any component of the study treatments (atezolizumab, or enzalutamide). Participants will be allocated to treatment arms according to the treatment they actually received (i.e., participants randomized to enzalutamide alone who received at least one full or partial dose of atezolizumab will be included in the atezolizumab arm for safety).

rPFS is defined as the time from randomization to the earliest occurrence of one of the following: * A participant is considered to have progressed by bone scan if: The first bone scan with ≥2 new lesions compared to baseline is observed \< 12 weeks from randomization and is confirmed by a second bone scan taken ≥6 weeks later showing ≥2 additional new lesions (a total of ≥4 new lesions compared to baseline); the date of progression is the date of the first post-treatment scan, OR After the first post-treatment scan, ≥2 new lesions are observed relative to the first post-treatment scan, which is confirmed on a subsequent scan ≥6 weeks later; the date of progression is the date of the post-treatment scan when ≥2 new lesions were first documented. * Progression of soft tissue lesions, as defined per PCWG3 modified RECIST v1.1 * Death from any cause

Time frame: Months 6, 12

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

OS (Overall Survival is defined as the time from randomization to death from any cause) probability at 6 and 12 months

Time frame: Months 6, 12

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

Verbatim description of adverse events will be coded to MedDRA preferred terms and graded according to NCI CTCAE v4.0.

Time frame: Baseline up to end of study (up to approximately 42 month)

Population: The safety population is defined as participants who received any amount of any component of the study treatments (atezolizumab, or enzalutamide). Participants will be allocated to treatment arms according to the treatment they actually received (i.e., participants randomized to enzalutamide alone who received at least one full or partial dose of atezolizumab will be included in the atezolizumab arm for safety)

PSA response rate, defined as a \> 50% decrease in PSA from baseline that is confirmed after ≥ 3 weeks by a consecutive confirmatory PSA measurement

Time frame: Baseline until disease progression (up to approximately 42 months)

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

Objective response rate in soft tissue lesions, defined as the percentage of participants with either a CR or PR on two consecutive occasions ≥ 6 weeks apart, as determined by the investigator through use of PCWG3 criteria

Time frame: Baseline until disease progression or death from any cause (up to approximately 42 months)

Population: * Participants were classified as missing or unevaluable if no post-baseline response assessments were available or all post-baseline response baseline assessments were unevaluable.~* Responders had to have a CR or PR on two consecutive occasions at least 6 weeks apart.

Safety visit-Discontinued participants had these visits within approximately 120 days after the last dosing; Study Completion Pre-dose visit-Participants had these visits at the last treatment dosing, samples were taken before the last dosing; Early Discontinuation Pre-dose visit-Participants who discontinued the study had these visits within the 30 days after their last dosing but samples were takes before their last dosing; Study Completion visit-Participants had these visits at their last treatment dosing but samples were taken after their last dosing; Early Discontinuation visit-Participants who discontinued the study had the visits within the 30 days after their last dosing but samples were taken after their last dosing; Unscheduled and Unscheduled Pre-dose Visits- At these visits, participants' samples were collected without dosing event and before dosing dosing at a visit unscheduled per protocol respectively. Enzalutamide Arm had no Atezolizumab dosing.

Time frame: Predose (0 hr) and 1 hr postdose on Day 1 Cycle 1 and 3 (Cycle length: 21 days); pre-dose (within 1 hr) on Day 1 Cycle 8

Population: PK evaluable population

Safety visit-Discontinued participants had these visits within approximately 120 days after the last dosing; Study Completion Pre-dose visit-Participants had these visits at the last treatment dosing, samples were taken before the last dosing; Early Discontinuation Pre-dose visit-Participants who discontinued the study had these visits within the 30 days after their last dosing but samples were takes before their last dosing; Study Completion visit-Participants had these visits at their last treatment dosing but samples were taken after their last dosing; Early Discontinuation visit-Participants who discontinued the study had the visits within the 30 days after their last dosing but samples were taken after their last dosing; Unscheduled and Unscheduled Pre-dose Visits- At these visits, participants' samples were collected without dosing event and before dosing dosing at a visit unscheduled per protocol respectively. Enzalutamide Arm had no Atezolizumab dosing.

Time frame: Predose (0 hr) and 1 hr postdose on Day 1 Cycle 1 and 3 (Cycle length: 21 days); pre-dose (within 1 hr) on Day 1 Cycle 8

Population: PK evaluable population

rPFS is defined as the time from randomization to the earliest occurrence of one of the following: * A participant is considered to have progressed by bone scan if: The first bone scan with ≥2 new lesions compared to baseline is observed \< 12 weeks from randomization and is confirmed by a second bone scan taken ≥6 weeks later showing ≥2 additional new lesions (a total of ≥4 new lesions compared to baseline); the date of progression is the date of the first post-treatment scan, OR After the first post-treatment scan, ≥2 new lesions are observed relative to the first post-treatment scan, which is confirmed on a subsequent scan ≥6 weeks later; the date of progression is the date of the post-treatment scan when ≥2 new lesions were first documented. * Progression of soft tissue lesions, as defined per PCWG3 modified RECIST v1.1 * Death from any cause

Time frame: Baseline until disease progression or death from any cause (up to approximately 42 months)

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

An SSE is defined as external beam radiation therapy to relieve skeletal symptoms (including initiation of radium-223 dichloride or other types of radionuclide therapy to treat symptoms of bone metastases), new symptomatic pathologic bone fracture, clinically apparent occurrence of spinal cord compression, or tumor related orthopedic surgical intervention.

Time frame: Baseline up to end of study (up to approximately 42 months)

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants from whom data was collected and analyzed.

In participants with no PSA decline from baseline, PSA progression is defined as a ≥25% increase and an absolute increase of ≥2 ng/mL above the baseline value, ≥12 weeks after baseline. In participants with an initial PSA decline from baseline, PSA progression is defined as a ≥25% increase and an absolute increase of ≥2 ng/mL above the nadir value, which is confirmed by a consecutive second value obtained ≥3 weeks later.

Time frame: Baseline until disease progression (up to approximately 42 months)

Population: The intent-to-treat (ITT) population is defined as all randomized participants regardless of whether the assigned study treatment was received. For efficacy analyses, participants were analyzed according to their randomized treatment assignment. Here, Number Analyzed refers to number of participants in the ITT population with measurable soft tissue lesions at baseline.