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

Percentage of Participants Who Survived at Month 6 and 12

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

Time to First Symptomatic Skeletal Event (SSE)

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)

Radiographic Progression-Free Survival (rPFS), as Assessed by the Investigator and Adapted From the PCWG3 Criteria

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)

Percentage of Participants Who Are Radiographic Progression-Free, as Assessed by the Investigator and Adapted From the PCWG3 Criteria

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

Percentage of Participants With Greater Than (>) 50 Percent (%) Decrease in Prostate-Specific Antigen (PSA) From Baseline

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)

Time to PSA Progression, Assessed as Per PCWG3 Criteria

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)

Percentage of Participant With Objective Response, as Determined by the Investigator Through Use of PCWG3 Criteria

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)

Percentage of Participants With Adverse Events

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)

Minimum Observed Serum Concentration (Cmin) of Atezolizumab

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)

Maximum Observed Serum Concentration (Cmax) of Atezolizumab

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])

Plasma Concentration of Enzalutamide

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

Plasma Concentration of N-Desmethyl Enzalutamide

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

Number of Participants With Anti-Drug Antibodies (ADAs) to Atezolizumab

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