A Study of Multiple Immunotherapy-Based Treatment Combinations in Hormone Receptor (HR)-Positive Human Epidermal Growth Factor Receptor 2 (HER2)-Negative Breast Cancer

Inclusion Criteria for Both Stages:

Measurable disease per RECIST v1.1
Adequate hematologic and end organ function
Disease progression during or after first- or second-line hormonal therapy with CDK4/6 inhibitor

Inclusion Criteria for Stage 1:

Eastern Cooperative Oncology Group (ECOG) performance status 0-1
Metastatic or inoperable, locally advanced, histologically or cytologically confirmed invasive HR-positive HER2-negative breast cancer
Recommended for endocrine therapy, and cytotoxic chemotherapy not indicated at study entry
Recurrence or progression following most recent systemic breast cancer therapy
Disease progression during or after first- or second-line hormonal therapy for locally advanced or metastatic disease
Postmenopausal according to protocol-defined criteria
Life expectancy >3 months
Available tumor specimen for determination of PD-L1 status

Inclusion Criteria for Stage 2:

ECOG performance status of 0-2
Ability to initiate treatment within 3 months after disease progression or unacceptable toxicity on a Stage 1 regimen

Exclusion Criteria for Both Stages:

Significant or uncontrolled comorbid disease as specified in the protocol
Uncontrolled tumor-related pain
Autoimmune disease except for stable/controlled hypothyroidism, Type 1 diabetes mellitus, or certain dermatologic conditions
Positive human immunodeficiency virus test
Active hepatitis B or C
Active tuberculosis
Severe infection within 4 weeks and/or antibiotics within 2 weeks prior to study treatment
Prior allogeneic stem cell or solid organ transplantation
History of malignancy other than breast cancer within 2 years prior to screening except those with negligible risk of metastasis/death
History of or known hypersensitivity to study drug or excipients
For patients entering Stage 2, recovery from all immunotherapy-related adverse events to Grade 1 or better or to baseline at the time of consent

Exclusion Criteria for Stage 1:

Prior fulvestrant or cytotoxic chemotherapy for metastatic breast cancer, or certain other agents as specified in the protocol
Unresolved AEs from prior anti-cancer therapy
Eligibility only for the control arm
Prior treatment with inhibitors as specified in the protocol

Exclusion Criteria for Stage 2:

Unacceptable toxicity with atezolizumab during Stage 1
Uncontrolled cardiovascular disease or coagulation disorder, including use of anticoagulants as specified in the protocol
Significant abdominal or intestinal manifestations within 6 months prior to treatment
Grade 2 or higher proteinuria