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