This research study is studying the combination of a drug called atezolizumab and a radiation procedure called stereotactic radiosurgery (SRS) as a possible treatment for triple-negative breast cancer that has spread to the brain.

The interventions involved in this study are:

• Atezolizumab
• Stereotactic radiosurgery (SRS)

This research study is a Phase II clinical trial. Phase II clinical trials test the safety and effectiveness of an investigational intervention to learn whether the intervention works in treating a specific disease. "Investigational" means that the intervention is being studied.

The FDA (the U.S. Food and Drug Administration) has not approved atezolizumab for this specific disease but it has been approved for other uses.

Atezolizumab is a protein that affects the immune system by blocking the PD-L1 pathway. The PD-L1 pathway controls the body's natural immune response, but tumors can interrupt this pathway and partially resist or escape the immune system. By blocking the PD-L1 pathway, Atezolizumab may help the immune system identify and catch tumor cells.

Stereotactic radiosurgery (SRS) is a standard procedure used to treat patients with cancer in the brain. SRS uses many precisely focused radiation beams to treat tumors. It is not surgery in the traditional sense because there's no incision. Instead, SRS uses 3-D imaging to target high doses of radiation to the affected area with minimal impact on the surrounding healthy tissue. Like other forms of radiation, SRS works by damaging the DNA of the targeted (tumor) cells. The affected cells then lose the ability to reproduce, which causes tumors to shrink.

When given separately, atezolizumab and SRS, work in different ways to help stop cancer cells from growing and spreading. However, it is not known if giving atezolizumab and SRS at the same time will have a better effect than giving each treatment on its own. It is hoped that SRS treatment will damage cancer cells and make them more visible to the immune system.

The researchers conducting this study are testing to see whether giving SRS with atezolizumab may boost the body's immune response to cancer, and therefore improve upon the effects of either SRS or atezolizumab given alone.

In this research study, the investigators will measure the length of time that the participant receive this study intervention without the disease getting worse. The investigators will also look at how well the disease responds to atezolizumab and SRS as well as the safety of the combination.

Inclusion Criteria:

• Participants must have histologically or cytologically confirmed Stage IV invasive breast cancer. Participants without pathologic or cytologic confirmation of metastatic disease should have unequivocal evidence of metastasis from physical examination or radiologic evaluation.

• Either the primary tumor and/or metastatic tumor must be triple-negative as defined below:

  • Hormone receptor status: the invasive tumor must be ER- and PR-negative, or staining present in <1% by immunohistochemistry (IHC)
  • HER2 status: the invasive tumor must be Human Epidermal Growth Factor Receptor 2 Negative (HER2-negative) by the ASCO CAP guidelines

• In cases where both primary tumor and metastatic sample(s) have been tested for ER, PR, and HER2, the triple-negative status of the most recent sample should be used.

• Participants must have a diagnosis of brain metastases for which SRS is indicated, as determined by a radiation oncologist.

• The contrast-enhancing intraparenchymal brain metastases(s) must be well circumscribed and must have a maximum diameter of ≤ 3.0 cm in any direction on the enhanced scan.

• Participants must not have more than 5 new or progressive lesions in the brain requiring SRS treatment (greater than 5 total brain lesions are allowed as long as no more than 5 lesions require SRS treatment).

• Participants must have measurable extracranial disease as defined by RECIST 1.1.

• Participants must be willing to undergo a research biopsy at baseline and at Cycle 2 Day 1 if extracranial metastases are safely accessible. Participants for whom biopsies cannot be safely performed must be willing to submit an archival primary and/or metastatic specimen. The biopsies may be waived with prior PI approval for the first 6 participants enrolled to the safety run in phase.

• Prior systemic therapy:

  • Participants must have discontinued systemic therapy at least 14 days prior to initiating protocol therapy.
  • There is no limit to the number of prior lines of systemic therapy. Participants who have not received any systemic therapy for metastatic disease are also eligible.
  • Participants may initiate or continue bisphosphonate therapy on study.

• Prior local therapy:

• Prior surgery, whole brain radiation or SRS is allowed as long as the most recent brain progression is amenable to SRS treatment.

• Resolution of all chemotherapy-related or radiation-related toxicities to Grade 1 severity or lower, except for stable sensory neuropathy (≤ Grade 2 allowed) and alopecia (of any grade).

• Participant is ≥18 years old.

• ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A)

• Stable dose of dexamethasone 2mg or less for at least 7 days prior to initiation of treatment

• Participants must have normal organ and marrow function as defined below:

  • absolute neutrophil count ≥1,000/μl
  • platelets ≥75,000/μl
  • hemoglobin ≥9 g/dL
  • total bilirubin ≤1.5mg/dL (≤2.0 in patients with known Gilberts syndrome)
  • AST(SGOT)/ALT(SGPT) ≤2.5 × institutional ULN. ≤5.0 × institutional ULN for patients with documented liver metastases.
  • albumin >2.5mg/dL
  • serum creatinine ≤1.5mg/dL or calculated GFR ≥60 mL/min

• Female subjects of childbearing potential must have a negative serum or urine pregnancy test within 8 days of initiating protocol therapy.

• For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of < 1% per year during the treatment period and for at least 90 days after the last dose of study treatment. A woman is considered to be of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state (≥ 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). Examples of contraceptive methods with a failure rate of < 1% per year include bilateral tubal ligation, male sterilization, established, proper use of hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception. The effects of atezolizumab on the developing human fetus are unknown and radiotherapy has known teratogenic effects so women of child-bearing potential and men must agree to use adequate contraception (barrier method of birth control; abstinence) prior to study entry and for the duration of study participation and 4 months after completion of atezolizumab administration.

• The subject is capable of understanding and complying with the protocol and has signed the informed consent document

Exclusion Criteria:

• CNS complications for whom urgent neurosurgical intervention is indicated (e.g., resection, shunt placement).
• Known leptomeningeal or brainstem metastases. The presence of leptomeningeal enhancement alone, without associated clinical manifestations and/or positive CSF cytology, will not be constituted as known leptomeningeal metastases.
• Treatment with high dose systemic corticosteroids defined as dexamethasone >2mg/day or bioequivalent within 7 days of initiating therapy.
• Patients unable to undergo gadolinium contrast-enhanced MRI or receive IV contrast for any reason (e.g., due to pacemaker, ferromagnetic implants, claustrophobia, extreme obesity, hypersensitity).
• Participants who are receiving any other investigational agents.
• Previous treatment with any anti-PD-1, PD-L1, or PD-L2 agent.
• Subjects with a history of hypersensitivity to compounds of similar biologic composition to atezolizumab or any constituent of the product
• The participant has an uncontrolled intercurrent illness, including, but not limited to uncontrolled hypertension, unstable angina pectoris, uncontrolled cardiac arrhythmia, congestive heart failure-New York Heart Association Class III or IV, active ischemic heart disease, myocardial infarction within the previous six months, uncontrolled diabetes mellitus, gastric or duodenal ulceration diagnosed within the previous 6 months, severe malnutrition or psychiatric illness/social situations that would limit compliance with study requirements.
• Participant has a medical condition that requires chronic systemic steroid therapy or on any other form of immunosuppressive medication. For example, patients with autoimmune disease that requires systemic steroids or immunosuppression agents should be excluded. Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment.
• Has evidence of active, noninfectious pneumonitis that requires treatment with steroids.
• Has a history of interstitial lung disease.
• The participant is known to be positive for the human immunodeficiency virus (HIV), HepBsAg, or HCV RNA. HIV-positive participants on combination antiretroviral therapy are ineligible because of the potential for pharmacokinetic interactions with atezolizumab.
• Individuals with a history of different malignancy are ineligible except for the following circumstances. Individuals with a history of other malignancies are eligible if they have been disease-free for at least 3 years or are deemed by the principal investigator to be at low risk for recurrence of that malignancy.
• Has received a live vaccine within 28 days of planned start of study therapy.
• The participant is pregnant or breast-feeding.

Adverse event data were collected on the first day of each treatment cycle, at the end of treatment, and up to 30 days post-end of treatment, for up to a maximum of 1.8 years. Death events were assessed for up to a maximum of 3.8 years.