G1T38, a CDK 4/6 Inhibitor, in Combination With Fulvestrant in Hormone Receptor-Positive, HER2-Negative Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: * Confirmed diagnosis of HR-positive, HER2-negative breast cancer, not amenable to curative therapy * Pre- or perimenopausal women can be enrolled if amenable to be treated with goserelin * Patients must satisfy 1 of the following criteria for prior therapy: * Progressed during treatment or within 12 months of completion of adjuvant therapy with an aromatase inhibitor or tamoxifen * Progressed during treatment or within 2 months after the end of prior aromatase inhibitor therapy for advanced/metastatic breast cancer, or prior endocrine therapy for advanced/metastatic breast cancer * Received ≤ 2 chemotherapy regimens (Part 1) or ≤ 1 chemotherapy regimen (Part 2) for advanced/metastatic disease * For Part 1, evaluable or measurable disease (bone only disease eligible for Part 1 only) * For Part 2, measurable disease as defined by RECIST, Version 1.1 * ECOG performance status 0 to 1 * Adequate organ function Exclusion Criteria: * For Part 1, prior treatment with fulvestrant * For Part 2, prior treatment with any CDK inhibitor or fulvestrant * Active uncontrolled/symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease * Chemotherapy within 21 days of first G1T38 dose * Investigational drug within 28 days of first G1T38 dose * Concurrent radiotherapy, radiotherapy within 14 days of first G1T38 dose, previous radiotherapy to the target lesion sites, or prior radiotherapy to \> 25% of bone marrow * Prior hematopoietic stem cell or bone marrow transplantation