G1T48, an Oral SERD, Alone and in Combination With Palbociclib in ER-Positive, HER2-Negative Advanced Breast Cancer

Inclusion Criteria: * For Part 1, postmenopausal women only * For Parts 2 and 3, any menopausal status * Confirmed diagnosis of ER-positive, HER2-negative advanced breast cancer, not amenable to curative therapy * For Part 1, prior treatment with less than 4 prior lines of chemotherapy * For Part 2, prior treatment with less than 2 prior line of chemotherapy * For Part 3, prior treatment with no more than 1 prior line of chemotherapy * For Parts 1 and 2, prior treatment with less than 4 prior endocrine therapies for metastatic breast cancer * For Part 3, prior treatment with no more than 1 prior line of endocrine therapies for metastatic breast cancer * For Parts 1 and 2, 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 * Progressed after the end of prior aromatase inhibitor therapy for advanced/metastatic breast cancer * For Part 3, patients must satisfy 1 of the following criteria for prior therapy: * Received ≥ 24 months of endocrine therapy in the adjuvant setting prior to recurrence or progression * Received ≥ 6 months of endocrine therapy in the advanced/metastatic setting prior to progression * For Part 1, evaluable or measurable disease * For Parts 2 and 3, evaluable (approximately 25%) or measurable disease (approximately 75%) as defined by RECIST, Version 1.1 including bone-only disease * ECOG performance status 0 to 1 * Adequate organ function Exclusion Criteria: * For Part 3, prior treatment with CDK4/6 inhibitor, investigational oral SERDs or SERCAs in any setting * Active uncontrolled/symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease * Anticancer therapy within 14 days of first G1T48 dose or within 28 days for antibody-based therapy * Concurrent radiotherapy, radiotherapy within 14 days of first G1T48 dose, previous radiotherapy to the target lesion sites, or prior radiotherapy to \> 25% of bone marrow * Prior hematopoietic stem cell or bone marrow transplantation