A Study To Assess The Tolerability And Clinical Activity Of Gedatolisib In Combination With Palbociclib/Letrozole Or Palbociclib/Fulvestrant In Women With Metastatic Breast Cancer

Inclusion Criteria: * Women 18 years of age or older, who are either: Postmenopausal or Pre/perimenopausal women with medically-induced menopause by treatment with agents to induce chemical menopause. * Histologically or cytologically proven diagnosis of breast cancer with evidence of metastasis. * Documentation of estrogen receptor positive ((ER+), human epidermal growth factor receptor 2 (HER2 negative (HER2-)) tumor. * Dose Escalation Portion: Patients must satisfy one of the following criteria: * Letrozole combination cohort (L): metastatic breast cancer (MBC) with progression who are candidates for a letrozole-containing regimen, with palbociclib. * Fulvestrant combination cohort (F): MBC with progression who are candidates for a fulvestrant containing regimen, with palbociclib. * Dose Expansion Portion: Patients must satisfy one of the following criteria: * Arm A: MBC with progression and no prior endocrine based systemic therapy in the metastatic setting; * Arm B: MBC with progression during or following one prior endocrine based systemic therapy in the metastatic setting, with no prior therapy with any cyclin-dependent kinase (CDK) inhibitor; * Arm C/Arm D: MBC with progression during or following one or two prior endocrine based systemic therapies in the metastatic setting, and following prior therapy with a CDK inhibitor. * Measurable disease as defined by Response Evaluation Criteria In Solid Tumors (RECIST) version 1.1. * Bone only patients during dos…