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 dose escalation portion.
* Availability of archival tumor biopsy sample or willing to provide fresh biopsy if not available.
* Eastern Cooperative Oncology Group \[ECOG\] performance must be 0 or 1.
* Adequate bone marrow, renal and liver function.

Exclusion Criteria:

* Prior treatment with a mechanistic target of rapamycin (mTOR) inhibitor or phosphoinositide 3-kinase (PI3K) inhibitor.
* More than 1 line of prior chemotherapy in the treatment of metastatic or locally advanced/recurrent disease.
* Bone only patients during expansion/efficacy portion.
* Patients with advanced/metastatic disease who have symptomatic visceral spread, and who have life threatening complications needing immediate therapy, such as massive uncontrolled effusions \[pleural, pericardial, peritoneal\], pulmonary lymphangitis, and over 50% liver replacement with tumor.
* Known active uncontrolled or symptomatic Central Nervous System (CNS) metastases.
* Active bacterial, fungal or viral infection.
* Uncontrolled or significant cardiovascular disease.
* Radiation therapy within 4 weeks of investigational product.
* Cytotoxic chemotherapy within 4 weeks of investigational product (6 weeks for mitomycin C or nitrosoureas) if immediate prior regimen was administered on an every 3 4 week schedule or 2 weeks of investigational product if immediate prior regimen consisted of weekly therapy.
* Any other anti cancer agents (eg, hormonal, biological, investigational) within 5 times the half life prior to investigational product.
* Impairment of gastro intestinal (GI) function or GI disease.
* Pregnant female patients; breastfeeding female patients; and female patients of childbearing potential who are unwilling or unable to use 2 highly effective methods of contraception as outlined in this protocol for the duration of the study and for 90 days.