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.