Inclusion Criteria:

          1. Female, Age ≥ 18 years

          2. Histologic or cytologic confirmation of human epidermal growth factor receptor 2
             (HER2)-positive breast cancer according to most recent biopsy (local testing
             permitted)

          3. Measurable or evaluable metastatic disease by Response Evaluation Criteria in Solid
             Tumors (RECIST) (v1.1)

          4. Previous treatment with ado-trastuzumab emtansine (T-DM1) for metastatic disease

             a. Prior therapy with pertuzumab is allowed but not required

          5. At least 1 but no more than 3 prior chemotherapy regimens for metastatic breast cancer
             (MBC)

          6. Life expectancy > 6 months

          7. Eastern Cooperative Group (ECOG) performance status ≤ 2

          8. Left Ventricular Ejection Fraction (LVEF) ≥ 50% at baseline as determined by either
             echocardiogram (ECHO) or Multi Gated Acquisition Scan (MUGA) and within normal limits
             per institutional guidelines

          9. Adequate bone marrow function as indicated by the following:

               1. Absolute Neutrophil Count (ANC) ≥1500/uL (or 1500 per microliter)

               2. Platelets ≥100,000/uL

               3. Hemoglobin >9 g/dL

         10. Adequate renal function, as indicated by creatinine <1.5 times upper limit of normal
             (ULN)

         11. Adequate liver function, as indicated by bilirubin <1.5 times ULN

         12. Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT) <2 x ULN unless
             liver metastases are present in which case AST and ALT up to 5.x ULN are allowed

         13. Negative serum pregnancy test within 72 hours before starting study medications for
             women of childbearing potential

         14. Women of childbearing potential must be willing to use an acceptable form of birth
             control (ie, intra-uterine device, diaphragm with spermicide, condom with spermicide,
             or abstinence) for the duration of the study Note: Women are considered postmenopausal
             and not of childbearing potential if they have had 12 months of natural (spontaneous)
             amenorrhea with an appropriate clinical profile (e.g. age appropriate, history of
             vasomotor symptoms), or if they have undergone surgical sterilization.

         15. Signed informed consent obtained prior to any screening procedures.

        Exclusion Criteria:

        Patients will be excluded from the study based on the following criteria:

          1. Prior treatment in the metastatic setting with the agent chosen as physician's choice
             of chemotherapy

          2. Active infection

          3. Uncontrolled central nervous system metastases, defined as clinical or radiologic
             evidence of progression of brain metastases or clinical signs of leptomeningeal
             disease

             a. Patients with treated brain metastases are eligible provided they do not have
             clinical or radiologic evidence of disease progression and have been off of
             dexamethasone for at least 3 weeks

          4. Patient is pregnant or lactating

          5. Prior chemotherapy within the last 3 weeks (last 6 weeks for nitrosureas/mitomycin)

          6. Prior radiation therapy within the last 2 weeks; prior radiation therapy to indicator
             lesion (unless objective disease recurrence or progression within the radiation portal
             has been documented since completion of radiation).

          7. Requirement for chronic steroid therapy with a requirement for > 5mg/day of prednisone
             or the equivalent.

             a. Treatment with physiologic doses of hydrocortisone up to 20 mg daily (QD) is
             allowed.

          8. Requirement for immunosuppressive therapy, such as those used to treat autoimmune
             disease.

          9. Concomitant malignancies or previous malignancies within the last 3 years, with the
             exception of adequately treated basal or squamous cell carcinoma of the skin or
             carcinoma in situ of the cervix.

         10. History of significant cardiac disease, cardiac risk factors or uncontrolled
             arrhythmias

         11. Ejection fraction <50% or below the lower limit of the institutional normal range,
             whichever is lower

         12. Known hypersensitivity to trastuzumab or pertuzumab

         13. Serious medical or psychiatric limitations likely to interfere with participation in
             this study.

         14. Symptomatic intrinsic lung disease or extensive tumor involvement of the lungs,
             resulting in dyspnea at rest or requiring supplemental oxygen

         15. Patient is currently part of or has participated in any clinical trial of an
             investigational agent within 1 month prior to enrollment in this study.