Inclusion Criteria:

          1. Be informed of the investigational nature of the study and all pertinent aspects of
             the trial

          2. Sign and provide written consent in accordance with institutional and federal
             guidelines.

          3. ECOG Performance status of 0-2

          4. Recurrent or metastatic HER2-positive breast cancer (HER2 positive is defined per
             ASCO-CAP guidelines)

          5. Adequate cardiac reserve (EF≥50%)

          6. Serum creatinine ≤ 1.5 x institutional upper limit of normal (IULN), bilirubin ≤ 2.0,
             and an SGOT/SGPT/alkaline phosphatase ≤ 2.0 x IULN

          7. Adequate bone marrow function (ANC ≥1000, Platelets ≥100,000/ml, Hemoglobin ≥10gm/dL)

          8. Be willing and able to comply with scheduled visits, treatment plan, laboratory tests
             and other trial procedures

          9. Been treated with pertuzumab previously (neoadjuvant or metastatic setting). Patients
             who weren't able to tolerate pertuzumab due to side effects can be eligible for study
             upon discussion with the study PI

         10. No more than 2 lines of therapy in the metastatic disease setting

        Exclusion Criteria:

          1. HER2 negative tumors

          2. Prior treatment with T-DM1

          3. Prior treatment with CDK 4/6 inhibitors

          4. Known active CNS metastases or carcinomatous meningitis. Patients with stable CNS
             metastases including brain metastases who have completed a course of radiotherapy are
             eligible for the study provided they are clinically stable. However, oral
             corticosteroids for control of CNS symptoms are not allowed on study

          5. Known documented or suspected hypersensitivity to the components of the study drug(s)
             or analogs.

          6. Uncontrolled systemic illness, including but not limited to ongoing or active
             infection

          7. Symptomatic congestive heart failure, unstable angina pectoris, stroke or myocardial
             infarction within 3 months

          8. Be pregnant or breast feeding. Female subjects must be surgically sterile or be
             postmenopausal, or must agree to use effective contraception during the period of
             therapy. All female subjects with reproductive potential must have a negative
             pregnancy test (serum or urine) prior to enrollment and must agree to use effective
             contraception during the period of therapy

          9. Concurrent hormonal or other anti-neoplastic therapy is not allowed. Patients can
             receive supportive therapy like bone-directed therapy including bisphosphonates or
             denosumab