• Men or women with HER2+\* metastatic breast cancer. \*HER2+ status will be defined in accordance with ASCO-CAP 2018 guidelines, and can be diagnosed at any time prior to enrolment;

• Evidence of LMD\* in the brain and/or spine (either positive cerebral spinal fluid cytology and/or magnetic resonance imaging evidence of LMD). Measurable disease in the central nervous system is not required. \* The diagnosis of LMD can occur at any time prior to enrolment;

• Age 18+ at time of consent;

• ECOG ≤ 2;

• More than 14 days or 5 half-lives from the last dose of any experimental agent is required, whichever is greater;

• All toxicity related to prior cancer therapies must have resolved to ≤ Grade 1 prior to enrollment, except for alopecia; neuropathy, must have resolved to ≤ Grade 2.

∙ Phase 2: Inclusion Criteria

• Left ventricular ejection fraction (LVEF) must be within institutional limits of normal as assessed by ECHO or MUGA documented within 2 weeks prior to starting systemic therapy on the study;

• Adequate hematologic, liver, and renal function within 2 weeks prior to phase 2 enrollment, as follows:

‣ Hemoglobin ≥ 9 g/dL

⁃ ANC ≥ 1 x109/L

⁃ Platelets ≥ 100 x109/L

⁃ Total bilirubin ≤ 1.5 X upper limit of normal (ULN)

⁃ AST and ALT ≤ 2.5X ULN

⁃ International normalized ratio (INR) and activated partial thromboplastin time (aPTT) ≤ 1.5 X ULN

⁃ Creatinine clearance (CrCL) ≥ 50 mL/min

• The last dose of prior therapy must have been completed 14 days prior to study enrollment. Prior chemotherapy, immunotherapy, endocrine therapy, targeted therapy and experimental agents are allowed (including prior use of trastuzumab or other antibody-based therapy). Prior use of capecitabine either alone or in combination with other HER2-targeted therapies (including other tyrosine kinase inhibitors) is permitted;