⁃ Histologically confirmed HER2+ breast carcinoma as determined by a sponsor-designated central laboratory

• History of prior treatment with a taxane and trastuzumab in any setting, separately or in combination

• Have progression of unresectable locally advanced/metastatic breast cancer after last systemic therapy, or be intolerant of last systemic therapy

• Measurable or non-measurable disease assessable by RECIST v1.1

• ECOG performance status score of 0 or 1

• CNS Inclusion - Based on screening contrast brain magnetic resonance imaging (MRI), participants must have at least one of the following:

⁃ (a) No evidence of brain metastases

⁃ (b) Untreated brain metastases not needing immediate local therapy

⁃ (c) Previously treated brain metastases

‣ Brain metastases previously treated with local therapy may either be stable since treatment or may have progressed since prior local CNS therapy, provided that there is no clinical indication for immediate re-treatment with local therapy

‣ Participants treated with CNS local therapy for newly identified lesions or previously treated and progressing lesions may be eligible to enroll if all of the following criteria are met:

∙ (i) Time since SRS is at least 7 days prior to first dose of study treatment, time since WBRT is at least 14 days prior to first dose, or time since surgical resection is at least 28 days.

∙ (ii) Other sites of evaluable disease are present

‣ Relevant records of any CNS treatment must be available to allow for classification of target and non-target lesions