Inclusion Criteria

• • Histologically and/or cytologically documented adenocarcinoma of the colon or rectum that is metastatic and/or unresectable
• • Unless contraindicated, participants must have received and failed regimens containing the following agents: fluoropyrimidine (e.g., 5-fluorouracil or capecitabine), oxaliplatin, irinotecan, an anti-VEGF monoclonal antibody (bevacizumab, ramucirumab, or ziv-aflibercept), and an anti-PD-(L)1 therapy (nivolumab or pembrolizumab) if tumor has deficient mismatch repair proteins or is MSI-High.
• • Have progression of unresectable or metastatic CRC after the last systemic therapy, or be intolerant of last systemic therapy
• • Have RAS wild-type in primary or metastatic tumor tissue, based on expanded RAS testing
• • Willing and able to provide the most recent tissue blocks obtained prior to treatment initiation. If archival tissue is not available, then a newly-obtained baseline biopsy of an accessible tumor
• • Have confirmed HER2-positive mCRC, as defined by having tumor tissue tested at a Clinical Laboratory Improvement Act (CLIA)-certified laboratory, meeting at least one of the following criteria:
• • HER2+ overexpression (3+ immunohistochemistry [IHC])by an FDA-approved HER2 ICH test
• • HER2 2+ IHC is eligible if the tumor is amplified by an FDA-approved HER2 in situ hybridization assay (FISH or chromogenic in situ hybridization [CISH]))
• • HER2 (ERBB2) amplification by CLIA-certified next generation sequencing (NGS) sequencing assay
• • Have radiographically measurable disease assessable by RECIST 1.1, with at least one site of disease that is measurable and that has not been previously irradiated; or, if the participant has had previous radiation to the target lesion(s), there must be evidence of progression since the radiation
• • Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0, 1, or 2
• • Life expectancy greater than 3 months
• • Have adequate hematological, hepatic, renal, coagulation, and cardiac function

Exclusion Criteria

• • Previous treatment with anti-HER2 targeting therapy
• • Previous treatment with any systemic anticancer therapy, non-central nervous system radiation, or experimental agent within 3 weeks of first dose of study treatment
• • Toxicity related to prior cancer therapies that has not resolved to = Grade 1, with the following exceptions:
• • Alopecia and neuropathy, which must have resolved to = Grade 2
• • Congestive heart failure (CHF), which must have been = Grade 1 in severity at the time of occurrence, and must have resolved completely
• • Anemia, which must have resolved to = Grade 2
• • Decreased ANC, which must have resolved to = Grade 2
• • Have clinically significant cardiopulmonary disease
• • Have known myocardial infarction, unstable angina, cardiac or other vascular stenting, angioplasty, or cardiac surgery within 6 months prior to first dose of study treatment
• • Major surgical procedure, open biopsy, or significant traumatic injury =28 days prior to enrollment (=56 days for hepatectomy, open thoracotomy, or major neurosurgery) or anticipation of need for major surgical procedure during the study
• • Serious, non-healing wound, ulcer, or bone fracture
• • Known to be positive for hepatitis B by surface antigen expression
• • Known to have active hepatitis C infection

o Exception for participants with a documented sustained virologic response of 12 weeks

• • Known to be positive for human immunodeficiency virus (HIV)
• • Subjects who are pregnant, breastfeeding, or planning a pregnancy
• • Inability to swallow pills or any significant gastrointestinal disease which would preclude the adequate oral absorption of medications
• • Have used strong CYP2C8 inhibitor within 5 half-lives of the inhibitor, or have used a CYP2C8 or CYP3A4 inducer within 5 days prior to first dose of study treatment
• • History of another malignancy within 3 years before the first dose of study drug, or any evidence of residual disease from a previously diagnosed malignancy.

o Exceptions are malignancies with a negligible risk of metastasis or death

• • Subjects with known active CNS metastasis o Irradiated or resected lesions are permitted, provided the lesions are fully treated and inactive, subject is asymptomatic, and no steroids have been administered for at least 30 days