• HER2 positive solid tumors with expression defined as IHC3+, IHC2+/ISH+, or low HER2 expression defined as IHC2+ (ISH-) or IHC1+ (ISH- /+ or untested).

• Participants with HR positive BC must have received prior treatment with a CDK4/6 inhibitor, in countries where this is standard therapy.

• Platelets ≥ 75,000 /mcL

• Hemoglobin ≥ 9.0 g/dL

• Absolute neutrophil count ≥ 1000/mcL

• No administration of granulocyte colony-stimulating factor (G-CSF) is allowed within 2 weeks prior to first study drug administration

• Creatinine clearance \> 45/mL/min (using the Cockcroft-Gault equation)

• Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) ≤ 3 x institutional upper limit of normal (ULN) ≤ 5 x ULN if liver metastases present

• Total bilirubin ≤ 1.5 x ULN if no liver metastases or \< 3 x ULN with Gilbert's Syndrome or liver metastases at baseline

• Albumin \> 2.5 g/dL

• Prothrombin time or international normalized ratio (INR) and either partial thromboplastin time (PTT) or activated (a) PTT ≤ 1.5 x ULN, ≤ 3 x institutional ULN if anticoagulated.

• Must have adequate treatment washout period before trial treatment, defined as: Major surgery (≥ 4 weeks) and radiation therapy (≥ 3 weeks; in case of palliative radiation ≥ 2 weeks)

• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1 (or equivalent Karnofsky PS)

• Part 2 Dose Expansion Cohorts May Include:

‣ Advanced or metastatic BC that is confirmed HER2-positive defined as IHC 3+ or IHC 2+ and evidence of HER2 amplification by ISH, as per ASCO-CAP and previously treated with at least two HER2 directed treatments.

⁃ Advanced or metastatic BC that has low HER2 expression defined as IHC2+ (ISH-) or IHC1+ (ISH-/+ or untested) and previously treated with at least 1 prior line of therapy which may include chemotherapy and/or a HER2 directed ADC.

⁃ Advanced or metastatic GC or GEJ cancer that is confirmed HER2-positive defined as IHC 3+ or IHC 2+ and evidence of HER2 amplification by ISH as per ASCO-CAP and previously treated with at least 1 prior line of therapy, which may include chemotherapy and/or a HER2 directed ADC.

⁃ Advanced or metastatic GC or GEJ cancer that has low HER2 expression defined as IHC2+ (ISH-) or IHC1+ (ISH-/+ or untested) and has been previously treated with at least one prior line of therapy.

⁃ Advanced or metastatic solid tumor that has any degree of HER2 expression (HER2 IHC3+, IHC2+, IHC1+ or ISH+) or a known activating HER2 mutation and has been treated with standard of care therapy relevant to the disease.