Key Inclusion Criteria:

          1. Disease diagnosis and prior treatment:

               1. Cohort 1 (the first-line breast cancer treatment cohort):

                    -  Female participants with histologically or cytologically confirmed
                       unresectable, locally advanced, recurrent or metastatic adenocarcinoma of
                       the breast and candidate for chemotherapy. Locally recurrent disease must
                       not be amenable to resection with curative intent.

                    -  Human epidermal growth factor receptor 2 (HER2) IHC 3+ or in situ
                       hybridization positive on the archival tumor tissue or fresh biopsy sample.

                    -  Have not received previous systemic anticancer therapy for locally advanced
                       unresectable or metastatic disease.

               2. Cohort 2 (the first-line gastric/gastroesophageal junction adenocarcinoma
                  treatment cohort):

                    -  Histologically or cytologically confirmed unresectable, locally advanced,
                       recurrent or metastatic adenocarcinoma of the stomach or gastroesophageal
                       junction

                    -  HER2 IHC 3+ or HER2 IHC 2+ together with in situ hybridization positive on
                       the archival tumor tissue or fresh biopsy sample.

                    -  Have not received previous systemic anticancer therapy for locally advanced
                       unresectable or metastatic disease, including any approved or
                       investigational estimated glomerular filtration rate (EGFR) or anti-HER2
                       agents or vaccines, cytotoxic chemotherapy or checkpoint inhibitors

          2. At least 1 measurable lesion as defined per RECIST Version 1.1

          3. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1

          4. Adequate organ function as indicated by the following laboratory values during
             screening:

          5. Left ventricular ejection fraction (LVEF) ≥ 50% at baseline as determined by either
             echocardiogram or multigated acquisition scan (MUGA) (echocardiogram is the preferred
             method) within 28 days before the first dose of study drug

        Key Exclusion Criteria:

          1. Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 or any other antibody or drug
             specifically targeting T-cell co-stimulation or checkpoint pathways

          2. History of approved or investigative tyrosine kinase/HER inhibitors in any treatment
             setting

             a. except trastuzumab with or without pertuzumab used in neoadjuvant or adjuvant
             setting for Cohort 1

          3. Active leptomeningeal disease or uncontrolled brain metastasis. Participants with
             equivocal findings or with confirmed brain metastases are eligible for enrollment
             provided that they are asymptomatic and radiologically stable without the need for
             corticosteroid treatment for ≥ 4 weeks before the first dose of study drug

          4. Any active malignancy ≤ 2 years before the first dose of study drug, except for the
             specific cancer under investigation in this trial and any localized cancer that has
             been treated curatively (eg, resected basal or squamous cell skin cancer, superficial
             bladder cancer, carcinoma in situ of the cervix or breast)

          5. Any condition that required systemic treatment with either corticosteroids (> 10 mg
             daily of prednisone or equivalent) or other immunosuppressive medication ≤ 14 days
             before the first dose of study drug

        Note: Participants who are currently or have previously been on any of the following
        steroid regimens are not excluded:

          1. Adrenal replacement steroid (dose ≤ 10 mg daily of prednisone or equivalent)

          2. Topical, ocular, intra-articular, intranasal, or inhaled corticosteroid with minimal
             systemic absorption

          3. Short course (≤ 7 days) of corticosteroid prescribed prophylactically (eg, for
             contrast dye allergy) or for the treatment of a non-autoimmune condition (eg,
             delayed-type hypersensitivity reaction caused by contact allergen)

        NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.