Key Inclusion Criteria

All participants must meet all of the following criteria, as well as all criteria from the relevant sub-protocol to be eligible for enrollment:

1. * At least 18 years or the minimum legal adult age (whichever is greater) at the time the ICF is signed.
2. * Has at least 1 measurable lesion based on investigator imaging assessment (computed tomography or magnetic resonance imaging) using RECIST v 1.1 at Screening.
3. * Is willing to provide an adequate tumor sample.
4. * Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1 at Screening.

Additional Key Inclusion for Sub-Protocol A:

* Diagnosed with pathologically documented breast cancer that:

1. 1. Is unresectable or metastatic.
2. 2. Has progressed on and would no longer benefit from endocrine therapy in hormone receptor-positive subjects in the opinion of the investigator.
3. 3. Has been treated with at least 1 and at most 2 prior lines of chemotherapy in the recurrent or metastatic setting.
4. 4. Has a history of low HER2 expression, defined as IHC 2+ /ISH-negative or IHC 1+ (ISH-negative or untested). ), as classified by the American Society of Clinical Oncology/College of American Pathologists 2018 HER2 testing guidelines.
5. 5. Was never previously HER2-positive (IHC 3+ or IHC 2+/ISH+) on prior pathology testing (per American Society of Clinical Oncology/College of American Pathologists guidelines

Additional Key Inclusion for Sub-Protocol B:

• Gastric or GEJ adenocarcinoma that is (a) unresectable or metastatic or (b) has progressed on trastuzumab or approved trastuzumab biosimilar-containing regimen.

1. Additional Key Inclusion for Sub-Protocol C:
2. * Pathologically documented Stage IIIB, IIIC, or IV non-squamous NSCLC with or without AGA at the time of enrollment.
3. * Must meet prior therapy requirements:

1. * Participants without AGA: (a) received platinum-based chemotherapy in combination with α-PD-1/α -PD-L1 mAb as a prior line of therapy or (b) received platinum-based chemotherapy and α -PD-1/ α -PD-L1 mAb (in either order) sequentially as 2 prior lines of therapy.
2. * Participants with AGA: (a) has been treated with at least 1 or 2 prior lines of applicable targeted therapy that is locally approved for participant's genomic alteration at the time of Screening, (b) participants who have received platinum-based chemotherapy as a prior line of cytotoxic therapy, (c) may have received α -PD-1/α -PD-L1 mAb alone or in combination with a cytotoxic agent

Key Exclusion Criteria

1. * Has previously been treated with any enhancer of zeste homolog inhibitors.
2. * Uncontrolled or significant cardiovascular disease.
3. * Has spinal cord compression or clinically active central nervous system metastases, defined as untreated and symptomatic, or requiring therapy with corticosteroids or anticonvulsants to control associated symptoms.
4. * Has leptomeningeal carcinomatosis or metastasis.
5. * Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses.
6. * Current use of moderate or strong cytochrome P450 (CYP)3A inducers.
7. * Systemic treatment with corticosteroids (\>10 mg daily prednisone equivalents).
8. * History of severe hypersensitivity reactions to other monoclonal antibodies (mAbs).
9. * Evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection requiring treatment with intravenous (IV) antibiotics, antivirals, or antifungals.
10. * Female who is pregnant or breastfeeding or intends to become pregnant during the study.
11. * Psychological, social, familial, or geographical factors that would prevent regular follow-up.

Additional Key Exclusion for Sub-Protocol A:

1. * Has previously received any anti-HER2 therapy in the metastatic setting.
2. * Has received prior treatment with an antibody-drug conjugate that consists of an exatecan derivative that is a topoisomerase I inhibitor, including either as part of prior treatment history or within prior participation in a clinical study.

Additional Key Exclusion for Sub-Protocol B:

\* Participants who have received an antibody-drug conjugate consisting of an exatecan derivative that is a topoisomerase I inhibitor.

Additional Key Exclusion for Sub-Protocol C:

\* Has received any agent, including an ADC, containing a chemotherapeutic agent targeting topoisomerase I or TROP2-targeted therapy including Dato-DXD