First-in-human Study of DB-1419 for Advanced/Metastatic Solid Tumors

Inclusion Criteria:

1. Adults aged ≥ 18 years at the time of voluntarily signing informed consent.

2. Histologically or cytologically confirmed unresectable advanced/metastatic solidtumor that has relapsed or progressed on or after standard systemic treatments, orrefused the standard treatment, or for which no standard treatment is available.

3. At least one measurable lesion as assessed by the Investigator according to RECISTv1.1 criteria (Only applicable to backfill participants in phase 1a and participantsin phase 1b/2a). CRPC participants with bone-only disease may be eligible on acase-by-case basis after discussion with the Medical Monitor.

4. Has a life expectancy of ≥ 3 months.

5. Has an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0-1.

6. Has LVEF ≥ 50% by either echocardiography (ECHO) or multiple-gated acquisition (MUGA) within 28 days before enrollment.

7. Has adequate organ function within 7 days prior to the first dose of studytreatment.

8. Has adequate treatment washout period prior to the first dose of study treatment.

9. Is willing to provide pre-existing resected tumor samples when available or undergofresh tumor biopsy if feasible for the measurement of B7-H3/PD-L1 level and otherbiomarkers if no contraindication. Note: there is no minimum B7-H3/PD-L1 expression level mandatory for entry into thestudy.

10. Is capable of comprehending study procedures and risks outlined in the informedconsent and able to provide written consent and agree to comply with therequirements of the study and the schedule of assessments.

Exclusion Criteria:

1. Prior treatment with B7-H3 targeted therapy.

2. Has a medical history of symptomatic congestive heart failure (New York HeartAssociation [NYHA] classes II-IV or serious cardiac arrhythmia requiring treatment.

3. Has a medical history of myocardial infarction or unstable angina within 6 monthsbefore enrollment.

4. Has an average of Fredericia's formula-QT corrected interval (QTcF) prolongation to > 470 millisecond (ms) in males and females based on a 12-lead electrocardiogram (ECG) in triplicate.

5. Has a medical history of interstitial lung diseases (e.g., non-infectiousinterstitial pneumonia, pneumonitis, pulmonary fibrosis, and severe radiationpneumonitis which needs glucocorticoids and antibiotics) or current interstitiallung diseases or who are suspected to have these diseases by imaging at screening.

6. Has a history of underlying pulmonary disorder including, but not limited to,pulmonary emboli within 3 months of the start of study treatment, severe asthma,severe COPD, restrictive lung disease, and other clinically significant pulmonarycompromise or requirement for supplemental oxygen.

7. Has an active autoimmune disease that has required systemic treatment in past 2years (i.e., with use of disease modifying agents, corticosteroids, orimmunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, orphysiologic corticosteroid replacement therapy for adrenal or pituitaryinsufficiency) is allowed.

8. Has an uncontrolled infection requiring intravenous injection of antibiotics,antivirals, or antifungals within 2 weeks before first dose of study treatment.

9. Know human immunodeficiency virus (HIV) infection.

10. Has spinal cord compression or clinically active central nervous system (CNS)metastases, defined as untreated and symptomatic, or requiring therapy withcorticosteroids or anticonvulsants to control associated symptoms. Participants withasymptomatic CNS metastases who are radiologically and neurologically stable for atleast 4 weeks following CNS-directed therapy (defined as 2 brain images, sameimaging modality, both of which are obtained after treatment to the brainmetastases; these imaging scans should be obtained at least 4 weeks apart and showno evidence of intracranial progression), and are on stable or decreasing doses ofcorticosteroids equivalent to ≤10 mg/day prednisone are eligible for study entry.

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