A Phase 1/2a, Multicenter, Open-Label, Non-Randomized First in Human Study to Assess the Safety, Tolerability, Pharmacokinetics, and Preliminary Antitumor Activity of DB-1310 in Subjects with Advanced/Metastatic Solid Tumors

This is a dose-escalation and dose-expansion Phase 1/2a trial to evaluate the safety and
tolerability of DB-1310 in subjects with advanced solid tumors.

Inclusion Criteria:

1. Male or female adults (defined as ≥ 18 years of age or acceptable age according to
local regulations at the time of voluntarily signing of informed consent).

2. Have relapsed or progressed on or after standard systemic treatments, or intolerable
with standard treatment, or for which no standard treatment is available. Documented
radiological disease progression during/after most recent treatment regimen for
advanced/unresectable, or metastatic disease.

3. At least one measurable lesion as assessed by the investigator according to response
evaluation criteria in solid tumors (RECIST) version 1.1 criteria. Subjects with
nonmeasurable disease only are allowed in Cohort 2c of Phase 2a.

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 functions within 7 days prior to Day 1 of Cycle 1.

8. Has adequate treatment washout period prior to Day 1 of Cycle 1.

9. Is willing to provide pre-existing resected tumor samples or undergo fresh tumor
biopsy for the measurement of HER3 level and other biomarkers if no contraindication.

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

11. Male and female subjects of reproductive/childbearing potential must agree to use
adequate contraceptive methods (e.g., double barrier or intrauterine contraceptive)
during the study and for at least 4 months and 7 months after the last dose of study
drug, respectively.

12. Male subjects must not freeze or donate sperm starting at screening and throughout the
study period, and at least 4 months after the final study drug administration.

13. Female subjects must not donate, or retrieve for their own use, ova from the time of
screening and throughout the study treatment period, and for at least 7 months after
the final study drug administration.

Exclusion Criteria:

1. Prior treatment with HER3 targeted therapy.

2. Prior treatment with antibody drug conjugate with topoisomerase I inhibitor (exclusive
of trastuzumab deruxtecan for Cohort 2e of Phase 2a).

3. Has a medical history of symptomatic congestive heart failure (CHF) (New York Heart
Association [NYHA] classes II-IV) or serious cardiac arrhythmia requiring treatment.

4. Has a medical history of myocardial infarction or unstable angina within 6 months
before enrollment.

5. Has any clinically important abnormalities in rhythm, conduction or morphology of
resting electrocardiogram (ECG), e.g., complete left bundle branch block, third-degree
heart block, second-degree heart block, or PR interval > 250 milliseconds (ms).

6. 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.

7. Unable or unwilling to discontinue concomitant drugs that are known to prolong the QT
interval.

8. Has a medical history of interstitial lung diseases (e.g., non-infectious interstitial
pneumonia, pneumonitis, pulmonary fibrosis, and severe radiation pneumonitis) or
current interstitial lung diseases or who are suspected to have these diseases by
imaging at screening.

9. Has an uncontrolled infection requiring intravenous injection of antibiotics,
antivirals, or antifungals.

10. Has clinically significant corneal disease.

11. Know human immunodeficiency virus (HIV) infection.

12. Subjects have active viral (any etiology) hepatitis are excluded. However, subjects
with positive hepatitis B surface antigen (HBsAg) who have the HBV DNA (viral load)
below the lower limit quantification or HBV DNA titer < 1000 cps/mL or 200 IU/mL per
local testing and are not currently on viral suppressive therapy may be eligible and
should be discussed with the Sponsor's Medical Monitor. However, subjects with a
history of hepatitis C virus (HCV) infection who have completed curative antiviral
treatment and have the HCV RNA below the lower limit of quantification per local
testing are eligible for study entry.

13. Is a lactating mother (women who are willing to temporarily interrupt breastfeeding
will also be excluded), or pregnant as confirmed by serum pregnancy tests performed
within 7 days prior to Cycle 1 Day 1.

14. Has clinically active brain metastases, defined as untreated and symptomatic, or
requiring therapy with steroids or anticonvulsants to control associated symptoms.
However, subjects with asymptomatic central nervous system (CNS) metastases who are
radiologically and neurologically stable for at least 4 weeks following CNS-directed
therapy, and who are on stable or decreasing doses of corticosteroids equivalent to
≤10 mg/day prednisone are eligible for study entry.

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