The goal of this clinical trial is to determine the MTD or OBED of DA-3501 given in Q3W to determine a wRP2D in patients with advanced CLDN18.2 expressing (CLDN18.2+) GC/GEJ and advanced CLDN18.2+ PDAC.

Participants will receive the assigned dose once every three weeks and, according to the study procedures, will undergo tumor assessments as well as safety assessments, PK evaluations, and ADA testing.

Inclusion Criteria:

1. Patients aged ≥ 19 years
2. Histologically or cytologically documented advanced CLDN18.2+ GC/GEJ adenocarcinoma or advanced CLDN18.2+ PDAC
3. Measurable disease as defined by Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1)
4. Life expectancy ≥12 weeks as judged by the Investigator

Exclusion Criteria:

1. Cardiac abnormalities
2. Gastrointestinal abnormalities
3. Active and clinically significant bacterial, fungal, or viral infection, including known hepatitis B virus (HBV), known hepatitis C virus (HCV), known human immunodeficiency virus (HIV).
4. Active systemic infection requiring IV antibiotics therapy.