A Study of ASP546C in Adults With Gastroesophageal Cancer, Pancreatic Cancer or Other Solid Tumors

Inclusion Criteria: * Participant has a histologically confirmed diagnosis of gastroesophageal (gastric/GEJ/esophageal) adenocarcinoma, pancreatic adenocarcinoma, or pan-tumor (cholangiocarcinoma, colorectal adenocarcinoma, NSCLC \[adenocarcinoma\], SCLC, ovarian mucinous carcinoma or invasive breast cancer \[ER/PR+HER2-; ER/PR-HER2+; ER/PR+HER2+ (triple positive); ER/PR-HER2- (triple negative)\]. * Participant has radiologically confirmed uLA/m gastroesophageal (gastric/GEJ/esophageal) adenocarcinoma, pancreatic adenocarcinoma or pan-tumor within 28 days prior to the first dose of study intervention. * Cohorts 1 to 3 only: Participant has measurable disease according to RECIST v1.1 within 28 days prior to the first dose of study intervention. For participants with only 1 measurable lesion and prior radiotherapy, the lesion must be outside the field of prior radiotherapy or must have documented progression following radiation therapy. * Cohort 4 only: Participant has radiologically evaluable disease (measurable and/or non-measurable) according to RECIST v1.1, within 28 days prior to the first dose of study intervention. For participants with only 1 evaluable lesion and prior radiotherapy, the lesion must be outside the field of prior radiotherapy or must have documented progression following radiation therapy. * Participant's tumor expresses CLDN18.2. * Participant has received at least 1 line of therapy for uLA/m disease. * Participant has an ECOG performance status of 0 or 1. * Participant has a predicted life expectancy \>= 12 weeks. * Female participant is not pregnant and at least 1 of the following conditions apply: * Not a women of childbearing potential (WOCBP) * WOCBP who has a negative urine or serum pregnancy test at screening (Specific to Japan: with a medical interview), and agrees to follow the contraceptive guidance from the time of informed consent through at least 5 half-lives (45 days) plus 6 months after final investigational study intervention administration. * Female participant must not be breastfeeding or lactating starting at screening and throughout the investigational period and for 5 half-lives (45 days) plus 6 months after final investigational study intervention administration. * Female participant must not donate ova starting at first administration of study intervention and throughout the investigational period and for 5 half-lives (45 days) plus 6 months after final investigational study intervention administration. * Male participant must agree to use contraception with female partner(s) of childbearing potential (including breastfeeding partner) throughout the treatment period and for 5 half-lives (45 days) plus 3 months after final investigational study intervention administration. * Male participant must agree to remain abstinent or use a condom with pregnant partner(s) for the duration of the pregnancy throughout the investigational period and for 5 half-lives (45 days) plus 3 months after final investigational study intervention administration. * Male participant must not donate sperm during the treatment period and for 5 half-lives (45 days) plus 3 months after final investigational study intervention administration. * Participant must meet all of the criteria based on the locally analyzed laboratory tests collected within 14 days prior to the first dose of study intervention. In case of multiple local laboratory tests within this period, the most recent data should be used. * Participant is willing to provide or has sufficient tumor tissue for central biomarker assessment. * Participant agrees not to participate in another interventional study while receiving study intervention in the present study. Exclusion Criteria: * Cohorts 1, 2 and 3 only: Participant's disease is of the non-adenocarcinoma histology or mixed histology containing adenocarcinoma. * Cohorts 1, 2 and 3 only: Participant has received \> 2 prior lines of therapy for uLA/m disease. * Participants in Cohort 4 (pan-tumor) may enroll regardless of the number of prior lines of therapy, if they are not eligible for, decline, or do not have any available standard of care treatment options. * Participant has complete gastric outlet syndrome or a partial gastric outlet syndrome with persistent recurrent vomiting. * Participant has significant gastric bleeding or had a significant bleeding episode from the gastrointestinal tract within 3 months prior to the first dose of study intervention and/or an untreated peptic ulcer disease that would preclude the participant from participation. * Participant has significant bleeding disorders or has had vasculitis within 3 months prior to the first dose of study intervention. * Participant has a history of gastrointestinal perforation and/or fistula within 6 months prior to the first dose of study intervention. * Participant has symptomatic, untreated brain metastases or meningeal carcinomatosis (carcinomatous meningitis) from the primary malignancy. A participant with stable central nervous system metastases for \> 3 months without need of steroids for \>= 2 weeks prior to the first dose of study intervention is eligible. * Participant has a past or current mental illness that is difficult to control. * Participant has unresolved pneumonitis or a history of non-infectious pneumonitis such as immune-related pneumonitis or radiation-induced pneumonitis for which the participant is taking glucocorticoids or needed glucocorticoids within 6 months prior to the first dose of study intervention. * Participant has a known history of a positive test for human immunodeficiency virus (HIV) infection or known active hepatitis B (positive hepatitis B surface antigen \[HBsAg\]) or hepatitis C infection. Screening for these infections should be conducted if indicated per local requirements. * If participant is negative for HBsAg, but hepatitis B core antibody (HBcAb) and/or hepatitis B surface antibody (HBsAb) positive, a hepatitis B DNA test will be