A Study of MGC030 in Participants With Advanced Solid Tumors

Inclusion Criteria: * Participants in dose escalation cohorts must have histologically proven unresectable, locally advanced or metastatic solid tumor limited to one of the following types: urothelial cancer, colorectal cancer (CRC), gastric or gastroesophageal junction (GEJ) cancer, esophageal cancer, or pancreatic carcinoma that is refractory to standard therapy, or for which standard therapy does not exist, has proven to be intolerable, or has been refused by the participant. * Participants in expansion cohorts must have histologically proven, unresectable, locally advanced or metastatic solid tumors * Urothelial cancer with progression following platinum-containing chemotherapy for metastatic or locally advanced/unresectable cancer, or within 12 months from completion of neo-adjuvant or adjuvant platinum-containing chemotherapy for localized muscle-invasive cancer. * Gastric/GEJ carcinoma with progression during or following at least 1 systemic therapy. * Esophageal cancer with progression during or following at least 1 systemic therapy. * CRC with progression during or following treatment with at least 2 prior lines of systemic therapy (containing a fluoropyrimidine plus a platinum analogue and/or irinotecan) for metastatic disease. * CRC harboring an activating EGFR mutation must have progressed during or following at least one EGFR targeted therapy, if available. * Participants should have received no more than 4 prior lines of systemic therapy for advanced or metastatic disease. * Pancreatic cancer with progression during or following at least 1 systemic therapy. Participants should have received no more than 2 prior lines of cytotoxic chemotherapy for advanced or metastatic disease. * Participants must have at least one lesion that meets the definition of measurable disease by RECIST v1.1. * Participants must have an available archival or formalin-fixed paraffin-embedded tumor tissue or be willing to undergo a biopsy procedure to obtain a fresh tumor sample. * Participants have acceptable physical condition and laboratory values. * Participants of childbearing potential must agree to use highly effective methods of birth control. * Participants must not be pregnant, planning to be pregnant, or breastfeeding. Exclusion Criteria: * Any underlying medical or psychiatric condition impairing participant's ability to receive, tolerate, or comply with the planned treatment or study procedures. * Active brain metastases or leptomeningeal metastases. * Prior stem cell, tissue, or solid organ transplant. * Another malignancy that required treatment within the past 2 years, with the exception of those with a negligible risk of metastasis or death such as adequately treated non-melanomatous skin cancer, localized prostate cancer (Gleason Score \< 6), or carcinoma in situ. * History of primary immunodeficiency. * Active viral, bacterial, or fungal infection * Prior treatment with Topoisomerase 1-based ADCs for cancer. * Prior treatment with major surgery, mediastinal or lung radiation, vaccination with live virus vaccines, systemic cancer treatment, chimeric antigen receptor (CAR)-T cell therapy, or experimental treatment within 4 weeks of the start of study treatment.