Study of ADCT-502 in Patients With Advanced Solid Tumors Withhuman Epidermal Growth Factor Receptor-2 (HER2) Expression

Main Inclusion Criteria: * Male or female age 18 years or older * Refractory to or intolerant of existing therapy(ies) known to provide clinical benefit for their condition. * Eastern Cooperative Oncology Group (ECOG) performance status: Part 1: 0-2, Part 2: 0-1 * Availability of formalin-fixed paraffin-embedded (FFPE) tumor tissue block or unstained slides to demonstrate HER2 expression. * Pathologic diagnosis of solid tumor malignancy that is locally advanced or metastatic at time of Screening with documented HER2 expression. * Part 2/Dose Expansion Only: Measurable disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 * Absolute neutrophil count (ANC) ≥ 1500/mm3 (≥1.5× 109/L). * Platelet count ≥100,000 //mm3 (≥100 × 109/L). * Hemoglobin ≥ 9 g/L (≥5.6 mmol/L). * Aspartate transaminase (AST) and alanine aminotransferase (ALT) ≤ 2.5 x upper limit of normal (ULN); or ≤ 5.0 × ULN if liver metastases are present. * Total bilirubin ≤ 1.5× ULN (or ≤ 3× ULN, with direct bilirubin ≤1.5 × ULN, in participants with known Gilbert syndrome). * Creatinine ≤ 1.5× ULN; or, if serum creatinine \> 1.5 × ULN, a measured creatinine clearance must be \>60mL/min/1.73m2 as calculated by the Cockcroft and Gault equation for participants to be eligible. * Women of childbearing potential must agree to use a highly effective method of contraception from the time of giving informed consent until at least 16 weeks after the last dose of ADCT-502. Men with female partners who are of childbearing potential must agree that they or their partners will use a highly effective method of contraception from the time of giving informed consent until at least 16 weeks after the participant receives his last dose of ADCT-502. Main Exclusion Criteria: * Known history of ≥ Grade 3 hypersensitivity to a therapeutic antibody. * Known history of positive serum human anti-drug antibody (ADA) to trastuzumab. * Major surgical procedure or significant traumatic injury, radiotherapy, chemotherapy, targeted therapy, hormone therapy, or other anticancer therapy. * Failure to recover to Grade 0 or Grade 1 from acute non-hematologic toxicity due to previous therapy, prior to screening (with the exception of alopecia). * Central Nervous System (CNS) disease only. * Symptomatic CNS metastases or evidence of leptomeningeal disease. * Active cardiovascular disease or significant history thereof. * Other active disease including but not limited to ulceration of the upper gastrointestinal tract, autoimmune disease, HIV infection, active Hepatitis B virus (HBV) and hepatitis C virus (HCV) infection. * Breastfeeding or pregnant. * Other concurrent severe and/or uncontrolled medical conditions.