The primary objectives of this study are to identify the recommended phase 2 dose (RP2D) and/or maximum tolerated dose (MTD), and to characterize the safety and the tolerability of ADCT-901.

Inclusion Criteria:

1. Pathologic diagnosis of selected solid tumor malignancy that is locally advanced or metastatic at time of Screening: cholangiocarcinoma, ovarian/fallopian tube cancers, prostate cancer, renal cell carcinoma, and triple negative breast cancer (TNBC).

   Note: Histologic variants of prostate cancer, including neuroendocrine features and small cell carcinoma of the prostate are permitted.

2. Participants who are refractory to or intolerant to existing therapy(ies) known to provide clinical benefit for their condition per Investigator judgment.

3. Participants with measurable disease as determined by Response Evaluation Criteria In Solid Tumors (RECIST) v1.1:

Note 1: Lytic bone lesions or mixed lytic-blastic lesions, with identifiable soft tissue components, that can be evaluated by cross sectional imaging techniques such as computed tomography (CT) or magnetic resonance imaging (MRI) can be considered as measurable lesions only if the soft tissue component meets the definition of measurability per RECIST v1.1.

Note 2: Prostate cancer participants without measurable lesions will be accepted, with evidence of bone metastatic disease on radiographic examination, whether from bone scan or other imaging modality, and prostate specific antigen (PSA) ≥2.0 ng/mL.

Exclusion Criteria:

1. History of active infection (requiring intravenous [IV] antibiotics, IV antiviral or IV antifungal treatment within 4 weeks of cycle 1, day 1 [C1D1]).

2. Symptomatic central nervous system (CNS) metastases or evidence of leptomeningeal disease (brain MRI or previously documented cerebrospinal fluid cytology). Previously treated asymptomatic CNS metastases are permitted provided that the last treatment (systemic anticancer therapy and/or local radiotherapy) was completed ≥4 weeks prior to C1D1 except usage of low dose of steroids on a taper (i.e., up to 10 mg prednisone or equivalent on Day 1 and consecutive days is permissible if being tapered down). Participants with discrete dural metastases are eligible.

3. Clinically significant third space fluid accumulation (i.e., ascites requiring drainage or any serosal effusion that is either requiring drainage or associated with shortness of breath).

4. Active diarrhea ≥ Common Terminology Criteria for Adverse Events (CTCAE) Grade 2 or a medical condition associated with chronic diarrhea (such as irritable bowel syndrome, inflammatory bowel disease).

5. Active or clinically significant ocular surface disease at baseline. An ocular evaluation is to be confirmed by an ophthalmologist at screening. Participants with any prior episode of cicatricial conjunctivitis (as evaluated by the investigator) are ineligible.

   Note: Mild dry eye syndrome or blepharitis managed with artificial tear drops, without injection or epithelial changes, are not exclusionary.

6. Use of any other experimental medication within 14 days prior to start of study drug (C1D1).