Dose Determining Study of EXS74539 (REC-4539) in Participants With Select Solid Tumors

Key Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) performance status of 0-1. * Histologically confirmed diagnosis of the following solid tumors: * SCLC * High grade neuroendocrine or small cell carcinomas of non-lung origin * Non-small cell lung cancer \[NSCLC\] * Prostate cancer * Ovarian cancer * Renal carcinoma \[clear and non-clear cell\] * Head and neck squamous cell carcinoma * Hepatic cancer * Gastric cancer * Triple-negative breast cancer \[TNBC\] * Participant disease should have progressed, relapsed or the participants should have been intolerant to at least one prior standard systemic therapy for their respective underlying malignancy. * Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Key Exclusion Criteria: * Any medical or psychiatric condition that, in the opinion of the Investigator, could jeopardize or would compromise the participant's safety or ability to participate in the study. * Symptomatic brain metastases, leptomeningeal metastases, or spinal cord compression due to disease. Participants with brain metastases must be clinically stable and off corticosteroids and anticonvulsants for ≥1 month prior to study treatment and not require any specific intervention for brain metastases. * Active and clinically significant infection requiring systemic antibacterial, antiviral, or antifungal therapy \<7 days of the first scheduled dose of the study treatment. * Significant cardiac…