The primary purpose of this study is to determine the safety, tolerability, and maximum tolerated dose (MTD) 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 abnormalities.
• Major surgery within 28 days prior to the start of study treatment.

Note: Other protocol-defined inclusion and exclusion criteria may apply.