This is a Phase 1/1b, multicenter, open label study to evaluate the Safety and Antitumor Activity of FS120, an OX40/CD137 Bispecific Antibody, Alone and in Combination with Pembrolizumab, in Subjects with Advanced Malignancies

Inclusion Criteria:

• Measurable disease
• Eastern Cooperative Oncology Group Performance Status 0-1.
• Highly effective contraception if risk of conception exists
• A female subject is eligible if not pregnant, not breastfeeding, not a woman of childbearing potential (WOCBP) or is a WOCBP that uses highly effective contraception
• Subjects with HIV who are healthy and how a low risk of acquired immunodeficiency syndrome related outcomes
• For combination part: Subjects must have histologically confirmed locally advanced, unresectable or metastatic solid tumours where there is regulatory approval for use of pembrolizumab as a monotherapy agent

Exclusion Criteria:

• Prior systemic anticancer therapy within 28 days or 5- half-lives, whichever is shorter, before the first dose of study drug.
• Prior therapy with any OX40 agonist, CD137 (4-1BB) agonist, CD40 agonist, GITR, or CD27 targeting therapy (single agent or combination).
• Prior radiotherapy within 2 weeks of start of study treatment.
• HIV infected participants with a history of Kaposi sarcoma and/or Multicentric Castleman Disease.
• Uncontrolled central nervous system (CNS) metastases and/or carcinomatous meningitis, primary CNS tumours, or solid tumours with CNS metastases as the only measurable disease.
• Prior history of any grade ≥3 immune-related AE (irAE) that has not improved to grade ≤1; any grade ≥3 irAE that resulted in discontinuation of treatment, significant (grade ≥3 NCI CTCAE Version 5.0) treatment-related cytokine release syndrome.
• Use of immunosuppressive agents, hypersensitivity or intolerance to monoclonal antibodies or their excipients, persistent grade ≥1 NCI CTCAE Version 5 toxicity related to prior therapy or any condition that would significantly impair and/or prohibit the participant's participation in the study, as per the investigator's judgement.
• Vaccination with a live vaccine within 30 days before first dose of study drug.
• Participants with a known additional malignancy that is progressing or has required active treatment in the past 3 years.
• Participants with severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.