FS120 Phase 1/1b Study in Patients 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.