A Phase 1/2 Study of FS118 in Patients With Advanced Malignancies

Inclusion Criteria: All participants: * Age ≥18 years; * Participants with histologically confirmed, locally advanced, unresectable, or metastatic solid tumors that progressed while on or after PD-1/PD-L1 containing therapy; * Measurable disease; * Eastern Cooperative Oncology Group (ECOG) Performance Status ≤1; * Life expectancy estimated to be at least 3 months; * Highly effective contraception; * Willing and able to provide written informed consent. Expansion cohort only: * Histologically and/or cytologically confirmed recurrent/metastatic (R/M) SCCHN that is not amenable to curative therapy by surgery or radiation; * Only 1 prior anti-PD-1 or anti-PD-L1 therapy and documented PD-L1 scoring ≥1% by combined positive score or tumor proportion score as part of their treatment; * An anti-PD-1 or anti-PD-L1 treatment regimen must be the last prior therapy before study enrollment, following no more than 2 prior systemic regimens for R/M SCCHN; * Acquired resistance to an anti-PD-1- or anti-PD-L1-containing therapy; * The participant agrees to undergo a pre-treatment and on-treatment core or excisional biopsy and the biopsy procedure is not judged to be high risk by the Investigator. Exclusion Criteria: All participants: * Participant is deemed at high risk of fatal outcome in case of COVID-19; * Participants with a history of COVID-19 and have not provided a negative test for SARS CoV-2 infection within 28 days of the planned first dose date with FS118; * Prior therapy: Received systemic anti-cancer therapy within 28 days or 5 half-lives, of the first dose of study drug, or prior treatment with a LAG-3 inhibitor; * Participants with active or documented history of autoimmune disease; * History of uncontrolled intercurrent illness; * Known infections; * Uncontrolled CNS metastases, primary CNS tumors, or solid tumors with CNS metastases as only measurable disease; * Prior history of or active interstitial lung disease or pneumonitis, encephalitis, seizures, severe immune related adverse events with prior PD-1/PD-L1 containing treatments; * Significant cardiac abnormalities; * Significant laboratory abnormalities; * Intolerance to the investigational product or its excipients, or any condition that would significantly impair and/or prohibit the participants's participation in the study, as per the Investigator's judgment. Expansion cohort only: * Participant has nasopharynx or thyroid primary tumor site; * History of severe immune-related toxicity during the prior treatment with checkpoint inhibitors.