RecruitingCombination productSTC-15 as a Part of Combination Therapy With Toripalimab in Selected Advanced Cancers and as Monotherapy in Participants With Selected SarcomasThis early phase oncology trial will be conducted at various study centers to investigate the safety, tolerability, and antitumor activity of STC-15 (a METTL3 inhibitor) in combination with toripalimab (anti- programmed cell death 1 \[PD-1\]) in advanced unresectable or metastatic tumors. The Phase 2 Monotherapy part is an open-label, non-randomized, multicenter Simon's 2-stage design that investigates the safety, tolerability, and antitumor activity of STC-15 in participants with selected, relapsed sarcomas subtypes, dedifferentiated (DD) liposarcoma and leiomyosarcoma (uterine and non-uterine).