This 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).

Key Inclusion Criteria:

• Estimated life expectancy ≥ 3 months.
• ECOG performance status 0 or 1.
• Measurable disease according to RECIST v1.1 as assessed by the local site investigator/radiology.
• Documented radiologic assessment of progression on the prior therapy before study entry.
• Have adequate organ function.
• Have the ability to swallow, retain, and absorb oral medication.

Inclusion Criteria (Phase 2 Monotherapy Cohorts):

• Have histologic or cytologic confirmation of advanced sarcoma of the selected histologic subtype that is not amenable to local curative therapy. Participant must have received at least 2, but no more than 4 prior lines of systemic therapy.
• Pre-treatment and on-treatment biopsy if medically feasible.

Key Exclusion Criteria:

• Pregnant and lactating women.
• Received prior systemic anticancer therapy including investigational agents within 4 weeks or 5 half-lives, whichever is shorter, prior to first IMP administration.
• Participants who have not recovered from all AEs due to previous therapies to Grade ≤ 1 or baseline, according to NCI-CTCAE v5.0. Exceptions include: alopecia, Grade ≤ 2 neuropathy, and endocrine-related AEs Grade ≤ 2 who are stable on treatment or hormone replacement.
• History of (non-infectious) pneumonitis/interstitial lung disease that required steroids or the presence of ongoing pneumonitis/interstitial lung disease).
• Clinically significant cardiovascular disease or condition.
• Known active CNS metastases and/or leptomeningeal disease.