STC-15 as a Part of Combination Therapy With Toripalimab in Selected Advanced Cancers

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 the ability to swallow, retain, and absorb oral medication. Inclusion Criteria (Expansion): * NSCLC (when applicable): Participants with AGAs (actionable genetic alterations) must have received targeted therapy unless contraindicated. Participants must not have received more than three previous lines of systemic treatment for unresectable locally advanced or metastatic disease. This must include at least a prior anti-PD-1/L1 alone or in combination with chemotherapy. Participants must have disease progression on prior anti-PD-1/L1. * HNSCC (when applicable): Participants must have progressed on following prior lines: 2L or 3L recurrent/metastatic disease; regardless of PD-L1 score. Participants must not have received more than three previous lines of systemic treatment for unresectable locally advanced or metastatic disease. This must include at least a prior anti-PD-1/L1 alone or in combination with chemotherapy. * Endometrial (when applicable): Participants must not have received more than three previous lines of systemic treatment for unresectable locally advanced or metastatic disease. This must include at least a prior anti-PD-1/L1 alone or in combination with chemotherapy. Participants must have disease progression on prior anti-PD-1/L1 * Melanoma (when applicable): Participants must not have received more than three previous lines of systemic treatment for unresectable locally advanced or metastatic disease. This must include at least a prior anti-PD-1/L1 alone or in combination with chemotherapy. Participants must have disease progression on prior anti-PD-1/L1. Key Exclusion Criteria: * Pregnant and lactating women. * Received prior systemic anticancer therapy including investigational agents within 4 weeks 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. * Major surgery less than 4 weeks prior to the first IMP administration or participants who have not recovered from the side effects of the surgery. * 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.