A Study to Learn About the Study Medicine (Called Ontorpacept or TTI-621) Given Alone and in Combination With Doxorubicin in People With Leiomyosarcoma

This trial will be conducted in 2 phases: Phase I (dose escalation of Ontorpacept in combination with fixed-dose doxorubicin) and Phase II (dose expansion of Ontorpacept in combination with fixed-dose doxorubicin). Phase I will enroll patients with soft-tissue sarcomas including leiomyosarcoma, undifferentiated pleomorphic sarcoma, myxofibrosarcoma, dedifferentiated liposarcoma, angiosarcoma or epithelioid sarcoma to evaluate escalating doses of Ontorpacept (TTI-621) administered in combination with fixed-dose doxorubicin for up to six cycles followed by Ontorpacept (TTI-621) monotherapy. Phase II will enroll patients with high-grade leiomyosarcoma and will evaluate two dose levels of Ontorpacept (TTI-621) in combination with fixed-dose doxorubicin for up to six cycles followed by Ontorpacept (TTI-621) monotherapy. .