Oral SM-88 in Patients With Metastatic HR+/HER2- Breast Cancer

This is a multicenter phase II single arm trial designed to evaluate the efficacy of SM-88 plus three subtherapeutic conditioning agents (methoxsalen, phenytoin, and sirolimus \[MPS\]) in patients with metastatic HR+/HER2- breast cancer. . Thirty patients will be enrolled in first stage of the study, and if 3 or more patients have an objective response (complete or partial response) then an additional 20 patients will be enrolled in the second stage of the study. Patients will receive the recommended phase 2 dose (RP2D) of SM-88 (460 mg by mouth \[PO\] twice a day \[BID\] D1 - 28) as well as three conditioning agents (MPS): methoxsalen (10 mg PO daily \[Qd\] D1 - 28), phenytoin (50 mg PO Qd D1 - 28), and sirolimus (0.5 mg PO Qd D1 - 28). Assessment of efficacy will be conducted every 3 cycles (approximately every 12 weeks) with CT chest/abdomen/pelvis using RECIST v1.1 criteria. Safety including clinic visits, and exams will occur every 4 weeks on Day 1 of each cycle. Laboratory testing will be performed every 2 weeks for the first 2 cycles and then on Day 1 of each subsequent cycle.