Open-label, multicenter, phase 1b/2 study of rebastinib in combination with paclitaxel to assess safety and efficacy in patients with advanced or metastatic endometrial cancer.

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5576 Background: Rebastinib is a first-in-class investigational, orally administered, potent and selective switch-control kinase inhibitor of tunica interna endothelial cell kinase (TIE2). This is a 2-part open-label, multicenter Phase 1b/2 study of rebastinib in combination with paclitaxel. Here we provide updated results (ASCO 2020) from the fully enrolled endometrial cancer (EC) cohort of the study. Methods: Part 2 of the study has five disease-specific cohorts (EC, platinum-resistant ovarian cancer, gynecological carcinosarcoma, TNBC and inflammatory breast cancer). Patients were treated at the RP2D and evaluated for efficacy (RECIST v1.1) and safety (CTCAE v5.0). Results: As of Jan 8, 2021, 38 EC patients were enrolled (median age of 66 years); 42% were of grade 2/3 endometroid histological subtype. All patients received at least 1 prior line of paclitaxel in combination with carboplatin and 79% of patients received ≥3 prior anti-cancer regimens. Sixteen of 38 patients were initially treated with a starting dose of rebastinib 100 mg BID, 11 of which dose reduced to 50 mg BID, and 22 patients were treated with a starting dose of rebastinib 50 mg BID, in combination with paclitaxel 80 mg/m2 IV weekly (days 1, 8, 15 of 28-day cycle). In 33 evaluable patients with median follow-up of 5.9 months, the ORR was 33% and clinical benefit rate at 8 and 16 weeks was 70% and 55%, respectively, including 11 PRs (8 confirmed) and 12 SDs. Treatment-emergent adverse events ( > 20% of patients; mostly ≤ grade 2) included fatigue (n = 18), constipation, peripheral edema (each at n = 16), peripheral sensory neuropathy, nausea (each at n = 15), dyspnea (n = 13), alopecia, hypokalemia (each at n = 11), diarrhea, hypomagnesemia (each at n = 10), dry mouth, dysgeusia (each at n = 9), arthralgia, hypertension, dehydration, GERD and muscular weakness (each at n = 8). Serious adverse events (SAE) at least possibly related to rebastinib included muscular weakness (n = 2 at 100 mg BID, n = 1 at 50 mg BID), nausea (n = 2), acute myocardial infarction, atrial flutter, dehydration, non-infective encephalitis, peritonsillitis, and stress cardiomyopathy (each at n = 1) and were resolved after dose interruption. Conclusions: The updated results of rebastinib at 50 mg BID in combination with paclitaxel showed encouraging preliminary anti-tumor activity and an acceptable safety profile in heavily pretreated EC patients, and supports further development in patients with EC (NCT03601897). Clinical trial information: NCT03601897.