Metronomic dosing of selinexor in select soft tissue sarcomas (STS)

11557 Background: Selinexor (S) is a first-in-class, oral, selective inhibitor of nuclear export (SINE). S has a demonstrated anti-tumor activity in STS. However, the use of S can be associated with toxicities, such as nausea, anorexia and fatigue. A modified dosing schedule may improve the tolerability of S without compromising its efficacy. Methods: We evaluated S when given metronomically in patients (pts) with advanced metastatic leiomyosarcoma, endometrial stromal sarcoma, and malignant peripheral nerve sheath tumors. S was administered daily for 4-days in a row followed by 3-days break, repeated weekly in a 28-days cycle. Dose levels (DL) were escalated using a 3+3 design to determine maximum tolerated dose. DL included 2.5mg (DL1), 5mg (DL2), 7.5mg (DL3), 10 mg (DL4), 12.5 mg (DL5), 15 mg (DL6) and 17.5 mg (DL7). S was administered until unacceptable toxicities or disease progression. Imaging was performed every 8 weeks. Primary objectives were safety and tolerability as well as the determination of the recommended phase II dose. Secondary objectives were anti-tumor activity, toxicity profile, and pharmacokinetics (PK) profile. Results: Twenty-five pts (22 females/3 males, median age 59 years [range 35-84]) were enrolled at different DL (3, 3, 4, 6, 6, 3 pts at DL1 to DL6 respectively). The most common adverse events (AE) of any grade were constipation (n = 11, 50%), nausea (n = 11, 50%), and dysgeusia (n = 10, 45%). Eight (36%) pts experienced G3/4 AE while on trial; most were hematological toxicities seen in 3 (14%) pts (anemia, neutropenia, and leukopenia, n = 1 each). Non-hematological G3/4 AE were seen in 5 (23%) pts (Including: colonic obstruction, atrial fibrillation, and spinal cord compression, n = 1 each). Two Dose limiting toxicities were experienced, at DL4 (thrombocytopenia) and DL5 (transaminitis). Twenty-two pts were evaluable for response; 10 (45%) pts had stable disease (SD) as best response. Eight pts with SD had disease stability of > 4 months. Twelve pts (55%) had progressive disease. Median progression-free survival was 2.4 months (95% CI 1.7-5.3 months). PK data is awaiting final analysis. Conclusions: Metronomic S demonstrated tolerability in selected sarcoma pts. Signs of potential clinical benefit was seen in the form of SD. Further data comparison with historical controls and PK analysis are needed to put this finding into perspective. Clinical trial information: NCT04811196 .