Safety and Preliminary Efficacy of Once-Weekly Split-Dose Selinexor in Soft Tissue Sarcoma: Results of the Phase Ib METSSAR Clinical Trial.

Background: The approved dose of Selinexor, 60 mg twice-weekly, is associated with several clinically relevant toxicities. Preclinical studies show that a sustained-release formulation of selinexor results in a lower toxicity profile.

Objective: The phase 1b METSSAR trial assessed the safety and tolerability of an alternative dosing schedule of selinexor (to mimic the sustained-release formulation) in advanced soft tissue sarcoma (STS) patients.

Patients and methods: Selinexor was administered in a split-dose schedule (40 mg, 20 mg, 20 mg in the morning, afternoon, and evening, respectively) on days 1, 8, 15, and 22 of a 28-day cycle, until unacceptable toxicity or disease progression. The primary endpoint was the rate of grade ≥ 3 treatment-related adverse events (TRAEs). Secondary objectives were EORTC QLQ-C30 quality of life (QoL) assessment, and preliminary efficacy.

Results: Twenty patients with 12 STS subtypes were enrolled and received a median of four cycles of treatment. There were no grade ≥ 3 TRAEs. Dysgeusia, nausea, fatigue, and thrombocytopenia were the most common grade ≤ 2 TRAEs. No treatments were discontinued due to TRAE, but four patients (20%) required dose reduction. Median change in global health status (GHS) score from baseline to cycle 2 (by QLQ-C30 v3.0) was - 8.33, and only 39% of patients reported a clinically meaningful decline in GHS score (≥ 10 points). Median symptom scale scores on treatment were increased for fatigue (+12.35), nausea/vomiting (+18.52), and anorexia (+16.67), but reduced for pain (- 3.70). The median progression-free survival (PFS) was 4.0 months (95% confidence interval 1.9-7.5).

Conclusions: Split-dose once-weekly selinexor was reasonably well tolerated in this heterogeneous group of advanced STS patients with a better, or at least similar, clinician- and patient-reported toxicity profile compared to the standard dosing regimen. Further clinical evaluation is warranted, as better dose delivery can lead to improved antitumor efficacy.