Phase 1 dose escalation study of XMT-1536, a novel NaPi2b-targeting antibody-drug conjugate (ADC), in patients (pts) with solid tumors likely to express NaPi2b.

Abstract

3010 Background: XMT-1536 is a Dolaflexin ADC targeting the sodium-phosphate cotransporter NaPi2b, expressed in ovarian, non-squamous lung, papillary thyroid, endometrial, papillary renal and salivary duct cancers. Methods: In this ongoing Phase 1 study, pts with solid tumors likely to express NaPi2b, who progressed on standard therapy, are treated with intravenous XMT-1536 using a 3+3 design with a modified Fibonacci escalation. NaPi2b expression by IHC is being examined retrospectively in archived tumors. Primary objectives in dose escalation are safety and tolerability and determination of maximum tolerated dose (MTD) and recommended Phase 2 dose (RP2D). (ClinicalTrials.gov NCT03319628). Results: As of Jan. 28, 2019, 36 pts (22 ovarian, 7 endometrial, 4 NSCLC, 3 other) have received treatment with XMT-1536. Treatment was initially given every 3 weeks (q3w); 20 pts were treated in dose cohorts from 3 to 40 mg/m2. There was one DLT of reversible AST elevation at 40 mg/m2. The dosing interval was then changed to every 4 weeks (q4w), and dose escalation was restarted at 20 mg/m2. There was one DLT of reversible AST elevation at 30 mg/m2 on the q4w schedule. Further followup and dose escalation are ongoing. The most common (≥10% of patients) treatment-related adverse events (TRAEs) have been nausea, fatigue, headache, increased AST, anorexia, increased alkaline phosphatase, fever, increased GGT, myalgia, and vomiting. Grade 3 TRAEs were reversible AST increases in 3 patients and increased GGT, decreased lymphocytes, and systolic congestive heart failure in 1 patient each. Treatment-related serious AEs of fever and systolic congestive heart failure occurred in 1 patient each. Among patients dosed at 20 mg/m2 or higher who had restaging scans (n=20), there were 2 PR, in ovarian cancer pts at 30 mg/m2 q3w and 20 mg/m2 q4w, and 11 SD, with disease control maintained for up to 24 weeks. Patient-level results for NaPi2b expression will be presented. The systemic exposure of total payload showed approximately dose-proportional increase. Plasma concentration of free drug payload and its active metabolite were low. Conclusions: XMT-1536 has been well-tolerated up to the 30 mg/m2 dose level with early signs of anti-tumor activity. Dose escalation continues in pts with advanced solid tumors likely to express NaPi2b. Clinical trial information: NCT03319628.