An investigator-initiated phase I study of ONX-0801, a first-in-class alpha folate receptor targeted, small molecule thymidylate synthase inhibitor in solid tumors.

2503 Background: ONX-0801 is a first-in-class alpha folate receptor (AFR) targeted thymidylate synthase inhibitor, engineered to differentially accumulate 6000-fold in AFR overexpressing cancer cells. Methods: A 3+3 dose escalation design was used and two IV schedules were explored. Schedule A, weekly dosing (QW) and schedule B, once every 2 weeks dosing (Q2W). A cycle consisted of 4 weeks and treatment was stopped after 6 cycles in both schedules. An expansion cohort to evaluate clinical activity in patients with AFR overexpressing high grade serous ovarian cancer (HGSOC) was planned. Results: 21 patients each were treated in schedule A and B exploring doses ranging from 1-6 mg/m2 and 2-12 mg/m2, respectively. The dose limiting toxicity on schedule A was G3 cellulitis; no dose limiting toxicity was seen on schedule B. The most common toxicities were fatigue 15/42 (36%), nausea 9/42 (21%) and dysgeusia 5/42 (12%). Within schedule A at 4 mg/m2, 2 patients developed suspected drug-related changes on pulmonary function tests (drop in Dlco > 15%) at cycles 5 and 6, respectively. No cases of suspected drug-related drop in Dlco were noted in patients treated in schedule B. No grade 3-4 diarrhea, mucositis or neutropenia were seen in either cohort. The Cmax, AUC and half-life at 12 mg/m2 were 4952 ng/mL, 85170 h*ng/mL and 26 h, respectively. Pre-clinical PK-PD modelling aimed to achieve concentrations between 0.05-1 µM and this was achieved for periods of 48 h at doses of 4 mg/m2and above. Based on safety and PK, the recommended phase II dose (RP2D) of ONX-0801 was 12 mg/m2 Q2W and an expansion in patients with HGSOC is ongoing. 5 patients with HGSOC had partial responses (PRs) in the dose escalation cohort. In the current expansion cohort in patients with HGSOC, 5/11 patients had PRs. Archival samples have been analyzed from 8/11 patients in the expansion cohort. 4/4 AFR+ve and 4/4 AFR-ve patients did and did not have a PR following treatment with ONX-0801, respectively. Conclusions: The RP2D of ONX-0801 is 12 mg/m2 Q2W. At the RP2D, multiple patients with AFR overexpressing HGSOC have had PRs and further randomized biomarker prespecified phase II trials are warranted. Clinical trial information: NCT02360345.