A phase I/II first-in-human trial of oral SRA737 (a Chk1 inhibitor) given in combination with low-dose gemcitabine in subjects with advanced cancer.

3095 Background: SRA737 is a potent, highly selective and orally-bioavailable inhibitor of checkpoint kinase 1 (Chk1). SRA737-02 was designed to investigate the safety, tolerability and preliminary activity of SRA737 in a novel combination with sub-therapeutic doses of gemcitabine (low dose gemcitabine; LDG) utilized to potentiate SRA737’s activity by induction of replication stress in subjects with advanced solid tumors. Methods: Phase 1 dose escalation investigated cohorts of 3 to 6 subjects receiving escalating doses of SRA737 for 2 days after LDG administration on days 1, 8, 15 of 28-day cycles. Phase 2 expansion cohorts explored the hypothesis that LDG strongly synergizes with SRA737 in subjects with genetically-defined tumors hypothesized to be sensitive to Chk1 inhibition: urothelial, high grade serous ovarian, small cell lung, soft tissue sarcoma, and cervical or anogenital cancers. Results: A total of 55 subjects received SRA737 in 13 dose escalation cohorts at doses of 40 to 600 mg SRA737 combined with LDG doses of 50 to 300 mg/m 2 . No protocol-defined dose limiting toxicities (DLTs) have been observed. The pharmacokinetic profile of SRA737 revealed an AUC 0-24 and C max of 3550 ng∙h/mL and 548 ng/mL at 150 mg SRA737. At this dose, the C min (52 ng/mL) exceeded that determined in preclinical models to be effective. Enrollment into expansion cohorts was initiated at 500 mg SRA737 plus 100 mg/m 2 LDG with intra-patient dose escalation permitted to 250 mg/m 2 LDG. Approximately 80 subjects were planned and 82 have been treated. Median treatment duration was 51 days (range 1 to 358). The most common treatment-emergent adverse events were nausea (53%), vomiting (45%), fatigue (40%), diarrhea (38%), and anemia (28%); the majority were of mild to moderate severity. Proof-of-concept clinical activity has been seen in tumor types such as anal, cervical, and rectal. Conclusions: The combination of LDG and SRA737 has been well tolerated. This first-in-human clinical study provides proof-of-concept that sub-therapeutic LDG effectively potentiates SRA737. This novel replication stress-targeted therapy warrants further evaluation in genetically pre-defined solid tumors. Clinical trial information: NCT02797977.