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Purpose: Checkpoint kinase 1 (CHK1) plays a central role in the response to replication stress through modulation of cell-cycle checkpoints and homologous recombination (HR) repair. In BRCA-deficient cancers with de novo or acquired PARP inhibitor resistance, the addition of the CHK1 inhibitor prexasertib to the PARP inhibitor olaparib compromises replication fork stability, as well as HR proficiency, allowing for sensitization to PARP inhibition. Patients and Methods: This study followed a 3+3 design with a 7-day lead-in of olaparib alone, followed by 28-day cycles with prexasertib administered on days 1 and 15 in combination with an attenuated dose of olaparib on days 1–5 and 15–19. Pharmacokinetic blood samples were collected after olaparib alone and following combination therapy. Patients enrolled to the expansion phase of the study underwent paired tumor biopsies for pharmacodynamic (PD) assessments. Results: Twenty-nine patients were treated. DLTs included grade 3 neutropenia and grade 3 febrile neutropenia. The MTD/recommended phase 2 dose (RP2D) was prexasertib at 70 mg/m2 i.v. with olaparib at 100 mg by mouth twice daily. Most common treatment-related adverse events included leukopenia (83%), neutropenia (86%), thrombocytopenia (66%), and anemia (72%). Four of 18 patients with BRCA1-mutant, PARP inhibitor–resistant, high-grade serous ovarian cancer (HGSOC) achieved partial responses. Paired tumor biopsies demonstrated reduction in RAD51 foci and increased expression of γ-H2AX, pKAP1, and pRPA after combination exposure. Conclusions: Prexasertib combined with olaparib has preliminary clinical activity in BRCA-mutant patients with HGSOC who have previously progressed on a PARP inhibitor. PD analyses show that prexasertib compromises HR with evidence of induction of DNA damage and replication stress.

Phase 1 Combination Study of the CHK1 Inhibitor Prexasertib and the PARP Inhibitor Olaparib in High-grade Serous Ovarian Cancer and Other Solid Tumors