Dose-finding study of the checkpoint kinase 1 inhibitor, prexasertib, in Japanese patients with advanced solid tumors

Prexasertib is a novel inhibitor of checkpoint kinase 1. The primary objective of this study was to evaluate prexasertib tolerability in Japanese patients with advanced solid tumors. This nonrandomized single-arm open-label phase 1 study of prexasertib consisted of 2 dose levels, 80 mg/m² and the global-recommended dose based on a US study of 105 mg/m² , administered intravenously once every 14 days (n = 6 for each dose). Transition to the higher dose proceeded if the frequency of dose-limiting toxicity observed in cycle 1 was <33% at the lower dose. Safety measures, pharmacokinetics and antitumor activity were assessed. A total of 12 patients were treated. Two patients, one in each dose group, experienced dose-limiting toxicities of febrile neutropenia, one grade 4 and the other grade 3; both patients recovered and continued the study treatment. The grade 4 treatment-emergent adverse events related to study treatment were neutropenia (6 patients [50.0%]), leukopenia (4 patients [33.3%]), and 1 instance each (8.3%) of anemia, febrile neutropenia and thrombocytopenia. Neutropenia was generally transient and reversible; 11 patients (91.7%) required granulocyte colony-stimulating factor treatment during the study. There were no discontinuations due to adverse events or deaths. The prexasertib pharmacokinetics displayed dose-independent and time-independent behavior across both dose levels, similar to the profile observed in the US-based phase 1 study. Eight patients had a best overall response of stable disease. These data are consistent with the known safety profile for prexasertib and confirm its tolerability in Japanese patients with advanced solid tumors.