Phase 1 dose-escalation study of oral tyrosine kinase inhibitor masitinib in advanced and/or metastatic solid cancers

Background: Masitinib is a tyrosine kinase inhibitor with a pre-clinical profile suggesting greater affinity and selectivity in vitro for the wild-type c-Kit receptor and its juxtamembrane mutation than imatinib. Methods: This dose-escalation study was conducted in patients with advanced and/or metastatic cancer to determine the maximum tolerated dose (MTD) for orally administered masitinib over a 12-week period. Secondary objectives were a clinical assessment of masitinib's activity in cancer patients and establishment of a pharmacokinetic profile. Results: Forty patients with various solid tumours (predominantly GIST, 19 patients) were treated with masitinib at doses ranging between 0.7 and 17.2 mg/kg/day. Although the MTD was not formally reached, an acceptable dose for chronic use was identified at 12 mg/kg/day. Treatment-related AEs were frequent (38/40 patients), however, the majority were grade 1 or 2 and demonstrated dose dependency at higher concentrations. Pharmacokinetic results showed a linear, dose-dependent increase of C_max and AUC. One of two GIST patients with imatinib intolerance had a partial response at 11.1 mg/kg/day. About 29% of the imatinib-resistant GIST population and 38% of the overall population had stable disease. Conclusions: The safety profile of masitinib at 12 mg/kg/day b.i.d. for the treatment of solid cancers appears favourable and compatible with a long-term regimen. Tumour control rate in imatinib-resistant patients was encouraging, hence, the activity of masitinib in c-Kit expressing tumours, such as GIST, warrants further exploration as first-line anticancer therapy as well as for imatinib-resistant patients.