Sorafenib inhibits the imatinib-resistant KITT670I gatekeeper mutation in gastrointestinal stromal tumor

Purpose: Resistance is commonly acquired in patients with metastatic gastrointestinal stromal tumor who are treated with imatinib mesylate, often due to the development of secondary mutations in the KIT kinase domain. We sought to investigate the efficacy of second-line tyrosine kinase inhibitors, such as sorafenib, dasatinib, and nilotinib, against the commonly observed imatinib-resistant KIT mutations (KITV654A, KITT670I, KITD820Y, and KITN822K) expressed in the Ba/F3 cellular system. Experimental Design: In vitro drug screening of stable Ba/F3 KIT mutants recapitulating the genotype of imatinib-resistant patients harboring primary and secondary KIT mutations was investigated. Comparison was made to imatinib-sensitive Ba/F3 KIT mutant cells as well as Ba/F3 cells expressing only secondary KIT mutations. The efficacy of drug treatment was evaluated by proliferation and apoptosis assays, in addition to biochemical inhibition of KIT activation. Results: Sorafenib was potent against all imatinib-resistant Ba/F3 KIT double mutants tested, including the gatekeeper secondary mutation KITWK557-8del/T670I, which was resistant to other kinase inhibitors. Although all three drugs tested decreased cell proliferation and inhibited KIT activation against exon 13 (KITV560del/V654A) and exon 17 (KITV559D/D820Y) double mutants, nilotinib did so at lower concentrations. Conclusions: Our results emphasize the need for tailored salvage therapy in imatinib-refractory gastrointestinal stromal tumors according to individual molecular mechanisms of resistance. The Ba/F3 KITWK557-8del/T670I cells were sensitive only to sorafenib inhibition, whereas nilotinib was more potent on imatinib-resistant KITV560del/V654A and KITV559D/D820Y mutant cells than dasatinib and sorafenib.