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Sorafenib Inhibits Imatinib-Resistant KIT and Platelet-Derived Growth Factor Receptor ß Gatekeeper Mutants

Authors' Affiliations: ¹ Istituto di Endocrinologia ed Oncologia Sperimentale del CNR, c/o Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università di Napoli Federico II, Naples, Italy and ² Bayer HealthCare Pharmaceuticals, West Haven, Connecticut

Requests for reprints: Massimo Santoro, Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università di Napoli Federico II, via S. Pansini 5, 80131 Naples, Italy. Phone: 39-81746-3056; Fax: 39-81746-3037; E-mail: masantor{at}unina.it.

Purpose: Targeting of KIT and platelet-derived growth factor receptor (PDGFR) tyrosine kinases by imatinib is an effective anticancer strategy. However, mutations of the gatekeeper residue (T670 in KIT and T681 in PDGFRß) render the two kinases resistant to imatinib. The aim of this study was to evaluate whether sorafenib (BAY 43-9006), a multitargeted ATP-competitive inhibitor of KIT and PDGFR, was active against imatinib-resistant KIT and PDGFRß kinases.

Experimental Design: We used in vitro kinase assays and immunoblot with phosphospecific antibodies to determine the activity of sorafenib on KIT and PDGFRß kinases. We also exploited reporter luciferase assays to measure the effects of sorafenib on KIT and PDGFRß downstream signaling events. The activity of sorafenib on interleukin-3–independent proliferation of Ba/F3 cells expressing oncogenic KIT or its imatinib-resistant T670I mutant was also tested.

Results: Sorafenib efficiently inhibited gatekeeper mutants of KIT and PDGFRß (IC₅₀ for KIT T670I, 60 nmol/L; IC₅₀ for PDGFRß T681I, 110 nmol/L). Instead, it was less active against activation loop mutants of the two receptors (IC₅₀ for KIT D816V, 3.8 µmol/L; IC₅₀ for PDGFRß D850V, 1.17 µmol/L) that are also imatinib-resistant. Sorafenib blocked receptor autophosphorylation and signaling of KIT and PDGFRß gatekeeper mutants in intact cells as well as activation of AP1-responsive and cyclin D1 gene promoters, respectively. Finally, the compound inhibited KIT-dependent proliferation of Ba/F3 cells expressing the oncogenic KIT mutant carrying the T670I mutation.

Conclusions: Sorafenib might be a promising anticancer agent for patients carrying KIT and PDGFRß gatekeeper mutations.

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