Antitumor Activity of RXDX-105 in Multiple Cancer Types with RET Rearrangements or Mutations

Purpose: While multikinase inhibitors with RET activity are active in RET-rearranged thyroid and lung cancers, objective response rates are relatively low and toxicity can be substantial. The development of novel RET inhibitors with improved potency and/or reduced toxicity is thus an unmet need. RXDX-105 is a small molecule kinase inhibitor that potently inhibits RET. The purpose of the preclinical and clinical studies was to evaluate the potential of RXDX-105 as an effective therapy for cancers driven by RET alterations.Experimental design: The RET-inhibitory activity of RXDX-105 was assessed by biochemical and cellular assays, followed by in vivo tumor growth inhibition studies in cell line- and patient-derived xenograft models. Antitumor activity in patients was assessed by imaging and Response Evaluation Criteria in Solid Tumors (RECIST).Results: Biochemically, RXDX-105 inhibited wild-type RET, CCDC6-RET, NCOA4-RET, PRKAR1A-RET, and RET M918T with low to subnanomolar activity while sparing VEGFR2/KDR and VEGFR1/FLT. RXDX-105 treatment resulted in dose-dependent inhibition of proliferation of CCDC6-RET-rearranged and RET C634W-mutant cell lines and inhibition of downstream signaling pathways. Significant tumor growth inhibition in CCDC6-RET, NCOA4-RET, and KIF5B-RET-containing xenografts was observed, with the concomitant inhibition of p-ERK, p-AKT, and p-PLCγ. Additionally, a patient with advanced RET-rearranged lung cancer had a rapid and sustained response to RXDX-105 in both intracranial and extracranial disease.Conclusions: These data support the inclusion of patients bearing RET alterations in ongoing and future molecularly enriched clinical trials to explore RXDX-105 efficacy across a variety of tumor types. Clin Cancer Res; 23(12); 2981-90. ©2016 AACR.