A Phase I/Ib Trial of the VEGFR-Sparing Multikinase RET Inhibitor RXDX-105
Drilon A, Fu S, Patel MR, Fakih M, Wang D, Olszanski AJ, Morgensztern D, Liu SV, Cho BC, Bazhenova L, Rodriguez CP, Doebele RC, Wozniak A, Reckamp KL, Seery T, Nikolinakos P, Hu Z, Oliver J, Trone D, McArthur K, Patel R, Multani PS, and Ahn MJ. A phase I/Ib trial of the vegfr-sparing multikinase ret inhibitor rxdx-105. Cancer Discov.2019 Mar;9(3):384-395.
RET fusions are oncogenic drivers of various tumors, including non-small cell lung cancers (NSCLC). The safety and antitumor activity of the multikinase RET inhibitor RXDX-105 were explored in a phase I/Ib trial. A recommended phase II dose of 275 mg fed daily was identified. The most common treatment-related adverse events were fatigue (25%), diarrhea (24%), hypophosphatemia (18%), maculopapular rash (18%), and nonmaculopapular rash (17%). In the phase Ib cohort of RET inhibitor-naïve patients with RET fusion-positive NSCLCs, the objective response rate (ORR) was 19% (95% CI, 8%-38%, n = 6/31). Interestingly, the ORR varied significantly by the gene fusion partner (P < 0.001, Fisher exact test): 0% (95% CI, 0%-17%, n = 0/20) with KIF5B (the most common upstream partner for RET fusion-positive NSCLC), and 67% (95% CI, 30%-93%, n = 6/9) with non-KIF5B partners. The median duration of response in all RETfusion-positive NSCLCs was not reached (range, 5 to 18+ months). SIGNIFICANCE: Although KIF5B-RET is the most common RET fusion in NSCLCs, RET inhibition with RXDX-105 resulted in responses only in non-KIF5B-RET-containing cancers. Novel approaches to targeting KIF5B-RET-containing tumors are needed, along with a deeper understanding of the biology that underlies the differential responses observed.