A Phase I/Ib Trial of the VEGFR-Sparing Multikinase RET Inhibitor RXDX-105

<jats:title>Abstract</jats:title> <jats:sec> <jats:title/> <jats:p>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 &lt; 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 RET fusion–positive NSCLCs was not reached (range, 5 to 18+ months).</jats:p> </jats:sec> <jats:sec> <jats:title>Significance:</jats:title> <jats:p>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.</jats:p> <jats:p>This article is highlighted in the In This Issue feature, p. 305</jats:p> </jats:sec>