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

Abstract

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.

Figure 1

The predicted binding mode of RXDX-105 with RET. A, Chemical structure of RXDX-105. B and C, Predicted binding mode by modeling.

Figure 2. In vitro

characterization of RET-inhibitory activity of RXDX-105. Inhibition of phosphorylation of RET and downstream pathways by RXDX-105 in (A) LC-2/ad cells (CCDC6-RET) and (B) TT cells (RET C634W). Apoptosis signals, cleaved caspase-3 and cleaved PARP, were measured after 24-hour incubation with indicated compounds in LC-2/ad cells (C) and TT cells (D).

Figure 3. In vivo

efficacy of RXDX-105 in a PDX model of NSCLC harboring KIF5B-RET fusion. A, Tumor growth inhibition of CTG-0838 by RXDX-105 orally administered BID at 30 mg/kg. Tumor sizes are presented as average ± SEM. (**, P < 0.01). B, At the end of the study (day 27), tumor samples randomly selected from two mice per group were collected two and 12 hours after the final treatment. Western blot was performed using antibodies against phospho- and total RET, ERK, PLCg, and AKT. C, Tumor growth inhibition of CTG-1048 by RXDX-105. Tumor sizes are presented as average ± SEM (**, P < 0.01).

Figure 4. In vivo

efficacy of RXDX-105 in a PDX model of CRC harboring CCDC6-RET fusion. A, Tumor growth inhibition by RXDX-105 orally administered BID at 10, 30 mg/kg, and QD at 60 mg/kg. Tumor sizes are presented as average ± SEM (**, P < 0.01). B, Plot of percentage change of individual tumor volume compared with baseline. The y-axis of individual tumor responses represents “% of change from start.” C, Plot of percentage body weight change over time during treatment.

Figure 5. In vivo

efficacy of RXDX-105 in a PDX model of CRC harboring NCOA4-RET fusion. A, Tumor growth inhibition by RXDX-105 orally administered BID at 10 and 30 mg/kg. Tumor sizes are presented as average ±SEM (*, P < 0.05; **, P < 0.01). B, Plot of percentage change of individual tumor volume compared with baseline. The y-axis of individual tumor responses represents “% of change from start.”

Figure 6

A rapid partial response was achieved after three weeks of therapy with RXDX-105. A, Baseline imaging prior to RXDX-105 treatment. Arrows indicate the lesions. B, Confirmation scan at week 8 with 64% decrease in tumor burden from the baseline, and almost complete resolution of subcentimeter asymptomatic brain metastases.