Background: RET kinase is dysregulated by activating mutations or fusion gene formation in multiple cancers, including medullary thyroid cancer (MTC) and non-small cell lung cancer (NSCLC). RET mutations are found in 50-95% of MTCs, with mutations at C634R (extracellular domain) and M918T (kinase domain) predominating. Recently, a translocation that results in formation of a KIF5B-RET fusion gene has been identified in 1-2% of NSCLCs. Ponatinib (AP24534) is a multi-targeted tyrosine kinase inhibitor (TKI) with potent activity against BCR-ABL being investigated in patients with chronic myeloid leukemia. Previously, ponatinib has been shown to potently inhibit the in vitro kinase activity of RET and the viability of Ba/F3 cells transformed to IL-3 independence through expression of a RET kinase domain artificially-activated via fusion to the TEL dimerization domain. Here, the activity of ponatinib was examined in Ba/F3 cells transformed with the most common, naturally-occurring activated variants of RET found in MTC and NSCLC. In addition, the activity of ponatinib was compared to that of 4 other multi-targeted TKIs with anti-RET activity in clinical development: vandetanib, cabozantinib, sunitinib and sorafenib.
Results: Ponatinib potently inhibited viability of Ba/F3 cells expressing RETC634R, RETM918T and KIF5B-RET, with IC50s of 2, 3, and 11 nM, respectively. Consistent with these effects being due to inhibition of RET, ponatinib inhibited RET phosphorylation with similar potency in each respective cell line (IC50s of 4, 2, and 9 nM). The TKIs vandetanib (IC50: 448, 357 and 773 nM, respectively), cabozantinib (226, 100 and 292 nM), sunitinib (299, 312 and 570 nM) and sorafenib (>1000, 372 and 861 nM) also inhibited viability of Ba/F3 cells expressing RETC634R, RETM918T and KIF5B-RET, however the potency of all 4 TKIs was substantially reduced compared to that of ponatinib. Importantly, trough ponatinib concentrations (64 nM) observed in patients treated once daily with ponatinib (45 mg) substantially exceed IC50s for inhibition of each of the mutationally-activated RET variants. Additional studies to further evaluate the anti-RET activity of ponatinib and other TKIs, including their susceptibility to mutation-based resistance, are ongoing and will be presented.
Conclusion: Ponatinib is a highly potent inhibitor of activated variants of RET found in MTC and NSCLC. The potency of ponatinib substantially exceeds that of other TKIs with anti-RET activity that are being evaluated in clinical trials. These results provide strong support for the clinical evaluation of ponatinib in patients with RET-driven cancers.
Citation Format: Joseph M. Gozgit, Tzu-Hsiu Chen, Tim Clackson, Victor Rivera. Ponatinib is a highly potent inhibitor of activated variants of RET found in MTC and NSCLC . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2084. doi:10.1158/1538-7445.AM2013-2084