Activity of novel RET inhibitors against RET genotypes associated with medullary thyroid cancer.

5559 Background: Point mutations in RET tyrosine kinase, most commonly C634R and M918T, result in the development of medullary thyroid cancer (MTC). C634R and M918T occur in the cysteine-rich and kinase domain, respectively, and therefore activate RET by differing mechanisms. V804L, a mutation at the putative RET gatekeeper residue, has demonstrated resistance to several tyrosine kinase inhibitors (TKIs) in vitro. Notably, with other tyrosine kinases, mutations in the kinase domains or gatekeeper residues alter drug sensitivity. We evaluated the effect of these RET mutations on the response to TKIs in a cell based system. Methods: CHO cells expressing WT and mutant RET proteins were treated with a panel of TKIs consisting of compounds in preclinical development (DP-2490 and DP-3636) and drugs being studied for treatment of MTC in clinical trials (axitinib, motesanib, sorafenib, sunitinib, vandetanib). Drug sensitivity was determined by inhibition of RET kinase phosphorylation and IC50s were calculated using photodensitometry. Results: The IC50s of each TKI against the different RET mutants are presented in the Table. WT RET was most sensitive to all of the TKIs. DP-2490 and DP-3636 had IC50s < 1 nM against WT RET, and demonstrated superior potency against all of the RET mutants tested. All drugs tested were at least 5-fold more potent against C634R than M918T. V804L demonstrated complete resistance to axitinib, motesanib, and vandetanib at 5,000nM, but demonstrated increased sensitivity to the other compounds when compared to M918T. Conclusions: RET kinase is a therapeutic target for the treatment of MTC. Here, we demonstrate that RET kinase inhibition differs by RET genotype and kinase inhibitor. These findings are similar to those in other malignancies, and suggest that MTC tumor genotype may be predictive of response to individual TKIs that target RET. DP-2490 and DP-3636 represent the most potent RET inhibitors described to date and should be studied further for the treatment of MTC. Inhibitor IC50 (nM) RET wild-type RET C634R RET V804L RET M918T Axitinib 50 130 >5,000 2,350 Motesanib 150 250 >5,000 1,870 Sorafenib 30 60 390 430 Sunitinib 13 70 40 360 Vandetanib 60 110 >5,000 560 DP-2490 0.4 10 30 86 DP-3636 0.1 0.7 1.4 10 Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Deciphera Deciphera