Use of ponatinib to inhibit kinase mutations associated with drug-resistant gastrointestinal stromal tumors (GIST).

10509 Background: Ponatinib (PO) is a multi-targeted tyrosine kinase inhibitor with potent pan-BCR-ABL activity that has recently been approved for treatment of CML and Ph+ ALL. PO also inhibits the kinase activity of KIT. Approximately 80% of gastrointestinal stromal tumors (GIST) contain primary activating KIT mutations, the majority of which cluster in exon 11. Imatinib (IM) is approved for the 1st line treatment of GIST; however, patients frequently relapse due to the acquisition of secondary resistance mutations located in either the KIT ATP-binding pocket or the activation (A) loop. Sunitinib (SU) is approved for 2nd line treatment of GIST but does not effectively inhibit A-loop mutants. Here we explored the activity of PO against major primary and secondary KIT mutants found in GIST. Methods: The drug sensitivity of KIT mutants was determined using engineered Ba/F3 cells harboring mutant forms of KIT exon 11 with or without ATP binding pocket or A-loop mutations. The abilities of PO, IM, SU, and regorafenib (RE) to inhibit viability and/or KIT kinase activity were compared using this system as well as an isogenic CHO cell system. We also profiled these same drugs using a panel of GIST cell lines, including cell lines with IM-resistant secondary KIT mutations. Results: In all in vitro systems, PO potently inhibited KIT exon 11 mutant kinases, with an IC50 of < 30 nM. PO also potently inhibited a range of secondary KIT mutants, including multiple A-loop mutant kinases. PO induced substantial tumor regression in Ba/F3 tumor models expressing a KIT exon 11 mutant with or without an A-loop mutation (D816H). Using GIST cell lines, PO inhibited the viability of those harboring primary KIT exon 11 and secondary resistance mutations more effectively than IM, SU, and RE. Importantly, in patients dosed once daily with 45 mg ponatinib, plasma concentrations achieved are predicted to lead to inhibition of all KIT mutants tested with the possible exception of V654A. Conclusions: PO potently inhibits the majority of clinically relevant KIT mutant kinases and has a broader spectrum of activity compared to IM, SU, or RE. Based on these data, a phase 2 study of PO in drug-resistant GIST is being initiated.