Sorafenib and Nilotinib Are Candidates to Overcome Imatinib Resistance in Myeloproliferation with FIP1L1-PDGFRA.

Abstract 2912

Poster Board II-888

Constitutively activated variants of PDGFRA, PDGFRB can be found in a subset of patients with myeloid neoplasms associated with eosinophilia. The most common is FIP1L1-PDGFRA (FP). Patients with PDGFR-A and -B rearranged myeloproliferation respond to treatment with imatinib. However, single cases of clinical resistance due to a secondary FP/T674I mutation have been reported. In CML, more than 40 different exchanges have been described that confer imatinib resistance, and sequential treatment with imatinib and novel Abl kinase inhibitors has become reality. Nilotinib and sorafinib are potent alternative inhibitors of PDGFR-A and -B. We therefore hypothesized that available PDGFR kinase inhibitors might produce specific profiles of secondary FP kinase domain mutations mediating inhibitor resistance. To this aim, we selected clones of FP expressing Ba/F3 cells resistant to rising concentrations of imatinib, nilotinib, and sorafinib. In these, we identified 27 different PDGFRA kinase domain mutations. Imatinib, nilotinib and sorafenib produced distinct profiles of resistance mutations. During selection with imatinib, FP/T674I predominated with rising concentrations. FP/T674I corresponds to Bcr-Abl/T315I, which is frequently found in imatinib resistant CML. In contrast to imatinib, nilotinib and sorafenib produced a significantly lower frequency of resistant cell clones. Also, T674I disappeared at therapeutic nilotinib concentrations in favour of T674I+T874I and D842V. Sorafinib displayed a distinct profile of mutations including D842V, but not T674I. Following cloning and expression of all FP variants, dose-response analysis indicated that full cross-resistance to all three inhibitors was limited to D842V, whereas the gatekeeper T674I exchange retained sensitivity to sorafenib and nilotinib, and T674I+T874I was responsive to sorafenib only. In silico structure modelling indicated that differences in inhibitor response observed in distinct clusters of FP mutations identified in drug-resistant clones are based on differences of sorafenib versus imatinib and nilotinib in key drug - protein target interactions in PDGFR family kinases. Besides direct inhibitor binding effects, we propose that the identified exchanges shift an inactive-active conformation equilibrium and thereby affect binding of type II inhibitors like imatinib, nilotinib and sorafenib. Our results predict PDGFR variants that might come up in patients with myeloproliferation positive for PDGFR-A or -B fusions treated with imatinib, nilotinib or sorafenib. These findings will help in selection of an appropriate second line PDGFR kinase inhibitor when resistance to imatinib emerges, and will guide drug design. Moreover, our data can be translated to other neoplasms driven by activated forms of PDGFR-A or -B including GIST, CMML, and dermatofibrosarcoma protuberans.