Abstract 2435: Anticipating clinical resistance to SAR302503, a JAK2 inhibitor in clinical development for the treatment of myelofibrosis

One of the challenges of using kinase inhibitors in the clinic is the potential emergence of resistance mechanisms that render them inefficacious in patients who were previously responsive. Imatinib is one of the best examples of this phenomenon. Despite initially impressive clinical responses, a significant proportion of CML patients become unresponsive to imatinib primarily due to the emergence of secondary missense mutations. Identifying these mutations and understanding resistance mechanisms are key to the design of second-generation inhibitors capable of overcoming resistance and for predicting potential clinical resistance to novel drugs. SAR302503 is a JAK2 inhibitor currently in Phase III clinical development as a treatment for myelofibrosis. In a Phase I/II trial, prolonged treatment (>4 years) with SAR302503 has resulted in durable responses (Pardanani et al. ASH 2011; Gotlib et al. EHA 2012). Because of the frequent emergence of resistance with other kinase inhibitors, we wanted to anticipate and understand potential resistance to JAK2 inhibitors in general and SAR302503 in particular. Resistance to ruxolitinib in vitro has recently been associated with reactivation of the JAK/STAT pathway due to the formation of JAK1/JAK2 and TYK2/JAK2 heterodimers (Koppikar et al. Nature 2012). To study potential clinical resistance to SAR302503 in vitro, two approaches were used: selective pressure and ENU induced mutagenesis using JAK2V617F expressing patient derived (SET-2 and HEL) and engineered (Ba/F3.JAK2V617F) cell lines. The results of these in vitro SAR302503 resistance screens indicate that: i) JAK2V617F cells are more prone to develop resistance to ruxolitinib than to SAR302503 after treatment with concentrations 4- or 10-fold over their respective IC50s; ii) resistance does not arise in cells treated with concentrations of SAR302503 above 3 μM even after a prolonged treatment (up to 12 weeks); iii) resistance to SAR302503 in the patient-derived cell lines SET-2 and HEL is accompanied by over-activation of the JAK/STAT pathway in the absence of JAK2 secondary mutations; and iv) resistance to SAR302503 in Ba/F3.JAK2V617F cells results from the mutation Y931C in all of the resistant clones analyzed. From these in vitro results and SAR302503 pharmacokinetic data in humans and preclinical species we can conclude: i) high-level resistance to SAR302503 in patients is unlikely because concentrations of free SAR302503 of approximately 6 μM are estimated to be reached and sustained in the tissues of myelofibrosis patients at a 500 mg daily dose level; ii) overactive JAK/STAT signaling in resistant clones can still be effectively inhibited by clinically achievable concentrations of SAR302503; and iii) clinical activity of SAR302503 may be less challenged than that of ruxolitinib in the event patients express the Y931C mutation. Citation Format: Raelene Hurley, Rita Greco, Fangxian Sun, Anlai Wang, Shih-Min A. Huang, Jenny Zhang, Claudia Lebedinsky, Pamela Cohen, Francisco Adrian. Anticipating clinical resistance to SAR302503, a JAK2 inhibitor in clinical development for the treatment of myelofibrosis. [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 2435. doi:10.1158/1538-7445.AM2013-2435