Comprehensive Kinase Profile of Pacritinib, a Non-Myelosuppressive JAK2 Kinase Inhibitor in Phase 3 Development in Primary and Post ET/PV Myelofibrosis

BackgroundPacritinib, a JAK2, JAK2 V617F and FLT3 kinase inhibitor is in Phase III development in myelofibrosis. In preclinical models and in Phase 1 studies in both non-Hodgkin lymphoma and myelofibrosis, it appeared unique among this class of agents in its lack of myelosuppression at doses that inhibited the JAK2/STAT3 pathway and resulted in significant clinical efficacy. This potentially important clinical observation was verified in two Phase II studies in myelofibrosis that had no exclusion restriction for low platelets. Pacritinib was effective at reducing spleen size and symptoms regardless of starting platelet counts and there was no apparent decline in either platelet count or hemoglobin levels, or an increase in transfusion requirements.

Aims and Methods: To help elucidate the mechanisms for pacritinib’s lack of myelosuppression, a kinome screening analysis against 429 recombinant kinases at 100 nM pacritinib concentration was performed, followed by titration from 1-100nM against those that were >50% inhibited at 100nM (Anastassiadis et al.; Nature Biotechnology; 2011). In humans, pacritinib achieves steady state free drug levels of approximately 200nM at the standard 400 mg daily dose.

Results:All kinases with IC50 values of <50nM are included in the table.

Of note, pacritinib did not affect JAK1 (82% control at 100nM). As the concentrations of pacritinib achieved clinically are likely inhibit signaling the kinases listed in the table, these data suggest that the lack of myelosuppression with pacritinib may be either due to inhibition of JAK2 without affecting JAK1 or to suppression of IRAK1, an IL-1 receptor kinase, downstream from Toll-Like Receptors, and associated with the inflammatory response and suppression of normal hematopoiesis in myelodysplasia models (Rhyasen et al.; Cancer Cell; 24:90, 2013).

Conclusions: The kinase profile for pacritinib is unique among agents in development for myelofibrosis combining lack of suppression of JAK1 with suppression of inflammatory signaling through IRAK1. Moreover, the kinase inhibition profile of pacritinib suggests its potential therapeutic utility in AML, MDS, and particularly in CMML due to CMML’s upregulation of JAK2, c-FMS, and IRAK1.