INCB018424, an Oral, Selective JAK2 Inhibitor, Shows Significant Clinical Activity in a Phase I/II Study in Patients with Primary Myelofibrosis (PMF) and Post Polycythemia Vera/Essential Thrombocythemia Myelofibrosis (Post-PV/ET MF).
Authors: Verstovsek Srdan , Kantarjian Hagop , Pardanani Animesh , Thomas Deborah , Cortes Jorge , Mesa Ruben , Redman John , Staschen Carl-Michael , Fridman Jordan , Vaddi Kris , Tefferi Ayalew 
Published: October 2019

Abstract
Background: A mutation in the Janus tyrosine kinase 2 gene (JAK2 V617F) has been recognized in Philadelphia chromosome-negative myeloproliferative disorders, including PV (∼95% of patients), ET (∼50%) and PMF (∼50%). INCB018424 is a potent and orally bioavailable selective JAK2 inhibitor with >80-fold selectivity against a broad panel of kinases, including JAK3. INCB018424 potently inhibits JAK2 V617F mediated signaling and malignant cell survival in vitro and in vivo in mice. Preclinical safety studies with INCB018424 demonstrated an excellent safety profile with no off-target toxicities.
Methods: A phase I/II trial of INCB018424 given orally BID is being conducted in patients with PMF and Post-PV/ET MF. Both JAK2 V617F and JAK2 wild type patients are eligible. PK and PD data are being collected. Responses are being evaluated using the International Working Group (IWG) consensus criteria for treatment response in myelofibrosis with myeloid metaplasia, for the IWG for Myelofibrosis Research and Treatment (IWG-MRT) (Tefferi et al., Blood, 108, 1497–1503, 2006).
Results: The initial dose of 25 mg PO BID resulted in a rapid and marked reduction in splenomegaly. All 3 patients in the first cohort of patients achieved a reduction in spleen size that is consistent with a clinical improvement response provided it is sustained for 8 weeks; spleen sizes of 25, 22, and 7 cm below the left costal margin have decreased to 8, 10, and 0 cm in the first month of therapy. At two months follow-up, the patient with a baseline spleen size of 22 cm is now down to 2 cm. All 3 patients also noted significant symptomatic improvement. The second cohort of 3 patients started therapy at an increased dose (50 mg PO BID) and at one week follow-up, the initial two patients (one JAK2 wild type, one JAK2 V617F) had decreases in spleen size from 22 cm to 17 cm and from 22 cm to 16 cm, respectively. No dose-limiting toxicities or other significant adverse events occurred in any patients to date. PK/PD analyses demonstrated that administration of INCB018424 resulted in plasma drug concentrations sufficient to markedly inhibit JAK2, as shown by suppression of phosphorylated STAT3 (a substrate of JAK2) in whole blood cells in all 3 patients in the first cohort. The 3 patients in the first cohort were all JAK2 V617F mutation positive: percentages of JAK2 V617F positive whole blood cells before therapy were 79%, 49% and 91%, and after one month of therapy, they were 59%, 48% and 78%, respectively.
Conclusions: Initial results show marked reduction in splenomegaly and symptomatic improvement, without significant toxicity. The percentage of blood cells with JAK2 V617F mutation appears to be decreasing in patients on therapy. Updated results on current and future patients will be presented at the meeting.