Momelotinib sulfate is an ATP-competitive JAK1/JAK2 inhibitor (IC50: 11 nM/18 nM). It has 10-fold selectivity versus JAK3.

JAK2:18 nM, JAK1:11 nM, JAK3:155 nM

Momelotinib sulfate inhibits growth of Ba/F3-JAK2V617F and Ba/F3-MPLW515L cells (IC50: 200 nM) or human erythroleukemia (HEL) cells (IC50: 1.5 μM). However, it has considerably less activity against BCR-ABL harboring K562 cells (IC50=58 μM) and FLT3 mutation harboring MV4-11 cells (IC50: 3 μM). Proliferation of parental Ba/F3 cells (Ba/F3-wt) stimulated with IL-3 is inhibited (IC50: 1.4 μM). This is same as the established role of IL-3-dependent signaling in the parental cell line [1].

Momelotinib sulfate at twice the dose used in disease model (50 and 100 mg/kg) has little to no effect on peripheral blood counts over a period of 8 weeks. Median plasma peak concentrations are 7.1 μM with the lower dose and 32.1μM with the higher dose, with a half-life of approximately 2 hours. Trough levels at 12 hours are 10nM for the 25 mg/kg and 900nM for the 50 mg/kg dose. After oral dosing, Momelotinib sulfate shows high plasma concentrations (Cmax= 40.4 μM; Tmax=4 h), with quantitative absolute oral bioavailability and an apparent half life of 2.4 h. The high oral bioavailability, can likely be partly ascribed to the low blood clearance of Momelotinib sulfate (6.3 mL/min/kg) and therefore low susceptibility to hepatic first pass metabolism [3]. At day 34 after transplantation, the mean white blood cell counts and hematocrit values of the entire cohort exceeded the normal range for Balb/c mice by more than 1 SD. At this point, 6 mice are sacrificed and subjected to autopsy. In the remaining animals, treatment is initiated with 25 mg/kg Momelotinib sulfate, 50 mg/kg Momelotinib sulfate, or vehicle, administered twice daily by oral gavage (12 mice per treatment group). A rapid drop of the white cell counts is apparent in both dose cohorts as early as 6 days after initiation of treatment and a decline of the hematocrit is apparent after 20 days [2].