Ruxolitinib is a potent and selective JAK1/2 inhibitor (IC50: 3.3/2.8 nM) and is relatively less selective for JAK3 (IC50: 322 nM).
Ruxolitinib (INCB018424) inhibited interleukin-6 signaling (IC50: 281 nM), and proliferation of JAK2V617F(+) Ba/F3 cells (IC50: 127 nM). In primary cultures, INCB018424 preferentially suppressed erythroid progenitor colony formation from JAK2V617F(+) polycythemia vera patients (IC50: 67 nM) versus healthy donors (IC50 > 400nM) [1].
In a mouse model of JAK2V617F(+) MPN, oral INCB018424 markedly reduced splenomegaly and circulating levels of inflammatory cytokines, and preferentially eliminated neoplastic cells, resulting in significantly prolonged survival without myelosuppressive or immunosuppressive effects [1]. A total of 28% of the patients in the ruxolitinib group had at least a 35% reduction in spleen volume at week 48, as compared with 0% in the group receiving the best available therapy; the corresponding percentages at week 24 were 32% and 0%. At 48 weeks, the mean palpable spleen length had decreased by 56% with ruxolitinib but had increased by 4% with the best available therapy [2]. The primary endpoint was reached in 41.9% of patients in the ruxolitinib group as compared with 0.7% in the placebo group. A reduction in spleen volume was maintained in patients who received ruxolitinib; 67.0% of the patients with a response had the response for 48 weeks or more. There was an improvement of 50% or more in the total symptom score at 24 weeks in 45.9% of patients who received ruxolitinib as compared with 5.3% of patients who received placebo [3].
The kinase domains of human JAK1 (837-1142), JAK2 (828-1132), JAK3 (781-1124), and Tyk2 (873-1187) were cloned by PCR with N-terminal epitope tags. Recombinant proteins were expressed using Sf21 cells and baculovirus vectors and purified with affinity chromatography. JAK kinase assays used a homogeneous time-resolved fluorescence assay with the peptide substrate (-EQEDEPEGDYFEWLE). Each enzyme reaction was carried out with test compound or control, JAK enzyme, 500nM peptide, adenosine triphosphate (ATP; 1mM), and 2.0% dimethyl sulfoxide (DMSO) for 1 hour. The 50% inhibitory concentration (IC50) was calculated as the compound concentration required for inhibition of 50% of the fluorescent signal. Biochemical assays for CHK2 and c-MET enzymes were performed using standard conditions (Michaelis constant [Km] ATP) with recombinantly expressed catalytic domains from each protein and synthetic peptide substrates. An additional panel of kinase assays (Abl, Akt1, AurA, AurB, CDC2, CDK2, CDK4, CHK2, c-kit, c-Met, EGFR, EphB4, ERK1, ERK2, FLT-1, HER2, IGF1R, IKKα, IKKβ, JAK2, JAK3, JNK1, Lck, MEK1, p38α, p70S6K, PKA, PKCα, Src, and ZAP70) was performed using standard conditions using 200nM INCB018424. Significant inhibition was defined as more than or equal to 30% (average of duplicate assays) compared with control values [1].
Cells were seeded at 2000/well of white bottom 96-well plates, treated with compounds from DMSO stocks (0.2% final DMSO concentration), and incubated for 48 hours at 37°C with 5% CO2. Viability was measured by cellular ATP determination using the Cell-Titer Glo luciferase reagent or viable cell counting. Values were transformed to percent inhibition relative to vehicle control, and IC50 curves were fitted according to nonlinear regression analysis of the data using PRISM GraphPad [1].
All of the procedures were conducted in accordance with the US Public Health Service Policy on Humane Care and Use of Laboratory Animals. Mice were fed standard rodent chow and provided with water ad libitum. Ba/F3-JAK2V617F cells (10^5 per mouse) were inoculated intravenously into 6- to 8-week-old female BALB/c mice. Survival was monitored daily, and moribund mice were humanely killed and considered deceased at time of death. Treatment with vehicle (5% dimethylacetamide, 0.5% methocellulose) or INCB018424 began within 24 hours of cell inoculation, twice daily by oral gavage. Hematologic parameters were measured using a Bayer Advia120 analyzed, and statistical significance was determined using Dunnett testing [1].
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