Abstract
BackgroundThe Janus kinase (JAK) family of tyrosine kinases includes JAK1, JAK2, JAK3 and TYK2, and is required for signaling through Type I and Type II cytokine receptors. CP-690,550 is a potent and selective JAK inhibitor currently in clinical trials for rheumatoid arthritis (RA) and other autoimmune disease indications. In RA trials, dose-dependent decreases in neutrophil counts (PBNC) were observed with CP-690,550 treatment. These studies were undertaken to better understand the relationship between JAK selectivity and PBNC decreases observed with CP-690,550 treatment.MethodsPotency and selectivity of CP-690,550 for mouse, rat and human JAKs was evaluated in a panel of in vitro assays. The effect of CP-690,550 on granulopoiesis from progenitor cells was also assessed in vitro using colony forming assays. In vivo the potency of orally administered CP-690,550 on arthritis (paw edema), plasma cytokines, PBNC and bone marrow differentials were evaluated in the rat adjuvant-induced arthritis (AIA) model.ResultsCP-690,550 potently inhibited signaling through JAK1 and JAK3 with 5-100 fold selectivity over JAK2 in cellular assays, despite inhibiting all four JAK isoforms with nM potency in in vitro enzyme assays. Dose-dependent inhibition of paw edema was observed in vivo with CP-690,550 treatment. Plasma cytokines (IL-6 and IL-17), PBNC, and bone marrow myeloid progenitor cells were elevated in the context of AIA disease. At efficacious exposures, CP-690,550 returned all of these parameters to pre-disease levels. The plasma concentration of CP-690,550 at efficacious doses was above the in vitro whole blood IC50 of JAK1 and JAK3 inhibition, but not that of JAK2.ConclusionResults from this investigation suggest that CP-690,550 is a potent inhibitor of JAK1 and JAK3 with potentially reduced cellular potency for JAK2. In rat AIA, as in the case of human RA, PBNC were decreased at efficacious exposures of CP-690,550. Inflammatory end points were similarly reduced, as judged by attenuation of paw edema and cytokines IL-6 and IL-17. Plasma concentration at these exposures was consistent with inhibition of JAK1 and JAK3 but not JAK2. Decreases in PBNC following CP-690,550 treatment may thus be related to attenuation of inflammation and are likely not due to suppression of granulopoiesis through JAK2 inhibition.
Highlights
The Janus kinase (JAK) family of tyrosine kinases includes JAK1, JAK2, JAK3 and TYK2, and is required for signaling through Type I and Type II cytokine receptors
The aim of the current study was to characterize the potency and selectivity of CP-690,550 for the JAK family members and to determine if PBNC reductions in the context of arthritis are related to the anti-inflammatory efficacy of CP-690,550, or due to inhibition of hematopoiesis through inhibition of JAK2 at efficacious exposures
Results of this study suggest that the CP-690,550 mediated reduction in PBNCs in the rat adjuvant-induced arthritis (AIA) model, and likely in human rheumatoid arthritis (RA) patients, is due to the antiinflammatory action of the compound by the suppression of cytokines and chemotactic factors which elevate neutrophil counts in the peripheral compartment
Summary
The Janus kinase (JAK) family of tyrosine kinases includes JAK1, JAK2, JAK3 and TYK2, and is required for signaling through Type I and Type II cytokine receptors. CP-690,550, a selective inhibitor of the JAK family of protein tyrosine kinases, is being developed as an immunosuppressive and anti-inflammatory agent for the treatment and prevention of acute allograft rejection, RA, psoriasis and other immune mediated diseases [1,2,3,4,5,6]. CP-690,550 administration resulted in a dose-related decrease in PBNCs in active RA patients [7,8] within 2 weeks of treatment, but not in psoriasis patients [9], renal allograft patients [10] or normal volunteers [11] for up to 14 and 28 days of treatment, respectively. Following treatment with CP-690,550 for 2 weeks, PBNCs in these patients were found to decrease to within the normal reference range, and showed a strong dose-related correlation with the anti-inflammatory activity of the compound [13]. JAK2 is required for signaling through several growth factor receptors and is important for myeloid and erythroid hematopoiesis [14,15,16]
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