Peficitinib (ASP015K) is an oral Janus kinase (JAK) inhibitor. It inhibits enzyme activities of JAK1, JAK2, JAK3 and Tyk2 with IC50 values of 3.9, 5.0, 0.71 and 4.8 nmol/L, respectively. It showed moderate selectivity for JAK3 inhibition [1].

JAK family of non-receptor protein tyrosine kinases includes JAK1, JAK2, JAK3 and tyrosine kinase 2 (Tyk2). JAK1, JAK2, JAK3 and Tyk2 are critically important in haematopoietic cells and immune cells [1].

Erythropoietin regulates haematopoiesis (the production of blood cells) [2]. Treatment with ASP015K inhibited the human T-cell proliferation induced by IL-2. And this effect was greater than the effect to inhibit EPO-induced human erythroleukemia cell proliferation. In human whole blood, ASP015K concentration-dependently inhibited STAT5 phosphorylation (pSTAT5) [3].

In healthy volunteers, STAT5-P was dose-dependently inhibited by ASP015K. When the doses of ASP015K were 60, 120, 200, or 300 mg, the mean peak percentage inhibitions of STAT5-P were 84%, 85%, 92%, and 93%, respectively. Plasma ASP015K inhibited STAT5-P with an EC50 value of 48 ng/mL, a shape factor (γ) of ~1.2, and an estimated Emax close to 100%. On days 1, 7, and 14, at ~2 hours after treatment with ASP015K at multiple doses, the peak of median percentage of STAT5-P inhibition appeared [4].

References:
[1].  Takeuchi T, Tanaka Y, Iwasaki M, et al. Efficacy and safety of the oral Janus kinase inhibitor peficitinib (ASP015K) monotherapy in patients with moderate to severe rheumatoid arthritis in Japan: a 12-week, randomised, double-blind, placebo-controlled phase IIb study. Annals of the rheumatic diseases, 2015: annrheumdis-2015-208279.
[2].  Digicaylioglu M, Lipton SA. Erythropoietin-mediated neuroprotection involves cross-talk between Jak2 and NF-κB signalling cascades. Nature, 2001, 412(6847): 641-647.
[3].  Yamazaki S, Morio H, Inami M, et al. THU0101 ASP015K: A Novel Jak Inhibitor Demonstrated Potent Efficacy in Adjuvant-Induced Arthritis Model in Rats. Annals of the Rheumatic Diseases, 2013, 72(Suppl 3): A197-A197.
[4].  Zhu T, Valluri U, Lewand M, et al. THU0256 Pharmacodynamics of ASP015K, A Novel Janus Kinase Inhibitor, in Healthy Volunteers. Annals of the Rheumatic Diseases, 2013, 72(Suppl 3): A252-A252.