PRT062607 Achieves Complete Inhibition of the Spleen Tyrosine Kinase at Tolerated Exposures Following Oral Dosing in Healthy Volunteers.

The spleen tyrosine kinase (SYK) regulates immune cell activation in response to engagement of a variety of receptors, making it an intriguing target for the treatment of inflammatory and autoimmune disorders as well as certain B-cell malignancies. We have previously reported on the discovery and preclinical characterization of PRT062607, a potent and highly selective inhibitor of SYK that exhibits robust anti-inflammatory activity in a variety of animal models. Here we present data from our first human studies aimed at characterizing the pharmacokinetics (PK), pharmacodynamics (PD), and safety of PRT062607 in healthy volunteers following single and multiple oral administrations. PRT062607 demonstrated a favorable PK profile and the ability to completely inhibit SYK activity in multiple whole-blood assays. The PD half-life in the more sensitive assays was approximately 24 hours and returned to predose levels by 72 hours. Selectivity for SYK was observed at all dose levels tested. Analysis of the PK/PD relationship indicated an IC50 of 324 nM for inhibition of B-cell antigen receptor-mediated B-cell activation and 205 nM for inhibition of FcεRI-mediated basophil degranulation. PRT062607 was safe and well tolerated across the entire range of doses. Clinical PK/PD was related to in vivo anti-inflammatory activity of PRT062607 in the rat collagen-induced arthritis model, which predicts that therapeutic concentrations may be safely achieved in humans for the treatment of autoimmune disease. PRT062607 has a desirable PK profile and is capable of safely, potently, and selectively suppressing SYK kinase function in humans following once-daily oral dosing.

Specific inhibition of spleen tyrosine kinase suppresses leukocyte immune function and inflammation in animal models of rheumatoid arthritis.

Based on genetic studies that establish the role of spleen tyrosine kinase (Syk) in immune function, inhibitors of this kinase are being investigated as therapeutic agents for inflammatory diseases. Because genetic studies eliminate both adapter functions and kinase activity of Syk, it is difficult to delineate the effect of kinase inhibition alone as would be the goal with small-molecule kinase inhibitors. We tested the hypothesis that specific pharmacological inhibition of Syk activity retains the immunomodulatory potential of Syk genetic deficiency. We report here on the discovery of (4-(3-(2H-1,2,3-triazol-2-yl)phenylamino)-2-((1R,2S)-2-aminocyclohexylamino) pyrimidine-5-carboxamide acetate (P505-15), a highly specific and potent inhibitor of purified Syk (IC50 1-2 nM). In human whole blood, P505-15 potently inhibited B cell antigen receptor-mediated B cell signaling and activation (IC50 0.27 and 0.28 µM, respectively) and Fcε receptor 1-mediated basophil degranulation (IC50 0.15 µM). Similar levels of ex vivo inhibition were measured after dosing in mice (Syk signaling IC50 0.32 µM). Syk-independent signaling and activation were unaffected at much higher concentrations, demonstrating the specificity of kinase inhibition in cellular systems. Oral administration of P505-15 produced dose-dependent anti-inflammatory activity in two rodent models of rheumatoid arthritis. Statistically significant efficacy was observed at concentrations that specifically suppressed Syk activity by ∼67%. Thus specific Syk inhibition can mimic Syk genetic deficiency to modulate immune function, providing a therapeutic strategy in P505-15 for the treatment of human diseases.