Novel immunosuppression: R348, a JAK3- and Syk-inhibitor attenuates acute cardiac allograft rejection.

Background: Janus kinase (JAK)3 is crucial for signal transduction downstream of various cytokine receptors in immune cells. This is the first report on the novel JAK3 inhibitor R348.

Methods: (1) Detailed pharmacokinetic data were obtained in rats; (2) multiple in vitro enzyme inhibition assays were performed to characterize the drug; (3) prevention of acute rejection was investigated in animals treated with different doses of R348 or rapamycin for 5 days; and (4) cardiac allograft survival after a 10-day treatment period was studied for various regimens of R348, tacrolimus, or rapamycin; combination indices were calculated to evaluate drug interactions.

Results: (1) Plasma levels of R348's active metabolite R333 sustained high for 8 hr or more, depending on the dose. (2) In vitro enzyme assays showed potent inhibition of JAK3- and Syk-dependent pathways. (3) R348 40 mg/kg preserved graft function, significantly reduced graft infiltration, and decreased histologic ISHLT rejection scores on postoperative day 5. Results were similar to those of rapamycin 3 mg/kg. Likewise, both drugs significantly reduced the cellular Th1 and Th2 immune responses, as determined by enzyme-linked immunosorbent assays. Intragraft inflammatory cytokine upregulation was similarly suppressed by R348 and rapamycin. R348 10 mg/kg was subtherapeutic. (4) Allograft survival was similar for R348 20 and 40 mg/kg, which was comparable with therapeutically dosed tacrolimus or rapamycin. In combination regimens, R348 demonstrated highly beneficial synergistic interactions with tacrolimus.

Conclusions: R348 is a promising novel JAK3/Syk-inhibitor with favorable pharmacokinetics and biological activity. It effectively diminishes acute cardiac allograft rejection and is suitable for combination regimens with tacrolimus.