Tetomilast (OPC-6535)

Abstract

Purpose: Tetomilast, an anti-inflammatory compound, is being developed for the treatment of inflammatory bowel disease (IBD). Although its mechanism of action has not been fully elucidated, it is known to inhibit phosphodiesterase 4 (PDE4) activity. To investigate the mechanisms involved in tetomilast's anti-inflammatory action, we compared its effects on CD4 T cells, which have been associated with the pathogenesis of IBD, with that of several known PDE4 inhibitors. Methods: The effect of tetomilast on recombinant human PDE4 subtypes was determined by PDE assay. IC50 was calculated from dose-response curves for tetomilast and for the PDE4 inhibitors cilomilast, rolipram, and roflumilast. Human CD4 T cells were purified from the blood of healthy donors using a commercial CD4 T cell isolation kit. CD4 T cells were stimulated using anti-CD3 and anti-CD28 antibodies and incubated in the absence or presence of test compounds for 72 hr at 37°C. For determination of T cell proliferation, [3H]-thymidine (0.2 μCi/well) was added for the last 18 hr of the incubation period. Interferon-γ (INF-γ) released into the culture medium was determined by ELISA. Results: Tetomilast was found to be a potent inhibitor of PDE4 subtypes. Its selectivity for inhibiting the high-affinity rolipram-binding (HARB) conformer of PDE4 was reduced compared with that of rolipram. Tetomilast's PDE4 inhibitory potency was similar to that of rolipram and cilomilast and was at least 100× less than that of roflumilast. At 10 μM, tetomilast caused a significantly greater inhibition of CD3/CD28-stimulated T cell proliferation than did the equivalent PDE4-inhibitory concentrations of 10 μM cilomilast, 10 μM rolipram, and 0.1 μM roflumilast. Tetomilast also demonstrated greater inhibition of INF-γ release from CD3/CD28-stimulated T cells as compared with the other PDE4 inhibitors. Conclusions: Compared with rolipram, tetomilast showed a reduced selectivity for inhibition of the HARB conformer of PDE4, predicting a reduction in side effects, which have been associated with binding of PDE4 inhibitors to the HARB site in the CNS and gastrointestinal tract. Tetomilast's inhibition of CD3/CD28-stimulated CD4 T cell proliferation and INF-γ release was significantly greater than that of the other PDE4 inhibitors tested, suggesting that other mechanisms beyond PDE4 inhibition may account for the pharmacologic activity of tetomilast.