Potential of p38 inhibitors in the treatment of rheumatoid arthritis.

Rheumatoid arthritis (RA) is a chronic debilitating disease estimated to afflict 13% of the world population. Although palliative treatments (nonsteroidal antiinflammatory drugs or NSAIDs) are widely prescribed, there are currently only a few treatments that can modify the insidious progression of the disease (disease-modifying antirheumatic drugs or DMARDs), which frequently leads to physical incapacitation and, on occasion, death. Proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) are implicated in the disease onset and in the progression of bone and joint destruction that characterizes chronic RA. p38 is an intracellular mitogen/stress-activated protein kinase (MAPK/SAPK) that regulates both the release and the actions of TNF-alpha and IL-1 beta. Inhibition of p38 kinase is thus an important potential target for novel DMARDs. Following the pioneering work conducted at SmithKline Beecham and elucidation of the roles of p38 with potent and selective inhibitors such as SB-203580, many pharmaceutical companies have embarked upon p38 synthetic programs, as indicated by the ever-increasing number of patents in this domain. At Aventis, a rapid parallel synthesis project led to the identification of RPR-200765A, a potent and selective p38 inhibitor of the lipopolysaccharide-induced release of TNF-alpha in vitro in mononuclear phagocytes and in vivo in the rat. It also reduces disease incidence and progression in the rat streptococcal cell wall arthritis model when administered orally in either a prophylactic or a therapeutic dosing regimen. Development of p38 inhibitors has been slow, probably because of toxicological problems, which might explain why only two oral p38 inhibitors, SB-242235 and VX-745, have advanced into clinical development. In the present article, the preclinical data exemplified in studies on RPR-200765A indicating why p38 inhibitors are attracting so much attention as potential novel anti-RA drugs are reviewed. Current information on the different structural classes of p38 inhibitors is presented and possible reasons for the delays in their development are critically discussed.