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A novel series of aminopyridine N-oxides were designed, synthesized, and tested for their ability to inhibit p38alpha MAP kinase. Some of these compounds showed a significant reduction in the LPS-induced TNFalpha production in human whole blood. Structure-activity relationship studies revealed that N-oxide oxygen was essential for activity and was probably a determinant factor for a marked selectivity against other related kinases. Compound 45 was identified as a potent and selective p38alpha inhibitor with an appropriate balance between potency and pharmacokinetics. In vivo efficacy of 45 was demonstrated in reducing TNFalpha levels in an acute murine model of inflammation (ED(50) = 1 mg/kg in LPS-induced TNFalpha production when dosed orally 1.5 h prior to LPS administration). The oral efficacy of 45 was further demonstrated in a chronic model of adjuvant arthritis in rats with established disease when administered orally (ED(50) = 4.5 mg/kg).