c-Jun N-terminal kinase 3␣1 (JNK3␣1) is a mitogen-activated protein kinase family member expressed primarily in the brain that phosphorylates protein transcription factors, including c-Jun and activating transcription factor-2 (ATF-2) upon activation by a variety of stress-based stimuli. In this study, we set out to design JNK3-selective inhibitors that had >1000-fold selectivity over p38, another closely related mitogen-activated protein kinase family member. To do this we employed traditional medicinal chemistry principles coupled with structurebased drug design. Inhibitors from the aminopyrazole class, such as SR-3576, were found to be very potent JNK3 inhibitors (IC 50 ؍ 7 nM) with >2800-fold selectivity over p38 (p38 IC 50 > 20 M) and had cell-based potency of ϳ1 M. In contrast, indazole-based inhibitors exemplified by SR-3737 were potent inhibitors of both JNK3 (IC 50 ؍ 12 nM) and p38 (IC 50 ؍ 3 nM). These selectivity differences between the indazole class and the aminopyrazole class came despite nearly identical binding (root mean square deviation ؍ 0.33 Å ) of these two compound classes to JNK3. The structural features within the compounds giving rise to the selectivity in the aminopyrazole class include the highly planar nature of the pyrazole, N-linked phenyl structures, which better occupied the smaller active site of JNK3 compared with the larger active site of p38.Because the initial reports on the discovery of p38 (1) and c-Jun N-terminal kinase (JNK) 2 (2-6) these mitogen-activated protein kinase family members have generated great interest as drug targets. p38 especially has garnered considerable interest, particularly for the treatment of rheumatoid arthritis and Crohn disease, and numerous compounds have entered clinical trials for these indications (7-11). Because most of the p38 inhibitors are competitive versus ATP (12-17), and there are 518 kinases in the genome, it was crucial to develop compounds that are selective against a broad panel of kinases so that compounds could be advanced to clinical development. The molecular basis that gives rise to selective p38 inhibitors from numerous structural classes has been reported (18 -20) and is centered on amino acid differences at the so-called "gatekeeper" Thr-106 residue in p38 (Met in all of the JNK isoforms and Gln in extracellular regulated kinase, the other mitogenactivated protein kinase family member). Many compounds have been synthesized that take advantage of this deeper hydrophobic pocket in p38, compared with JNK3, and the structures of the compounds have included trifluoromethyl and other large moieties, which all contribute to p38 selectivity (21). In contrast to p38, there have been fewer reports for selective JNK inhibitors, and the clinical development of JNK inhibitors also lags that of p38. Despite the paucity of highly selective JNK inhibitors that have advanced to clinical development, numerous recent reports have begun to emerge that show compounds from various structural classes (benzothiazole pyrimidines, aminopyr...