Structure-Activity Relationships and X-ray Structures Describing the Selectivity of Aminopyrazole Inhibitors for c-Jun N-terminal Kinase 3 (JNK3) over p38*

c-Jun N-terminal kinase 3α1 (JNK3α1) 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 structure-based drug design. Inhibitors from the aminopyrazole class, such as SR-3576, were found to be very potent JNK3 inhibitors (IC50 = 7 nm) with >2800-fold selectivity over p38 (p38 IC50 > 20 μm) and had cell-based potency of ∼1 μm. In contrast, indazole-based inhibitors exemplified by SR-3737 were potent inhibitors of both JNK3 (IC50 = 12 nm) and p38 (IC50 = 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.