Kinetics of small molecule inhibitor binding to p38 kinase

p38 mitogen-activated protein kinase (MAPK) (p38/p38-α/CSBP2/RK) has been implicated in the regulation of many proinflammatory pathways. Because of this, it has received much attention as a potential drug target for controlling diseases such as rheumatoid arthritis, endotoxic shock, inflammatory bowel disease, osteoporosis, and many others. A number of small molecule inhibitors of this kinase have been described, and in this paper we have used surface plasmon resonance to directly measure and quantitate their binding to p38. Despite the relatively low molecular mass (≈ 400 Da) of these inhibitors, specific binding can be observed. For the two most potent inhibitors studied, SB 203580 and RWJ 67657, dissociation constants, K_d's, of 22 and 10 nM, respectively, were obtained. These values closely match the IC₅₀ values observed in a cell-based TNFα release assay implying that p38 plays a major role in TNFα release. The association and dissociation rates for the binding of these inhibitors to p38 have also been quantitated. SB 203580 and RWJ 67657 have very similar association rates of around 8 × 10⁵ M^-1·S^-1, and the differences in affinity are determined by different dissociation rates. The weaker binding compounds have dissociation rates similar to SB 203580, but the association rates vary by an order of magnitude or more. The direct measurement of compounds binding to p38 may help in understanding the difference between potency and efficacy for these inhibitors. This in turn may yield clues on how to develop better inhibitors.