Page - (000065) - in Biomedical Chemistry: Current Trends and Developments

designing reversible, cysteine-targeted covalent inhibitors. Based on a report that revealed that simple thiols reacted instantaneously with 2- cyanoacrylates at physiological pH, although the corresponding products could not be isolated or structurally characterized (Pritchard, 1968), Taunton hypothesized that this scaffold underwent Michael-type conjugate addition via a rapid-equilibrium process (Serafimova, 2012). Using a simple model reaction of different Michael acceptors with β- mercaptoethanol (BME) monitored by NMR, the group showed that, while compounds with a single electron-withdrawing group (e.g. 3- phenyl acrylonitrile, Fig. 1.2.3A) led to a stable adduct, their counterparts with two electron-withdrawing groups (e.g. ethyl 3-phenyl- 2-cyanoacrylate, Fig. 1.2.3A) formed an adduct that rapidly reverted to the starting material upon ten-fold dilution with phosphate saline buffer (PBS). Combining this result with the predicted binding orientation of the pyrrolopyrimidine scaffold, similar to the irreversible fluoromethylketone-based inhibitor developed previously by the same group for the C-terminal domain of the p90 ribosomal protein S6 kinase RSK2, led to the discovery of the first reversible covalent kinase inhibitor (Fig. 1.2.3B) (Serafimova, 2012). Additional scaffolds were discovered de novo using a fragment-based ligand approach, where a library of low molecular-weight cyanoacrylamides was screened against several kinase assays (Fig. 1.2.3B) (Miller, 2013).