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Expanding the Armory: Predicting and Tuning Covalent Warhead Reactivity.

Targeted covalent inhibition is an established approach for increasing the potency and selectivity of potential drug candidates, as well as identifying potent and selective tool compounds for target validation studies. It is evident that identification of reversible recognition elements is essential for selective covalent inhibition, but this must also be achieved with the appropriate level of inherent reactivity of the reactive functionality (or "warhead"). Structural changes that increase or decrease warhead reactivity, guided by methods to predict the effect of those changes, have the potential to tune warhead reactivity and negate issues related to potency and/or toxicity. The half-life to adduct formation with glutathione (GSH t1/2) is a useful assay for measuring the reactivity of cysteine-targeting covalent warheads but is limited to synthesized molecules. In this manuscript we assess the ability of several experimental and computational approaches to predict GSH t1/2 for a range of cysteine targeting warheads, including a novel method based on pKa. Furthermore, matched molecular pairs analysis has been performed against our internal compound collection, revealing structure-activity relationships between a selection of different covalent warheads. These observations and methods of prediction will be valuable in the design of new covalent inhibitors with desired levels of reactivity.