Irreversible protein kinase inhibitors

Targeting cancer with small molecule irreversible inhibitors of kinases represents an emerging challenge in drug discovery. Irreversible inhibitors bind to kinase active site in a covalent and irreversible form, most frequently by reacting with a nucleophilic cysteine residue, located near the ATP binding pocket. The most common mechanism is the Michael reaction, that refers to the addition of a nucleophile, such as cysteine, to an α,β-unsaturated carbonyl. The nucleophile reacts at the electrophilic β-position to form an adduct; as a result the inhibitor irreversibly blocks binding of ATP to the kinase, rendering the kinase inactive. Different cysteine-reactive groups have been evaluated, an acrylamide or a substituted acrylamide moiety are the Michael acceptors of choice. There are some advantages for the irreversible kinase inhibition. These compounds are highly selective because they target a specific cysteine and only a limited number of kinases has a cysteine at the corresponding position. Another advantage is that covalent bond formation can overcome competition with the high endogenous concentration of ATP. A further motivation for designing irreversible inhibitors is their longer duration of action respect to conventional inhibitors. In fact, once bound to enzyme, these compounds do not readily dissociate and the inhibition continues even after the inhibitor leaves the circulation. Moreover, these inhibitors have the potential to overcome and prevent the emergence of acquired resistance conferred by mutations. In this review examples of irreversible inhibitors are reported, focusing on chemical structures, SAR and biological activities. The great potential of these compounds could open new and promising perspectives for a broader application of this approach.