Modulating the Binding Kinetics of Bruton's Tyrosine Kinase Inhibitors through Transition-State Effects

Optimization exercises strive toward increasing the efficacy and selectivity of small molecules toward the target of interest while simultaneously phasing out design elements that lead to off-target interactions. Given the nonequilibrium nature of biological systems, greater reliance should be placed on engineering kinetic selectivity in addition to equilibrium thermodynamic selectivity; however, the rational design of kinetic selectivity is a challenging endeavor. This study presents a systematic knowledge-based approach to the design of inhibitors that vary in their binding kinetics for Bruton's tyrosine kinase (BTK), a target for treating B-cell malignancies and autoimmune diseases. A detailed kinetic assessment was performed on existing BTK inhibitors, which, together with structural studies, provided critical insights into BTK-inhibitor interactions that control the kinetics of enzyme inhibition. Subsequently, a series of pyrazolopyrimidines was designed with the objective of modifying interactions between the inhibitor and the regulatory (R) spine in the kinase back pocket, which were hypothesized to modulate the stability of the transition state on the binding reaction coordinate. This resulted in the development of BTK inhibitors with extended residence time in which the variation in k_on and k_off was uncoupled from equilibrium thermodynamic affinity.