Osimertinib’s Proton-Catalyzed, Pseudoconcerted EGFR Inhibition Guides Next-Generation Inhibitor Design

Third-generation
inhibitors such as osimertinib irreversibly
inhibit
the epidermal growth factor receptor tyrosine kinase (EGFR-TK) via
Michael addition to Cys797, yet the mechanism of covalent bond formation
remains unsettled. Here, density functional theory-level quantum mechanics/molecular
mechanics (QM/MM) computations delineate a proton-catalyzed, pseudoconcerted
mechanism for covalent inhibition of EGFR by osimertinib. The computed
Gibbs energy profile features a single transition state (TS1) in which
Cys797 deprotonation, nucleophilic attack at osimertinib’s
Michael acceptor, and protonation of the acceptor’s carbonyl
oxygen by osimertinib’s terminal aliphatic aminium occur simultaneously,
in contrast to previous stepwise proposals. Natural bond orbital (NBO)
analysis shows that carbonyl O-protonation attenuates carbonyl π
bonding, increases β-carbon (Cβ) electrophilicity,
and thereby facilitates formation of the Cys797–Cβ bond. The resulting path proves to be more favorable in both kinetics
and thermodynamics than stepwise alternatives, consistent with experimental
trends. The calculations also rationalize osimertinib’s preferential
inhibition of the T790M mutant. Electrostatic potential (ESP) analysis
demonstrates that T790M subtly redistributes charge within osimertinib’s
pyrimidine–indole scaffold, enhancing the cationic character
at the indole N-methyl substituent and strengthening
its electrostatic interaction with Asp855 to stabilize the reactant
complex. Together, these results provide a mechanistic framework for
covalent bond formation underlying EGFR inhibitionhighlighting
inhibitor-derived proton catalysis and Asp855 engagementand
offer design principles for next-generation covalent inhibitors with
improved potency and resistance-breaking potential.