Structure-Based Discovery of Imidazo[4,5‑c]pyridine SARM1 Modulators Showing Paradoxical Activation

Sterile Alpha and TIR Motif Containing
1 (SARM1) is an
NAD+ hydrolase enzyme implicated in neurological diseases
with
prominent axonopathies. A reported method for SARM1 inhibition involves
the design of small molecules bearing reactive heterocyclic warheads,
which intercept the hydrolysis of NAD+ in the active site
of SARM1 and subsequently inhibit enzymatic function of the TIR domain.
Herein, we describe the discovery of a series of bicyclic SARM1 inhibitors,
initially identified via a unique workflow for free-energy perturbation
(FEP+) simulations. Subsequent hit expansion efforts identified potent
and cell-active inhibitors with slow off-rates, which impart a unique
conformational state of W662 in the SARM1 catalytic site, as assessed
via X-ray crystallography. Finally, we discuss an identified liability
associated with substrate-based SARM1 inhibitors such as 19, whereby insufficient target engagement results in an increase in
biomarkers of neurodegeneration at low doses in vivo and exacerbates neuronal degeneration and cell death in
vitro.