Unbinding of Translocator Protein 18 kDa (TSPO) Ligands: From in Vitro Residence Time to in Vivo Efficacy via in Silico Simulations

Translocator
protein 18 kDa (TSPO) is a validated pharmacological
target for the development of new treatments for neurological disorders. N,N-Dialkyl-2-phenylindol-3-ylglyoxylamides
(PIGAs) are effective TSPO modulators and potentially useful therapeutics
for the treatment of anxiety, central nervous system pathologies featuring
astrocyte loss, and inflammatory-based neuropathologies. For this
class of compounds, no correlation exists between the TSPO binding
affinity and the corresponding functional efficacy. Rather, their
biological effectiveness correlates with the kinetics of the unbinding
events and more specifically with the residence time (RT). So far,
the structural reasons for the different recorded RT of congeneric
PIGAs remain elusive. Here, to understand the different kinetics of
PIGAs, their unbinding paths were studied by employing enhanced-sampling
molecular dynamics simulations. Results of these studies revealed
how subtle structural differences between PIGAs have a substantial
effect on the unbinding energetics. In particular, during the egress
from the TSPO binding site, slow-dissociating PIGAs find tight interactions
with the protein LP1 region thereby determining a long RT. Further
support to these findings was achieved by in vivo studies, which demonstrated
how the anxiolytic effect observed for the inspected PIGAs correlated
with their RT to TSPO.