Closely related molecules can help test competing explanations of how a drug fits a receptor. A 2018 benzodiazepine study used stereoisomer pairs to investigate proposed binding orientations at GABAA receptors.
Computational docking into receptor homology models suggested that a chiral methyl group could discriminate among candidate orientations. Researchers then studied three pairs of R and S isomers from diazepam, imidazobenzodiazepine, and triazolam derivative chemotypes. Radioligand-displacement experiments assessed binding, while two-electrode voltage-clamp experiments measured functional effects in receptors expressed in Xenopus oocytes.
Both imidazobenzodiazepine isomers retained comparable binding affinities. The other two chemotypes showed a substantial R-versus-S difference, with reduced binding by the R isomers and retained activity by the S forms. The pattern supported different binding modes among the tested chemical families.
The binding-mode conclusion integrated modeling and experimental behavior. Radioligand displacement measures competition under assay conditions, and electrophysiology measures a receptor response. Neither experiment directly images a drug’s orientation. The convergence of these approaches provides support for the proposed poses while preserving the distinction between measured effects and structural interpretation. For forensic toxicologists evaluating a molecular-mechanism claim, identify the tested derivative, stereochemistry, receptor subtype, and endpoint. Generalizing the result to an untested compound or predicting impairment from a blood concentration requires additional evidence linking those different levels of observation.
Read the full scientific paper. DOI 10.1021/acschembio.8b00144.