Background: R-etomidate possesses unique desirable properties but potently suppresses adrenocortical function. Consequently, efforts are being made to define structure–activity relationships with the goal of designing analogues with reduced adrenocortical toxicity. The authors explored the pharmacological impact of modifying etomidate’s chiral center using R-etomidate, S-etomidate, and two achiral etomidate analogues (cyclopropyl etomidate and dihydrogen etomidate).
Methods: The γ-aminobutyric acid type A receptor modulatory potencies of drugs were assessed in oocyte-expressed α1(L264T)β3γ2L and α1(L264T)β1γ2L γ-aminobutyric acid type A receptors (for each drug, n = 6 oocytes per subtype). In rats, hypnotic potencies and durations of action were measured using a righting reflex assay (n = 26 to 30 doses per drug), and adrenocortical potencies were quantified by using an adrenocorticotropic hormone stimulation test (n = 20 experiments per drug).
Results: All four drugs activated both γ-aminobutyric acid type A receptor subtypes in vitro and produced hypnosis and suppressed adrenocortical function in rats. However, drug potencies in each model ranged by 1 to 2 orders of magnitude. R-etomidate had the highest γ-aminobutyric acid type A receptor modulatory, hypnotic, and adrenocortical inhibitory potencies. Respectively, R-etomidate, S-etomidate, and cyclopropyl etomidate were 27.4-, 18.9-, and 23.5-fold more potent activators of receptors containing β3 subunits than β1 subunits; however, dihydrogen etomidate’s subunit selectivity was only 2.48-fold and similar to that of propofol (2.08-fold). S-etomidate was 1/23rd as potent an adrenocortical inhibitor as R-etomidate.
Conclusion: The linkage between the structure of etomidate’s chiral center and its pharmacology suggests that altering etomidate’s chiral center may be used as part of a strategy to design analogues with more desirable adrenocortical activities and/or subunit selectivities.