Etomidate and Etomidate Analog Binding and Positive Modulation of γ-Aminobutyric Acid Type A Receptors: Evidence for a State-dependent Cutoff Effect.

WHAT WE ALREADY KNOW ABOUT THIS TOPIC: WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Naphthalene-etomidate, an etomidate analog containing a bulky phenyl ring substituent group, possesses very low γ-aminobutyric acid type A (GABAA) receptor efficacy and acts as an anesthetic-selective competitive antagonist. Using etomidate analogs containing phenyl ring substituents groups that range in volume, we tested the hypothesis that this unusual pharmacology is caused by steric hindrance that reduces binding to the receptor's open state. METHODS: The positive modulatory potencies and efficacies of etomidate and phenyl ring-substituted etomidate analogs were electrophysiology defined in oocyte-expressed α1ß3γ2L GABAA receptors. Their binding affinities to the GABAA receptor's two classes of transmembrane anesthetic binding sites were assessed from their abilities to inhibit receptor labeling by the site-selective photolabels [H]azi-etomidate and tritiated R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid. RESULTS: The positive modulatory activities of etomidate and phenyl ring-substituted etomidate analogs progressively decreased with substituent group volume, reflecting significant decreases in both potency (P = 0.005) and efficacy (P < 0.0001). Affinity for the GABAA receptor's two ß - α anesthetic binding sites similarly decreased with substituent group volume (P = 0.003), whereas affinity for the receptor's α - ß/γ - ß sites did not (P = 0.804). Introduction of the N265M mutation, which is located at the ß - α binding sites and renders GABAA receptors etomidate-insensitive, completely abolished positive modulation by naphthalene-etomidate. CONCLUSIONS: Steric hindrance selectively reduces phenyl ring-substituted etomidate analog binding affinity to the two ß - α anesthetic binding sites on the GABAA receptor's open state, suggesting that the binding pocket where etomidate's phenyl ring lies becomes smaller as the receptor isomerizes from closed to open.

Dimethoxy-etomidate: A Nonhypnotic Etomidate Analog that Potently Inhibits Steroidogenesis.

Cushing's syndrome is characterized by the overproduction of adrenocortical steroids. Steroidogenesis inhibitors are mainstays of medical therapy for Cushing's syndrome; unfortunately, adverse side effects and treatment failures are common with currently available drugs. The general anesthetic induction agent etomidate is among the most potent inhibitors of adrenocortical steroidogenesis. However, its use as a treatment of Cushing's syndrome is complicated by its sedative-hypnotic activity and ability to produce myoclonus, central nervous system actions thought to be mediated by the GABAA receptor. Here, we describe the pharmacology of the novel etomidate analog (R)-ethyl 1-(1-(3,5-dimethoxyphenyl)ethyl)-1H-imidazole-5-carboxylate (dimethoxy-etomidate). In contrast to etomidate, dimethoxy-etomidate minimally enhanced GABA-evoked GABAA receptor-mediated currents even at a near-saturating aqueous concentration. In Sprague-Dawley rats, dimethoxy-etomidate's potency for producing loss of righting reflexes-an animal model of sedation/hypnosis-was 2 orders of magnitude lower than that of etomidate, and it did not produce myoclonus. However, similar to etomidate, dimethoxy-etomidate potently suppressed adrenocortical steroid synthesis primarily by inhibiting 11ß-hydroxylase. [3H]etomidate binding to rat adrenocortical membranes was inhibited by dimethoxy-etomidate in a biphasic manner with IC50 values of 8.2 and 3970 nM, whereas that by etomidate was monophasic with an IC50 of 22 nM. Our results demonstrate that, similar to etomidate, dimethoxy-etomidate potently and dose-dependently suppresses adrenocortical steroid synthesis by inhibiting 11ß-hydroxylase. However, it is essentially devoid of etomidate's GABAA receptor positive modulatory and sedative-hypnotic activities and produces no myoclonus, providing proof of concept for the design of etomidate analogs without important central nervous system actions for the pharmacologic treatment of Cushing's syndrome.

Competitive Antagonism of Anesthetic Action at the γ-Aminobutyric Acid Type A Receptor by a Novel Etomidate Analog with Low Intrinsic Efficacy.

BACKGROUND: The authors characterized the γ-aminobutyric acid type A receptor pharmacology of the novel etomidate analog naphthalene-etomidate, a potential lead compound for the development of anesthetic-selective competitive antagonists. METHODS: The positive modulatory potencies and efficacies of etomidate and naphthalene-etomidate were defined in oocyte-expressed α1ß3γ2L γ-aminobutyric acid type A receptors using voltage clamp electrophysiology. Using the same technique, the ability of naphthalene-etomidate to reduce currents evoked by γ-aminobutyric acid alone or γ-aminobutyric acid potentiated by etomidate, propofol, pentobarbital, and diazepam was quantified. The binding affinity of naphthalene-etomidate to the transmembrane anesthetic binding sites of the γ-aminobutyric acid type A receptor was determined from its ability to inhibit receptor photoaffinity labeling by the site-selective photolabels [H]azi-etomidate and R-[H]5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid. RESULTS: In contrast to etomidate, naphthalene-etomidate only weakly potentiated γ-aminobutyric acid-evoked currents and induced little direct activation even at a near-saturating aqueous concentration. It inhibited labeling of γ-aminobutyric acid type A receptors by [H]azi-etomidate and R-[H]5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid with similar half-maximal inhibitory concentrations of 48 µM (95% CI, 28 to 81 µM) and 33 µM (95% CI, 20 to 54 µM). It also reduced the positive modulatory actions of anesthetics (propofol > etomidate ~ pentobarbital) but not those of γ-aminobutyric acid or diazepam. At 300 µM, naphthalene-etomidate increased the half-maximal potentiating propofol concentration from 6.0 µM (95% CI, 4.4 to 8.0 µM) to 36 µM (95% CI, 17 to 78 µM) without affecting the maximal response obtained at high propofol concentrations. CONCLUSIONS: Naphthalene-etomidate is a very low-efficacy etomidate analog that exhibits the pharmacology of an anesthetic competitive antagonist at the γ-aminobutyric acid type A receptor.