Etomidate and Etomidate Analog Binding and Positive Modulation of γ-Aminobutyric Acid Type A Receptors

Abstract

Editor’s Perspective 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 α1β3γ2L 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 3[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 β+ − α– anesthetic binding sites similarly decreased with substituent group volume (P = 0.003), whereas affinity for the receptor’s α+ – β–/γ+ – β– sites did not (P = 0.804). Introduction of the N265M mutation, which is located at the β+ − 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 β+ − 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.

Behavioral and Steroidogenic Pharmacology of Phenyl Ring Substituted Etomidate Analogs in Rats

“…40 mg/kg) or a 2-hr etomidate analog infusion lost their righting reflexes. Such a lack of sedative-hypnotic activity among the etomidate analogs is consistent with previous electrophysiological work showing that these analogs are essentially devoid of the GABA A receptor positive modulatory actions thought to mediate the sedative-hypnotic effects of etomidate [24,38,39].…”

“…However, the active site cavity volumes of these two enzymes are estimated to be 360 Å 3 (11b-hydroxylase) and 334 Å 3 (18hydroxylase), which are only modestly larger than the molecular volume of etomidate (269.7 Å 3 ) [31]. The addition of the various bulky substituents groups to etomidate (to form our four etomidate analogs) increases molecular volume by between 39.9 Å 3 and 52.7 Å 3 , potentially introducing steric hindrance and reducing the binding affinity to these enzymes [24]. Such a reduction in affinity could explain -at least in part -why the four phenyl ring substituent-containing analogs tended to produce smaller reductions in plasma corticosterone and aldosterone concentrations than etomidate.…”

“…As part of a strategy to overcome this limitation we developed (R)-ethyl 1-(1-(3,5dimethoxyphenyl)ethyl)-1H-imidazole-5-carboxylate (dimethoxy-etomidate), a phenyl ring substituted etomidate analog that retains the potent 11β-hydroxylase inhibitory activity of etomidate but is essentially devoid of the drug's GABA A receptor modulatory and sedative-hypnotic actions [23]. In the current manuscript, we describe studies to further define the pharmacology of this compound along with three additional phenyl ring substituted etomidate analogs that are similarly lacking in significant GABA A receptor modulatory activity (Fig 2) [24]. We also characterized the pharmacology of etomidate to provide context.…”

Towards a Comprehensive Understanding of Anesthetic Mechanisms of Action: A Decade of Discovery

“…The degree of selectivity for such general sites has been surprising. Ligand-gated ion channels, such as the GABA A -R or Gloeobacter Ligand-gated Ion Channel (GLIC) for example, show considerable selectivity for sites - [3,[6][7][8][9] -despite producing similar changes in activity ( Figure 1). Even volatile anesthetics may exhibit a degree of non-overlapping site occupancy as suggested by recent photolabeling studies [3].…”

“…This approach has verified a large number of plausible targets, including, but not limited to: specific ligand-gated ion channels [147,148], voltage-gated channels [149,150], hyperpolarizationactivated cyclic nucleotide (HCN)-gated and tandem pore potassium (K 2 P) channels [151], and mitochondrial proteins [152]. Attempts to distinguish functional significance of these many targets has used sensitivity, stereoselectivity [153], alkanol cutoff effect [9,154], and other phenomenological observations, some of which has caused further ambiguity. Nevertheless, this has been a powerful approach that has constrained the degree of potential complexity.…”