Analogues of etomidate: modifications around etomidate's chiral carbon and the impact on in vitro and in vivo pharmacology

New insights for screening etomidate analogues in the human H295R cell model

Etomidate is a sedative-hypnotic with excellent pharmacological effects, including rapid onset and hemodynamic stability. However, etomidate causes adrenocortical toxicity via binding to 11β-hydroxylase. Therefore, developing an approach to screen new etomidate analogues without endocrine-disrupting effects is urgently warranted. In this study, we employed the adrenocortical tumour cell line, NCI-H295R, as an in vitro system for etomidate analogues screening and detected the hormone levels in these cells using a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. After obtaining the concentration-response curves of hormone release, the "Adrenocortical Inhibitory Index" was used to evaluate the adrenocortical inhibitory potency of each compound. In summary, we demonstrate the benefits of our methods for screening of etomidate analogues that lack adrenocortical suppression, especially when this in vitro system is combined with in vivo testing.

Allosteric GABAA Receptor Modulators-A Review on the Most Recent Heterocyclic Chemotypes and Their Synthetic Accessibility

GABAA receptor modulators are structurally almost as diverse as their target protein. A plethora of heterocyclic scaffolds has been described as modulating this extremely important receptor family. Some made it into clinical trials and, even on the market, some were dismissed. This review focuses on the synthetic accessibility and potential for library synthesis of GABAA receptor modulators containing at least one heterocyclic scaffold, which were disclosed within the last 10 years.

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 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 [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

Background

Cushing’s syndrome is an endocrine disorder characterized by the overproduction of adrenocortical steroids. Steroidogenesis enzyme inhibitors are the mainstays of pharmacological treatment. Unfortunately, they produce significant side effects. Among the most potent inhibitors is the general anesthetic etomidate whose GABA_A receptor-mediated sedative-hypnotic actions restrict use. In this study, we defined the sedative-hypnotic and steroidogenesis inhibiting actions of etomidate and four phenyl-ring substituted etomidate analogs (dimethoxy-etomidate, isopropoxy-etomidate, naphthalene-etomidate, and naphthalene (2)-etomidate) that possess negligible GABA_A receptor modulatory activities.

Methods

In the first set of experiments, male Sprague-Dawley rats were assessed for loss of righting reflexes (LoRR) after receiving intravenous boluses of either etomidate (1 mg/kg) or an etomidate analog (40 mg/kg). In the second set of experiments, rats were assessed for LoRR and their abilities to produce adrenocortical and androgenic steroids after receiving 2-h infusions (0.5 mg kg^− 1 min^− 1) of either etomidate or an etomidate analog.

Results

All rats that received etomidate boluses or infusions had LoRR that persisted for minutes or hours, respectively. In contrast, no rat that received an etomidate analog had LoRR. Compared to rats in the vehicle control group, rats that received etomidate analog infusions had plasma corticosterone and aldosterone concentrations that were reduced by 80–84% and 68–94%, respectively. Rats that received etomidate infusions had plasma corticosterone and aldosterone concentrations that were also significantly reduced (by 92 and 96%, respectively). Rats that received etomidate or isopropoxy-etomidate had significant reductions (90 and 57%, respectively) in plasma testosterone concentrations whereas those that received naphthalene-etomidate had significant increases (1400%) in plasma dehydroepiandrosterone concentrations. Neither etomidate nor any etomidate analog significantly affected plasma androstenedione and dihydrotestosterone concentrations.

Conclusions

Our studies demonstrate that the four phenyl-ring substituted etomidate analogs form a novel class of compounds that are devoid of sedative-hypnotic activities and suppress plasma concentrations of adrenocortical steroids but vary in their effects on plasma concentrations of androgenic steroids.

Toxicologic and Inhibitory Receptor Actions of the Etomidate Analog ABP-700 and Its Metabolite CPM-Acid

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

The etomidate analog ABP-700 produces involuntary muscle movements that could be manifestations of seizures. To define the relationship (if any) between involuntary muscle movements and seizures, electroencephalographic studies were performed in Beagle dogs receiving supra-therapeutic (~10× clinical) ABP-700 doses. γ-aminobutyric acid type A (GABAA) and glycine receptor studies were undertaken to test receptor inhibition as the potential mechanism for ABP-700 seizures.

Methods

ABP-700 was administered to 14 dogs (6 mg/kg bolus followed by a 2-h infusion at 1 mg · kg-1 · min-1, 1.5 mg · kg-1 · min-1, or 2.3 mg · kg-1 · min-1). Involuntary muscle movements were documented, electroencephalograph was recorded, and plasma ABP-700 and CPM-acid concentrations were measured during and after ABP-700 administration. The concentration-dependent modulatory actions of ABP-700 and CPM-acid were defined in oocyte-expressed α1β3γ2L GABAA and α1β glycine receptors (n = 5 oocytes/concentration) using electrophysiologic techniques.

Results

ABP-700 produced both involuntary muscle movements (14 of 14 dogs) and seizures (5 of 14 dogs). However, these phenomena were temporally and electroencephalographically distinct. Mean peak plasma concentrations were (from lowest to highest dosed groups) 35 μM, 45 μM, and 102 μM (ABP-700) and 282 μM, 478 μM, and 1,110 μM (CPM-acid). ABP-700 and CPM-acid concentration–GABAA receptor response curves defined using 6 μM γ-aminobutyric acid exhibited potentiation at low and/or intermediate concentrations and inhibition at high ones. The half-maximal inhibitory concentrations of ABP-700 and CPM-acid defined using 1 mM γ-aminobutyric acid were 770 μM (95% CI, 590 to 1,010 μM) and 1,450 μM (95% CI, 1,340 to 1,560 μM), respectively. CPM-acid similarly inhibited glycine receptors activated by 1 mM glycine with a half-maximal inhibitory concentration of 1,290 μM (95% CI, 1,240 to 1,330 μM).

Conclusions

High dose ABP-700 infusions produce involuntary muscle movements and seizures in Beagle dogs via distinct mechanisms. CPM-acid inhibits both GABAA and glycine receptors at the high (~100× clinical) plasma concentrations achieved during the dog studies, providing a plausible mechanism for the seizures.

High-throughput Screening in Larval Zebrafish Identifies Novel Potent Sedative-hypnotics

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Many general anesthetics were discovered empirically, but primary screens to find new sedative-hypnotics in drug libraries have not used animals, limiting the types of drugs discovered. The authors hypothesized that a sedative-hypnotic screening approach using zebrafish larvae responses to sensory stimuli would perform comparably to standard assays, and efficiently identify new active compounds.

METHODS

The authors developed a binary outcome photomotor response assay for zebrafish larvae using a computerized system that tracked individual motions of up to 96 animals simultaneously. The assay was validated against tadpole loss of righting reflexes, using sedative-hypnotics of widely varying potencies that affect various molecular targets. A total of 374 representative compounds from a larger library were screened in zebrafish larvae for hypnotic activity at 10 µM. Molecular mechanisms of hits were explored in anesthetic-sensitive ion channels using electrophysiology, or in zebrafish using a specific reversal agent.

RESULTS

Zebrafish larvae assays required far less drug, time, and effort than tadpoles. In validation experiments, zebrafish and tadpole screening for hypnotic activity agreed 100% (n = 11; P = 0.002), and potencies were very similar (Pearson correlation, r > 0.999). Two reversible and potent sedative-hypnotics were discovered in the library subset. CMLD003237 (EC50, ~11 µM) weakly modulated γ-aminobutyric acid type A receptors and inhibited neuronal nicotinic receptors. CMLD006025 (EC50, ~13 µM) inhibited both N-methyl-D-aspartate and neuronal nicotinic receptors.

CONCLUSIONS

Photomotor response assays in zebrafish larvae are a mechanism-independent platform for high-throughput screening to identify novel sedative-hypnotics. The variety of chemotypes producing hypnosis is likely much larger than currently known.

Anesthetic Drug Discovery and Development: A Case Study of Novel Etomidate Analogs

All currently available general anesthetic agents possess potentially lethal side effects requiring their administration by highly trained clinicians. Among these agents is etomidate, a highly potent imidazole-based intravenous sedative-hypnotic that deleteriously suppresses the synthesis of adrenocortical steroids in a manner that is both potent and persistent. We developed two distinct strategies to design etomidate analogs that retain etomidate's potent hypnotic activity, but produce less adrenocortical suppression than etomidate. One strategy seeks to reduce binding to 11β-hydroxylase, a critical enzyme in the steroid biosynthetic pathway, which is potently inhibited by etomidate. The other strategy seeks to reduce the duration of adrenocortical suppression after etomidate administration by modifying the drug's structure to render it susceptible to rapid metabolism by esterases. In this chapter, we describe the methods used to evaluate the hypnotic and adrenocortical inhibitory potencies of two lead compounds designed using the aforementioned strategies. Our purpose is to provide a case study for the development of novel analogs of existing drugs with reduced side effects.

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 GABA_A 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 GABA_A 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 11β-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 11β-hydroxylase. However, it is essentially devoid of etomidate's GABA_A 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 α1β3γ2L 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.

Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes

IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing γ-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity also show remarkably selective interactions with distinct interfacial sites. These results suggest strategies for developing new drugs that selectively modulate distinct GABAA receptor subtypes.

Silver-Catalyzed Isocyanide-Isocyanide [3+2] Cross-Cycloaddition Involving 1,2-Group Migration: Efficient Synthesis of Trisubstituted lmidazoles

Imidazole ring is an important five-membered aromatic heterocycle that is widely present in natural products and synthetic molecules. The isocyanide-isocyanide [3+2] cross-cycloaddition reaction constitutes a straightforward method to access imidazoles starting from the easily available chemicals. So far, only three successive reports are known and all lead to the formation of 1,4-disubstituted imidazoles. Here, we report the first isocyanide-isocyanide [3+2] cross-cycloaddition reaction allowing for the formation of 1,4,5-trisubstituted imidazoles under silver catalysis. An unexpected 1,2-migration of sulfonyl, alkoxycaybonyl, and carbamoyl groups took place during the cyclization process that is responsible for the formation of trisubstituted imidazoles. This report displayed a mechanistically novel synthetic method toward a variety of imidazole derivatives, which are otherwise difficult to access by conventional methods.

Distinct Hypnotic Recoveries After Infusions of Methoxycarbonyl Etomidate and Cyclopropyl Methoxycarbonyl Metomidate: The Role of the Metabolite

BACKGROUND

Methoxycarbonyl etomidate (MOC-etomidate) and cyclopropyl methoxycarbonyl metomidate (CPMM) are rapidly metabolized "soft" etomidate analogs. CPMM's duration of hypnotic effect is context insensitive, whereas MOC-etomidate's is not. In this study, we tested the hypothesis that CPMM's effect is context insensitive because, unlike MOC-etomidate, its metabolite fails to reach physiologically important concentrations in vivo even with prolonged continuous infusion.

METHODS

We compared the potencies with which MOC-etomidate and CPMM activate α1(L264T)β3γ2 γ-aminobutyric acid type A receptors and induce loss-of-righting reflexes (i.e., produce hypnosis) in tadpoles with those of their metabolites (MOC-etomidate's carboxylic acid metabolite [MOC-ECA] and CPMM's carboxylic acid metabolite [CPMM-CA], respectively). We measured metabolite concentrations in the blood and cerebrospinal fluid of Sprague-Dawley rats on CPMM infusion and compared them with those achieved with MOC-etomidate infusion. We measured the rates with which brain tissue from Sprague-Dawley rats metabolize MOC-etomidate and CPMM.

RESULTS

Both analogs and their metabolites enhanced γ-aminobutyric acid type A receptor function and induced loss-of-righting reflexes in a concentration-dependent manner. However, in these 2 assays, CPMM-CA's potency relative to its parent hypnotic was approximately 1:4900 and 1:1900, respectively, whereas MOC-ECA's was only approximately 1:415 and 1:390, respectively. With 2-hour CPMM infusions, CPMM-CA reached respective concentrations in the blood and cerebrospinal fluid that were 2 and >3 orders of magnitude lower than that which produced hypnosis. CPMM was metabolized by the brain tissue at a rate that is approximately 1/15th that of MOC-etomidate.

CONCLUSIONS

Hypnotic recovery after CPMM administration is context insensitive because its metabolite does not accumulate to hypnotic levels in the central nervous system. This reflects the very large potency ratio between CPMM and CPMM-CA and the resistance of CPMM to metabolism by esterases present in the brain.

γ-Aminobutyric Acid Type A Receptor Modulation by Etomidate Analogs

BACKGROUND

Etomidate is a highly potent anesthetic agent that is believed to produce hypnosis by enhancing γ-aminobutyric acid type A (GABAA) receptor function. The authors characterized the GABAA receptor and hypnotic potencies of etomidate analogs. The authors then used computational techniques to build statistical and graphical models that relate the potencies of these etomidate analogs to their structures to identify the specific molecular determinants of potency.

METHODS

GABAA receptor potencies were defined with voltage clamp electrophysiology using α1β3γ2 receptors harboring a channel mutation (α1[L264T]) that enhances anesthetic sensitivity (n = 36 to 60 measurements per concentration-response curve). The hypnotic potencies of etomidate analogs were defined using a loss of righting reflexes assay in Sprague Dawley rats (n = 9 to 21 measurements per dose-response curve). Three-dimensional quantitative structure-activity relationships were determined in silico using comparative molecular field analysis.

RESULTS

The GABAA receptor and hypnotic potencies of etomidate and the etomidate analogs ranged by 91- and 53-fold, respectively. These potency measurements were significantly correlated (r = 0.72), but neither measurement correlated with drug hydrophobicity (r = 0.019 and 0.005, respectively). Statistically significant and predictive comparative molecular field analysis models were generated, and a pharmacophore model was built that revealed both the structural elements in etomidate analogs associated with high potency and the interactions that these elements make with the etomidate-binding site.

CONCLUSIONS

There are multiple specific structural elements in etomidate and etomidate analogs that mediate GABAA receptor modulation. Modifying any one element can alter receptor potency by an order of magnitude or more.

Differential Potency of 2,6-Dimethylcyclohexanol Isomers for Positive Modulation of GABAA Receptor Currents

GABAA receptors meet all of the pharmacological requirements necessary to be considered important targets for the action of general anesthetic agents in the mammalian brain. In the following patch-clamp study, the relative modulatory effects of 2,6-dimethylcyclohexanol diastereomers were investigated on human GABAA (α1β3γ2s) receptor currents stably expressed in human embryonic kidney cells. Cis,cis-, trans,trans-, and cis,trans-isomers were isolated from commercially available 2,6-dimethylcyclohexanol and were tested for positive modulation of submaximal GABA responses. For example, the addition of 30 μM cis,cis-isomer resulted in an approximately 2- to 3-fold enhancement of the EC20 GABA current. Coapplications of 30 μM 2,6-dimethylcyclohexanol isomers produced a range of positive enhancements of control GABA responses with a rank order for positive modulation: cis,cis > trans,trans ≥ mixture of isomers > > cis,trans-isomer. In molecular modeling studies, the three cyclohexanol isomers bound with the highest binding energies to a pocket within transmembrane helices M1 and M2 of the β3 subunit through hydrogen-bonding interactions with a glutamine at the 224 position and a tyrosine at the 220 position. The energies for binding to and hydrogen-bond lengths within this pocket corresponded with the relative potencies of the agents for positive modulation of GABAA receptor currents (cis,cis > trans,trans > cis,trans-2,6-dimethylcyclohexanol). In conclusion, the stereochemical configuration within the dimethylcyclohexanols is an important molecular feature in conferring positive modulation of GABAA receptor activity and for binding to the receptor, a consideration that needs to be taken into account when designing novel anesthetics with enhanced therapeutic indices.

Effect of Etomidate Versus Combination of Propofol-Ketamine and Thiopental-Ketamine on Hemodynamic Response to Laryngoscopy and Intubation: A Randomized Double Blind Clinical Trial

BACKGROUND

Laryngoscopy and intubation frequently used for airway management during general anesthesia, is frequently associated with undesirable hemodynamic disturbances.

OBJECTIVES

The aim of this study was to compare the effects of etomidate, combination of propofol-ketamine and thiopental-ketamine as induction agents on hemodynamic response to laryngoscopy and intubation.

PATIENTS AND METHODS

In a double blind, randomized clinical trial a total of 120 adult patients of both sexes, aged 18 - 45 years, scheduled for elective surgery under general anesthesia were randomly assigned into three equally sized groups. Patients in group A received etomidate (0.3 mg/kg) plus normal saline as placebo. Patients in group B and C received propofol (1.5 mg/kg) plus ketamine (0.5 mg/kg) and thiopental sodium (3 mg/kg) plus ketamine (0.5 mg/kg), respectively for anesthesia induction. Before laryngoscopy and tracheal intubation, immediately after, and also one and three minutes after the procedures, hemodynamic values (SBP, DBP, MAP and HR) were measured.

RESULTS

A repeated measurement ANOVA showed significant changes in mean SBP and DBP between the time points (P < 0.05). In addition, the main effect of MAP and HR were statistically significant during the course of study (P < 0.05). Furthermore, after induction of anesthesia, the three study groups had significantly different SBP, DBP and MAP changes overtime (P < 0.05). However, HR changes over time were not statistically significant (P > 0.05). Combination of propofol-ketamine had superior hemodynamic stability compared to other induction agents.

CONCLUSIONS

Combination of propofol-ketamine may be recommended as an effective and safe induction agent for attenuating hemodynamic responses to laryngoscopy and intubation with better hemodynamic stability. Although, further well-designed randomized clinical trials to confirm the safety and efficacy of this combination, especially in critically ill patients or patients with cardiovascular disease, are warranted.

Mutations at beta N265 in γ-aminobutyric acid type A receptors alter both binding affinity and efficacy of potent anesthetics

Etomidate and propofol are potent general anesthetics that act via GABAA receptor allosteric co-agonist sites located at transmembrane β+/α- inter-subunit interfaces. Early experiments in heteromeric receptors identified βN265 (M2-15') on β2 and β3 subunits as an important determinant of sensitivity to these drugs. Mechanistic analyses suggest that substitution with serine, the β1 residue at this position, primarily reduces etomidate efficacy, while mutation to methionine eliminates etomidate sensitivity and might prevent drug binding. However, the βN265 residue has not been photolabeled with analogs of either etomidate or propofol. Furthermore, substituted cysteine modification studies find no propofol protection at this locus, while etomidate protection has not been tested. Thus, evidence of contact between βN265 and potent anesthetics is lacking and it remains uncertain how mutations alter drug sensitivity. In the current study, we first applied heterologous α1β2N265Cγ2L receptor expression in Xenopus oocytes, thiol-specific aqueous probe modification, and voltage-clamp electrophysiology to test whether etomidate inhibits probe reactions at the β-265 sidechain. Using up to 300 µM etomidate, we found both an absence of etomidate effects on α1β2N265Cγ2L receptor activity and no inhibition of thiol modification. To gain further insight into anesthetic insensitive βN265M mutants, we applied indirect structure-function strategies, exploiting second mutations in α1β2/3γ2L GABAA receptors. Using α1M236C as a modifiable and anesthetic-protectable site occupancy reporter in β+/α- interfaces, we found that βN265M reduced apparent anesthetic affinity for receptors in both resting and GABA-activated states. βN265M also impaired the transduction of gating effects associated with α1M236W, a mutation that mimics β+/α- anesthetic site occupancy. Our results show that βN265M mutations dramatically reduce the efficacy/transduction of anesthetics bound in β+/α- sites, and also significantly reduce anesthetic affinity for resting state receptors. These findings are consistent with a role for βN265 in anesthetic binding within the β+/α- transmembrane sites.

Anesthetic Drugs in Congenital Heart Disease

The structural defects associated with the various forms of congenital heart disease lead to pathological and functional changes that place patients at risk for adverse events, and in fact the perioperative incidence of morbidity and mortality has been documented to be increased in children with congenital heart disease. Patients with congenital heart disease can present to the anesthesiologist in a relatively precarious state of balance of several hemodynamic factors, including preload, ventricular contractility, systemic vascular resistance, pulmonary vascular resistance, heart rate, and cardiac rhythm. Anesthetic drugs can affect each of these, and an ideal anesthetic drug for such patients does not exist. The purpose of this article is to review the hemodynamic effects of anesthetic drugs and how they may contribute to the occurrence of adverse events in children with congenital heart disease.