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.

Sedative-hypnotic Binding to 11ß-hydroxylase.

BACKGROUND: Etomidate potently suppresses adrenocortical steroid synthesis with potentially deleterious consequences by binding to 11ß-hydroxylase and inhibiting its function. The authors hypothesized that other sedative-hypnotics currently in clinical use or under development (or their metabolites) might bind to the same site at clinically relevant concentrations. The authors tested this hypothesis by defining etomidate's affinity for this site and the potencies with which other sedative-hypnotics (and their metabolites) inhibit etomidate binding. METHODS: H-etomidate's binding to adrenal membranes from Sprague-Dawley rats was characterized with a filtration assay, and its dissociation constant was defined using saturation and homologous ligand competition approaches. Half-inhibitory concentrations of sedative-hypnotics and metabolites were determined from the reduction in specific H-etomidate binding measured in the presence of ranging sedative-hypnotic and metabolite concentrations. RESULTS: Saturation and homologous competition studies yielded H-etomidate dissociation constants of 40 and 21 nM, respectively. Half-inhibitory concentrations of etomidate and cyclopropyl methoxycarbonyl metomidate (CPMM) differed significantly (26 vs. 143 nM, respectively; P < 0.001), and those of the carboxylic acid (CA) metabolites etomidate-CA and CPMM-CA were greater than or equal to 1,000× higher than their respective parent hypnotics. The half-inhibitory concentration of dexmedetomidine was 2.2 µM, whereas those of carboetomidate, ketamine, and propofol were greater than or equal to 50 µM. CONCLUSION: Etomidate's in vitro dissociation constant for 11ß-hydroxylase closely approximates its in vivo adrenocortical half-inhibitory concentration. CPMM produces less adrenocortical suppression than etomidate not only because it is metabolized faster but also because it binds to 11ß-hydroxylase with lower affinity. Other sedative-hypnotics and metabolites bind to 11ß-hydroxylase and inhibit etomidate binding only at suprahypnotic concentrations.

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)ß3γ2 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 α1ß3γ2 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.

Cyclopropyl-methoxycarbonyl Metomidate: Studies in a Lipopolysaccharide Inflammatory Model of Sepsis.

BACKGROUND: Cyclopropyl-methoxycarbonyl metomidate (CPMM) is a rapidly metabolized etomidate analog that is currently in clinical trials. The goal of this study is to assess CPMM's potential value as an anesthetic agent for use in patients with sepsis by defining its actions in an acute inflammatory model of sepsis. METHODS: Escherichia coli lipopolysaccharide (1 mg/kg) was injected intravenously into Sprague-Dawley rats. Thirty minutes later, CPMM, etomidate, or vehicle (n = 8 per group) was infused for 1 h. Plasma adrenocorticotropic hormone, corticosterone, and cytokine (interleukin-1ß, interleukin-6, interleukin-10, and tumor necrosis factor-α) concentrations were measured before, during, and after infusion. RESULTS: After lipopolysaccharide injection, adrenocorticotropic hormone concentrations changed similarly over time in all three groups. Compared with vehicle group rats, CPMM group rats had significantly lower corticosterone concentrations at only a single study time point during infusion and no significant differences in cytokine concentrations at any time during the study period. Compared with etomidate group rats, CPMM group rats had significantly higher corticosterone concentrations (up to nine-fold) during and after hypnotic infusion. Cytokine concentrations in CPMM group rats and vehicle group rats were not significantly different, but they were significantly lower than those in etomidate group rats. Postinfusion mortality was 40% in etomidate group rats and 0% in CPMM and vehicle group rats. CONCLUSION: Compared with etomidate, CPMM produces less adrenocortical suppression, lower plasma cytokine concentrations, and improved survival in a lipopolysaccharide inflammatory model of sepsis. These results suggest that CPMM may be a safer alternative to etomidate in patients with sepsis.

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

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)ß3γ2L and α1(L264T)ß1γ2L 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 ß3 subunits than ß1 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.

The pharmacology of cyclopropyl-methoxycarbonyl metomidate: a comparison with propofol.

BACKGROUND: Cyclopropyl-methoxycarbonyl metomidate (CPMM) is a "soft" etomidate analogue currently being developed as a propofol alternative for anesthetic induction and maintenance. METHODS: We compared the potencies of CPMM and propofol by assessing their abilities to directly activate α1(L264T)ß3γ2 gamma-aminobutyric acid type A (GABAA) receptors and induce loss of righting reflexes in tadpoles. We also measured the rates of encephalographic recovery in rats after CPMM and propofol infusions ranging in duration from 5 to 120 minutes. RESULTS: CPMM and propofol activate GABAA receptors and induce loss of righting reflexes in tadpoles with respective 50% effective concentrations (EC50s) of 3.8 ± 0.4 and 3.9 ± 0.2 µM (GABAA receptor) and 2.6 ± 0.19 and 1.3 ± 0.04 µM (tadpole). Encephalographic recovery after prolonged infusion was faster with CPMM and lacked propofol's context sensitivity. CONCLUSION: CPMM and propofol have similar potencies in GABAA receptors and tadpoles; however, CPMM provides more rapid and predictable recovery than propofol, particularly after prolonged infusion.

Adrenocortical suppression and recovery after continuous hypnotic infusion: etomidate versus its soft analogue cyclopropyl-methoxycarbonyl metomidate.

INTRODUCTION: Etomidate is no longer administered as a continuous infusion for anesthetic maintenance or sedation, because it results in profound and persistent suppression of adrenocortical steroid synthesis with potentially lethal consequences in critically ill patients. We hypothesized that rapidly metabolized soft analogues of etomidate could be developed that do not produce persistent adrenocortical dysfunction even after prolonged continuous infusion. We hope that such agents might also provide more rapid and predictable anesthetic emergence. We have developed the soft etomidate analogue cyclopropyl-methoxycarbonyl etomidate (CPMM). Upon termination of 120-minute continuous infusions, hypnotic and encephalographic recoveries occur in four minutes. The aims of this study were to assess adrenocortical function during and following 120-minute continuous infusion of CPMM and to compare the results with those obtained using etomidate. METHODS: Dexamethasone-suppressed rats were randomized into an etomidate group, CPMM group, or control group. Rats in the etomidate and CPMM groups received 120-minute continuous infusions of etomidate and CPMM, respectively. Rats in the control group received neither hypnotic. In the first study, adrenocortical function during hypnotic infusion was assessed by administering adrenocorticotropic hormone (ACTH) 90 minutes after the start of the hypnotic infusion and measuring plasma corticosterone concentrations at the end of the infusion 30 minutes later. In the second study, adrenocortical recovery following hypnotic infusion was assessed by administering ACTH every 30 minutes after infusion termination and measuring plasma corticosterone concentrations 30 minutes after each ACTH dose. RESULTS: During hypnotic infusion, ACTH-stimulated serum corticosterone concentrations were significantly lower in the CPMM and etomidate groups than in the control group (100 ± 64 ng/ml and 33 ± 32 ng/ml versus 615 ± 265 ng/ml, respectively). After hypnotic infusion, ACTH-stimulated serum corticosterone concentrations recovered to control values within 30 minutes in the CPMM group but remained suppressed relative to those in the control group for more than 3 hours in the etomidate group. CONCLUSIONS: Both CPMM and etomidate suppress adrenocortical function during continuous infusion. However, recovery occurs significantly more rapidly following infusion of CPMM.

Azo-propofols: photochromic potentiators of GABA(A) receptors.

Shine and rise! GABA(A) receptors are ligand-gated chloride ion channels that respond to γ-aminobutyric acid (GABA), which is the major inhibitory neurotransmitter of the mammalian central nervous system. Azobenzene derivatives of propofol, such as compound 1 (see scheme), increase GABA-induced currents in the dark form and lose this property upon light exposure and thus function as photochromic potentiators. Compound 1 can be employed as a light-dependent general anesthetic in translucent tadpoles.

Electroencephalographic and hypnotic recoveries after brief and prolonged infusions of etomidate and optimized soft etomidate analogs.

BACKGROUND: Methoxycarbonyl etomidate is the prototypical soft etomidate analog. Because it has relatively low potency and is extremely rapidly metabolized, large quantities must be infused to maintain hypnosis. Consequently with prolonged infusion, metabolite reaches sufficient concentrations to delay recovery. Dimethyl-methoxycarbonyl metomidate (DMMM) and cyclopropyl-methoxycarbonyl metomidate (CPMM) are methoxycarbonyl etomidate analogs with higher potencies and slower clearance. Because of these properties, we hypothesized that dosing would be lower and electroencephalographic and hypnotic recoveries would be faster - and less context-sensitive - with DMMM or CPMM versus methoxycarbonyl etomidate or etomidate. METHODS: Etomidate, DMMM, and CPMM where infused into rats (n = 6 per group) for either 5 min or 120 min. After infusion termination, electroencephalographic and hypnotic recovery times were measured. The immobilizing ED50 infusion rates were determined using a tail clamp assay. RESULTS: Upon terminating 5-min infusions, electroencephalographic and hypnotic recovery times were not different among hypnotics. However, upon terminating 120-min infusions, recovery times varied significantly with respective values (mean ± SD) 48 ± 13 min and 31 ± 6.5 min (etomidate), 17 ± 7.0 min and 14 ± 3.4 min (DMMM), and 4.5 ± 1.1 min and 4.2 ± 1.6 min (CPMM). The immobilizing ED50 infusion rates were (mean ± SD) 0.19 ± 0.03 mg · kg · min (etomidate), 0.60 ± 0.12 mg · kg · min (DMMM), and 0.89 ± 0.18 mg · kg · min (CPMM). CONCLUSIONS: Electroencephalographic and hypnotic recoveries following prolonged infusions of DMMM and CPMM are faster than those following methoxycarbonyl etomidate or etomidate. In the case of CPMM infusion, recovery times are 4 min and context-insensitive.

Modifying methoxycarbonyl etomidate inter-ester spacer optimizes in vitro metabolic stability and in vivo hypnotic potency and duration of action.

BACKGROUND: Methoxycarbonyl etomidate is the prototypical very rapidly metabolized etomidate analog. Initial studies suggest that it may be too short acting for many clinical uses. We hypothesized that its duration of action could be lengthened and clinical utility broadened by incorporating specific aliphatic groups into the molecule to sterically protect its ester moiety from esterase-catalyzed hydrolysis. To test this hypothesis, we developed a series of methoxycarbonyl etomidate analogs (spacer-linked etomidate esters) containing various aliphatic-protecting groups and spacer lengths. METHODS: Spacer-linked etomidate esters were synthesized and their hypnotic potencies and durations of action following bolus administration were measured in rats using a loss-of-righting reflexes assay. Octanol:water partition coefficients and metabolic half-lives in pooled rat blood were determined chromatographically. RESULTS: All spacer-linked etomidate esters produced hypnosis rapidly and in a dose-dependent manner. ED50s for loss of righting reflexes ranged from 0.69 ± 0.04 mg/kg for cyclopropyl-methoxycarbonyl metomidate to 11.1 ± 0.8 mg/kg for methoxycarbonyl metomidate. The slope of a plot of the duration of loss of righting reflexes versus the logarithm of the dose ranged 12-fold among spacer-linked etomidate esters, implying widely varying brain clearance rates. The in vitro metabolic half-lives of these compounds in rat blood varied by more than two orders of magnitude and were diastereometrically selective. CONCLUSIONS: We created 13 new analogs of methoxycarbonyl etomidate and identified two that have significantly higher potency and potentially address the too-brief duration of action for methoxycarbonyl etomidate. This work may provide a blueprint for optimizing the pharmacological properties of other soft drugs.

Carboetomidate inhibits alpha4/beta2 neuronal nicotinic acetylcholine receptors at concentrations affecting animals.

BACKGROUND: Carboetomidate is an etomidate derivative that produces hypnosis without inhibiting adrenal corticosteroid synthesis. Similar to etomidate, carboetomidate modulates γ-aminobutyric acid type A receptors, but its effects on other ion channel targets of general anesthetics are unknown. METHODS: We compared etomidate and carboetomidate effects on human N-methyl-d-aspartate receptors or neuronal nicotinic acetylcholine receptors (nnAChRs) expressed in Xenopus oocytes, using 2-microelectrode voltage clamp electrophysiology. RESULTS: Etomidate did not affect either type of receptor at clinically relevant concentrations, whereas carboetomidate concentrations near 50% effective concentration for anesthesia significantly inhibited nnAChRs. CONCLUSIONS: Compared with etomidate, carboetomidate's higher hydrophobicity is associated with greater inhibition of nnAChRs.

Electroencephalographic recovery, hypnotic emergence, and the effects of metabolite after continuous infusions of a rapidly metabolized etomidate analog in rats.

BACKGROUND: Methoxycarbonyl etomidate is an ultrarapidly metabolized etomidate analog. It is metabolized to methoxycarbonyl etomidate carboxylic acid (MOC-ECA), which has a hypnotic potency that is 350-fold less than that of methoxycarbonyl etomidate. The authors explored the relationships between methoxycarbonyl etomidate infusion duration, recovery time, metabolite concentrations in blood and cerebrospinal fluid (CSF), and methoxycarbonyl etomidate metabolism in brain tissue and CSF to test the hypothesis that rapid metabolism of methoxycarbonyl etomidate may lead to sufficient accumulation of MOC-ECA in the brain to produce a pharmacologic effect. METHODS: A closed-loop system with burst suppression ratio feedback was used to administer methoxycarbonyl etomidate infusions of varying durations to rats. After infusion, recovery of the electroencephalogram and righting reflexes were assessed. MOC-ECA concentrations were measured in blood and CSF during and after methoxycarbonyl etomidate infusion, and the in vitro half-life of methoxycarbonyl etomidate was determined in rat brain tissue and CSF. RESULTS: Upon termination of continuous methoxycarbonyl etomidate infusions, the burst suppression ratio recovered in a biexponential manner with fast and slow components having time constants that differed by more than 100-fold and amplitudes that varied inversely with infusion duration. MOC-ECA concentrations reached hypnotic concentrations in the CSF with prolonged methoxycarbonyl etomidate infusion and then decreased during a period of several hours after infusion termination. The metabolic half-life of methoxycarbonyl etomidate in brain tissue and CSF was 11 and 20 min, respectively. CONCLUSION: In rats, methoxycarbonyl etomidate metabolism is sufficiently fast to produce pharmacologically active MOC-ECA concentrations in the brain with prolonged methoxycarbonyl etomidate infusion.

In vivo and in vitro pharmacological studies of methoxycarbonyl-carboetomidate.

BACKGROUND: We previously developed 2 etomidate analogs that retain etomidate's favorable hemodynamic properties but whose adrenocortical effects are reduced in duration or magnitude. Methoxycarbonyl (MOC)-etomidate is rapidly metabolized and ultrashort acting whereas (R)-ethyl 1-(1-phenylethyl)-1H-pyrrole-2-carboxylate (carboetomidate) does not potently inhibit 11ß-hydroxylase. We hypothesized that MOC-etomidate's labile ester could be incorporated into carboetomidate to produce a new agent that possesses favorable properties individually found in each agent. We describe the synthesis and pharmacology of MOC-(R)-ethyl 1-(1-phenylethyl)-1H-pyrrole-2-carboxylate (MOC-carboetomidate), a "soft" analog of carboetomidate. METHODS: MOC-carboetomidate's octanol:water partition coefficient was determined chromatographically and compared with those of etomidate, carboetomidate, and MOC-etomidate. MOC-carboetomidate's 50% effective concentration (EC(50)) and 50% effective dose for loss of righting reflexes (LORR) were measured in tadpoles and rats, respectively. Its effect on γ-aminobutyric acid A (GABA(A)) receptor function was assessed using 2-microelectrode voltage clamp electrophysiological techniques and its metabolic stability was determined in pooled rat blood using high performance liquid chromatography. Its duration of action and effects on arterial blood pressure and adrenocortical function were assessed in rats. RESULTS: MOC-carboetomidate's octanol:water partition coefficient was 3300 ± 280, whereas those for etomidate, carboetomidate, and MOC-etomidate were 800 ± 180, 15,000 ± 3700, and 190 ± 25, respectively. MOC-carboetomidate's EC(50) for LORR in tadpoles was 9 ± 1 µM and its EC(50) for LORR in rats was 13 ± 5 mg/kg. At 13 µM, MOC-carboetomidate enhanced GABA(A) receptor currents by 400% ± 100%. Its metabolic half-life in pooled rat blood was 1.3 min. The slope of a plot of the duration of LORR in rats versus the logarithm of the hypnotic dose was significantly shallower for MOC-carboetomidate than for carboetomidate (4 ± 1 vs 15 ± 3, respectively; P = 0.0004123). At hypnotic doses, the effects of MOC-carboetomidate on arterial blood pressure and adrenocortical function were not significantly different from those of vehicle alone. CONCLUSIONS: MOC-carboetomidate is a GABA(A) receptor modulator with potent hypnotic activity that is more rapidly metabolized and cleared from the brain than carboetomidate, maintains hemodynamic stability similar to carboetomidate, and does not suppress adrenocortical function.

Differential effects of etomidate and its pyrrole analogue carboetomidate on the adrenocortical and cytokine responses to endotoxemia.

OBJECTIVE: We developed a novel pyrrole analog of etomidate, (R)-ethyl 1-(1-phenylethyl)-1H-pyrrole-2-carboxylate (carboetomidate), which retains etomidate's desirable anesthetic and hemodynamic properties but lacks its potent inhibitory affect on adrenocorticotropic hormone-stimulated steroid synthesis. The objective of this study was to test the hypothesis that in contrast to etomidate, carboetomidate neither suppresses the adrenocortical response to endotoxemia nor enhances the accompanying production of proinflammatory cytokines. DESIGN: Animal study. SETTING: University research laboratory. SUBJECTS: Male Sprague-Dawley rats. INTERVENTIONS: For both single and multiple anesthetic dose studies, rats were injected with Escherichia coli lipopolysaccharide immediately followed by a hypnotic dose of etomidate, carboetomidate, or vehicle alone (dimethyl sulfoxide) as a control. For single-dose studies, no additional anesthetic (or vehicle) was administered. For multiple anesthetic dose studies, additional doses of anesthetic (or vehicle) were administered every 15 mins for a total of eight anesthetic (or vehicle) doses. MEASUREMENTS AND MAIN RESULTS: Plasma adrenocorticotropic hormone, corticosterone, and cytokine concentrations were measured before lipopolysaccharide administration and intermittently throughout the 5-hr experiment. In single anesthetic dose studies, plasma adrenocorticotropic hormone and cytokine concentrations were not different at any time point among the etomidate, carboetomidate, and vehicle groups, whereas plasma corticosterone concentrations were briefly (60-120 mins) reduced in the etomidate group. In multiple anesthetic dose studies, plasma corticosterone concentrations were persistently lower and peak plasma interleukin-1ß and interleukin-6 concentrations were higher in the etomidate group vs. the carboetomidate and control groups. Peak plasma interleukin-10 concentrations were similarly elevated in the etomidate and carboetomidate groups vs. the control group. CONCLUSIONS: Compared with etomidate, carboetomidate produces less suppression of adrenocortical function and smaller increases in proinflammatory cytokine production in an endotoxemia model of sepsis. These findings suggest that carboetomidate could be a useful alternative to etomidate for maintaining anesthesia for a prolonged period of time in patients with sepsis.

Pharmacological studies of methoxycarbonyl etomidate's carboxylic acid metabolite.

BACKGROUND: Methoxycarbonyl etomidate (MOC-etomidate) is a rapidly metabolized and ultrashort-acting etomidate analog that does not produce prolonged adrenocortical suppression after bolus administration. Its metabolite (MOC-ECA) is a carboxylic acid whose pharmacology is undefined. We hypothesized that MOC-ECA possesses significantly lower pharmacological activity than MOC-etomidate, accounting for the latter's very brief duration of hypnotic action and inability to produce prolonged adrenocortical suppression after bolus administration. To test this hypothesis, we compared the potencies of MOC-ECA and MOC-etomidate in 3 biological assays. METHODS: The hypnotic potency of MOC-ECA was assessed in tadpoles using a loss-of-righting reflexes assay. The γ-aminobutyric acid type A (GABA(A)) receptor modulatory potencies of MOC-ECA and MOC-etomidate were compared by defining the concentrations of each required to directly activate α(1)(L264T)ß(2)γ(2L) GABA(A) receptors. The adrenocortical inhibitory potencies of MOC-ECA and MOC-etomidate were compared by defining the concentrations of each required to inhibit in vitro cortisol production by adrenocortical cells. RESULTS: MOC-ECA's 50% effective concentration for loss-of-righting reflexes in tadpoles was 2.8 ± 0.64 mM as compared with a previously reported value of 8 ± 2 µM for MOC-etomidate. The 50% effective concentrations for direct activation of GABA(A) receptors were 3.5 ± 0.63 mM for MOC-ECA versus 10 ± 2.5 µM for MOC-etomidate. The half-maximal inhibitory concentration for inhibiting in vitro cortisol production by adrenocortical cells was 30 ± 7 µM for MOC-ECA versus 0.10 ± 0.02 µM for MOC-etomidate. CONCLUSIONS: In all 3 biological assays, MOC-ECA's potency was approximately 300-fold lower than that of MOC-etomidate.

Closed-loop continuous infusions of etomidate and etomidate analogs in rats: a comparative study of dosing and the impact on adrenocortical function.

BACKGROUND: Etomidate is a sedative-hypnotic that is often given as a single intravenous bolus but rarely as an infusion because it suppresses adrenocortical function. Methoxycarbonyl etomidate and (R)-ethyl 1-(1-phenylethyl)-1H-pyrrole-2-carboxylate (carboetomidate) are etomidate analogs that do not produce significant adrenocortical suppression when given as a single bolus. However, the effects of continuous infusions on adrenocortical function are unknown. In this study, we compared the effects of continuous infusions of etomidate, methoxycarbonyl etomidate, and carboetomidate on adrenocortical function in a rat model. METHODS: A closed-loop system using the electroencephalographic burst suppression ratio as the feedback was used to administer continuous infusions of etomidate, methoxycarbonyl etomidate, or carboetomidate to Sprague-Dawley rats. Adrenocortical function was assessed during and after infusion by repetitively administering adrenocorticotropic hormone 1-24 and measuring serum corticosterone concentrations every 30 min. RESULTS: The sedative-hypnotic doses required to maintain a 40% burst suppression ratio in the presence of isoflurane, 1%, and the rate of burst suppression ratio recovery on infusion termination varied (methoxycarbonyl etomidate > carboetomidate > etomidate). Serum corticosterone concentrations were reduced by 85% and 56% during 30-min infusions of etomidate and methoxycarbonyl etomidate, respectively. On infusion termination, serum corticosterone concentrations recovered within 30 min with methoxycarbonyl etomidate but persisted beyond an hour with etomidate. Carboetomidate had no effect on serum corticosterone concentrations during or after continuous infusion. CONCLUSIONS: Our results suggest that methoxycarbonyl etomidate and carboetomidate may have clinical utility as sedative-hypnotic maintenance agents when hemodynamic stability is desirable.