In vivo and in vitro pharmacological studies of methoxycarbonyl-carboetomidate

Abstract

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 11β-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.

Figure 1

Chemical structures of etomidate, MOC-etomidate, carboetomidate, and MOC-carboetomidate. Their structural differences are highlighted by the dashed boxes.

Figure 2

Synthesis of MOC-carboetomidate.

Figure 3

MOC-carboetomidate concentration-response curve for loss of righting reflexes (LORR) in tadpoles. Each point represents data from a single tadpole. The curve is a fit of the concentration-response data to a logistic equation using the method of Waud. The calculated EC50 for LORR was 9 ± 1 µM.

Figure 4

Loss of righting reflexes (LORR) in rats. (A) MOC-carboetomidate and carboetomidate dose-responses curves for LORR in rats. The curve is a fit of dose-response data to a logistic equation using the method of Waud. The calculated ED50s for LORR were 13 ± 4 mg/kg for MOC-carboetomidate and 7.7 ± 0.8 mg/kg for carboetomidate. (B) Time to righting after bolus administration of MOC-carboetomidate or carboetomidate at the indicated doses. For both drugs, the time to righting increased logarithmically with dose. However, the slope of this relationship was significantly shallower for MOC-carboetomidate than for carboetomidate (4 ± 1 vs. 15 ± 3, respectively; p = 0. 0004123).

Figure 5

Representative traces showing the enhancing effect of MOC-carboetomidate on human γ-aminobutyric acid type A receptor function. The first and last traces show the control electrophysiological responses elicited with 3 µM γ-aminobutyric acid alone. The middle trace demonstrates the enhancing effect of 13 µM MOC-carboetomidate on currents elicited with 3 µM γ-aminobutyric acid in the same oocyte.

Figure 6

Metabolic stability of sedative-hypnotics in rat blood. Each point is the average value determined in two separate experiments. Each experiment used blood pooled from a different group of three rats. The metabolic half-lives of MOC-carboetomidate and MOC-etomidate were 1.3 min and 0.35 min, respectively.

Figure 7

The differential effects of MOC-carboetomidate (27 mg/kg) and etomidate (2 mg/kg) on adrenocortical function in the rat. These doses correspond to twice the respective ED50s for loss of righting reflexes. Adrenocorticotropic hormone_1–24 was given simultaneously with sedative-hypnotic to stimulate corticosterone production and serum corticosterone concentrations were measured 30 minutes later. Four rats were studied in each group. Average corticosterone concentrations (±SD) were 460 ± 140 ng/ml in the vehicle control group, 90 ± 44 ng/ml in the etomidate group, and 610 ± 280 ng/ml in the MOC-carboetomidate group. *, P < 0.05; **, P < 0.01; NS, no significant difference.

Figure 8

The effects of MOC-carboetomidate on mean arterial blood pressure in the rat. MOC-carboetomidate (27 mg/kg or 54 mg/kg) in dimethyl sulfoxide vehicle or vehicle alone was administered IV at time 0. In all three groups, there was a significant decrease in mean arterial blood pressure 30 s after injection. However among the three groups, the mean blood pressure was not significantly different at any time point during the experiment.