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In Vitro Remifentanil Metabolism: The Effects of Whole Blood Constituents and Plasma Butyrylcholinesterase

Davis, Peter J., MD*†,; Stiller, Richard L., PhD*‡,; Wilson, Annette S., PhD; McGowan, Francis X., MD*†#,; Egan, Talmage D., MD§,; Muir, Keith T., PhD

doi: 10.1097/00000539-200211000-00038

We designed this in vitro study to determine whether the half-life of remifentanil was altered in butyrylcholinesterase-deficient patients. Test tubes containing Krebs buffered solution, whole blood, plasma, or red cells from both normal and butyrylcholinesterase-deficient patients were incubated with remifentanil. Remifentanil concentrations were determined by using gas chromatography and mean half-lives were calculated by using a nonlinear regression analysis. There were no differences in whole blood, red cells, or plasma half-life between normal and butyrylcholinesterase-deficient volunteers. In both normal and butyrylcholinesterase-deficient volunteers, whole blood and plasma had a significantly longer half-life than the red cell component. Extrapolation to the in vivo setting would suggest that a butyrylcholinesterase-deficient patient should not have altered remifentanil kinetics.

Departments of *Anesthesiology, †Pediatrics, ‡Pharmacology, and ¶Environmental and Occupational Health, University of Pittsburgh, School of Medicine, Pittsburgh, Pennsylvania; §Department of Anesthesiology, University of Utah, School of Medicine, Salt Lake City, Utah; ∥Clinical Pharmacokinetics, Glaxo-SmithKline; and #Department of Anesthesia, Harvard Medical School, Boston, Massachusetts

Supported by a grant from Glaxo-Wellcome Pharmaceuticals, Inc.

This work was done at the University of Pittsburgh School of Medicine, Children’s Hospital of Pittsburgh.

July 9, 2002.

Address correspondence to Peter J. Davis, MD, Department of Anesthesiology, Children’s Hospital of Pittsburgh, 3705 5th Ave., Pittsburgh, PA 15213-2583. Address e-mail to davispj@anes.

Remifentanil is a recently introduced 4- anilidopiperidine opioid analgesic that was specifically designed to be short acting. Emulating a strategy used in the development of esmolol, a short-acting, β-adrenergic blocker, the goal of remifentanil’s development program was to discover a compound that would loose its potent μ receptor agonist activity upon ester hydrolysis (1). Incorporation of a methyl ester into the N-acyl moiety of the basic 4-anilidopiperidine skeleton forms the structural basis of remifentanil’s vulnerability to metabolism by esterases. Initial animal studies confirmed remifentanil’s potent μ agonist activity (2) and susceptibility to esterase metabolism in human whole blood. Subsequent investigation in early human clinical trials confirmed remifentanil’s short-acting μ receptor activity (3–6).

Because of remifentanil’s esterase-dependent metabolic pathway, questions regarding remifentanil metabolism in pseudocholinesterase-deficient patients inevitably arise. Aside from the demonstration that remifentanil undergoes rapid hydrolysis in human whole blood (1), there have been no laboratory investigations examining the details of remifentanil’s metabolism in humans (that have appeared beyond preliminary, abstract form). However, a case report suggesting that remifentanil metabolism is not influenced by pseudocholinesterase deficiency has been published (7).

In this study, we aimed to examine the influence of pseudocholinesterase deficiency on remifentanil’s metabolism in vitro. We hypothesized that pseudocholinesterase deficiency would have no impact on the in vitro rate of remifentanil metabolism in human blood.

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Materials and Methods

These experiments were IRB approved, and informed written consent was obtained from all volunteers. We determined the metabolism of remifentanil in whole blood, plasma, and red blood cells in volunteers with normal and deficient levels of plasma butyrylcholinesterase. All volunteers with plasma butyrylcholinesterase deficiency had been previously identified by a history that included an ambulatory surgical procedure in which succinylcholine was administered, and the procedure was complicated by prolonged neuromuscular blockade that required postoperative mechanical ventilation. In all the subjects at the time of this laboratory study, plasma butyrylcholinesterase levels were determined by spectrophotometric assay with propionylthiocholine, and the presence or absence of diminished enzyme activity was confirmed. The pharmacokinetic variables of remifentanil were estimated by using nonlinear regression assuming a single-compartment model. Statistical analysis included unpaired Student’s t-test and one- and two-way analysis of variance with the Student-Newman-Keuls test. Statistical significance was considered for P < 0.05.

Fifty milliliters of whole blood was obtained from each of the seven adult volunteers with normal plasma butyrylcholinesterase and five adult volunteers with plasma butyrylcholinesterase deficiency. The blood was collected in heparinized syringes, and the volunteers’ hematocrit was determined. The blood was then partitioned into 10-mL aliquots of whole blood, plasma, and red blood cells, and each component was further divided into 1-mL aliquots. The red blood cells were washed three times in normal saline, and Krebs buffer was added to the red blood cell sample to maintain the hematocrit at the volunteer’s original hematocrit. A true buffer control of Krebs buffer solution was used to determine the spontaneous breakdown of remifentanil. To each 1-mL test tube of either Krebs buffer, whole blood, plasma, or red blood cells, 50 ng of remifentanil was added. The test tubes were gently mixed, the samples were placed in a 37°C bath under a nitrogen/oxygen/carbon dioxide (75:20:5) atmosphere and the pH was adjusted to 7.4. Remifentanil concentrations were determined at 0, 10, 20, 30, 60, 120, and 240 min for the whole blood and red blood cell components, whereas the plasma was sampled at 0, 5, 10, 20, 30, 60, 120, 240, 720, and 1440 min. All samples were immediately placed in acetonitrile and extracted with methylene chloride to prevent further remifentanil metabolism. The samples were stored at −70°C, and the remifentanil concentration was later determined by gas chromatography/mass spectroscopy (8). The lower limit of quantitation of this method is 0.100 ng/mL and the upper limit of quantification is 100 ng/mL. The correlation coefficient of this assay was >0.99. All samples were run in triplicate except for 1 patient with plasma butyrylcholinesterase deficiency in which there was only enough blood for 1 determination. The mean half-lives were calculated by using nonlinear regression analysis from curves fitted to the individual concentration-time data. There was no weighting used in the nonlinear regression analysis.

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By design, there were significant differences in the plasma butyrylcholinesterase activity between the normal and butyrylcholinesterase-deficient volunteers (mean ± sd) (3165 ± 208 versus 865 ± 115 U/L). There were no significant differences in age between the 2 groups (mean ± sd) (35 ± 8.0 versus 45 ± 17.4 yr). All of the volunteers in the butyrylcholinesterase-deficient group were women, whereas in the group of normal volunteers, four of seven were women. All of the volunteers had normal hematocrits.

In the Krebs buffered control, the half-life was 72.0 ± 3.6 min. In the normal volunteers (control), the remifentanil half-life (mean ± sd) of the whole blood (91 ± 6.1 min), red blood cells (75 ± 11.0 min), and plasma (164 ± 34.5 min) were significantly different from one another (Fig. 1, left). The findings in the butyrylcholinesterase-deficient volunteers were similar to the normal volunteers in that the half-life for the whole blood (99 ± 5.7 min), red blood cells (66 ± 10.0 min), and plasma (208 ± 41.7 min) were significantly different from one another (Fig. 1, right). There were no significant differences in the whole blood, red blood cell, or plasma half-life between the normal and butyrylcholinesterase-deficient volunteers.

Figure 1

Figure 1

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The essential finding of this study is that plasma butyrylcholinesterase deficiency does not influence the metabolism of remifentanil. When comparing published remifentanil kinetics in patients or volunteers with the results of our in vitro study, it is clear that tissue metabolism is the major site in remifentanil hydrolysis (3–5,9). Because most of the remifentanil is metabolized in tissues, it is not surprising that plasma butyrylcholinesterase deficiency has little effect on its metabolism. Thus, this in vitro laboratory study of remifentanil supports the single case report of Manullang and Egan (7) in which the clinical effects of remifentanil were not prolonged in a patient with plasma butyrylcholinesterase deficiency.

Although the blood component seems to contribute little to remifentanil metabolism, nonetheless, the different components of blood affect remifentanil metabolism differently. Both whole blood and plasma prolong the half-life of remifentanil compared with red blood cells. Remifentanil’s prolonged half-life observed in whole blood and plasma when compared with the Krebs buffer solution suggests that remifentanil may undergo protein binding which “protects” remifentanil from hydrolysis.

The half-life of remifentanil in the red blood cell component seems similar to its half-life in the Krebs buffer solution. At present, there are few data on the role of red cells in drug metabolism. Both the membrane and the cytosol of red blood cells contain esterase activity. Hemoglobin and carbonic anhydrase have also been shown to have esterase activity; however, this esterase activity is limited (10), and preliminary work suggests that carbonic anhydrase I and II do not affect remifentanil metabolism. 1 For the β agonist, esmolol, the cytosol of the red cell has been shown to be the major site of its metabolism, and in rats, the coadministration of esmolol has no effect on remifentanil pharmacokinetics (11). Whether the coadministration of esmolol would affect red cell metabolism of remifentanil in patients is unknown. Our in vitro studies did not address the red cell site of remifentanil or the interactive effects (if any) of remifentanil and esmolol in red cells.

We investigated the effects of cholinesterase deficiency on the in vitro hydrolysis rate of remifentanil. Extrapolation to the in vivo setting would suggest that butyrylcholinesterase-deficient patients should not demonstrate an atypical response to remifentanil and that dosage alterations are unnecessary. Our findings that the buffer solution has a faster half-life than whole blood and plasma suggest that protein binding may influence remifentanil metabolism in blood.

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1 Selinger K, Nation RL, Smith GA. Enzymatic and chemical hydrolysis of remifentanil [abstract]. Anesthesiology 1995;83:A385.
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