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The Pharmacokinetics and Neuromuscular Effects of Rocuronium Bromide in Patients with Liver Disease

Magorian, Toni MD; Wood, Paul MB, ChB; Caldwell, James MB, ChB; Fisher, Dennis MD; Segredo, Veronica MD; Szenohradszky, Janos MD; Sharma, Manohar PhD; Gruenke, Larry PhD; Miller, Ronald MD

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To determine the effect of liver disease on the pharmacokinetics of rocuronium, the authors administered 0.6 mg/kg (twice the ED95) to 10 patients with liver disease and compared these results to values in 10 healthy surgical patients. Anesthesia was induced with thiopental and maintained with isoflurane (0.9%-1.1% end-tidal concentration) and nitrous oxide (60%). Venous blood samples were obtained for 6 h after rocuronium injection and plasma concentrations were measured using gas chromatography. Pharmacokinetic differences between groups were determined using a population-based pharmacokinetic analysis (NONMEM). Hepatic impairment did not alter the plasma clearance of rocuronium (217 +/- 21.8 mL/min, mean +/- SE, for both groups), but did increase the volume of the central compartment (5.96 +/- 1.01 L for controls, 7.87 +/- 1.33 L for patients with liver disease) and volume of distribution at steady state (16.4 L for controls, 23.4 L for patients with liver disease). In turn, elimination half-life was longer in patients with liver disease (111 min) compared to controls (75.4 min). The authors conclude that liver disease alters the pharmacokinetics of rocuronium by increasing its volume of distribution. The longer elimination half-life might result in a longer duration of action of rocuronium in patients with liver disease, particularly after prolonged administration.

(Anesth Analg 1995;80:754-9)

Department of Anesthesia, University of California, San Francisco, California.

This work was supported by a grant from Organon, Inc.

Accepted for publication November 10, 1994.

Address correspondence and reprint requests to Toni Magorian, MD, Department of Anesthesia, University of California, Mount Zion, Box 1610, 1600 Divisadero, San Francisco, CA 94115.

Rocuronium bromide (ORG 9426) is a new steroidal nondepolarizing neuromuscular blocking drug that has a faster onset of action than other nondepolarizing neuromuscular blocking drugs [1]. In animals, rocuronium appears to be eliminated primarily via the liver. For example, in cats, 54% of an intravenous (IV) dose is found in the bile and 21% in liver homogenate [2]. In addition, when the liver is excluded from the circulation (via a portal vein-to-inferior vena cava shunt), rocuronium's clinical duration of action increases almost threefold [2]. Finally, only 10% of an IV dose is excreted in urine [2,3]. These findings suggest that rocuronium's pharmacokinetics, and possibly its time course of neuromuscular effects, might be altered by liver disease. To examine this possibility, we determined the pharmacokinetics and the onset and clinical duration of rocuronium in patients with liver disease and a group of patients with normal hepatic and renal function.

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Methods

With approval from our Committee on Human Research, we obtained written informed consent from 20 surgical patients--10 healthy patients (ASA class I-II) and 10 with hepatic dysfunction (ASA class III). Hepatic dysfunction was defined as the presence of one or more of the following: 1) hepatic cirrhosis diagnosed by biopsy or ultrasound; 2) ascites and history of ethanol abuse; and, 3) alcohol-related increase of serum transaminases (five times normal aspartate aminotransferase or alanine aminotransferase) within 36 h of surgery. Patients also were assigned the appropriate Child's classification based on their laboratory data and physical findings [4]. Exclusion criteria were evidence of neuromuscular disease, use of medications known to influence neuromuscular function, or obesity (>30% above ideal body weight). Although the control and hepatic disease patients were studied concurrently, pharmacokinetic data for the control patients have been reported previously [3].

After administration of midazolam, 0.02-0.05 mg/kg IV, an intraarterial or IV catheter was placed to permit sampling of blood for determination of plasma rocuronium concentration. Anesthesia was induced with thiopental, 1-5 mg/kg IV, followed by inhalation of 1%-4% isoflurane in oxygen. The trachea was intubated without administration of a muscle relaxant. Anesthesia was maintained with 60% nitrous oxide and isoflurane (end-tidal concentrations of 1.0% +/- 0.1% in hepatic patients, 1.2% +/- 0.5% in control patients), monitored by mass spectrometry. Ventilation was controlled to maintain end-tidal PCO2 between 30 and 40 mm Hg. Esophageal temperature was maintained at 35-37 degrees C. Routine monitoring consisted of electrocardiography, automated blood pressure, and pulse oximetry.

Prior to administration of rocuronium, the mechanical evoked response of the adductor pollicis muscle to the first stimulus (T1) in a train-of-four sequence was determined. A Grass S88 nerve stimulator (Grass Medical Instruments, Quincy, MA) delivered supramaximal, train-of-four square-wave impulses of 0.2-ms duration (2 Hz) via surface electrodes near the ulnar nerve at the wrist. The stimulus sequence was repeated at intervals of 12 s and the evoked mechanical response of the adductor pollicis muscle was quantified by a force-displacement transducer (APMX Registered Trademark; Professional Instruments, Houston, TX) and recorded. When the amplitude of T1 was stable for > 15 min, this peak amplitude response was used as the baseline for comparison of all subsequent responses.

After anesthetic conditions and twitch tensions were stable for >or=to 15 min, rocuronium, 0.6 mg/kg [twice the ED95[5,6]], was injected rapidly. Twitch tension was recorded until the end of surgery or until additional muscle relaxation was required and provided by atracurium (which does not interfere with measurement of rocuronium). Residual neuromuscular blockade at the end of surgery was antagonized by administration of neostigmine and glycopyrrolate (40-70 micro gram/kg and 8-15 micro gram/kg, respectively). The times from injection of rocuronium to complete ablation of T1 (onset) and T1 return to 25% of baseline (clinical duration) were recorded.

Venous or arterial blood samples (4 mL each) were drawn from the contralateral arm before and at 2, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, 60, 75, 90, 120, 150, 180, 210, 240, 300, and 360 min after administration of rocuronium. In the first seven control patients, samples were obtained for 240 min only. Blood samples were anti-coagulated with heparin, iced, and centrifuged within 1 h of collection. Plasma was mixed with an equivalent volume of 0.8 M sodium dihydrogen phosphate and was stored at -30 degrees C until analysis. After methylene chloride extraction and derivatization, plasma concentrations of rocuronium were determined using gas chromatography with nitrogen-phosphorus detection [7]. This method is sensitive to 10 ng/mL with a coefficient of variation of < 10% at 20 ng/mL. Chromatograms also were examined for the presence and magnitude of peaks consistent with ORG 9943 (17-desacetylrocuronium), a putative metabolite of rocuronium.

A population pharmacokinetic analysis was performed using NONMEM [8] to estimate "typical values" of the pharmacokinetic variables, standard errors of these estimates, and interindividual variability within the population. Two- and three-compartment pharmacokinetic models were fit to the rocuronium plasma concentration data for all patients; models were parameterized in terms of volume of the central (V1), second (V2), and, if appropriate, third compartments (V3); clearance (Cl); rapid distributional clearance (Clrapid, equal to V1 centered dot k12 where k12 is the intracompartmental rate constant for drug movement from the central to the second compartment); and, if appropriate, slow distributional clearance--Clslow, the intracompartmental rate constant for drug movement from the central to third compartment (equal to V1 centered dot k13). Volume of distribution at steady state was calculated as the sum of V1, V2, and V3. Distribution half-lives (t 1/2 pi for the three-compartment model, t 1/2 alpha for both models) and elimination half-life (t 1/2 beta) were determined iteratively (Excel Solver User's Guide, Version 3.0a, Redmond, WA, Microsoft Corporation, 1991).

Variability among patients in Cl, V1, V2, and V3 was modeled by assuming that individual pharmacokinetic values for each variable may be expressed as the sum of the typical value for the population and a factor for the individual. Because interindividual variability tends to be skewed (i.e., log-normally distributed), interindividual variability for Cl was modeled as: Equation 1 where Cli is the estimate for clearance for the ith individual, Cl is the typical value for the population, and eta is a random variable with a mean value of 0.0. Equation 1 can be rewritten as: Equation 2 Interindividual variability for V1, V2, and V3 was modeled similarly, but interindividual variability for V2 and V3 was assumed to be the same.

The effect of liver disease on rocuronium pharmacokinetics was determined by comparing pharmacokinetic models having the same or different "typical values" for Cl, V1, V2, and V3 (alone or in combination) in patients with and without liver disease. These models were compared by their effects on the objective function (NONMEM's equivalent of the residual sum of squares for traditional nonlinear regression analysis), the residual differences between observed and predicted plasma concentrations, and interindividual variability. Liver disease was entered into the model as a dichotomous variable, i.e., absent or present. The influence of liver disease on Cl, V1, V2, and V3 (alone or in combination) was included in the model if it improved the objective function significantly (P < 0.05, i.e., the larger model improved the objective function by 3.8) and by the appearance of the residual differences between measured plasma concentrations and those predicted by the model. Whether the addition of a third pharmacokinetic compartment was justified [9] and whether estimates of pharmacokinetic variables were improved by normalizing for weight were determined.

Physical characteristics and neuromuscular variables in the two groups were compared by Mann-Whitney U-test. Unless otherwise identified, results are expressed as mean +/- SD.

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Results

There were no differences between groups in age, body weight, or gender Table 1. Nine of 10 patients in the hepatic group had alcoholic liver disease (cirrhosis) and the remaining patient, had a diagnosis of hepatoma with >2 L of ascites. Patients underwent general, gynecologic, or orthopedic procedures lasting 3.2 +/- 2.0 h. For technical reasons, blood samples could not be obtained in one patient with liver disease, onset could not be determined in a different patient, and clinical duration could not be determined in three patients. Onset times were similar in the control and hepatic patients. T1 was completely abolished in all patients. Clinical duration tended to be longer in patients with liver disease (P = 0.06).

Table 1

Table 1

"Average" plasma concentrations [determined] using a smoother, Supersmoother [10]; Statistical Sciences, Inc., Seattle, WA] were similar for control and hepatic patients for the first 30 min after rocuronium administration; thereafter, plasma concentrations declined more slowly in patients with liver disease Figure 1 and Figure 2. When a two-compartment pharmacokinetic model was fit to the plasma concentration data, residual differences between the observed and predicted plasma concentrations suggested that this model failed to describe the data adequately. A three-compartment model markedly improved the objective function (P < 0.005), and examination of the residuals indicated no model-misspecification. Neither weight-normalization of pharmacokinetic values nor the influence of liver disease on Cl further improved the objective function. However, objective function was improved by permitting V2 and V3 to differ between patients with normal liver function and those with liver disease; permitting V1, V2, and V3 to differ between groups further improved the fit Table 2. Therefore, "optimal" modeling of the effect of liver disease on rocuronium pharmacokinetics permitted V1, V2, and V3 (and, thus, the derived variables, volume of distribution at steady state, t 1/2 pi, t 1/2 alpha, and t 1/2 beta) to differ between control and hepatic patients. Cl, Clrapid, and Clslow were identical for control and hepatic patients Table 3 and Table 4.

Figure 1

Figure 1

Figure 2

Figure 2

Table 2

Table 2

Table 3

Table 3

Table 4

Table 4

The putative metabolite of rocuronium, 17-desacetylrocuronium (ORG 9943), was not detected.

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Discussion

We found that hepatic dysfunction increased the volume of distribution of rocuronium but did not alter its clearance. Although rocuronium's onset of action was not affected by liver disease, its clinical duration tended to be longer in those patients.

Khalil et al. [11] reported a similar clinical duration of rocuronium, 0.6 mg/kg, in cirrhotic patients compared to patients with normal liver function (42 +/- 16 vs 41 +/- 7 min, respectively). However, their recovery index (time for twitch tension to recover from 25% to 75% of control) was longer for cirrhotic patients (35 +/- 14 min) than for control patients (17 +/- 5 min). We observed a longer clinical duration in cirrhotic patients compared to control patients (median values of 62 and 42 min, respectively), although these differences did not attain statistical significance (presumably a function of the small sample size). Although our results differ from those of Khalil et al., findings from both studies suggest that initial recovery after a usual clinical dose of rocuronium (which is presumably a function of distribution rather than elimination) is not affected by liver disease. However, later recovery after usual clinical doses, or recovery after larger initial doses or repeated administration (which are presumably a function of elimination rather than distribution), may be prolonged in patients with liver disease. This is a result of the longer elimination half-life of rocuronium in cirrhotic patients (as well as the longer mean residence time in these patients reported by Khalil et al.).

Our findings regarding onset differ from those of Khalil et al.: we observed similar onset times in both groups whereas Khalil et al. reported that onset was slower in patients with cirrhosis. In addition, their onset times were markedly longer than ours. This latter finding may be explained by differences in anesthetic conditions in the two studies--we administered isoflurane to all subjects whereas they used an opioid-based anesthetic. Because isoflurane potentiates rocuronium relative to opioid-based anesthesia [12] and the same absolute dose of rocuronium, 0.6 mg/kg, was administered in the two studies, our dose represents a larger multiple of the ED95, and this relatively larger dose should result in a faster onset of paralysis [13]. We are unable to explain why Khalil et al. observed a longer onset time in cirrhotics although we saw no difference between groups.

The pharmacokinetic analyses suggest that the volume of distribution of rocuronium is larger in patients with liver disease than in control patients but that clearance is the same in both groups. The larger volume of distribution may result from an increase in the extracellular fluid volume that occurs in patients with liver disease [14], particularly those with ascites, as in our subjects. Whether altered protein binding in liver disease also contributes to the larger volume of distribution is unknown. However, for drugs of low protein binding (rocuronium's protein binding is approximately 30% [personal communication, Mitchell Weinberger, PhD, Organon, Inc., 1992]), protein binding minimally influences volume of distribution [15]. Based on previous reports that rocuronium is eliminated predominantly by the liver and is excreted intact in bile, we expected clearance of rocuronium to be smaller in patients with liver disease than in control patients. In contrast, clearance was similar in the two groups. There are two possible explanations for this finding. First, clearance of rocuronium is markedly less than hepatic plasma flow suggesting that rocuronium [like vecuronium [16]] has a low extraction ratio; this may lessen the effect of liver disease. Second, biliary excretory capacity may be preserved despite extensive hepatocellular disease [17].

The findings of the present study can be compared with those for vecuronium, another steroidal muscle relaxant that is eliminated by the liver [18-22]. In cirrhotic patients, vecuronium's onset is delayed [19], its clearance is decreased [20], and the duration of large doses is prolonged compared to controls [20-22]. The present findings suggest that rocuronium is affected by liver disease differently than vecuronium.

We did not model the relationship between concentration and effect as has been done in previous studies of muscle relaxants [23]. This decision was based on the variability of isoflurane concentrations in the control group. Although we could have determined the concentration-effect relationship in patients with liver disease, the absence of appropriate control data limits the utility of that estimate. In addition, plasma samples from all control patients and approximately half of the patients with liver disease were venous rather than arterial. The only time at which venous and arterial concentrations differ significantly is during onset, the time during which optimal estimates (i.e., arterial concentrations) are needed.

In summary, liver disease increases the volume of distribution of rocuronium and its elimination half-life, but not its clearance. The longer elimination half-life in patients with liver disease should prolong rocuronium's duration of action, particularly with larger initial doses or prolonged administration. However, onset time is not altered by the presence of liver disease, preserving a desirable property of rocuronium. These results suggest that rocuronium may be used cautiously in patients with impaired liver function. Further studies will indicate whether rocuronium's clinical effects are markedly influenced by liver disease.

The authors wish to thank James Arden, MD, for his critical review of the manuscript and substantive comments and also Winifred von Ehrenburg for her editorial expertise and enthusiasm in the preparation of this manuscript.

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