Determination of rocuronium and its putative metabolites in body fluids and tissue homogenates

Application of ICH Guidelines for Studying the Degradation Behavior of Rocuronium Bromide Coupled with Stability-Indicating RP-LC Method

Non-depolarizing neuromuscular blocking agent Rocuronium bromide was quantified in drug substance and drug product using reversed-phase liquid chromatographic method. Forced degradation studies were conducted for Rocuronium bromide in drug substance under acidic (2MHCl), basic (2MNaOH), oxidative (3% H2O2), thermal (135°C) and photolytic (254 nm) stress conditions. An Agilent H12 C18 column was used for separation using diammonium hydrogen phosphate buffer (pH 8; 0.04M)- acetonitrile (50:50; v/v) as mobile phase at flow rate of 1 mL/min. The quantification was done using UV detection at 210 nm. The limit of quantification and detection was 11.1 and 3.66 μg/mL, respectively, and the recovery percentage was 99% in drug substance and drug product. ICH guidelines were adopted for method validation. The proposed LC method monitored the degradation profile for Rocuronium bromide under various stress conditions and provided a specific LC method for its routine analysis. Besides, the MS data were used to identify all Rocuronium bromide degradation products and the possible degradation pathway was designated.

The SLCO1A2 -189_-188InsA polymorphism reduces clearance of rocuronium in patients submitted to elective surgeries

PURPOSE

Rocuronium (ROC) is a neuromuscular blocker mainly eliminated by biliary excretion dependent on organic anion transporting polypeptide 1A2 (OATP1A2) hepatocellular uptake. However, the influence of SLCO1A2 (gene encoding OATP1A2) genetic polymorphism on ROC pharmacokinetics was never described before. The objective of this work was to evaluate the influence of genetic polymorphisms of SLCO1A2 on the pharmacokinetics of rocuronium (ROC).

METHODS

Patients undergoing elective surgeries under general anesthesia using rocuronium as a neuromuscular blocker were genotyped for SLCO1A2 polymorphisms in the coding region (41A>G, 382A>T, 404A>T, 502C>T, 516A>C, 559G>A, 830C>A, and 833delA) and in the promoter region (-1105G>A, -1032G>A, -715T>C, -361G>A, and -189_-188insA). Rocuronium pharmacokinetic parameters were estimated by non-compartmental analysis.

RESULTS

None of the patients had heterozygous or homozygous variant of 404A>T, 382A>T, 502C>T, 833delA, 830C>A, 41A>G, and -715T>C. A linkage disequilibrium was found between -1105G>A and -1032G>A genotypes. Patients genotyped as -A or AA (n = 17) for SLCO1A2 -189_-188InsA showed reduced total clearance of ROC compared to patients genotyped as -/- (n = 13) (151.6 vs 207.1 mL/min, p ≤ 0.05). The pharmacokinetics parameters of ROC were not significantly different between other SLCO1A2 genotypes.

CONCLUSION

SLCO1A2 -189_-188InsA polymorphism is related to the reduced clearance of rocuronium in patients submitted to elective surgeries under general anesthesia.

TRIAL REGISTRATION

NCT 02399397 ( ClinicalTrials.gov ).

Pharmacokinetics and pharmacodynamics of rocuronium in young adult and elderly patients undergoing elective surgery

Objective

To evaluate the impact of advanced age on rocuronium kinetic disposition in ASA I–III patients undergoing elective surgeries.

Methods

Young adult (20–50 years, n = 15) and elderly patients (65–85 years, n = 14) submitted to surgery under general anaesthesia were investigated. All patients were induced with individual intravenous doses of midazolam, rocuronium, fentanyl and propofol. Rocuronium-induced neuromuscular block was monitored by train of four stimulations of the adductor muscle of the thumb on the ulnar nerve. The pharmacokinetic parameters were calculated by non-compartmental analysis. The relationship between rocuronium plasma concentration and the neuromuscular blockade was described by a sigmoidal Emax model.

Key-findings

Elderly patients presented decreased Cl (2.1 ml/kg per min vs 2.8 ml/kg per min; P = 0.0123); increased AUC/dose (507.8 μg min/ml (mg/kg) vs 392.2 μg min/ml/(mg/kg); P = 0.0168) and reduced volume of distribution (285.4 ml/kg vs 435.6 ml/kg, P = 0.0434) compared to young adults. The concentrations required to achieve 50% of maximum neuromuscular block (EC50) were similar for young adult (338.8 ng/ml) and elderly (462.7 ng/ml) patients (P > 0.05).

Conclusions

Elderly patients showed increased AUC/D and reduced total Cl compared to young adult patients due to the age-related reduced renal function. Differences in the PK-PD properties of rocuronium in elderly population are due to changes in drug disposition rather than to alterations in the sensitivity to the drug.

Development of SPME method for concomitant sample preparation of rocuronium bromide and tranexamic acid in plasma

A high-throughput method using solid-phase microextraction coupled to liquid chromatography-tandem mass spectrometry (SPME-LC-MS/MS) for determination of tranexamic acid and rocuronium bromide in human plasma was developed and validated. Standard analytical approaches employ acidification of the sample due to the instability of rocuronium bromide in collected plasma samples. However, acidification affects the binding equilibrium of the drug and consequently no information on the free/bound concentration can be obtained. Contrary to these protocols, the proposed method requires minimum sample handling and no ion pairing and/or derivatization procedure. A weak cation exchange coating was chosen as the best extracting phase for selected drugs, guaranteed a good recovery, minimum carry-over, reusability and reproducibility. SPME procedure met all Food and Drug Administration acceptance criteria for bioanalytical assays at three concentration levels, for both selected drugs. Post-extraction addition experiments showed that matrix effect was less than ±3%. Here, a weak cation exchange thin-film solid-phase microextraction (WCX TF-SPME) approach is presented, offering effective cleanup procedure and full quantitation of the drugs in plasma, undoubtedly one the most challenging matrices with regards to its complexity. In addition, the 96-well plate format of WCX TF-SPME system provides considerable advantages, such as high throughput analysis for up to 96 samples in 35min (22s/sample), requirement of small amounts of plasma samples (0.8mL), and a simple sample preparation protocol, all of which shows a promise for possible on-site application in hospitals to monitor concentrations of the drugs in close to real time.

Analysis of rocuronium in human plasma by liquid chromatography-tandem mass spectrometry with application in clinical pharmacokinetics

Rocuronium (ROC) is a neuromuscular blocking agent used in surgical procedures which is eliminated primarily by biliary excretion. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for analysis of ROC in human plasma. Separation of ROC and IS (verapamil) was performed using an endcapped C-18 column and a mixture of water:acetonitrile:trifluoracetic acid (50:50:0.1, v/v) as mobile phase. Aliquots of 100 μL of human plasma were extracted at pH 3, using dichloromethane. The lower limit of quantification of 5 ng/mL shows the high sensitivity of this method. Intra- and inter-assay precision (as relative standard deviation) was all ≤14.2% and accuracy (as relative standard error) did not exceed 10.1%. The validated method was successfully applied to quantify ROC concentrations in patients under surgical procedures up to 6h after the administration of the 0.4-0.9 mg/kg ROC. The pharmacokinetic parameter estimations of ROC showed AUC/dose of 563 μg min/mL, total clearance of 2.5 mL/min/kg, volume of distribution at steady state of 190 mL/kg and mean residence time of 83 min.

Pharmacokinetic-pharmacodynamic modeling of the influence of chronic phenytoin therapy on the rocuronium bromide response in patients undergoing brain surgery

BACKGROUND

Antiepileptic drugs decrease the intensity of the effect of neuromuscular blocking agents. The objective of this study was to evaluate the influence of chronic phenytoin therapy (CPT) on the pharmacokinetics (PK) and pharmacodynamics (PD) of rocuronium.

METHODS

A total of 21 patients undergoing intracranial surgery were enrolled in the study. Ten of these were under CPT. Rocuronium was administered intravenously. Arterial blood samples were drawn, and the T1% (percentage change from the response to the supramaximal stimulus) derived from electromyogram was continuously recorded. NONMEM: software was used to construct, evaluate and validate the PKPD models.

RESULTS

The PKPD of rocuronium was described using a three-compartment PK model and effect compartment model. The CPT therapy was found to increase the total plasma clearance from 0.26 to 0.75 L min(-1). The PD model parameter estimates were k(e0)= 0.073 min(-1), IC(50) (the steady-state plasma concentration eliciting half of the maximum response) = 836 ng mL(-1) and gamma = 3.13.

CONCLUSIONS

Chronic phenytoin therapy increases the clearance of rocuronium from 0.26 to 0.75 L min(-1) but has no effect on the k(e0), IC(50) or gamma parameters.

Simultaneous versus sequential pharmacokinetic-pharmacodynamic population analysis using an Iterative Two-Stage Bayesian technique

A method for simultaneous pharmacokinetic-pharmacodynamic (PK-PD) population analysis using an Iterative Two-Stage Bayesian (ITSB) algorithm was developed. The method was evaluated using clinical data and Monte Carlo simulations. Data from a clinical study with rocuronium in nine anesthetized patients and data generated by Monte Carlo simulation using a similar study design were analysed by sequential PK-PD analysis, PD analysis with nonparametric PK data and simultaneous PK-PD analysis. Both PK and PD data sets were 'rich' with respect to the number of measurements per individual. The accuracy and precision of the estimated population parameters were evaluated by comparing their mean error (ME) and root mean squared error (RMSE), respectively. The influence of PD model misspecification on the results was also investigated. The simultaneous PK-PD analysis resulted in slightly more precise population parameter estimates than the sequential PK-PD analysis and the nonparametric PK method. In the presence of PD model misspecification, however, simultaneous analysis resulted in poor PK parameter estimates, while sequential PK-PD analysis performed well. In conclusion, ITSB is a valuable technique for PK-PD population analysis of rich data sets. The sequential PK-PD method is better suited for the analysis of rich data than the simultaneous analysis.

Target-controlled infusion of rocuronium in infants, children, and adults: a comparison of the pharmacokinetic and pharmacodynamic relationship

Target-controlled infusion (TCI) of rocuronium can be used to maintain a stable blood concentration (Cp). At steady-state, the pharmacokinetics (PK) are compensated for by TCI, and a stable effect can be observed. The theoretical Cp may represent effect-site concentration (EC). We used the EC-effect relationship to study pharmacodynamics (PD) in infants, children, and adults. After giving their written, informed consent, 14 infants, 23 children, and 21 adults scheduled for elective surgical procedures received 3 to 6 ascending Cp targets of TCI rocuronium according to PD data. Just before each increase of TCI, venous blood samples were taken to measure Cp. Neuromuscular block was evaluated acceleromyographically. Individual effect data and measured Cp were fitted to the Hill equation. Maximum block during TCI targets-1000, 1300, and 1600 ng/mL-was smaller in children in comparison with infants and adults. The concentration in the effect compartment associated with a 50% drug effect (EC(50)) was significantly smaller in infants (mean [SD]) (652 [215] ng/mL) than in adults (954 [276] ng/mL) and was the largest in children (1200 [295] ng/mL). Calculated mean EC(90) values were 1705, 2230, and 2035 ng/mL, respectively, in infants, children, and adults. TCI rocuronium established steady-state PK/PD at different TCI targets and allowed us to define PK/PD relationships in a standardized way. Steady-state TCI rocuronium revealed the most potency of rocuronium in infants and the least in children.

IMPLICATIONS

Target-controlled infusion (TCI) of rocuronium in infants, children, and adults was used to analyze the pharmacokinetic/pharmacodynamic relationship. Steady-state TCI rocuronium revealed the most potency of rocuronium in infants and the least in children.

Pharmacokinetic/pharmacodynamic modeling of rocuronium in myasthenic patients is improved by taking into account the number of unbound acetylcholine receptors

Patients with myasthenia gravis are more sensitive than healthy patients to nondepolarizing neuromuscular blocking drugs. We performed a pharmacokinetic/pharmacodynamic modeling study of rocuronium in eight myasthenic patients and eight matched control patients. Patients were anesthetized with propofol and sufentanil and a mixture of nitrous oxide/oxygen. Mechanomyographical monitoring of the adductor pollicis was applied. Rocuronium was infused at a rate of 25 micro g. kg(-1). min(-1) in myasthenic patients and 116.7 micro g. kg(-1). min(-1) in control patients and was terminated at 70% neuromuscular block. Arterial blood samples were drawn during onset and offset of the block and for 4 h after the administration of rocuronium. Plasma concentrations were determined by high-performance liquid chromatography. Pharmacokinetic/pharmacodynamic modeling was performed by using the Sheiner model and the unbound receptor model (URM), which takes into account the number of unbound acetylcholine receptors. The effective concentration at 50% effect and the steepness of the concentration-effect relationship were significantly decreased in myasthenic patients. Both the URM and the Sheiner model provided an adequate fit in myasthenic patients. The acetylcholine receptor concentration was significantly decreased in myasthenic patients. The URM explains the observed differences in time course and potency, whereas the Sheiner model does not.

IMPLICATIONS

We performed a pharmacokinetic/pharmacodynamic modeling study in myasthenic patients and control patients. The unbound receptor model, which takes into account the number of unbound acetylcholine receptors in the biophase, was introduced and compared with the model proposed by Sheiner.

Gas chromatographic-mass spectrometric assay for rocuronium with potential for quantifying its metabolite, 17-desacetylrocuronium, in human plasma

A rapid, sensitive and selective method has been developed for the quantification of plasma concentrations of neuromuscular blocking drug, rocuronium, using gas chromatography with mass spectrometric detection. 3-Desacetylvecuronium served as the internal standard. The method involved iodide ion pair formation and a single-step liquid-liquid extraction with dicholoromethane. This method also permits simultaneous determination of its putative metabolite, 17-desacetylrocuronium, although the high detection limit for the metabolite limits the practical application of this method in pharmacokinetic study of the metabolite. The extraction efficiency was approximately 75% for rocuronium and approximately 50% for 17-desacetylrocuronium. The limit of quantification was 26 ng/ml for rocuronium and 870 ng/ml for its metabolite. The assay was used successfully in a patient undergoing liver transplantation and receiving rocuronium as a constant rate infusion and in a patient undergoing general elective surgery receiving the drug as an intravenous bolus. This assay is a time-saving alternative to published gas or liquid chromatographic methods for assaying rocuronium.

Quantitative determination of rocuronium in human plasma by liquid chromatography-electrospray ionization mass spectrometry

Liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) was used for the quantification of the neuromuscular blocking agent rocuronium in human plasma. Verapamil was used as internal standard. The samples were subjected to a dichloromethane liquid-liquid extraction after ion pairing of the positively charged ammonium compound with iodide prior to LC-MS. Optimized conditions involved separation on a Symmetry Shield RP-18 column (50 x 2.1 mm, 3.5 microm) using a 15-min gradient from 10 to 90% acetonitrile in water containing 0.1% trifluoroacetic acid at 250 microl/min. Linear detector responses for standards were observed from 25 to 2,000 ng/ml. The extraction recovery averaged 59% for rocuronium and 83% for the internal standard. The limit of quantification (LOQ), using 500 microl of plasma, was 25 ng/ml. Precision ranged from 1.3 to 19% (LOQ), and accuracy was between 92 and 112%. In plasma samples, at 20 and 4 degrees C, rocuronium was stable at physiological pH for 4 h; frozen at -30 degrees C it was stable for at least 75 days. The method was found suitable for the analysis of samples collected during pharmacokinetic investigations in humans.

Urinary, biliary and faecal excretion of rocuronium in humans

The excretion of rocuronium and its potential metabolites was studied in 38 anaesthetized patients, ASA I-III and 21-69 yr old. Rocuronium bromide was administered as an i.v. bolus dose of 0.3 or 0.9 mg kg-1. In Part A of the study, the excretion into urine and bile, and the liver content were studied. Plasma kinetics (n = 19) were similar to those reported previously. Urinary recovery within 48 h after administration was 26 (8)% (mean (SD)) (n = 8) of the dose. In bile obtained from T-drains, the recovery within 48 h was 7 (6)% (n = 11). The rocuronium concentration in bile declined bi-exponentially, with half-lives of 2.3 (0.7) and 16 (11) h respectively (n = 6). In three patients from whom stoma fluid was collected, the amount of rocuronium recovered ranged from 0.04 to 12.0% of the dose. In liver tissue obtained from four patients undergoing hemihepatectomy, the estimated amount of rocuronium at 2-5 h after administration ranged between 6.3 and 13.2% (n = 4). In the second part of the study (Part B), urine and faeces were collected over 4-8 days and the recovery was 27 (13)% and 31 (23)% of the dose respectively (n = 10). In most samples, irrespective of the type of biological material, only small amounts of the metabolite 17-desacetyl-rocuronium was found. The results demonstrate that rocuronium is taken up by the liver and excreted into bile in high concentrations. The faecal and urinary excretion of unchanged rocuronium are the major routes of rocuronium elimination.

Pharmacokinetics of rocuronium bromide in obese female patients

Following administration of 0.6 mg kg-1 rocuronium, the pharmacokinetics and the pharmacodynamics were studied in six obese and six control (normal weight) patients receiving balanced anaesthesia. Twelve gynaecological patients were allocated into two groups, according to body mass index (normal weight: body mass index: 20-24, obese weight: body mass index > 28). Venous plasma concentrations were determined by high-pressure liquid chromatography before administration of rocuronium, at 1, 2, 4, 6, 8, 10, 15, 20, 25, 30, 35, 40, 48, 60, 75, 120, 180, 240, 300, 360 and 420 min after administration of rocuronium and at recovery of single twitch to 25% and 75% of control twitch height. Onset time was shorter (NS) in the obese compared with normal weight (obese weight: 65 +/- 16, normal weight: 100 +/- 39 s, mean +/- SD). Duration 25% (obese weight: 29.5 +/- 5.3, normal weight: 28.4 +/- 5.3 min) and spontaneous recovery time (obese weight: 12.6 +/- 2.7, normal weight: 12.5 +/- 2.3 min) did not show any differences between the two groups. The pharmacokinetics of rocuronium were comparable in the two groups. The volume of distribution at steady state Vss (mL kg-1) was 208 +/- 56 in normal weight and 169 +/- 37 in obese weight. Distribution (T1/2 alpha) and elimination half-life (T1/2 beta) as well as mean residence time were 15.6 +/- 3.7, 70.3 +/- 23.9 and 53.2 +/- 9.8 min in normal weight and 16.9 +/- 3.8, 75.5 +/- 25.5 and 51.1 +/- 18.9 min in obese weight, respectively. Also, no differences were observed in plasma clearance (3.89 +/- 0.58 in normal weight and 3.62 +/- 1.42 mL kg-1 min-1 obese weight). This study indicates that the pharmoacodynamics and pharmacokinetics of rocuronium are in female patients not altered by obesity.

Do plasma concentrations obtained from early arterial blood sampling improve pharmacokinetic/pharmacodynamic modeling?

In pharmacokinetic/pharmacodynamic (PK/PD) modeling the first blood sample is usually taken 1 to 2 min after drug administration (late sampling). Therefore, investigators have to extrapolate the plasma concentration to Time 0. Extrapolation, however, erroneously assumes instantaneous and complete mixing of drug in the central volume of distribution. We investigated whether plasma concentrations obtained from early arterial blood sampling would improve PK/PD modeling. In 14 pigs, one of five neuromuscular blocking agents (NMBAs) was administered into the right ventricle within 1 sec and arterial sampling was performed every 1.2 sec (1st min). The response of the tibialis muscle was measured mechanomyographically. The influence of inclusion of data from early arterial sampling on PK/PD modeling was determined. Furthermore, the concentrations in the effect compartment at 50% block (EC50) derived from modeling were compared to the measured concentration in plasma during a steady state 50% block. A very high peak in arterial plasma concentration was seen within 20 sec after administration of the NMBA. Extensive modeling revealed that plasma concentrations obtained from early arterial blood sampling improve PK/PD modeling. Independent of the type of modeling, the EC50 and KeO based on data sets that include early arterial blood sampling were, for all five NMBAs, significantly higher and lower respectively, than those based on data sets obtained from late sampling. Early arterial sampling shows that the mixing of the NMBA in the central volume of distribution is incomplete. A parametric PD (sigmoid Emax) model could not describe the time course of effect of the NMBAs adequately.

Quantification of the neuromuscular blocking agent rocuronium and its putative metabolite 17-desacetylrocuronium in heparinized plasma by capillary gas chromatography using a nitrogen sensitive detector

We have developed a sensitive and specific capillary GC (cGC) assay for the quantification of the quarternary aminosteroidal compound rocuronium (roc), a neuromuscular blocking agent, and its putative metabolite 17-desacetylrocuronium (17OH-roc), using 3-desacetylvecuronium (3OH-vec) as an internal standard (I.S.). This novel method has been applied to a pharmacokinetic study with roc, monitoring sixty patients who were classified according to four different body mass index (BMI) groups. The isolation of these drugs from plasma was carried out using a dichloromethane liquid-liquid extraction after ion-pairing of the positively charged ammonium compounds with iodide. To achieve thermal stability, tert.-butyldimethylsilyl-ethers were formed at the 3OH- and 17OH-steroidal positions by reaction with N-methyl-N-(tert.-butyldimethylsilyl)-trifluoroacetamide at 70 degrees C overnight. An automated cGC system fitted with a nitrogen sensitive detector with a specially prepared glass phase bead and a computer controlled data handling system was used to analyze and quantify the compounds, which were separated on a DB1 capillary column with helium as the carrier gas and a temperature program ranging from 120 to 300 degrees C. The method is linear for 50-6400 ng/ml for roc and 80-6400 ng/ml for 17OH-roc. The detection limits were 10 ng/ml for roc and 50 ng/ml for 17OH-roc. The lower limit of quantification was 50 ng/ml for roc and 80 ng/ml for 17OH-roc. Intra-assay coefficients of variation (C.V.s) were 10% and 15% and the inter-assay C.V.s 8-18% and 16-21% for roc and 17OH-roc, respectively.

Determination of dopaminergic prodrugs by high-performance liquid chromatography followed by post-column ion-pair extraction

One possibility to optimize the therapeutic application of dopaminergic compounds with a catechol function is the reversible protection of this moiety using a prodrug approach. Important features in this respect are a proper chemical stability in the gastrointestinal tract, an adequate release rate after arrival in the blood stream or the possibility to cross the blood-brain barrier. A HPLC method was developed to measure the hydrolysis of prodrugs of dopamine and epinine directly. The method is based on reversed-phase separation followed by post-column ion-pair extraction with a fluorescent counter-ion. The separation of di-isobutyryl esters of dopamine and epinine is obtained within 10 min while the more hydrophobic dopaminergic esters, di-benzoyl and di-pivaloyl dopamine, are retained for 30 min. The precision of the assay measuring 160 ng dibudop and 100 ng ibopamine was 1.2 and 1.0%, respectively. The detection limit of all prodrugs tested was approximately 10 ng.

Clinical pharmacokinetics of rocuronium bromide

A new aminosteroidal neuromuscular blocking agent, rocuronium bromide, has recently been introduced into clinical practice. Its main advantage over other currently used drugs of this kind is its fast onset of action, which could render rocuronium the muscle relaxant of choice for rapid facilitation of tracheal intubation. A further advantage of the new compound over vecuronium bromide is the less extensive formation of breakdown products, reducing the contribution of active metabolites to the neuromuscular blocking effects of the parent compound. Thorough knowledge of the pharmacokinetics of any new drug is highly desirable for the anaesthesiologist because absorption, distribution to the tissue, as well as elimination by biotransformation and excretion, are closely related to its effects. Due to its chemical relationship to other aminosteroidal neuromuscular blocking agents such as pancuronium bromide or vecuronium, rocuronium is expected to display pharmacokinetic behaviour similar to that of its predecessors. Hepatic and renal disease may prolong the effect of rocuronium, but to a lesser extent than seen with pancuronium or vecuronium, because the plasma clearance of rocuronium is not significantly influenced by dysfunction of the liver or kidneys. On the contrary, in elderly or hypothermic patients the reduction in plasma clearance results in a prolonged duration of the action of rocuronium. All information on the pharmacokinetics of this new nondepolarising neuromuscular blocking agent which has been made available to date is presented in this review, with a discussion of the significance of these data for clinical use of the drug.

Effects of hypothermic cardiopulmonary bypass on the pharmacodynamics and pharmacokinetics of rocuronium

OBJECTIVE

To study the influence of hypothermic cardiopulmonary bypass (CPB) on the pharmacodynamics and pharmacokinetics of rocuronium.

DESIGN

Prospective, descriptive study.

SETTING

Operating room at a university hospital.

PARTICIPANTS

Ten ASA class III and IV patients, ranging in age from 35 to 75 years, scheduled for elective coronary artery bypass grafting.

INTERVENTIONS

Neuromuscular transmission was monitored mechanomyographically. The time course of action of maintenance doses and plasma concentration-response relationships were determined before, during, and after CPB. The plasma concentration decay and renal elimination were studied simultaneously. Plasma and urine concentration of rocuronium were determined by high-performance liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Hypothermic CPB prolonged the duration of action of maintenance doses and coincided with a lower plasma concentration at a twitch response of 5% of control. The duration of action of maintenance doses returned to prehypothermic CPB level after rewarming to a nasopharyngeal temperature of 37 degrees C. The plasma concentration-response relationship did not return to precooling control value, probably owing to persisting peripheral hypothermia. Both the renal elimination of rocuronium and the plasma concentration decay after the last maintenance dose under normothermic conditions resembled values obtained in patients not undergoing hypothermic CPB.

CONCLUSIONS

Hypothermic CPB prolongs the duration of action of maintenance doses and alters the plasma concentration-response relationship of rocuronium. These changes may be the result of, on the one hand, an increased sensitivity of the neuromuscular transmission and/or decreased muscle contractility and, on the other hand, the result of a reduced plasma clearance during hypothermia.

Pharmacokinetics and pharmacodynamics of rocuronium at the vocal cords and the adductor pollicis in humans

The pharmacokinetic-pharmacodynamic relationship of rocuronium at the laryngeal adductor muscles and the adductor pollicis was determined in eight patients during general anesthesia. Rocuronium was administered as an infusion at a rate of 100 micrograms.kg-1.min-1 over 5 minutes. The half-life of transport between plasma and biophase (effect compartment) was significantly shorter at the adductor laryngeal muscles (2.7 +/- 0.6 minutes, mean +/- SD) than at the adductor pollicis (4.4 +/- 1.5 minutes, p = 0.003). The concentration in the effect compartment producing 50% of the maximum effect was significantly greater at the adductor laryngeal muscles (1424 +/- 148 micrograms.L-1) than at the adductor pollicis (823 +/- 157 micrograms.L-1, p = 0.0001). The shorter onset of neuromuscular blockade at the laryngeal muscles than at the adductor pollicis may be explained by a faster transfer rate at the laryngeal adductor muscles neuromuscular junction than at the adductor pollicis neuromuscular junction.

Pharmacodynamic behaviour of vecuronium in primary hyperparathyroidism

This study evaluated the potency and time course of action of vecuronium in patients with primary hyperparathyroidism (HPT) and marked hypercalcaemia during nitrous oxide-opioid anaesthesia. Twenty ASA physical status I and II patients were studied by measuring the force of contraction of the adductor pollicis in response to stimulation of the ulnar nerve: ten control patients and ten patients with HPT and ionized calcium concentration over 2.80 mEq.L-1. After induction of anaesthesia with thiopentone and maintenance with N2O/O2 and fentanyl, vecuronium was administered to determine cumulative dose-response curves. When maximum block had been obtained, twitch height was maintained at 10% of baseline value over 20 min by adjusting the infusion rate of a syringe-pump containing vecuronium and vecuronium plasma concentration (EC90ss) was determined. During spontaneous recovery, after termination of infusion, the recovery index, the time from 25 to 75% recovery, was measured. The dose to produce 90% block was greater in the HPT than in control group: 69 (24) vs 54 (18) micrograms.kg-1 (P < 0.02). The calculated ED50 was also greater in HPT: 42 (4) vs 31 (5) micrograms.kg-1 in controls (P < 0.001). (Values are given as mean and coefficient of variation). The slope of the dose-response curve, the dose necessary to maintain 90% block, and the EC90ss did not differ. The RI25-75 was slower in the HPT group although the difference did not reach statistical significance. It is concluded that hyperparathyroidism with hypercalcaemia increases vecuronium requirement; only during the onset of neuromuscular blockade.

Preliminary investigations of the clinical pharmacology of three short-acting non-depolarizing neuromuscular blocking agents, Org 9453, Org 9489 and Org 9487

Three muscle relaxants, Org 9453, Org 9489 and Org 9487, short-acting in animals, were investigated to establish their profiles in humans. Potency, time course of action, and pharmacokinetic behaviour were studied in 90 healthy patients during fentanyl/halothane/N2O anaesthesia. Neuromuscular function was monitored mechanomyographically. Plasma and urine concentrations (three patients per compound) were measured by HPLC, and these data were analyzed by iterative linear least square regression analysis. The ED90 values for Org 9453, Org 9489 and Org 9487 were 1.4, 0.45 and 1.15 mg.kg-1 respectively. The onset times of Org 9453 (1.5 mg.kg-1, 1.1 X ED90), Org 9489 (0.9 mg.kg-1, 2 X ED90) and Org 9487 (1.5 mg.kg-1, 1.3 X ED90) were 1.2, 1.6 and 1.5 min, and the durations until 25% twitch recovery were 8.6, 22.0 and 8.9 min, respectively. Clearances of these doses were 6.9, 5.8, and 11.1 ml.kg-1.min-1, and mean residence times 26, 79, and 41 min, respectively. Mean renal excretion (parent compound and metabolites) within 24 hr amounted to 5, 11.3 and 12.2% respectively. No side effects other than a moderate short-lasting decrease of blood pressure and a concomittant increase in heart rate were noted. It is concluded that Org 9453 and Org 9487 are short-acting muscle relaxants in humans.