Vecuronium Pharmacokinetic-Pharmacodynamic Modelling With and Without a Receptor Concentration in the Effect Compartment in Anaesthetised Patients

Pharmacokinetic studies of neuromuscular blocking agents: good clinical research practice (GCRP)

In September 1997, an international consensus conference on standardization of studies of neuromuscular blocking agents was held in Copenhagen, Denmark. Based on the conference, a set of guidelines for good clinical research practice (GCRP) in pharmacokinetic studies of neuromuscular blocking agents is presented. Guidelines include: design of the study; relevant patient groups to investigate; test drug administration, sampling and analysis; pharmacokinetic analysis; pharmacokinetic/pharmacodynamic modeling; population pharmacokinetics; statistics; and presentation of pharmacokinetic data. The guidelines are intended to aid those working in this research area; it is hoped that they will assist researchers, editors of scientific papers, and pharmaceutical companies in improving the quality of pharmacokinetic studies.

Pharmacokinetic-pharmacodynamic modeling of mivacurium in rats

The pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of the neuromuscular blocking agent mivacurium were evaluated separately in two groups of rats receiving 0.6 mg kg-1 of mivacurium in a 2.5-min intravenous continuous (iv) infusion. The PK parameters for mivacurium were determined in the first group. A two-compartment model describes the kinetics of mivacurium in plasma. The estimates of the apparent volume of distribution at steady-state and plasma clearance [mean(SE)] were 650 (123) mL kg-1 and 9.9 (0.75) mL min-1 kg-1, respectively. In the second group, the evoked tibialis anterior muscle tension was monitored. The PK parameters derived from the first group were used to compute mivacurium plasma concentrations (C) at the times the PD measurements were recorded in the second group. The concentration-neuromuscular effect [% depression of initial twitch tension (E)] relationship was analyzed by two approaches. (1) The relationship of estimated effect site concentrations versus E; a sigmoidal Emax model described the effect compartment concentrations versus E relationship. The estimate [mean(SE)] of Cess50 (steady-state plasma concentration eliciting half of maximum E) was 0.65 (0.01) microgram mL-1. The value [mean-(SE)] of Keo (rate constant of equilibration between plasma and effect site) was estimated at 0.32 (0.03) min-1. (2) The relationship of descending limb C versus E; a sigmoidal Emax model described such relationship. The estimate [mean(SE)] of C50 (post-infusion C eliciting half of maximum E) was 0.57(0.03) microgram mL-1. The PD properties of mivacurium were also evaluated in another two groups of animals receiving either 5- or 10-min continuous iv infusion; PK and PD parameters obtained from the 2.5-min infusion experiments were used to predict the time course of E in the groups receiving 0.6 mg kg-1 of mivacurium in 5- and 10-min infusions; simulations using the estimated parameters adequately describe the time course of E in those groups. The effect of mivacurium on the mean arterial blood pressure (MAP) was also investigated; a 10% nonsignificant decrease (p > 0.05) in MAP was found in all groups.

An extended pharmacokinetic/pharmacodynamic model describing quantitatively the influence of plasma protein binding, tissue binding, and receptor binding on the potency and time course of action of drugs

An extended pharmacokinetic/pharmacodynamic (PK/PD) model is presented, in which the effect of binding of the drug to plasma proteins and to tissue binding sites in a peripheral compartment, and nonspecific and receptor binding in the effect compartment are taken into account. It represents an extension of the classical Sheiner model, and the model proposed by Donati and Meistelman. The present model is characterized by the following parameters: Kue (exit rate constant of unbound drug from the effect compartment), Pue (ratio of the unbound clearances to and from the effect compartment), fue (fraction of drug in effect compartment that is not bound to nonspecific binding sites), Kd (equilibrium dissociation constant of drug-receptor binding), and Rtot (concentration of receptor binding sites in effect compartment). The rate of association and dissociation of the drug-receptor complex can be incorporated in the model. The influence of the pharmacokinetic parameters (V1, V2, fu, fu2, CLu10, CLu20, CLu12, CLu21) and the PK/PD model parameters (kue, Pue, fue, Kd, Rtot) on various dynamic parameters is analyzed. These include potency (single dose needed to produce 90% effect, ED90), constant infusion dosing rate needed to maintain a constant effect of 90%, time to maximum effect (onset time), and duration to 90% recovery. The neuromuscular blocking agent vecuronium is used as an example. It is shown that both potency and time course of action are strongly dependent on the ratio V1/fu, CLu10, kue, Pue (at equipotent doses the time course is not affected by Pue), fue, Kd, and Rtot (only if Rtot is high), whereas they are less affected by the ratio V2/fu2, CLu20, CLu12, and CLu21. In general, the model parameters affect the ED90 and the time course of action in the same direction, e.g., an increase of V1 results in an increase of ED90 and an increase of onset time and duration. However, the unbound clearance CLu10, the intercompartmental unbound clearance CLu12 and the receptor affinity Kd have an opposite effect on ED90 and the time course parameters, e.g., an increase of CLu10 results in an increase of ED90 and a decrease of onset time and duration. This effect may be responsible for the inverse relationship between onset time and potency of neuromuscular blocking drugs observed in animal experiments and clinical studies. We demonstrate that PK/PD analysis using the traditional effect compartment model (Sheiner model) results in an apparent value of keo, which is a function of kue, fue, Kd, Rtot, as well as the unbound drug concentration in the effect compartment Cue. On the other hand, the model proposed by Donati and Meistelman gives correct values of keo (equal to the product fue.kue), but the receptor affinity Kd and the receptor density Rtot obtained by this method are apparent values, which depend on fu, fue, and Pue.

Measurement of diurnal variations in urinary cystine saturation

In an attempt to improve the diagnostic value of urine analysis in patients with homozygous cystinuria, we studied the diurnal variation in urine composition. A simplified estimate of the ion-activity product of cystine was used to increase the probability of identifying patients with a particular risk of stone formation. Eight 6-h urine samples were collected during two 24-h periods. The highest urinary excretion of cystine was recorded between 1200 and 1800 hours and the lowest between 0000 and 0600 hours, whereas the urinary cystine concentration was highest between 0000 and 0006 hours and lowest between 1200 and 1800 hours. The approximate ion-activity product of cystine had a maximal level between 0000 and 0600 hours but a minimal level between 0600 and 1200 hours. The differences between different periods were numerically more pronounced in terms of the ion-activity product of cystine than in terms of concentration. The peak concentrations of cystine in 6-h samples were about 90% higher than the corresponding concentrations in 24-h urine samples. It is concluded that the analysis of cystine in 6-h urine samples reveals transient episodes of cystine supersaturation that otherwise will remain undetected. Further studies are, however, needed to establish its usefulness in clinical practice.

Clinical course and cystine stone formation during tiopronin treatment

The formation of stones in patients with cystinuria can be counteracted by reducing the urinary concentration of cystine and by increasing its solubility. Thirty-one patients with homozygous cystinuria and treated with tiopronin (2-mercaptopropionylglycine) were followed for between 0.4 and 12 years (median 8.8). With the aim of avoiding cystine concentrations above 1200 mumol/l, the daily dose varied between 500 and 3000 mg (median 1500). The therapeutic effect was evaluated from the clinical symptoms and repeated radiographic examinations. The rate of stone formation during the treatment period was reduced by 60% in comparison with the pretreatment period (P < 0.001). The frequency of active stone removal was reduced by 72% (P < 0.05). The formation of new stones was associated with a higher cystine concentration than was the case during periods when stone formation and stone growth were excluded (P < 0.05). The probability of new stone formation increased with increasing concentrations of cystine up to 1100 mumol/l, but stone formation was not accentuated above 1200 mumol/l. There was no significant relationship between the 24 h excretion of cystine and stone formation. It is concluded that the formation of cystine stones can be efficiently counteracted during treatment with tiopronin, guided by analysis of the concentration of urinary cystine.