Pharmacokinetic-pharmacodynamic modeling of doxacurium: effect of input rate

One of the basic assumptions in pharmacokinetic-pharmacodynamic modeling (PK-PD) is that drug equilibration rate constant between plasma concentration and effect (Ke0) is not changed by input rate. To test this assumption in a clinical setting, a 25 micrograms/kg i.v. dose of doxacurium was administered either by bolus injection or 10-min infusion to 15 anesthetized patients. Neuro-muscular function was monitored using train-of-four stimulation of the ulnar nerve. For the short infusion dose, arterial concentrations were measured at I-min intervals during infusion and at frequent intervals thereafter. Following the iv bolus dose, the early PK profile of doxacurium was investigated by measuring doxacurium arterial concentrations every 10 sec during the first 2 min and at frequent intervals thereafter. PK-PD modeling was performed using nonparametric approach with and without including a finite receptor concentration (Rtot) in the effect compartment. Kinetic parameters were unchanged. For the bolus and the infusion, Ke0 values were 0.053 +/- 0.006 and 0.056 +/- 0.009 min-1, respectively. Using the Rtot model, corresponding Ke0 values were 0.148 +/- 0.016 and 0.150 +/- 0.024, respectively. The relatively faster Ke0 obtained with the Rtot model is compatible with the high potency of doxacurium. Our results show that PK-PD parameters derived with either a bolus or an infusion mode of administration are equally reliable.

Anticonvulsant pharmacodynamics and disposition of triazolam in rats

Triazolam (TZ) is a triazolobenzodiazepine used in the treatment of insomnia that possesses significant anticonvulsant properties. Despite the widespread use of this drug, detailed pharmacokinetic-pharmacodynamic information is lacking, especially with respect to inhibition of seizure activity. TZ disposition has been described previously by methods with limited specificity, and the concentration-anticonvulsant effect relationship has not been characterized. The current studies were undertaken to examine TZ disposition with a specific HPLC method, and to evaluate the relationship between anticonvulsant effect and concentration in Sprague-Dawley rats. TZ pharmacokinetics were characterized after bolus or infusion administration; in a separate experiment, TZ pharmacodynamics were assessed with pentylenetetrazol-induced seizures. The systemic disposition of TZ could be described with a two-compartment model; systemic clearance ranged from 2.45 to 5.30 L/h/ kg, steady-state volume of distribution ranged from 2.10 to 4.02 L/kg, and mean residence time ranged from 47 to 65 min. The concentration-effect relationship was well described by a simple Emax model: Emax, expressed as the ratio of post-TZ to pre-TZ threshold convulsant doses of pentylenetetrazol, was 9.9 +/- 0.7, and the EC50 values were 10.0 +/- 4.6 ng/mL and 34.8 +/- 9.0 ng/g in serum and whole brain tissue, respectively. Under single-dose conditions, TZ is a very potent anticonvulsant in the rat pentylenetetrazol seizure model.

Pharmacokinetic origin of carbamazepine-induced resistance to vecuronium neuromuscular blockade in anesthetized patients

BACKGROUND

Patients receiving chronic carbamazepine therapy have shortened recovery times from a neuromuscular block induced by vecuronium. The current study investigates the pharmacokinetic or pharmacodynamic mechanisms responsible for this observation.

METHODS

Pharmacokinetics and pharmacodynamics of 0.1 mg/kg intravenous bolus vecuronium in ten epileptic patients receiving chronic carbamazepine therapy were compared to that of ten control subjects. All patients were scheduled for neurosurgery while anesthetized with isoflurane and sufentanil. Arterial blood samples were collected for 6 h. Plasma vecuronium concentrations were measured by high-performance liquid chromatography coupled to electrochemical detection. The adductor pollicis force of contraction was recorded after supramaximal ulnar nerve stimulation. Plasma vecuronium concentrations were fitted to a two-compartment pharmacokinetic model, and the effect compartment equilibration rate constant was derived with a nonparametric link model. The effect compartment concentrations were fitted to a sigmoid Emax model. Results were compared using Student's t-test for independent samples.

RESULTS

In the carbamazepine group, the mean recovery times to T(1) 25% were shorter (28.1 +/- 3.4 vs. 47.3 +/- 5.1 min in control subjects; P=0.007), and the T(1) 25% to T(1) 75% recovery index was decreased (7.6 +/- 1.2 vs. 21.9 +/- 6.8 min in control subjects; P=0.025). No changes in onset times were observed. Clearance was 9.0 +/- 1.2 ml x kg-1 x min-1 versus 3.8 +/- 0.3 in the control group (P=0.003), whereas no changes in volumes of distribution at steady-state were observed. Therefore, the mean residence time was halved (17.8 +/- 2.5 vs. 31.9 +/- 2.5 min in control subjects; P=0.001). No differences in the effect compartment equilibration rate constant, vecuronium effect compartment concentration present at a 50% block (EC50), or slope of the sigmoid between the two groups were found.

CONCLUSIONS

The twofold increase in clearance provides evidence of a pharmacokinetic origin to the carbamazepine-vecuronium interaction; however, the possibility of a concurrent pharmacodynamic alteration cannot be assessed. Greater knowledge of protein drug binding needs to be acquired to give a meaningful interpretation to the similar EC50 values observed in the two groups.

Pharmacokinetics and anticonvulsant effect of a new hypnotic, CL 284,846, in rats

PURPOSE

CL 284,846 (CL846) is an investigational non-benzodiazepine agent with hypnotic, anxiolytic, myorelaxant and anticonvulsant properties. This study assessed the pharmacokinetics and anticonvulsant action of CL846 in female Sprague-Dawley rats.

METHODS

CL846 pharmacokinetics were examined after either an iv bolus dose (2.5 mg/kg) or a 6-hr infusion (0.4 mg/kg/hr). CL846 pharmacodynamics were evaluated with a pentylenetetrazol (PTZ) infusion 5 min after a CL846 in bolus dose (0 to 10 mg/kg). CL846 and the derived metabolite CL 284,859 (CL859) concentrations in serum and brain tissue were determined by HPLC with fluorescence detection.

RESULTS

Both the steady-state volume of distribution (1636 +/- 162 and 1804 +/- 293 ml/kg, after bolus and infusion administration, respectively) and systemic clearance (19.1 +/- 7.1 and 22.2 +/- 4.3 ml/min/kg for bolus and infusion administration, respectively) were high. No differences in pharmacokinetic parameters were noted between the two modes of administration. The relationship between anticonvulsant effect and brain/serum concentrations was well described by an Emax model. CL846 was as effective as triazolam in antagonizing PTZ-induced seizures.

CONCLUSIONS

Under the conditions of the present study, CL846 pharmacokinetics were linear and stationary. Further evaluation of the anticonvulsant properties of CL846 is warranted, including the potential development of tolerance, which is well known for benzodiazepines.

In vitro plasma protein binding of neuromuscular blocking agents in different subpopulations of patients

In vitro protein binding of several neuromuscular blocking agents (NMBAs) was measured by ultrafiltration in plasma from patients susceptible to demonstrate changes in their protein constituents. First, the relationship between the free fraction of atracurium and plasma lipoproteins levels in young volunteers (22-32 yr old, n = 6) and hyperlipidemic patients (44-68 yr old, n = 13) was studied, and second, the free fraction of atracurium, mivacurium, doxacurium, and vecuronium was determined in plasma of healthy young (27-47 yr old, n = 10), elderly (72-89 yr old, n = 11) and obese (21-57 yr old, n = 9, 200%-360% ideal body weight) patients scheduled for elective surgery. In hyperlipidemic patients, atracurium free fraction was significantly less than in young volunteers (40% +/- 5% vs 50% +/- 5%, mean +/- SD), and decreased as total cholesterol, low-density lipoprotein cholesterol and triglycerides increased (P < 0.05). In young individuals, NMBA free fraction was 25% +/- 5% for vecuronium, 58% +/- 8% for doxacurium, 52% +/- 6% for atracurium, and 72% +/- 3%, 70% +/- 3%, 70% +/- 4% for mivacurium trans-trans, cis-trans, and cis-cis isomers, respectively. Higher triglyceride concentrations in obese patients and lower high-density-lipoprotein cholesterol concentrations in both obese and elderly patients were observed when compared with young subjects. However, there was no significant difference in protein binding of NMBAs among these three groups. We conclude that, in otherwise healthy patients, age and weight are not likely to alter the free fraction of NMBAs.

High-performance liquid chromatographic assays with fluorometric detection for mivacurium isomers and their metabolites in human plasma

Two high-performance liquid chromatographic assays coupled with fluorometric detection have been developed for the determination of mivacurium isomers (trans-trans, cis-trans and cis-cis) and their monoester and alcohol metabolites in human plasma. A novel solid-phase extraction procedure allowed good recovery of the mivacurium isomers (mean 98%) and their monoester metabolites (mean 83%), whereas the alcohol metabolites were analyzed after direct precipitation of plasma proteins. For all analytes, these assays proved to be sensitive (LOQ 3.9-15.6 ng/ml), reproducible (C.V. < 15%) and accurate (> 94%) over the therapeutic range of concentrations of mivacurium and its metabolites. These two methods were applied successfully to a pharmacokinetic study of mivacurium after a bolus dose of 0.15 mg/kg in anesthetized patients.

Bretylium decreases and verapamil increases defibrillation threshold in pigs

BACKGROUND

Patients with ischemic heart disease may require antianginal and/or antiarrhythmic regimes. These patients may also be candidates for implantable defibrillators. The effects of antiarrhythmics, such as bretylium, or calcium antagonists, such as verapamil, nifedipine, or diltiazem on internal defibrillation efficacy have been inconsistent or are unknown.

METHODS AND RESULTS

The effects of bretylium and verapamil on the energy requirements for ventricular defibrillation threshold (DFT) were determined in 92 open-chest anesthetized pigs. Triplicate DFTs were determined before and after intravenous administration of saline or one of four doses of verapamil, or saline or one of three doses of bretylium, in a balanced random order. Bretylium elicited a dose dependent reduction of DFT (F = 2.72 at 3 degrees and 36 degrees of freedom). DFT was significantly reduced with the highest dose of bretylium, (from 5.9 +/- 0.6 J to 4.7 +/- 0.6 J, mean +/- S.E.M.; P < 0.01). However, cardiac massage was sometimes needed at this dose due to low blood pressure immediately after defibrillation. In contrast, there was a positive correlation between DFT and serum verapamil concentration (r = 0.54, P < 0.001). The highest dose of verapamil significantly increased DFT (from 6.3 +/- 0.6 J to 8.2 +/- 1.1 J; P < 0.05), at a serum verapamil concentration of 86.6 +/- 6.8 ng/mL.

CONCLUSIONS

These data indicate that bretylium decreases while verapamil increases the minimum energy requirement for internal defibrillation. Caution is warranted in patients who may be hemodynamically comprised and may be candidates for bretylium therapy or in patients who have marginal DFT value who might be candidates for verapamil therapy.

Determination of midazolam and two of its metabolites in human plasma by high-performance liquid chromatography

A high-performance liquid chromatographic assay coupled with UV detection (254 nm) has been developed for the determination of midazolam and two of its hydroxylated metabolites, 1-hydroxymidazolam (1-OH) and 4-hydroxymidazolam (4-OH), in human plasma. Following a novel solid-phase extraction procedure, midazolam and its metabolites are well recovered from plasma. The analytes were extracted with C1 cartridges and the extracts were evaporated to dryness. The dry residues were dissolved in 200 microliters of mobile phase [0.02 M ammonium phosphate monobasic buffer-methanol-acetonitrile (60:35:5, v/v) (300 ml), 600 microliters of 0.2 M tetrabutylammonium bromide solution, adjusted to a pH* (apparent pH) of 4.10]. The separation of the analytes was performed on a Spherisorb C8 column (10 cm x 4.6 mm I.D.) maintained at 30 degrees C. The mobile phase was pumped at a flow-rate of 1.5 ml/min. The method has a lower limit of quantitation of 15 ng/ml of plasma for midazolam and proved to be reproducible (inter-assay precision 5.4%) and accurate (94 +/- 5%) over the therapeutic range of concentrations.

Importance of early blood sampling on vecuronium pharmacokinetic and pharmacodynamic parameters

The effect of early blood sampling on the description of the vecuronium pharmacokinetic-pharmacodynamic relationship was studied following a bolus injection. Sample collection every 10 sec during the first 2 min showed a high concentration peak at 30 to 40 sec, accounting for an important proportion of the total area under the plasma concentration-time curve (AUC). Neglecting it, using only blood samples drawn at 1 and 2 min (limited sampling), led to a significant overestimation of noncompartmentally derived values of mean residence time, clearance, volume of distribution at steady-state and rate of transfer of vecuronium into the effect compartment. Compartmental pharmacokinetics could not be applied to concentration-time curves constructed with early samples, but limited sampling data were fitted to a 2-compartment model. Derived compartmental pharmacokinetic and pharmacokinetic-pharmacodynamic parameters were similar to those obtained noncompartmentally with complete sampling every 10 sec, because back-extrapolation to time zero contributed to the increase in the AUC. However, compartmental analysis does not provide an accurate description of concentration changes following injection.

Influence of aging on the pharmacokinetics and pharmacodynamics of doxacurium

Doxacurium (30 micrograms/kg) pharmacokinetics and pharmacodynamics were evaluated in nine elderly (age range, 70 to 83 years) and nine young (age range, 19 to 39 years) patients under nitrous oxide-isoflurane anesthesia. The force of contraction of the adductor pollicis was monitored and plasma samples were collected for an 8-hour period. In the elderly group, doxacurium elimination half-life was prolonged (119.7 versus 75.9 minutes) and plasma clearance was significantly reduced (1.75 versus 2.54 ml/min/kg) without any change in volume of distribution. Onset (12.9 versus 8.9 minutes) and recovery times (113.4 versus 48.1 minutes) were longer in the elderly group. The equilibrium rate constant to the effect compartment (kco) was decreased in the elderly (0.039 versus 0.051 min-1), whereas the effect compartment concentration at 50% block was similar in both groups (44.7 versus 54.1 ng/ml). An age-related reduction in muscle blood flow may be responsible for the decrease in kco. The pharmacokinetic changes observed in the elderly are consistent with a decreased function in the organs of elimination.

Simple, rapid and selective method using high-performance liquid chromatography for the determination of bretylium in plasma

A high-performance liquid chromatographic method with ultraviolet detection has been developed for the determination of bretylium in plasma. Following a single-step solid-phase extraction procedure, bretylium is selectively isolated and well recovered from plasma. The assay sensitivity is 0.156 micrograms/ml from 250-microliters plasma samples and its linearity was assessed up to 40 micrograms/ml. The method is accurate (101.0 +/- 5.4%) and precise (maximum coefficient of variation of 8%). It provides a simple and time-saving alternative to existing methods and is particularly suitable for pharmacokinetic studies.

High-performance liquid chromatographic determination of spironolactone and its metabolites in human biological fluids after solid-phase extraction

A simple and sensitive high-performance liquid chromatographic procedure to determine spironolactone and its three major metabolites in biological specimens is described. The assay involves sequential extraction on C18 and CN solid phases, and subsequent separation on a reversed-phase column. In plasma samples, spironolactone and its metabolites were completely separated within 8 min using an isocratic mobile phase, while in urine samples a methanol gradient was necessary to achieve a good separation within 14 min. Recoveries for all analytes were greater than 80% in plasma and 72% in urine. Linear responses were observed for all compounds in the range 6.25-400 ng/ml for plasma and 31.25-2000 ng/ml for urine. The plasma and urine methods were precise (coefficient of variation from 0.8 to 12.5%) and accurate (-12.1% to 7.4% of the nominal values) for all compounds. The assay proved to be suitable for the pharmacokinetic study of spironolactone in healthy human subjects.

High-performance liquid chromatography—electrochemical detection of vecuronium and its metabolites in human plasma

A high-performance liquid chromatographic assay coupled with electrochemical detection has been developed for the determination of vecuronium and its three putative deacetylated metabolites in human plasma. A novel solid-phase extraction procedure allowed good recovery of both vecuronium and its metabolites, together with ease and speed of execution. This method was sensitive, reproducible and accurate over the therapeutic range of concentrations of vecuronium and its metabolites, and was applied successfully to a study of the pharmacokinetics of vecuronium in anaesthetized patients.

Pharmacokinetics of sufentanil in normal children

The pharmacokinetic variables of sufentanil were studied in 20 healthy children between two and eight years of age. The plasma concentrations of sufentanil were measured for up to 480 min after administration of a bolus of sufentanil, 1-3 micrograms.kg-1. The distribution half-life (t1/2 alpha) was 5.2 +/- 2.2 (mean +/- SD) min and the elimination half life (t1/2 beta) was 97.0 +/- 42.0 min. The volume of distribution at steady state (Vdss) was 2.9 +/- 0.6 L.kg-1 and the clearance was 30.5 +/- 8.8 ml.kg-1.min-1. The Vdss was one and a half times greater than that reported in adults when expressed as a function of body weight but similar to that of adults when expressed as a function of body surface area. According to our results, the clearance of sufentanil in normal children between two and eight years of age is twice as rapid as that described in adults and adolescents. A greater clearance of sufentanil in children suggests that they would require relatively greater maintenance doses than adults.

Residual curarization in the neonate after caesarean section

The transplacental transfer and the neonatal effects of atracurium 0.3 mg.kg-1 (ED95) were compared with those of d-tubocurarine at the usual clinical dose of 0.3 mg.kg-1 (ED90) in 46 patients undergoing elective Caesarean section. The atracurium group (25 patients) was similar to the d-tubocurarine group (21 patients) as far as age, parity and time intervals between precurarization, induction, skin incision, muscle relaxant administration, hysterotomy and birth. The transplacental transfer of atracurium was lower than that of d-tubocurarine, with a feto-maternal ratio of 9 +/- 3% for atracurium and 12 +/- 5% for d-tubocurarine (P less than 0.05). The transplacental transfer of laudanosine was low at 14 +/- 5%, with blood levels of 0.101 +/- 0.032 microM.L-1 in the umbilical vein. Newborns in the two groups were comparable in terms of Apgar scores at one, five and ten minutes, as well as for NACS scores (neurological and adaptive capacity scoring test) at two and 24 hours after birth. However, at 15 min after birth, only 55% of newborns in whom the mothers received atracurium had a normal NACS score (greater than or equal to 35/40) compared with 83% of newborns in whom the mothers received d-tubocurarine (P less than 0.05). Further analysis of the five variables related to active muscle tone revealed that the modal score for active extension of the neck of newborns from the atracurium group was lower than for newborns from the d-tubocurarine group (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and pharmacodynamics of atracurium obtained with arterial and venous blood samples

To determine the influence of sampling site on atracurium pharmacokinetic-pharmacodynamic relationships, blood was drawn simultaneously from the radial artery and peripheral vein during a 20-minute period after injection of atracurium, 0.2 mg/kg, in eight patients. Atracurium and laudanosine concentrations were measured by HPLC. Neuromuscular blockade was measured at the adductor pollicis, after stimulation of the ulnar nerve. Venous levels were lower than corresponding arterial values for up to 20 minutes, and this difference was marked for the early samples. Neuromuscular blockade was maximum after 5 to 7 minutes, much later than the peak venous concentration (1 to 3 minutes). Nonparametric analysis yielded (mean +/- SEM) a rate constant, concentration for 50% blockade, and slope of the effect-concentration relationship of 0.092 +/- 0.01 min-1, 379 +/- 27 ng/ml, and 7.3 +/- 1.67, respectively, when based on arterial samples. The values were statistically different (0.135 +/- 0.011 min-1, 235 +/- 42 ng/ml, and 3.41 +/- 0.37, respectively) when venous levels were used (p less than 0.05). It is concluded that forearm venous levels do not correspond to adductor pollicis neuromuscular blockade and the kinetics and kinetic-dynamic relationship for atracurium are heavily dependent on sampling site.

Pharmacokinetics and pharmacodynamics of atracurium with and without previous suxamethonium administration

Suxamethonium increases neuromuscular block produced by non-depolarizing agents administered subsequently. To determine if this effect has a pharmacokinetic or pharmacodynamic origin, 18 ASA physical status I or II adults received atracurium 0.2 mg kg-1, with (n = 10) or without (n = 8) previous injection of suxamethonium 1 mg kg-1, during a thiopentone-nitrous oxide-isoflurane (0.5% end-tidal) anaesthetic. Arterial blood samples were obtained and plasma atracurium concentration measured by HPLC. Train-of-four stimulation was applied to the ulnar nerve and the force of contraction of the adductor pollicis muscle was recorded. Mean (SEM) volume of distribution was slightly greater with previous suxamethonium (143 (13) ml kg-1) than without (109 (5) ml kg-1) (P less than 0.04). Mean elimination half-life was unaffected (20.3 (0.8) min and 20.4 (1.6) min, respectively). Neuromuscular block was more intense and recovery was slower with previous administration of suxamethonium. Atracurium concentration at 50% block (Cpss50) was 305 (30) ng ml-1 with and 454 (25) ng ml-1 without previous suxamethonium (P less than 0.01). It is concluded that suxamethonium may be associated with a slight increase in the volume of distribution of atracurium, but this effect is more than compensated by a decrease in atracurium concentration required for a given effect.

Potentiation of atracurium neuromuscular blockade by enflurane: time-course of effect

This study was designed to determine the time required for potentiation of atracurium neuromuscular blockade after the introduction of enflurane. Ten ASA physical status I and II adults anesthetized with thiopental, nitrous oxide, and alfentanil were given 0.4 mg/kg atracurium besylate. The force of contraction of the adductor pollicis muscle in response to train-of-four stimulation of the ulnar nerve was recorded. When the first twitch (T1) of the train-of-four recovered to 10% of control, an atracurium infusion was started and adjusted to keep the level of blockade constant. After 15 min of stable blockade, 1.6%-1.7% end-tidal enflurane was started and maintained for up to 2 h. Venous blood samples were drawn and plasma atracurium concentrations were measured 15 min before and 0, 5, 10, 15, 30, 45, 60, 90, and 120 min after the introduction of enflurane. Atracurium plasma concentrations were 730 +/- 127 (SEM) ng/mL at time 0. During the first 30 min, no significant decrease in plasma levels occurred; but at 45 min, concentrations were only 67% +/- 8% of their initial value (P less than 0.01) and 48% +/- 2% at 120 min (P less than 0.01). This suggests that the interaction between enflurane and atracurium is time-dependent. Clinically, the interaction between atracurium and enflurane is negligible during procedures of less than 45 min.

Isolation of 6-mercaptopurine in human plasma by aluminum ion complexation for high-performance liquid chromatographic analysis

A sample preparation technique and a high-performance liquid chromatographic method for 6-mercaptopurine (6-MP) that is simple, sensitive and without interference from its metabolites is described. 6-Thioguanine (6-TG) is added as an internal standard to the plasma sample, which is then treated with an aqueous solution of aluminum perchlorate to denature the plasma proteins and form complexes with 6-TG, 6-MP and its major metabolite, 6-thiouric acid (6-TUA). These complexes coprecipitate with proteins on centrifugation. 6-MP and its analogues are then extracted from the precipitate with perchloric acid containing sodium hydrosulfite and the extract is chromatographed on an Ultrasphere ODS column eluted with 0.1 M phosphoric acid and 0.001 M dithiothreitol in deionized water. The eluate is monitored at 340 nm. No interfering peak was encountered in over 300 clinical plasma samples. 6-TUA was separated from 6-MP and was found to be present in much higher concentration than 6-MP itself throughout the sampling time (6 h) following oral administration of the drug.

Determination of atracurium and laudanosine in human plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method coupled with fluorometric detection has been developed for the determination of atracurium and its major end-product laudanosine in human plasma. The method enables good separation of atracurium from its metabolites after direct precipitation of plasma proteins. The assay is sensitive, reproducible and linear for atracurium concentrations ranging from 31.25 to 8000 ng/ml. In a clinical setting, drugs commonly administered during anesthesia did not interfere with the assay. This method provides a simple and time-saving alternative to existing methods.

Influence of extreme obesity on the body disposition and neuromuscular blocking effect of atracurium

The pharmacokinetics and pharmacodynamics of atracurium, a nondepolarizing neuromuscular blocking agent, were compared between morbidly obese patients and nonobese patients. Atracurium besylate (0.2 mg/kg) was administered intravenously as a bolus to patients who had received anesthesia. The force of contraction of the adductor pollicis was measured and plasma samples were collected for a 2-hour period. The concentrations of atracurium and its major end product, laudanosine, were determined by use of a chromatographic method. The pharmacokinetic-pharmacodynamic relationship was characterized by use of several models. No difference was observed between obese patients and nonobese patients in atracurium elimination half-life (19.8 +/- 0.7 versus 19.7 +/- 0.7 minutes), volume of distribution at steady state (8.6 +/- 0.7 versus 8.5 +/- 0.7 L), and total clearance (444 +/- 29 versus 404 +/- 25 ml/min). However, if values were expressed on a total body weight basis, there was a difference between obese and nonobese patients in the volume of distribution at steady state (0.067 versus 0.141 L/kg) and total clearance (3.5 +/- 0.2 versus 6.6 +/- 0.5 ml/min/kg). Although atracurium concentrations were consistently higher in obese patients than in nonobese patients, there was no difference in the time of recovery from neuromuscular blockade between the two groups. Consequently, the median effective concentration was higher in obese than in nonobese patients (470 +/- 46 versus 312 +/- 33 ng/ml).

Influence of food on the bioavailability of diltiazem and two of its metabolites following the administration of conventional tablets and slow-release capsules

The influence of food on the bioavailability of a conventional tablet and of a slow-release capsule of diltiazem was investigated in two separate groups of 24 healthy volunteers in two open crossover studies. Diltiazem, as a conventional tablet (2 x 30 mg, first group) or as a slow-release capsule (120 mg SR, second group), was administered in a fasting condition and 30 min after a breakfast of 784 kcal (23 per cent proteins, 55 per cent lipids, and 22 per cent of carbohydrates). Multiple blood samples were withdrawn during the next 24 h and diltiazem, desmethyldiltiazem, and deacetyldiltiazem were assayed by HPLC. Neither the rate of absorption, assessed by the rate constant of absorption, the peak plasma concentration, and the time required to reach the peak, nor the amount of drug reaching the systemic circulation, assessed by the area under the plasma concentration time curve (AUC infinity) were influenced by food, and that independently of the formulation. Compared to the fasting experiment, food did not affect either the rate of formation or the AUC infinity of desmethyldiltiazem or deacetyldiltiazem. The results of the present study show that the relative bioavailability of the single dose of diltiazem administered as a slow-release capsule is significantly higher (69 per cent) than that estimated after the administration of diltiazem in a conventional tablet. It was concluded that food does not influence the bioavailability of diltiazem administered as a conventional tablet or as a slow-release formulation.