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Biomedical subjects

J W Mandema

Publications and source records attributed to J W Mandema.

At least 19 recordsLinked to original sources
PubMed2005

Therapeutic benefit of eletriptan compared to sumatriptan for the acute relief of migraine pain--results of a model-based meta-analysis that accounts for encapsulation.

A novel model-based meta-analysis was used to quantify the dose-response relationship of sumatriptan and eletriptan for the proportion of patients that achieve migraine pain relief up to 4 h after treatment. The proportion of patients that became pain free was also evaluated. This analysis includes some unique features, allowing comparison of sumatriptan and eletriptan doses that have not been directly compared in a head to head study and also permitting comparison between the two drugs at multiple time points up to 4 h after treatment. Because the analysis allows comparison of response to blinded sumatriptan with that to marketed sumatriptan and contains timepoints as early as 0.5 h, it is especially suited to detection of possible effects of encapsulation on sumatriptan's therapeutic effectiveness and thus was employed to assess this also. Data from 19 randomized placebo controlled clinical trials were jointly analysed using a random-effects logistic regression model. The results of this analysis show a significant clinical benefit of eletriptan 40 mg compared to sumatriptan 100 mg at any point in time up to 4 h after treatment. The benefit of eletriptan 40 mg is greatest around 1.5-2 h after treatment with an absolute difference at 2 h of 9.1% (7.4-11.5%) more patients achieving pain relief and 7.3% (5.8-8.6%) more patient achieving pain free when compared to sumatriptan 100 mg. An absolute benefit of more than 5% of patients is maintained from 45 min up to 4 h after treatment for pain relief and from 1.5 h up to 4 h for pain free. Eletriptan 20 mg was superior to sumatriptan 50 mg and similar to sumatriptan 100 mg for pain relief while it was similar to sumatriptan 50 mg for pain free. The benefit of eletriptan 20 mg when compared to sumatriptan 50 mg is greatest around 1.5-2 h after treatment with an absolute difference at 2 h of 5.0% (2.9-8.1%) more patients achieving pain relief. An absolute benefit of more than 3% of patients was maintained from 1 h up to 3 h after treatment. No significant difference was found between eletriptan 20 mg and sumatriptan 50 mg for the fraction of patients that became pain free. No significant effect of encapsulation of sumatriptan was found on the time course of response up to 4 h after treatment when compared to commercial sumatriptan.

Capsules↗
PubMed2000

Quantification of pharmacodynamic interactions between dexmedetomidine and midazolam in the rat.

The pharmacodynamic (PD) interaction between the benzodiazepine agonist midazolam and the alpha(2)-adrenergic agonist dexmedetomidine was characterized for defined measures of anesthetic action and cardiovascular and ventilatory side effects in 33 rats. For various combinations of constant plasma concentrations of midazolam (0.1-20 microg/ml) and dexmedetomidine (0.3-19 ng/ml) obtained by target-controlled infusion, the whisker reflex (WR), righting reflex (RR), startle reflex to noise (SR), tail clamp response (TC), and corneal reflex (CR) were assessed. EEG (power in 0.5-3.5-Hz frequency band), mean arterial pressure, and heart rate were recorded continuously. Blood gas values and arterial drug concentrations were determined regularly. The nature and extent of PD interaction was quantified by the model parameter synergy (SYN < 0, antagonism; SYN = 0, additivity; and SYN > 0, synergy). With increasing drug concentrations WR was lost first, followed by RR, SR, TC, and CR. These effects were accompanied by an increase of the EEG measure. The drug interaction was synergistic for all stimulus-response measures and the degree of synergy increased with deeper levels of central nervous system depression (SYN was 7.3, 145, 560, 374, and 1490 for WR, RR, SR, TC, and CR, respectively). The cardiovascular side effects of dexmedetomidine, evaluated at similar PD endpoints, were reduced in the presence of midazolam. Ventilatory side effects were minor for all drug combinations. The nature and extent of the PD interactions were not reflected in the EEG measure.

Adrenergic alpha-Agonists↗
PubMed1999

Characterization of the pharmacodynamic interaction between parent drug and active metabolite in vivo: midazolam and alpha-OH-midazolam.

The pharmacodynamic interaction between midazolam and its active metabolite alpha-OH-midazolam was investigated to evaluate whether estimates of relevant pharmacodynamic parameters are possible after administration of a mixture of the two. Rats were administered 10 mg/kg of midazolam, 15 mg/kg of alpha-OH-midazolam, or a combination of 3.6 mg/kg of midazolam and 35 mg/kg of alpha-OH-midazolam. Increase in the 11.5- to 30-Hz frequency band of the electroencephalogram was used as the pharmacodynamic endpoint. The pharmacodynamics of midazolam and alpha-OH-midazolam after combined administration were first analyzed according to an empirical and a competitive interaction model to evaluate each model's capability in retrieving the pharmacodynamic estimates of both compounds. Both models failed to accurately estimate the true pharmacodynamic estimates of midazolam and alpha-OH-midazolam. The pharmacodynamic interaction was subsequently analyzed according to a new mechanism-based model. This approach is based on classical receptor theory and allows estimation of the in vivo estimated receptor affinity and intrinsic in vivo drug efficacy. The relationship between stimulus and effect is characterized by a monotonically increasing function f, which is assumed to be identical for midazolam and alpha-OH-midazolam. The pharmacodynamic interaction is characterized by the classical equation for the competition between two substrates for a common receptor site. This mechanism-based interaction model was able to estimate the pharmacodynamic parameters of both midazolam and alpha-OH-midazolam with high accuracy. It is concluded that pharmacodynamic parameters of single drugs can be estimated after a combined administration when a mechanistically valid interaction model is applied.

Algorithms↗
PubMed1999

Anesthetic profile of dexmedetomidine identified by stimulus-response and continuous measurements in rats.

This study characterizes the anesthetic profile of dexmedetomidine on the basis of steady-state plasma concentrations using defined stimulus-response, ventilatory, and continuous electroencephalographic (EEG) and cardiovascular effect measures in rats. At constant plasma concentrations of dexmedetomidine (range, 0.5-19 ng/ml), targeted and maintained by target-controlled infusion, the whisker reflex, righting reflex, startle reflex (to noise), tail clamp response, hot water tail-flick latency, and attenuation of heart rate (HR) increase associated with tail-flick (sympathoadrenal block) and corneal reflex, were assessed in 22 rats. EEG (power in 0.5- to 3.5-Hz frequency band), mean arterial pressure, and HR were recorded continuously. Blood gas values and arterial drug concentrations were determined regularly. The following steady-state plasma EC(50) values of dexmedetomidine (mean +/- S.E. nanograms per milliliter) were estimated: HR decrease (0.51 +/- 0.04), EEG (1.02 +/- 0.08), whisker reflex (1.09 +/- 0.10), sympathoadrenal block (1.85 +/- 0.80), mean arterial blood pressure increase (1.99 +/- 0.44), righting reflex (2.13 +/- 0.15), tail-flick latency (3.65 +/- 0.87), startle reflex (3.75 +/- 0.64), tail clamp (5.49 +/- 1.34), and corneal reflex (24.5 +/- 12.3). At the EC(50) value of tail clamp, ventilatory depression was minor. In rats, dexmedetomidine creates bradycardia, sedation/hypnosis, sympathoadrenal blocking effects, and blood pressure-increasing effects at plasma concentrations below 2.5 ng/ml. Higher plasma concentrations are needed to loose the startle reflex, tail-flick, tail clamp, and corneal reflex responses. Ventilatory depressant effects are minor. The applied EEG measure seems to reflect sedation/hypnosis but seems to have limited value to predict the deeper levels of analgesia and anesthesia of dexmedetomidine.

Analgesics, Non-Narcotic↗
PubMed1998

Relevance of arteriovenous concentration differences in pharmacokinetic-pharmacodynamic modeling of midazolam.

In the present investigation, the extent of arteriovenous concentration differences of midazolam in rats was quantified, and the consequences of these differences on the pharmacodynamic estimates were determined. The arterial concentration-effect relationships where analyzed by a traditional-effect compartment model that characterizes the delay between blood and the effect site with the rate constant k(eo). Venous concentration-effect relationships where analyzed according to the traditional model and an extended-effect compartment model that, by incorporating an additional rate constant k(vo), can characterize the delay between the arterial and venous sampling site. Significant hysteresis was observed in the arterial but not the venous concentration-effect relationships. Rate constants for k(eo), k(vo) and terminal half-life were (mean +/- S.E.M.) 0.32 +/- 0.062, 0.093 +/- 0.013 and 0.0217 +/- 0.0008 min-1, respectively, indicating the existence of significant arteriovenous concentration differences. Pharmacodynamic estimates as determined on basis of the arterial concentrations and the traditional-effect compartment model were EC50 = 104 +/- 1 ng/ml, Emax = 151 +/- 4 microV/sec and gamma = 0.83 +/- 0.06. Analysis of the venous concentration-effect relationships on basis of the traditional- or extended-effect compartment model led to similar pharmacodynamic estimates, indicating that the observed arteriovenous concentration differences did not result in biased pharmacodynamic estimates. This is due to the fact that the effect relevant elimination rate constant of midazolam is relatively small compared with its k(eo). The observed results are consistent with earlier reports based on computer simulations.

Animals↗
PubMed1998

Pharmacokinetic-pharmacodynamic modeling of the anticonvulsant and electroencephalogram effects of phenytoin in rats.

In this study a pharmacokinetic-pharmacodynamic model is proposed for drugs with nonlinear elimination kinetics. We applied such an integrated approach to characterize the pharmacokinetic-pharmacodynamic relationship of phenytoin. In parallel, the anticonvulsant effect and the electroencephalogram (EEG) effect were used to determine the pharmacodynamics. Male Wistar-derived rats received a single intravenous dose of 40 mg . kg-1 phenytoin. The increase in the threshold for generalized seizure activity (TGS) was used as the anticonvulsant effect and the increase in the total number of waves in the 11.5 to 30 Hz frequency band was taken as the EEG effect measure. Phenytoin pharmacokinetics was described by a saturation kinetics model with Michaelis-Menten elimination. Vmax and Km values were, respectively, 386 +/- 31 microg . min-1 and 15.4 +/- 2.2 microg . ml-1 for the anticonvulsant effect in the cortical stimulation model and 272 +/- 31 microg . min-1 and 5.9 +/- 0.7 microg . ml-1 for the EEG effect. In both groups, a delay to the onset of the effect was observed relative to plasma concentrations. The relationship between phenytoin plasma concentrations and effect site was estimated by an equilibration kinetics routine, yielding mean ke0 values of 0.108 and 0.077 min-1 for the anticonvulsant and EEG effects, respectively. The EEG changes in the total number of waves could be fitted by the sigmoid Emax model, but Emax values could not be estimated for the nonlinear relationship between concentration and the increase in TGS. An exponential equation (E = E0 + Bn . Cn) derived from the sigmoid Emax model was applied to describe the concentration-anticonvulsant effect relationship, under the assumption that Emax values cannot be reached within acceptable electric stimulation levels. This approach yielded a coefficient (B) of 2.0 +/- 0.4 microA . ml . microg-1 and an exponent (n) of 2.7 +/- 0.9. The derived EC50 value of 12.5 +/- 1. 3 microg . ml-1 for the EEG effect coincides with the "therapeutic range" in humans.

Animals↗
PubMed1997

Determination of dexmedetomidine in rat plasma by a sensitive [3H]clonidine radioreceptor assay.

This paper describes the development and implementation of a sensitive radioreceptor assay (RRA) for determining concentrations of dexmedetomidine, an alpha-2 adrenergic agonist with anesthetic properties, in rat plasma. Calf retina membranes were selected as the alpha-2 adrenergic receptor source, and the alpha-2 antagonist [3H]RX821002 and the alpha-2 agonist [3H]clonidine were evaluated as radioligands. We optimized the binding conditions for both radioligands and chose a radioligand for implementation in the RRA based on the characteristics of the inhibition binding curves with dexmedetomidine. The final method is based on competition between the radioligand [3H]clonidine and dexmedetomidine for high-affinity binding sites present in calf retina membranes. The assay has a coefficient of variation of 8% in the range 23.7-592 pg for 0.2 mL of plasma. This assay can be applied to pharmacokinetic-pharmacodynamic studies of dexmedetomidine.

Adrenergic alpha-2 Receptor Agonists↗
PubMed1997

The impact of arteriovenous concentration differences on pharmacodynamic parameter estimates.

In many pharmacodynamic investigations venous drug concentrations are measured and linked to effect-site concentrations by means of a traditional first-order effect-compartment model to estimate pharmacodynamic (PD) parameters. This analysis ignores the underlying physiology that arterial blood supplies both the venous sampling site and effect site. Recently, an extended effect-compartment model has been proposed that reflects physiology by postulating a first-order rate constant of equilibrium between arterial and effect-site concentrations (ke0) as well as first-order rate constant between arterial and venous concentrations (kv0). In the current paper, we evaluate the bias in PD parameter estimates if venous drug concentrations are measured and linked to effect-site concentrations by a traditional effect compartment as a function ke0, kv0, and the drug's elimination half-life (T1/2); we present an analytical solution to the differential equations characterizing the extended effect-compartment model; and we evaluate the performance of the extended effect-compartment model to estimate pharmacodynamic parameters on the basis of venous drug concentrations. Time profiles of venous drug concentrations and drug effect were simulated for a wide range of different values of the half-life of ke0 (T1/2,e0), the half-life of kv0 (T1/2,v0), and T1/2. The simulations showed that a significant bias (up to 90%) in PD parameter estimates occurred for certain values of T1/2,e0, T1/2,v0, and T1/2 if venous drug concentrations are linked to effect-site concentrations by a traditional effect-compartment model. This model misspecification is not apparent from the results of the fitting procedure. The extended effect-compartment model provided unbiased but imprecise PD parameter estimates. The extended effect-compartment model was also able to analyze instances in which the venous concentrations equilibrate slower with the arterial concentrations than the effect-site concentrations, and proteresis is observed in the concentration--effect relationship. It is concluded that if the apparent T1/2 of the drug in the time period in which the decline in pharmacological effect is most pronounced is greater than 5 times T1/2,e0 and T1/2,e0 is greater than T1/2,v0 there is no need to model the underlying arteriovenous equilibrium delay. Under these conditions a traditional first-order link between venous and effect-site concentrations will yield accurate and reliable (less than 10% bias) estimates of the PD parameters such as Emax, EC50 and N. If T1/2 is less than 5 times T1/2,e0 or if T1/2,v0 is greater than T1/2,e0, the underlying arteriovenous equilibration delay needs to be taken into account in the model to obtain unbiased estimates of the PD parameters. This applies for almost all values of T1/2.v0. Arteriovenous equilibration delay can be best taken into account by measuring arterial blood concentrations. If this is not possible, the extended effect-compartment link model can be used. However, a large number of effect measurements needs to be obtained to estimate the model parameters accurately.

Arteries↗
PubMed1997

Influence of age and gender on the pharmacokinetics and pharmacodynamics of remifentanil. I. Model development.

BACKGROUND: Previous studies have reported conflicting results concerning the influence of age and gender on the pharmacokinetics and pharmacodynamics of fentanyl, alfentanil, and sufentanil. The aim of this study was to determine the influence of age and gender on the pharmacokinetics and pharmacodynamics of the new short-acting opioid remifentanil. METHODS: Sixty-five healthy adults (38 men and 27 women) ages 20 to 85 y received remifentanil by constant-rate infusion of 1 to 8 micrograms.kg-1.min-1 for 4 to 20 min. Frequent arterial blood samples were drawn and assayed for remifentanil concentration. The electroencephalogram was used as a measure of drug effect. Population pharmacokinetic and pharmacodynamic modeling was performed using the software package NONMEM. The influence of volunteer covariates were analyzed using a generalized additive model. The performances of the simple (without covariates) and complex (with covariates) models were evaluated prospectively in an additional 15 healthy participants ages 41 to 84 y. RESULTS: The parameters for the simple three-compartment pharmacokinetic model were V1 = 4.98 l, V2 = 9.01 l, V3 = 6.54 l, Cl1 = 2.46 l/min, Cl2 = 1.69 l/min, and Cl3 = 0.065 l/min. Age and lean body mass were significant covariates. From the ages of 20 to 85 y, V1 and Cl1 decreased by approximately 25% and 33%, respectively. The parameters for the simple sigmoid Emax pharmacodynamic model were Ke0 = 0.516 min-1, E0 = 20 Hz, Emax = 5.62 Hz, EC50 = 11.2 ng/ml, and gamma = 2.51. Age was a significant covariate of EC50 and Ke0, with both decreasing by approximately 50% for the age range studied. The complex pharmacokinetic-pharmacodynamic model performed better than did the simple model when applied prospectively. CONCLUSIONS: This study identified (1) an effect of age on the pharmacokinetics and pharmacodynamics of remifentanil; (2) an effect of lean body mass on the pharmacokinetic parameters; and (3) no influence of gender on any pharmacokinetic or pharmacodynamic parameter.

Adult↗
PubMed1997

Pharmacokinetic-pharmacodynamic characterization of the cardiovascular, hypnotic, EEG and ventilatory responses to dexmedetomidine in the rat.

This study characterizes the pharmacokinetic-pharmacodynamic (PK-PD) relationships of the cardiovascular, EEG, hypnotic and ventilatory effects of the alpha-2 adrenergic agonist dexmedetomidine in rats. Dexmedetomidine was administered by a single rapid infusion (n = 6) and by an infusion regimen of gradually increasing rate (n = 8). HR, mean arterial pressure (MAP) and EEG signals were recorded continuously, as was the time at which the rats woke up spontaneously from drug-induced sleep, a measure of hypnosis. Arterial concentrations of dexmedetomidine and blood gases were determined regularly. A sigmoidal Emax model was used to describe the HR, MAP and EEG concentration-effect relationships, with the EEG effect (activity in 0.5-3.5-Hz frequency band) linked to an effect-site model. The PK of dexmedetomidine could be described by a two-compartment model, with similar PK parameters for both infusion regimens. Plasma protein binding was 84.1[0.7]%. Because of complex cardiovascular homeostatic reflex mechanisms, HR and MAP could only be analyzed during gradually increasing infusions. The maximal decrease in HR was 35(2)%, and the maximal increase in MAP was 37(2)%. For both infusion regimens, similar PD parameters were found for the EEG and the hypnotic measure. These data suggest the absence of active metabolites or tolerance of the EEG and hypnotic effects. Judging on the basis of concentrations of dexmedetomidine (mean (S.E. M.)), HR decrease was the most sensitive response [EC50 of 0.65(0. 09) ng/ml], followed by increase in MAP [EC50 of 2.01(0.14) ng/ml], change in EEG activity [EC50 of 2.24(0.16) ng/ml] and the hypnotic measure [Cwake-up of 2.64(0.10) ng/ml]. Ventilatory effects were minor.

Adrenergic alpha-Agonists↗
PubMed1996

High-performance liquid chromatographic assay of propofol in human and rat plasma and fourteen rat tissues using electrochemical detection.

This paper describes a sensitive HPLC-electrochemical detection analytical method for determining the concentration of the intravenous anesthetic, propofol, in human or rat plasma or serum and a variety of rat tissues. Internal standard and drug are extracted from serum or plasma and other tissues with pentane. 2,6-tert.-Butylmethylphenol is used as internal standard. It includes a novel steam distillation procedure for separating the highly lipophilic propofol from skin and fat. The plasma/serum assay has a precision of 1-4% (C.V.) in the range 10 ng/ml to 1 microgram/ml and permits the assay of assay of 5 ng/ml from 0.1 ml of plasma/serum. The tissue procedure allows the estimation of 50 ng/g in 0.1 g of tissue for most of the major organs with less than 2% (C.V.) precision. This assay was used to measure propofol concentrations in plasma/serum and tissue samples in support of a project to develop a physiological pharmacokinetic model for propofol in the rat.

Animals↗
PubMed1996

Population pharmacodynamic model for ketorolac analgesia.

OBJECTIVE: To derive a population pharmacokinetic-pharmacodynamic model that characterizes the distribution of pain relief scores and remedication times observed in patients receiving intramuscular ketorolac for the treatment of moderate to severe postoperative pain. BACKGROUND: The data analysis approach deals with the complexities of analyzing analgesic trial data: (1) repeated measurements, (2) ordered categorical response variables, and (3) nonrandom censoring because the patients can take a rescue medication if their pain relief is insufficient. METHODS: Patients (n = 522) received a single oral or intramuscular administration of placebo or a single intramuscular dose of 10, 30, 60, or 90 mg ketorolac for postoperative pain relief. Pain relief was measured periodically with use of a five-category ordinal scale up to 6 hours after dosing. In this period, 288 patients received additional medication because of insufficient pain relief. Pharmacokinetic data was available for 85 subjects. Models were fitted to the data with the NONMEM program. RESULTS: The pharmacokinetic data was best described by a two-compartment model with first-order absorption. Pain relief was found to be a function of drug concentration (Emax model), time (waxing and waning of placebo effect), and an individual random effect. The drug concentration at half-maximal effect (EC50) and the first-order rate constant (keo) half-life for pain relief were 0.37 mg/L and 24 minutes. The probability of remedication was found to be a function of the observed level of pain relief and was found to increase with time. Monte Carlo simulations showed that adequate pain relief was achieved in 50% of the patients at 41, 27, 23, and 21 minutes after 10, 30, 60, or 90 mg of intramuscular ketorolac. Adequate pain relief was maintained up to 6 hours in 50%, 70%, 78%, and 81% of patients after these four doses. Only 25% of the patients achieved adequate pain relief with placebo. CONCLUSIONS: A population pharmacokinetic-pharmacodynamic model for the analgesic efficacy of intramuscular ketorolac was derived. The simulated relationship between dose, time, and percentage of patients with adequate pain relief suggested that 30 mg intramuscular ketorolac was the optimal initial dose for postoperative pain relief.

Administration, Oral↗
PubMed1996

Characterization and validation of a pharmacokinetic model for controlled-release oxycodone.

1. Oxycodone is a strong opioid agonist that is currently available in immediate-release (IR) formulations for the treatment of moderate to severe pain. Recently, controlled-release (CR) oxycodone tablets were developed to provide the benefits of twice-a-day dosing to patients treated with oxycodone. The purpose of this investigation was to develop and validate a pharmacokinetic model for CR oxycodone tablets in comparison with IR oxycodone solution. 2. Twenty-four normal male volunteers were enrolled in a single-dose, randomized, analytically blinded, two-way crossover study designed to compare the pharmacokinetics of two 10 mg CR oxycodone tablets with 20 mg IR oxycodone oral solution. Pharmacokinetic models describing the oxycodone plasma concentration vs time profiles of CR tablets and IR solution were derived using NONMEM version IV. The predictive performance of the models was assessed by comparison of predicted oxycodone plasma concentrations with actual oxycodone plasma concentrations observed in a separate group of 21 volunteers who received repeated doses of IR and CR oxycodone for 4 days. 3. The unit impulse disposition function of oxycodone was best described by a one-compartment model. Absorption rate of the IR solution was best described by a mono-exponential model with a lag time, whereas absorption rate of the CR tablet was best described using a bi-exponential model. The absorption profile of the CR tablets was characterized by a rapid absorption component (t1/2abs = 37 min) accounting for 38% of the available dose and a slow absorption phase (t1/2abs = 6.2 h) accounting for 62% of the available dose. Two 10 mg tablets of oral CR oxycodone hydrochloride were 102.7% bioavailable relative to 20 mg of IR oxycodone hydrochloride oral solution. The population model derived after administration of a single dose accurately predicted both the mean and range of oxycodone concentrations observed during 4 days of repeated dosing. The mean prediction error was 2.7% with a coefficient of variation of 54%. 4. The absorption characteristics of CR oxycodone tablets should allow effective plasma concentrations of oxycodone to be reached quickly and for effective concentrations to be maintained for a longer period after dosing compared with the IR oral solution. The CR dosage form has pharmacokinetic characteristics that permit 12 hourly dosing.

Adult↗
PubMed1996

Thiopental uncouples hippocampal and cortical synchronized electroencephalographic activity.

BACKGROUND: Thiopental produces a concentration-dependent continuum of effects on the cortical electroencephalogram (EEG) that has been linked to behavioral measures of anesthetic depth. The complexity of the response, however, limits a clear insight into the neurophysiologic actions of thiopental. The current study investigated thiopental actions on cortical EEG and hippocampal electrical activity, to determine whether similar effects occur on both structures and to compare synchronized activity between these structures. METHODS: Thiopental was administered intravenously via an implanted catheter in freely moving rats. Arterial blood oxygen/carbon dioxide concentration, thiopental concentrations, and temperature were monitored and controlled. Neocortical EEG was recorded from implanted dural surface electrodes and hippocampal neuron electrical activity was recorded from stereotaxically placed microelectrodes. Pharmacokinetic models were used to determine effect site concentrations. RESULTS: Thiopental produced an increase in EEG frequency and amplitude at low concentrations (15-20 micrograms/ml total plasma, approximately 10 microM unbound), which produced a loss of righting reflex. This was followed by a frequency decrease and burst suppression activity at higher concentrations (50-80 micrograms/ml, approximately 60 microM), which produced a loss of tail pinch and corneal reflexes. Higher concentrations of thiopental ( > 60 micrograms/ml) uncoupled synchronized burst discharges recorded in hippocampus and cortex. Isoelectric EEG activity was associated with concentrations of 70-90 micrograms/ml (approximately 80 microM) and a deep level of anesthesia; motor reflexes were abolished, although cardiovascular reflexes remained. In all frequency bands, similar concentration-EEG effect relationships were observed for cortical and hippocampal signals, only differing in the magnitude of response. A reversed progression of effects was observed on recovery. CONCLUSIONS: The results confirm earlier findings in humans and animals and demonstrate that both the hippocampus and neocortex exhibit burst suppression and isoelectric activity during thiopental anesthesia. Thiopental-induced synchronized burst activity was depressed by progressively higher concentrations. The lost synchronization suggests a depression of synaptic coupling between cortical structures contributes to anesthesia.

Anesthetics, Intravenous↗
PubMed1996

Population pharmacodynamic modeling and covariate detection for central neural blockade.

BACKGROUND: In spinal anesthesia, often a large interindividual variability in analgesic response is observed after administration of a certain fixed dose of anesthetic to a patient population. To improve therapeutic outcome it is important to characterize the variability in response by means of a population model (e.g., mixed-effects models or two-stage approaches). The purpose of this investigation is to derive a population model for spinal anesthesia with plain bupivacaine. Based on the population models, a description of a patient's time course of drug action is obtained, the influence of patient covariates on clinically important endpoints is examined, and the success of Bayesian forecasting of the offset of effect in a specific patient from the data obtained during onset is evaluated. METHODS: The level of central neural blockade after intrathecal injection of plain bupivacaine was assessed by testing analgesia to pinprick. A total of 714 measurements in 96 patients (4-10 per subject) were available for analysis. Two pharmacodynamic models, based on the understanding of the physiology of the spread of local anesthetic in the spinal fluid, were evaluated to characterize the time course of analgesia in a specific patient. The first model is a combination of a biexponential pharmacokinetic model, describing the onset and offset of effect and a linear pharmacodynamic model. The second model combines the biexponential pharmacokinetic model with an Emax type pharmacodynamic model. The interindividual variability in model parameters was modeled by an exponential variance model. An additional term characterized the residual error. The population mean parameters, interindividual variance, and residual variance were estimated using the first-order conditional estimate method in the NONMEM software package. Clinically important endpoints such as onset time, time to reach the maximal level, the maximal level, and the duration of analgesia were estimated from the Bayesian fit of each subject's data and correlated with patient-specific covariates. Using Bayesian forecasting, the offset of spinal analgesia was predicted for each patient based on the population model and measurements from the first 30 min and from the first 60 min, respectively. RESULTS: The Emax type pharmacodynamic model was superior based on the improvement in likelihood (P < 0.001) and on visual inspection of the fits. The estimates of the population mean parameters (coefficient of variation) were: (1) maximal effects: T4, which was coded for the purpose of the calculation as 18 (14%); (2) rate of offset of effect: 0.0118 (26%) min-1; (3) rate of onset of effect: 0.061 (45%) min-1. The standard deviation of the residual error was 1.4. Large interindividual differences were observed in the time course of analgesic response and clinically important endpoints. The mean onset time; that is, time to reach T10 (interindividual variability) was 4.2 min (90%), the mean time to maximal level was 35.5 min (29%), the mean duration of effect was 172 min (28%), and the mean maximal achieved level was T6 (12%). Significant correlations between onset time and height and weight, between time to maximal level and age, between maximal level and weight and height, and between duration and height were found. Bayesian regression using the population model and data from the first 30 min and from the first 60 min predicted the offset of effect in each patient reasonably well, with coefficients of determination (R2) of 0.71 and 0.72. This is a significant improvement over the population mean prediction. CONCLUSION: A population model was derived for the description of the time course of central neural blockade. Based on the population model, a continuous effect profile over time was obtained for each person...

Adolescent↗
PubMed1996

Pharmacokinetic/pharmacodynamic relationship of benzodiazepines in the direct cortical stimulation model of anticonvulsant effect.

The in vivo concentration-anticonvulsant effect relationships of six benzodiazepines, midazolam, clonazepam, oxazepam, flunitrazepam, diazepam and clobazam were quantified in individual rats and correlated with the affinity to the GABAA-benzodiazepine receptor complex. Furthermore the interaction between midazolam and the benzodiazepine antagonist flumazenil was characterized. All benzodiazepines exhibited a nonlinear relationship between concentration and anticonvulsant effect without ceiling of the effect at higher concentration. The potency of most benzodiazepines was similar with values of the EC250, between 0.067 +/- 0.01 mg. l-1 for midazolam and 0.21 +/- 0.03 mg. l-1 for diazepam. The EC250,u of clobazam was 2.8 +/- 0.9 mg. l-1. These values were considerably larger than the Ki for binding at the GABAA-benzodiazepine receptor complex. No correlation was observed between EC250,u and Ki. Antagonism of the anticonvulsant effect of midazolam by flumazenil was associated with a remarkable change in the concentration-anticonvulsant effect relationship. Analysis of these data on basis of a composite model provided evidence for two separate effects of which only one is antagonized by flumazenil. The anticonvulsant effect at low midazolam concentration was characterized on basis of the sigmoid E maximal effect pharmacodynamic model. The value of the EC50,u was 0.0086 +/- 0.0013 mg. l-1 which is similar to the Ki for binding at the GABAA-benzodiazepine receptor complex. The second more pronounced anticonvulsant effect occurred at higher concentration and was described by an exponential function. The findings of this study indicate that the effect of benzodiazepines against seizures induced by cortical stimulation in vivo cannot be fully accounted for by an interaction at the GABAA-benzodiazepine receptor complex.

Animals↗
PubMed1995

Pharmacodynamic interaction between midazolam and a low dose of ethanol in vivo.

The pharmacokinetic and pharmacodynamic interactions between midazolam and ethanol were studied in the rat in vivo. Ethanol was given as a constant rate intravenous infusion (1.85 mg/min). The pharmacokinetics and pharmacodynamics of midazolam were determined following an intravenous dose of 5 mg/kg in 15 minutes. Amplitudes in the 11.5-30 Hz (beta) frequency band of the EEG was used as a measure of the pharmacological effect. Ethanol infusion resulted in a constant plasma alcohol concentration of 0.44 +/- 0.04 g/l (Mean +/- SE) and had no effect on the baseline value of the EEG effect parameter. Also the pharmacokinetics of midazolam were unchanged. However, a significant parallel shift of the midazolam concentration-EEG effect relationship to lower concentrations was observed. These findings show that there is a pharmacodynamic interaction between midazolam and ethanol in vivo.

Animals↗