Simultaneous modeling of the pharmacokinetic and pharmacodynamic properties of benzodiazepines. I: Lorazepam

Population Pharmacokinetic/ Pharmacodynamic Modelling of Auditory-Evoked Event-Related Potentials with Lorazepam

Event-related potentials (ERPs) are commonly used in Neuroscience research, particularly the P3 waveform because it is associated with cognitive brain functions and is easily elicited by auditory or sensory inputs. ERPs are affected by drugs such as lorazepam, which increase the latency and decrease the amplitude of the P3 wave. In this study, auditory-evoked ERPs were generated in 13 older healthy volunteers using an oddball tone paradigm, after administration of single 0.5 and 2 mg doses of lorazepam. Population pharmacokinetics (PK)/pharmacodynamics (PD) models were developed using nonlinear mixed-effects methods in order to assess the effect of lorazepam on the latency and amplitude of the P3 waveforms. The PK/PD models showed that doses of 0.3 mg of lorazepam achieved approximately half of the maximum effect on the latency of the P3 waveform. For P3 amplitude, half the maximum effect was achieved with a dose of 1.2 mg of lorazepam. The PK/PD models also predicted an efficacious dose range of lorazepam, which was close to the recommended therapeutic range. The use of longitudinal P3 latency data allowed better predictions of the lorazepam efficacious dose range than P3 amplitude or aggregate exposure-response data, suggesting that latency could be a more sensitive parameter for drugs with similar mechanisms of action as lorazepam and that time course rather than single time-point ERP data should be collected. Overall, the results suggest that P3 ERP waveforms could be used as potential non-specific biomarkers for functional target engagement for drugs with brain activity, and PK/PD models can aid trial design and choice of doses for development of new drugs with ERP activity.

Pharmacokinetics of intravenous lorazepam in pediatric patients with and without status epilepticus

OBJECTIVE

To evaluate the single dose pharmacokinetics of an intravenous dose of lorazepam in pediatric patients treated for status epilepticus (SE) or with a history of SE.

STUDY DESIGN

Ten hospitals in the Pediatric Emergency Care Applied Research Network enlisted patients 3 months to 17 years with convulsive SE (status cohort) or for a traditional pharmacokinetics study (elective cohort). Sparse sampling was used for the status cohort, and intensive sampling was used for the elective cohort. Non-compartmental analyses were performed on the elective cohort, and served to nest compartmental population pharmacokinetics analysis for both cohorts.

RESULTS

A total of 48 patients in the status cohort and 15 patients in the elective cohort were enrolled. Median age was 7 years, 2 months. The population pharmacokinetics parameters were: clearance, 1.2 mL/min/kg; half-life, 16.8 hours; and volume of distribution, 1.5 L/kg. On the basis of the pharmacokinetics model, a 0.1 mg/kg dose is expected to achieve concentrations of approximately 100 ng/mL and maintain concentrations >30 to 50 ng/mL for 6 to 12 hours. A second dose of 0.05 mg/kg would achieve desired therapeutic serum levels for approximately 12 hours without excessive sedation. Age-dependent dosing is not necessary beyond using a maximum initial dose of 4 mg.

CONCLUSIONS

Lorazepam pharmacokinetics in convulsive SE is similar to earlier pharmacokinetics measured in pediatric patients with cancer, except for longer half-life, and similar to adult pharmacokinetics parameters except for increased clearance.

Pharmacodynamic differentiation of lorazepam sleepiness and dizziness using an ordered categorical measure

Categorical measures of lorazepam sleepiness and dizziness were modeled to identify differences in pharmacodynamic (PD) parameters between these adverse events (AEs). Differences in data-derived PD parameters were compared with relative incidence rates in the drug label (15.7% and 6.9%, respectively). Healthy volunteers (n = 20) received single oral doses of 2 mg lorazepam or placebo in a randomized, double-blind, cross-over fashion. A seven-point categorical scale measuring the intensity of AEs was serially administered over 24 h. The maximum score (MaxS), and area under the effect curve (AUEC) were determined by noncompartmental methods and compared using a paired t-test. Individual scores were modeled using a logistic function implemented in NONMEM. AUEC and MaxS for sleepiness were significantly higher than dizziness (20.35 vs. 9.76, p < 0.01) and (2.35 vs. 1.45, p < 0.01). Model slope estimates were similar for sleepiness and dizziness (0.21 logits x mL/ng vs. 0.19 logits x mL/ng), but baseline logits were significantly higher for sleepiness (-2.81 vs. -4.34 logits). Data-derived PD parameters were in concordance with label incidence rates. The higher intensity of sleepiness may be directly related to baseline (no drug present) while the increase in intensity as a result of drug was relatively similar for both AEs.

Contribution of the active metabolite M1 to the pharmacological activity of tesofensine in vivo: a pharmacokinetic-pharmacodynamic modelling approach

BACKGROUND AND PURPOSE

Tesofensine is a centrally acting drug under clinical development for Alzheimer's disease, Parkinson's disease and obesity. In vitro, the major metabolite of tesofensine (M1) displayed a slightly higher activity, which however has not been determined in vivo. The aims of this investigation were (i) to simultaneously accomplish a thorough characterization of the pharmacokinetic (PK) properties of tesofensine and M1 in mice and (ii) to evaluate the potency (pharmacodynamics, PD) and concentration-time course of the active metabolite M1 relative to tesofensine and their impact in vivo using the PK/PD modelling approach.

EXPERIMENTAL APPROACH

Parent compound, metabolite and vehicle were separately administered intravenously and orally over a wide dose range (0.3-20 mg kg(-1)) to 228 mice. Concentrations of tesofensine and M1 were measured; inhibition of the dopamine transporter was determined by co-administration of [(3)H]WIN35,428 as the pharmacodynamic measure.

KEY RESULTS

Pharmacokinetics of tesofensine and M1 were best described by one-compartment models for both compounds. Nonlinear elimination and metabolism kinetics were observed with increasing dose. The PK/PD relationship was described by an extended E(max) model. Effect compartments were used to resolve observed hysteresis. EC(50) values of M1, as an inhibitor of the dopamine transporter, were 4-5-fold higher than those for tesofensine in mice.

CONCLUSIONS AND IMPLICATIONS

The lower potency of M1 together with approximately 8-fold higher through steady-state concentrations suggest that M1 did contribute to the overall activity of tesofensine in mice.

Premedication with orally administered lorazepam in adults undergoing ERCP: a randomized double-blind study

BACKGROUND

Restlessness often complicates ERCP and may be a reason for premature termination of the procedure.

OBJECTIVE

Our purpose was to evaluate whether a premedication with orally administered lorazepam could reduce the need for sedatives and improve sedation quality.

DESIGN

Randomized double-blind trial.

SETTING

Therapeutic ERCP with an intravenous sedation containing midazolam, propofol, and S(+)-ketamine.

PATIENTS

95 inpatients (aged 20-91 years).

INTERVENTIONS

1 mg of lorazepam (n=47) or placebo (n=48) given orally before ERCP.

MAIN OUTCOME MEASUREMENT

Total amount of administered propofol.

RESULTS

Heart rate, blood pressure, number of oxygen desaturations, and states of restlessness did not differ between the groups. The same amount of midazolam was administered in both groups. There was no significant difference in the total amount of propofol to achieve adequate sedation (lorazepam vs placebo: 71+/-5 vs 63+/-4 microg/kg/min, mean+/-SE). Paradoxically, patients pretreated with lorazepam even needed more propofol in the early phase of sedation (275+/-39 vs 159+/-37 microg/kg in minutes 5-10, P<.05) and the total amount of ketamine administered was higher in this group as well (15.8+/-1.4 vs 11.3+/-1.2 microg/kg/min, P<.05). In both groups there were high rates of satisfaction with the course of the procedure evaluated both by the endoscopists and the patients.

CONCLUSION

The trial failed to show an advantage of an oral premedication with lorazepam. The amount of sedatives administered in the lorazepam group even tended to be higher. A premedication with lorazepam may be counterproductive when followed by sedation containing another benzodiazepine.

Lorazepam induces an atypical dissociation of visual and auditory event-related potentials

Lorazepam has been reported to atypically disrupt visual processing compared to other benzodiazepines (BZs), but it is not known to what extent this effect extends to impairment in other modalities. Our objective was to compare the effects of lorazepam with those of flunitrazepam, a BZ with standard effects, on visual and auditory event-related potentials (ERPs) using the same paradigm. The study followed a placebo-controlled, double-blind, parallel group-design and involved single oral doses of lorazepam (2.0 mg), flunitrazepam (1.2 mg) and placebo. Thirty-six young, healthy subjects completed a test battery before and after treatment including classic behavioural tests, visual and auditory ERPs. Both drugs led to comparable alterations on behavioural tests and double-dissociations were found, indicating that the doses used were equipotent: lorazepam was more deleterious than flunitrazepam and placebo in fragmented shape identification, while simple reaction times were prolonged for flunitrazepam in comparison to lorazepam and placebo. Effects on P3 latencies were also distinct: alterations in both modalities for flunitrazepam were equivalent and greater than placebo's. In contrast, lorazepam at the frontal and central electrode sites led to greater changes in visual than in auditory latency, and also to longer visual latencies than flunitrazepam and placebo, but lorazepam's auditory latency effects were only different to placebo's at the parietal electrode site. Peripheral visual changes were not responsible for these effects. Differences in the impairment profile between equipotent doses of lorazepam and flunitrazepam suggests that lorazepam induces atypical central visual processing changes.

Relationship between cerebral pharmacokinetics and anxiolytic activity of diazepam and its active metabolites after a single intra-peritoneal administration of diazepam in mice

The relationship between the cerebral pharmacokinetics of diazepam and its active metabolites (desmethyldiazepam, oxazepam) and the anxiolytic effect evaluated by the four-plates test and the light/dark test were investigated after a single intra-peritoneal injection of diazepam (1 mg/kg or 1.5 mg/kg). For up to 30 min after administration, the sedative effect interfered with the anxiolytic effect, thus the results of the anxiolytic effect were not interpretable. From 30 min to 60 min after administration, this interference disappeared, the cerebral level of benzodiazepines was stable (the brain elimination of diazepam was compensated for by the appearance of desmethyldiazepam followed by oxazepam) but the anxiolytic effect decreased dramatically in all the tests with diazepam 1 mg/kg or 1.5 mg/kg. The acute tolerance to benzodiazepines and the difference of affinity for subtypes of GABA(A) receptors between diazepam, desmethyldiazepam, oxazepam could explain this result.

Cortical excitability changes induced by deafferentation of the contralateral hemisphere

Short-term deprivation of sensory input by ischaemic nerve block (INB) leads to functional reorganization in the deafferented motor cortex. Here, we show that INB also elicits functional changes in homotopic regions of the cortex contralateral to the deafferented one. We measured motor evoked potential (MEP) amplitudes elicited by transcranial magnetic stimulation (TMS) in small hand and biceps brachii muscles before, during and after INB of the right hand. INB increased excitability of the cortical representation of (i) the intact hand and (ii) body parts proximal to the deafferented hand (upper arm), in the absence of excitability changes in other body part representations such as thorax or leg muscles. This effect persisted throughout the entire period of deafferentation and returned to baseline values afterward. Motor responses to brainstem electrical stimulation remained unchanged during INB, indicating that the effect is probably of cortical origin. Lorazepam, a GABA(A) receptor agonist, blocked this increased excitability. Interhemispheric inhibition between hand muscles decreased during INB. After chronic deafferentation in amputees, MEP amplitudes and motor output curves in small hand muscles were depressed and motor thresholds were elevated compared with aged-matched controls. These results indicate that acute hand deafferentation can elicit a focal increase in excitability in the hand motor representation contralateral to the deafferented cortex that is influenced by transcallosal interactions and GABAergic transmission, and is balanced in the setting of chronic deafferentation.

Modulation of human corticomotor excitability by somatosensory input

In humans, somatosensory stimulation results in increased corticomotoneuronal excitability to the stimulated body parts. The purpose of this study was to investigate the underlying mechanisms. We recorded motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) from abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles. MEP amplitudes, recruitment curves (RC), intracortical inhibition (ICI), intracortical facilitation (ICF), resting (rMT) and active motor thresholds (aMT) were recorded before and after a 2-h period of ulnar nerve electrical stimulation at the wrist. Somatosensory input was monitored by recording somatosensory evoked potentials. To differentiate excitability changes at cortical vs. subcortical sites, we recorded supramaximal peripheral M-responses and MEPs to brainstem electrical stimulation (BES). In order to investigate the involvement of GABAergic mechanisms, we studied the influence of lorazepam (LZ) (a GABA(A) receptor agonist) relative to that of dextromethorphan (DM) (an NMDA receptor antagonist) and placebo in a double-blind design. We found that somatosensory stimulation increased MEP amplitudes to TMS only in the ADM, confirming a previous report. This effect was blocked by LZ but not by either DM or placebo and lasted between 8 and 20 min in the absence of (i) changes in MEPs elicited by BES, (ii) amplitudes of early somatosensory-evoked potentials or (iii) M-responses. We conclude that somatosensory stimulation elicited a focal increase in corticomotoneuronal excitability that outlasts the stimulation period and probably occurs at cortical sites. The antagonistic effect of LZ supports the hypothesis of GABAergic involvement as an operating mechanism.

A double blind parallel group placebo controlled comparison of sedative and mnesic effects of etifoxine and lorazepam in healthy subjects [corrected]

This paper describes the psychomotor and mnesic effects of single oral doses of etifoxine (50 and 100 mg) and lorazepam (2 mg) in healthy subjects. Forty-eight healthy subjects were included in this randomized double blind, placebo controlled parallel group study [corrected]. The effects of drugs were assessed by using a battery of subjective and objective tests that explored mood and vigilance (Visual Analog Scale), attention (Barrage test), psychomotor performance (Choice Reaction Time) and memory (digit span, immediate and delayed free recall of a word list). Whereas vigilance, psychomotor performance and free recall were significantly impaired by lorazepam, neither dosage of etifoxine (50 and 100 mg) produced such effects. These results suggest that 50 and 100 mg single dose of etifoxine do not induce amnesia and sedation as compared to lorazepam.

Pharmacokinetic and pharmacodynamic analysis of sedative and amnesic effects of lorazepam in healthy volunteers

This study describes for the first time the pharmacokinetic and pharmacodynamic modeling of the psychomotor and amnesic effects of a single 2-mg oral dose of lorazepam in healthy volunteers. Twelve healthy volunteers were included in this randomized, double-blinded, placebo-controlled two-way crossover study. The effect of lorazepam was examined for a battery of tests that explored mood, vigilance, psychomotor performance, and memory. The pharmacokinetic and pharmacodynamic modeling of these tests was performed using the indirect response model. Vigilance and psychomotor performance were significantly impaired. Short-term memory was not affected, but a paradoxical tendency to improvement of the score was observed 0.4 hours after drug intake. Significant impairment was observed for immediate and delayed cued verbal recall, for immediate and delayed free recall, and for picture recognition as well as for visual-verbal recall, but not for cued visual-spatial recall or priming. Globally, the different effects were greatest between 0.4 to 3 hours after dosing. However, the time course profile of the recovery period suggests a possible dissociation between the kinetics of the effects of lorazepam on vigilance, psychomotor performance, and visual episodic memory, on the one hand, and on verbal episodic memory, on the other. The pharmacokinetic and pharmacodynamic model used two compartments with first-order absorption to describe the lorazepam concentrations and an indirect response model with inhibition or stimulation of Kin to describe the effects. The mean values for calculated median effective concentration (EC50) derived from the pharmacokinetic and pharmacodynamic modeling of the different tests ranged from 11.3 to 39.8 ng/mL. According to these EC50 values, lorazepam seemed to be more potent on the delayed-recall trials than on the immediate-recall trials; similar observations were made concerning the free-recall versus cued-recall trials. The previously stated results suggest that the tests performed in this study represent sensitive measurements of the effects of lorazepam on the central nervous system. Moreover, the parameter values derived from pharmacokinetic and pharmacodynamic modeling, especially, the EC50 values, may provide sensitive indices that can be used to compare the central nervous system effects of benzodiazepines.

Kinetics and dynamics of lorazepam during and after continuous intravenous infusion

OBJECTIVE

To evaluate the kinetics and dynamics of lorazepam during administration as a bolus plus an infusion, using electroencephalography as a pharmacodynamic end point.

METHODS

Nine volunteers received a 2-mg bolus loading dose of lorazepam, coincident with the start of a 2 microg/kg/hr zero-order infusion. The infusion was stopped after 4 hrs. Plasma lorazepam concentrations and electroencephalographic activity in the 13- to 30-Hz range were monitored for 24 hrs.

RESULTS

The bolus-plus-infusion scheme rapidly produced plasma lorazepam concentrations that were close to those predicted to be achieved at true steady state. Mean kinetic values for lorazepam were as follows: volume of distribution, 126 L; elimination half-life, 13.8 hrs; and clearance, 109 mL/min. Electroencephalographic effects were maximal 0.5 hr after the loading dose, were maintained essentially constant during infusion, and then declined in parallel with plasma concentrations after the infusion was terminated. There was no evidence of tolerance. Plots of pharmacodynamic electroencephalographic effect vs. plasma lorazepam concentration demonstrated counterclockwise hysteresis, consistent with an effect-site equilibration delay. This was incorporated into a kinetic-dynamic model in which hypothetical effect-site concentration was related to pharmacodynamic electroencephalographic effect via the sigmoid Emax model. The analysis yielded the following mean estimates: maximum electroencephalographic effect, 12.7% over baseline; 50% effective concentration, 13.1 ng/mL; and effect-site equilibration half-life, 8.8 mins.

CONCLUSION

Despite the delay in effect onset, continuous infusion of lorazepam, preceded by a bolus loading dose, produces a relatively constant sedative effect on the central nervous system, which can be utilized in the context of critical care medicine.

Pharmacokinetic-pharmacodynamic analysis of mnesic effects of lorazepam in healthy volunteers

AIMS

To describe the pharmacokinetic-pharmacodynamic modelling of the psychomotor and mnesic effects of a single 2 mg oral dose of lorazepam in healthy volunteers.

METHODS

This was a randomized double-blind, placebo-controlled two-way cross-over study. The effect of lorazepam was examined with the following tasks: choice reaction time, immediate and delayed cued recall of paired words and immediate and delayed free recall and recognition of pictures.

RESULTS

The mean calculated EC50 values derived from the PK/PD modelling of the different tests ranged from 12.2 to 15.3 ng ml-1. On the basis of the statistical comparison of the EC50 values, the delayed recall trials seemed to be more impaired than the immediate recall trials; similar observations were made concerning the recognition vs recall tasks.

CONCLUSIONS

The parameter values derived from PK/PD modelling, and especially the EC50 values, may provide sensitive indices that can be used, rather than the raw data derived from pharmacodynamic measurements, to compare CNS effects of benzodiazepines.

A double-blind comparison of lorazepam and oxazepam in psychomotor retardation and mutism

BACKGROUND

An increasing number of case reports indicate a superior therapeutic response of catatonialike symptoms, such as severe psychomotor disturbance and mutism, associated with psychiatric disorder to the benzodiazepine lorazepam (LO). Equivocal results, however, are also reported with regard to other benzodiazepines for the treatment of this syndrome. The purpose of this study was to compare the effects of LO and oxazepam (OX), benzodiazepines with comparable pharmacokinetics, on psychomotor retardation and mutism associated with psychiatric disorder.

METHODS

Twenty-one hospitalized patients with severe psychomotor retardation and mutism were treated with 2 mg LO and 60 mg OX in a double-blind crossover study design.

RESULTS

Both benzodiazepines significantly reduced psychomotor symptoms. When administered for the first time, 4 of 7 patients with LO and 6 of 10 patients with OX improved at least 50% on visual analog scale (VAS) rating. Reduction in symptoms was significant with LO and OX treatment on either day of treatment. The second time, however, LO was significantly better compared with OX in alleviating the target symptoms.

CONCLUSIONS

Both OX and LO are effective for the treatment of psychomotor retardation. Thus, the beneficial effect of LO on psychomotor retardation and mutism is not a unique pharmacodynamic property but more likely due to its pharmacokinetic profile. The differential effect of the two benzodiazepines on the second day of treatment warrants further clarification. Several hypotheses are evaluated.

Assessment of the interaction between a partial agonist and a full agonist of benzodiazepine receptors, based on psychomotor performance and memory, in healthy volunteers

Potential interactions between the imidazopyridine anxiolytic alpidem and the full benzodiazepine agonist lorazepam were assessed in a randomized, double-blind, four-way cross-over, placebo-controlled study in 16 healthy young male volunteers. Each volunteer received alpidem, 50 mg, or a placebo twice daily for 8 days with a 1- week wash-out interval. The interaction between alpidem, at the steady state, and a single oral dose of lorazepam 2 mg or a placebo was assessed after concomitant administration on days 7 or 9 of each treatment period. Psycho motor performance and cognitive function were evaluated before and 2, 4, 6 and 8 h post-dose, using objective tests [critical flicker fusion threshold (CFF), choice reaction time (CRT), digit-symbol substitution (DSST), body sway and short-term memory (Sternberg memory scanning)] and self-ratings [line analogue rating scales: (LARS)]. Long-term memory (delayed free recall and recognition of pictures) was assessed before the dose and 2 and 4 h post-dose. Pharmacodynamic interactions were evaluated by applying repeated measures ANOVA to a 2 x 2 factorial interaction model. Alpidem, 50 mg twice daily at the steady state, was free of any clinically relevant detrimental effects on skilled performance, information processing or memory. In contrast, a single 2 mg dose of lorazepam induced marked impairment of psychomotor performance and cognitive function (significant reductions in CFF and DSST and increases in CRT and body sway), as well as subjective sedation from 2 to 8 h post-dose, depending on the test used. In addition, lorazepam induced anterograde amnesia, characterized by a decrease in delayed free recall and recognition, and a deficit in short-term memory. Finally, alpidem 50 mg did not potentiate the detrimental effects of lorazepam 2 mg. On the contrary, alpidem significantly antagonized the lorazepam-induced CRT increase and anterograde amnesia, and produced similar trends on most of the other cognitive parameters; thus, the results obtained with the combination of alpidem and lorazepam consistently indicated less impairment than those measured after lorazepam alone. These results are consistent with the suggested partial agonsist properties of alpidem at the benzodiazepine receptor and indicate that such properties can be assessed in humans based on antagonism of the effects of a full agonist.

Lack of interaction between a new antihistamine, mizolastine, and lorazepam on psychomotor performance and memory in healthy volunteers

1. The possible interaction between a new H1 antihistamine, mizolastine, and lorazepam was assessed in a randomised, double-blind, cross-over, placebo-controlled study involving 16 healthy young male volunteers who received mizolastine 10 mg or placebo once daily for 8 days with a 1 week wash-out interval. The interaction of mizolastine, at steady-state, with a single oral dose of lorazepam or placebo was assessed on days 6 or 8 of each treatment period. 2. Psychomotor performance and cognitive function were evaluated using objective tests (critical flicker fusion threshold, choice reaction time, tapping, arithmetic calculation, body sway) and self-ratings (visual analogue scale, ARCI) before and at 2, 4, 6 and 8 h after dosing. Short-term memory (Sternberg memory scanning immediate free recall of a word list) and long-term memory (delayed free recall and recognition of words and pictures) were assessed before and at 3 h after dosing. Pharmacodynamic interactions were evaluated by repeated measures ANOVA in a 2 x 2 factorial interaction model. 3. Mizolastine, 10 mg once daily, at steady-state, was devoid of sedation and detrimental effect on skilled performance and memory. 4. In contrast, a single 2 mg dose of lorazepam produced marked impairment of psychomotor performance, cognitive functions (significant reduction in flicker fusion threshold, tapping and arithmetic calculation and increase in reaction times and body sway) and subjective sedation from 2 to 8 h after dosing. In addition, lorazepam induced an anterograde amnesia, characterised by a decrease in delayed free recall and recognition, and a deficit in short term memory.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the relationship between structure and CNS effects for lorazepam, clonazepam and alprazolam

Eight healthy young males were administered single doses of lorazepam (4 mg), clonazepam (4 mg), alprazolam (2 mg) or placebo, and their performance on behavioral tasks was monitored for 7 h. Lorazepam and clonazepam impaired performance on subcritical tracking, a primarily neuromotor task, for 2-4 h longer than alprazolam. Although the duration of impairment of the digit symbol substitution task was less discrepant for the three drugs, clonazepam and lorazepam still affected performance for a longer period of time than alprazolam. The rapid development of acute tolerance was indicated by clockwise hysteresis curves for all the drugs on the SCT task and for clonazepam and alprazolam on the DSS task. The maximum effect, effect offset rate and duration of the drug effect are discussed in relation to molecular structure and receptor affinity.

Modeling the pharmacokinetics and pharmacodynamics of dexamethasone in depressed patients

Changes in time course effected by cortisol suppression and the relationship of these changes to the plasma dexamethasone concentration of suppressor and non-suppressor patients are described in this report on a combined pharmacokinetic-pharmacodynamic model. Thirteen depressed patients (8 suppressors and 5 non-suppressors) received an intravenous dose (1.5 mg) of dexamethasone. The drug-induced effect changes are found to lag behind, in time, the plasma drug level changes. To accurately relate the temporal relationship of effect changes to plasma dexamethasone levels, a pharmacodynamic model (sigmoid-Emax) was combined with a pharmacokinetic model that incorporated an effect compartment. The magnitude of the time-lag was quantified by the half-time of equilibration between concentrations in the hypothetical effect compartment and the plasma dexamethasone levels (t1/2keo). The t1/2keo of the nonsuppressing group was about 50% of that of the suppressing group, indicating that for a given plasma level the onset and termination of effect for the nonsuppressing group is about two times more rapid than for the suppressing group. Moreover, the model can estimate the effect-site concentration that causes one-half of the maximal predicted effect (EC50), a measure of an individual's sensitivity to dexamethasone. The receptor sensitivity (as determined from the EC50 ratio) of the suppressing group was about twice that of the nonsuppressing group.

Drug metabolism and interactions in abuse liability assessment

The interpretation of acute abuse liability studies and drug interaction studies would be importantly strengthened by the routine inclusion of drug concentration measurements at appropriate sampling times. Reliance on mean kinetic data misrepresents the variation in drug kinetics and fails to take experimental advantage of the natural differences in the population which may represent the extremes of abuse risk. Pharmacokinetic-pharmacodynamic studies to better understand the relationship of plasma drug concentration, drug concentration in the receptor biophase and specific drug reinforced behaviour will ensure proper study design and yield useful theoretic information. Multi- and poly-drug abuse (including heavy smoking and heavy ethanol use) are very common. Such patterns of use can have quite large effects on drug kinetics. Because of the potentially large qualitative and quantitative differences in drug metabolism and kinetics between pre-clinical species and the human, data should be gathered at the earliest possible time with respect to human metabolic rates, patterns, and identification of inhibitors. The availability of human liver microsomes facilitates such studies.

Simultaneous modeling of the pharmacokinetic and pharmacodynamic properties of benzodiazepines. II. Triazolam

This study compares the time course of triazolam effects on psychomotor and cognitive skills with triazolam plasma concentrations in a combined pharmacokinetic-pharmacodynamic (sigmoid-Emax) model. Ten male subjects received a single oral dose (1 mg) of triazolam or placebo. The CNS impairment effects were measured by using computerized tracking, body sway, and digit symbol substitution tests, and triazolam plasma concentration was measured by gas chromatography. The drug-induced effect changes lagged behind the plasma drug level changes. The magnitude of the time lag was quantified by the half-time of equilibration between concentrations in the hypothetical effect compartment and the plasma triazolam levels (t 1/2 keo). Essentially the same t 1/2 keo (approximately 6 min) was found for subcritical tracking, body sway, and digit symbol substitution tests. When using the predicted drug concentrations at the effect site, the hysteresis of the plasma concentration-effect disappears, suggesting that the hysteresis is not caused by drug induced tolerance. Moreover, the model allows for estimation of the effect site concentration that causes one-half of the maximal predicted effect (EC50, approximately 5 ng/ml) which is a measure of an individual's sensitivity to triazolam. On the basis of the EC50 values of the effect measures, body sway was slightly less sensitive to triazolam than subcritical tracking and digit symbol substitution tests.