Theodore Crews

Simultaneous modeling of the pharmacokinetics and pharmacodynamics of midazolam and diazepam.

The pharmacokinetics and pharmacodynamics of midazolam and diazepam were compared after intravenous infusions of 0.03 and 0.07 mg/kg midazolam and 0.1 and 0.2 mg/kg diazepam on four separate occasions in 12 healthy male subjects in a randomized four‐way crossover design. The Digit Symbol Substitution Test (DSST) was used as a measure of drug effect. Subjects performed three practice tests before dosing to account for any effects caused by familiarization (“learning curve”) with the testing procedure. Pharmacokinetic and pharmacodynamic data were simultaneously fitted to a semiparametric model. In this model, a pharmacokinetic model related dose to plasma concentrations, a link model related plasma concentrations to the concentration at the effect site, and a pharmacodynamic model related the effect site concentration to the observed effect. The plasma—effect site equilibrium half‐life was approximately 2½ times longer for midazolam than for diazepam, which is in good agreement with previously published data. Based on the estimated effect site concentration at which half of the maximal effect was reached, midazolam had approximately a sixfold greater intrinsic potency than diazepam. This difference in potency was also observed in a previous study that used transformed electroencephalographic (EEG) data to assess pharmacodynamic activity. The findings reported here with a clinically relevant pharmacodynamic marker (DSST) confirm the utility of surrogate drug effect measures such as EEG. This work also shows the feasibility of conducting pharmacokinetic pharmacodynamic analysis during the drug development process.

Influence of Food on Midazolam Absorption

The influence of food on the absorption of midazolam, a new benzodiazepine derivative, was investigated in 18 healthy volunteers in a four‐way, randomized, crossover study with a one‐week washout period between treatments. Single 15‐mg oral doses of midazolam were administered one hour before, with, and one hour after a standard meal as well as under fasting conditions (control). Following serial blood sampling over the next 24‐hour period, midazolam plasma concentrations were determined by gas chromatography and mass spectrometry for pharmacokinetic evaluation. The maximum plasma concentration (Cmax), time of maximum concentration (tmax), lag time prior to absorption (tlag), area under the plasma concentration‐time curve (AUC), and elimination rate constant of midazolam and 1‐hydroxymethylmidazolam were determined. Significant changes in these parameters were not found when midazolam was taken one hour before or with a meal as compared with the control condition. Significant changes in the Cmax, tmax, and AUC parameters for both midazolam and its metabolite were seen when midazolam was ingested one hour after a meal: There was a delayed and reduced rate of absorption as well as a small reduction in the extent of absorption. Thus, ingestion of midazolam within one hour after a meal may result in a delay in the onset of the pharmacologic effect These changes may be of some clinical significance in that they may potentially delay the onset of sleep.

The pharmacokinetic-pharmacodynamic (Digit Symbol Substitution Test) relationship of flumazenil in a midazolam steady-state model in healthy volunteers.

To characterize the plasma concentration–effect relationship of flumazenil in the presence of a predefined midazolam level, a double‐blind, placebo‐controlled, randomized two‐way crossover study was conducted in nine healthy male subjects. After reaching a criterion level of midazolam‐induced depression of the Digit Symbol Substitution Test (DSST), volunteers received a dose of flumazenil (1.0 mg) or placebo over 1 minute, with the infusion of midazolam continued. Blood samples were collected, simultaneously with the DSST assessment, at predetermined intervals and were assayed for flumazenil and/or midazolam plasma concentrations. Pharmacokinetic‐pharmacodynamic modeling techniques were used to estimate the equilibration rate constant (keo) between plasma concentration and effect for flumazenil; a sigmoidal maximum‐effect model was used to relate the DSST score to the flumazenil plasma concentration. Flumazenil exhibited a rapid onset (the half‐life of equilibration between drug concentration in the blood and drug effect was 3.3 minutes) and short duration of action (the flumazenil plasma concentration causing half‐maximal effect was 7.4 ng/ml, which was reached about 1 hour after dosing). The results of this study also show the competitive nature of flumazenil as a midazolam antagonist.

Preparative HPLC for the facile isolation of drug glucuronide conjugates from crude extracts of urine

Abstract Reverse-phase preparative HPLC has been used to advantage for the isolation of crystalline quantities of drug glucuronide conjugates from crude urinary extracts. Following chronic administration of large doses of diazepam, levorphanol and hydroxyethylflurazepam to dogs, urine was collected and the water soluble drug conjugates adsorbed on a column of Amberlite XAD-2 resin. In each instance elution with methanol and solvent evaporation yielded a crude oil (approx. 3 g) which was chromatographed in one portion on either PrepPAK 500 C18 (Waters) or Magnum 40 ODS-3 (Whatman) columns using aqueous methanol solvent systems. A greater than 90% purification was achieved in this single initial chromatographic step. Employing a combination of subsequent semi-preparative HPLC steps on either C18 or silica gel columns, milligram quantities of the glucuronides of oxazepam, levorphanol and hydroxyethylflurazepam were isolated. The isolation procedures provide a general approach for obtaining milligram quantiti...

Chlordiazepoxide metabolism as related to the reduction in the aggressive behaviour of cynomolgus primates.

1. Blood levels and tissue distribution of chlordiazepoxide and its major metabolites have been correlated with observed changes in ‘aggression” and ‘activity” following oral administration of the compound to cynomolgus (Macaca fascicularis) primates.2. Distribution of 14C after a single oral dose of [2-14C] chlordiazepoxide (5 mg/kg) is indicative of rapid absorption, approximately 30% and 80% dose being absorbed by 0.25 h and 2 h respectively. Highest blood concentrations are observed 2-6 h after dosing. The relatively high content of 14C in the tissues and the corresponding low content in urine and faeces at 2-12 h after dosing are suggestive of tissue storage and/or enterohepatic recirculation.3. Analysis of blood, brain and muscle for chlordiazepoxide and its major metabolites indicate that these are present 0.25 h after dosing. Chlordiazepoxide and its N-desmethylated derivative reach peak concentrations in all three tissues 2 h after dosing while the peak for ‘lactam” is attained at 6 h. In all 3 t...

Determination of Ro 19–6327 (Lazabemide) in human plasma and urine by gas chromatography-negative chemical ionization mass spectrometry

A sensitive and specific analytical method was developed for determination of Ro 19-6327 (Lazabemide) in human plasma and urine samples to provide pharmacokinetic data from clinical trials. The new method employs a simple liquid-liquid extraction to isolate the drug from biological samples. The extract is reacted to form the trifluoroacetyl derivative of Ro 19-6327 and then analyzed by gas chromatography-negative chemical ionization mass spectrometry (GC-NCIMS). The lower limit of quantitation of the assay is 0.05 ng/ml for plasma and 5.0 ng/ml for urine, based on 1-ml aliquots. No interferences from anticoagulants, collection devices, or endogenous constituents of plasma and urine were observed. Recovery (64.3%), inter-assay precision (< 8% R.S.D.), and accuracy (> 85%) of the method were considered acceptable. The assay proved reliable enough to be automated for unattended sample analysis of approximately 50 samples daily. In an additional series of tests, Ro 19-6327 was shown to be stable under conditions that might be encountered during the analysis of samples from clinical trials.

Effect of noise on peak heights calculated using an exponentially modified gaussian peak shape model

Abstract Most computer-based methods for finding chromatographic peak heights are relatively crude, relying on finding an appropriate baseline, then measuring a maximum signal height relative to the baseline. The error in finding a precise signal height of a weak signal can be increased by noise spikes. In this article, data are presented to show that the use of the exponentially modified gaussian peak shape model can effectively increase the quality of height measurements of peaks deliberately degraded to near undiscernability by dilution.