Probe of CYP3A by a single-point blood measurement after oral administration of midazolam in healthy elderly volunteers.

BACKGROUND: Midazolam (MDZ) is used as an assessment of human cytochrome P450 3A (CYP3A) activity. A single blood measurement is used as a marker of its activity based on an observed correlation between MDZ clearance and the 1'-hydroxymidazolam (1'-OH-MDZ): MDZ plasma ratio is assessed at 0.5 h followig the intake of a single 7.5 mg oral dose of MDZ in healthy young volunteers. In addition, a 4-h plasma MDZ measurement has been found to be an excellent predictor of AUC and CYP3A activity. OBJECTIVES: The main aim of this study was to define a single-point blood sampling in healthy elderly volunteers. The secondary objective was to investigate the pharmacological effects of a low oral dose of MDZ (5 mg) and its potential psychometric changes. METHODS: Eight healthy elderly Caucasian volunteers participated in a single-dose, open-label, non-comparative study. Each subject received a single 5 mg oral dose of MDZ. Plasma concentrations of MDZ and its major metabolite, 1'-OH-MDZ, were assayed over 12 h. Secondary assessments of critical flicker fusion (CFF), body sway and mini-mental state examination were also carried out during the 12-h post-administration period. RESULTS: A moderate correlation was observed between MDZ clearance and the 1'-OH-MDZ: MDZ plasma concentration ratio at 9 h post-dosing (Rho=0.81; p=0.04), but an even better correlation (Rho=0.99; p<0.009) was found between MDZ AUC and MDZ plasma concentration at 6 h post-dosing, with the latter value corresponding approximately to the average mean residence time (MRT) determined in our trial. This study was well-tolerated despite a significant transitory decrease (relative to baseline) in cortical arousal at 1 h post-dosing, as assessed by CFF, and a non-significant decrease (relative to baseline) in balance and vigilance also measured at 1 h and assessed on body sway, compared to baseline values. CONCLUSION: Despite the small sample size, based on the results of healthy, elderly volunteers, a single MDZ plasma measurement taken at 6 h post-oral administration may represent an accurate marker of CYP3A phenotype. This single-time-point method could be used safely for predicting drug-drug or diet interactions and identifying individuals with genetic polymorphism that affect CYP3A activity.

Ignoring pharmacokinetics may lead to isoboles misinterpretation: illustration with the norfloxacin-theophylline convulsant interaction in rats.

PURPOSE: To investigate the norfloxacin-theophylline convulsant interaction in vivo, with an experimental approach distinguishing between pharmacodynamics and pharmacokinetics contributions to the observed effect. METHODS: Male Sprague Dawley rats (n = 38) were infused each compound separately or in different combination ratios. Infusion was maintained until the onset of maximal seizures. Cerebrospinal fluid and plasma samples were collected for high performance liquid chromatography drug determination. The nature and intensity of the pharmacodynamics interaction between drugs was quantified with an isobolographic approach. RESULTS: Isobolograms suggested a relatively marked antagonism between norfloxacin and theophylline at the cerebrospinal fluid (previously shown to be part of the biophase) and dose levels, but not at the plasma (free and total concentrations) levels. These apparent discrepancies could be explained by nonlinear distribution or/and distribution desequilibrium phenomenon. CONCLUSIONS: These findings showed that the quantitative isobolographic approach is appropriate to assess the nature and intensity of the pharmacodynamic interaction between two drugs when data are collected within the biophase, but that data interpretation outside the biophase can be risky due to further pharmacokinetic complexities, in particular slow or/and nonlinear diffusion into the biophase.

Pharmacokinetic-pharmacodynamic modeling of the electroencephalogram effect of norfloxacin in rats.

A previously developed pharmacokinetic-pharmacodynamic (PK-PD) modeling approach was used to investigate the epileptogenic activity of norfloxacin as a representative antibiotic with concentration-dependent antimicrobial activity. Rats received an intravenous infusion of norfloxacin at a rate of 5 mg kg of body weight(-1) min(-1) over 30 min. Blood samples were collected for drug assay, and an electroencephalogram (EEG) was recorded during infusion and postinfusion. An important delay was observed between concentrations of norfloxacin in plasma and the EEG effect. Indirect effect models failed to describe these data, which were successfully fitted by using an effect compartment model with a spline function to describe the relationship between effect and concentration at the effect site, as previously observed with imipenem. The robustness of the PK-PD model was then assessed by keeping the dose constant but increasing the duration of infusion to 120 and 240 min. Although this was accompanied by PK modifications, PD parameters did not vary significantly, and the PK-PD model still applied. In conclusion, the successful PK-PD modeling of the norfloxacin EEG effect in rats should be considered to predict and reduce the epileptogenic risk associated with this antibiotic as a representative fluoroquinolone (E. Fuseau and L. B. Sheiner, Clin. Pharmacol. Ther. 35:733-741, 1984).

Antidepressant response and fluvoxamine plasma concentrations: a pilot study.

The objective of this pilot study was to examine the relation between fluvoxamine (FVX) plasma concentrations, therapeutic response and side effects during a four-week treatment period. Twenty-two patients who met the DSM-IV criteria for major depression received 100 mg FVX during the first 2 days of treatment and then 150 or 200 mg/day. No clear relationship between plasma concentrations and side effects was detectable. A relationship between plasma concentrations and clinical efficacy was detectable after 21 days but not after 28 days of treatment. These preliminary results indicate that therapeutic drug monitoring might be useful for patients treated with FVX.