APPLICATION OF A GENERIC PHYSIOLOGICALLY BASED PHARMACOKINETIC MODEL TO THE ESTIMATION OF XENOBIOTIC LEVELS IN HUMAN PLASMA

Estimation of xenobiotic kinetics in humans frequently relies upon extrapolation from experimental data generated in animals. In an accompanying paper, we have presented a unique, generic, physiologically based pharmacokinetic model and described its application to the prediction of rat plasma pharmacokinetics from in vitro data alone. Here we demonstrate the application of the same model, parameterized for human physiology, to the estimation of plasma pharmacokinetics in humans and report a comparative evaluation against some recently published predictive methods that involve scaling from in vivo animal data. The model was parameterized through an optimization process, using a training set of in vivo data taken from the literature, and validated using a separate test set of published in vivo data. On average, the vertical divergence of the predicted plasma concentrations from the observed data, on a semilog concentration-time plot, was 0.47 log unit. For the training set, more than 80% of the predicted values of a standardized measure of the area under the concentration-time curve were within 3-fold of the observed values; over 70% of the test set predictions were within the same margin. Furthermore, in terms of predicting human clearance for the test set, the model was found to match or exceed the performance of three published interspecies scaling methods, all of which showed a distinct bias toward overprediction. We conclude that the generic physiologically based pharmacokinetic model, as a means of integrating readily determined in vitro and/or in silico data, is potentially a powerful, cost-effective tool for predicting human xenobiotic kinetics in drug discovery and risk assessment.

The determination of biperiden in plasma using gas chromatography mass spectrometry: pharmacokinetics after intramuscular administration to guinea pigs

A gas chromatographic-mass spectrometric (GC-MS) method has been developed for the analysis of the biperiden from plasma. The method utilizes 290 microl of plasma and a simple hexane extraction/clean-up procedure. Standard curves were linear over the range of 1.9-250 ng/mL. The range of correlation coefficients for the individual standard curves was 0.9984-0.9999; the largest coefficient of variation expressed as a percentage (% CV) was 11.5%. Precision and accuracy were examined by assessing between-day and within-day variability. For between-day precision, the % CVs ranged from 2.86 to 5.17%. Accuracy as expressed by percentage error ranging from -2.16 to 5.83%. The study for within-day precision demonstrated % CVs from 0.95 to 5.55% with accuracy from -3.37 to 2.45%. Applicability of the method was demonstrated by examining the pharmacokinetics of intramuscular (i.m.) biperiden as an anticonvulsant treatment in a guinea pig model for organophosphate (OP)-induced seizure activity. Mean pharmacokinetic parameter estimates were similar to literature values; selected mean pharmacokinetic parameter estimates were: apparent volume of distribution, 13.9 L/kg; half-life of elimination, 93 min; time to maximal plasma concentration, 27.4 min; and maximal plasma concentration, 32.22 eta g/mL. The time to maximal plasma concentration was found to be similar to the onset time for terminating OP-induced seizure activity in guinea pigs receiving biperiden as an anticonvulsant treatment. The studies indicate that the method affords the required precision, accuracy and sensitivity to assay biperiden at the doses utilized for these pharmacokinetic studies after i.m. administration to guinea pigs.

Comparison of central and peripheral pharmacologic effects of biperiden and trihexyphenidyl in human volunteers

In this double-blind, randomized study, indices of central (memory, sedation) and peripheral (salivation, ratio of R-R interval on electrocardiogram) muscarinic function were evaluated in 14 healthy volunteers who received trihexyphenidyl, biperiden, and placebo. Additionally, serum drug levels were obtained 2 hours after oral administration. All subjects participated in three study sessions. During each session, subjects received two doses of biperiden (4 mg), trihexyphenidyl (5 mg), or placebo, and four series of tests were administered. The tests included the determination of cardiac response to standing (R-R ratio), mouth salivation, finger-tapping speed, digit span (forward and backward), a selective reminding task, and visual analog scales (VAS). On the VAS, subjects rated biperiden as significantly more sedating than either trihexyphenidyl or placebo, and both biperiden and trihexyphenidyl were associated with more dizziness than was placebo. Saliva production was significantly reduced by both trihexyphenidyl and biperiden compared with placebo. Digit span performance was significantly decreased in only the backward direction. The selective reminding task revealed highly significant decrements in the number of words recalled and consistent long-term retrieval after both biperiden and trihexyphenidyl. Delayed recall was significantly decreased by both active drugs. Both trihexyphenidyl and biperiden caused a significant increase in the R-R ratio comparison with placebo. With the exception of the VAS measurement of sedation, the effects caused by biperiden and trihexyphenidyl did not differ. The results of this study do not support the hypothesis that the side effect profile of biperiden is significantly different from that of trihexyphenidyl.

Muscarinic receptor occupancy by biperiden in living human brain

Anticholinergic drug is often used to treat extrapyramidal symptoms. We measured muscarinic cholinergic receptor (mAchR) occupancy by the oral administration of biperiden in eight healthy subjects using positron emission tomography (PET) and [11C]N-methyl-4-piperidylbenzilate (NMPB). After the baseline scan each subject underwent one or two post-dose PET scans. mAchR occupancy was 10-45% in the frontal cortex three hours after the oral administration of 4 mg of biperiden. The occupancy correlated with the plasma concentration of biperiden in a curvilinear manner.

Influence of ammonium chloride on the tissue distribution of anticholinergic drugs in rats

Ammonium chloride (NH4Cl) increases lysosomal pH and thereby abolishes intralysosomal accumulation of drugs. Its effect on the tissue distribution of biperiden and trihexyphenidyl in rats has been investigated. The tissue-plasma concentration ratios (Kp) of these drugs in various tissues were determined by infusion studies at steady-state in the presence or absence of NH4Cl. Treatment with NH4Cl reduced the Kp values for both drugs, causing the largest reduction in Kp in the lung (52.1 to 11.8 for biperiden and 59.5 to 18.9 for trihexyphenidyl; ratios of decrease 0.77 and 0.68, respectively), followed by the heart and kidneys, with relatively small reductions in the brain, gut, muscle and fat. Subcellular fractionation studies in the lung indicated that the subcellular fraction-plasma concentration ratio of each drug at the steady state (K(p,sf)) was reduced by treatment with NH4Cl, with the largest decrease in the post-nuclear fraction (ratio of decrease 0.82 for biperiden and 0.74 for trihexyphenidyl), followed by the nucleus, microsomes and supernatant, in that order. A strong correlation was found between the ratio of decrease in K(p,sf) after NH4Cl treatment and the specific activity of acid phosphatases, a marker of lysosomes, in each fraction (biperiden, r = 0.948; trihexyphenidyl, r = 0.945). These results suggest that acidic organelles contribute significantly to the distribution kinetics of anticholinergic drugs.

Biperiden and haloperidol plasma levels and extrapyramidal side effects in schizophrenic patients

Anticholinergic drugs such as biperiden are used for the treatment of extrapyramidal side effects (EPS) induced by neuroleptics such as haloperidol. The effects of biperiden and haloperidol plasma levels on EPS were studied in 29 chronically ill schizophrenics. The results show relationships between biperiden dose and biperiden plasma levels (BPL), and between BPL and haloperidol plasma levels (HPL). Neither BPL nor HPL seem to influence EPS.

A capillary gas-liquid chromatographic method for the assay of the neuroleptic drug zotepine in human serum or plasma

A capillary gas-liquid chromatographic method suitable for the assay of the atypical neuroleptic drug zotepine in human serum or plasma was developed. A liquid-liquid extraction with three subsequent extraction steps was applied for sample preparation. The minimum detectable concentration was 1.0 ng ml-1. The within-day relative standard deviation (RSD) (n = 6) was 5.3% at 5 ng ml-1, 3.6% at 10 ng ml-1 and 6.1% at 100 ng ml-1. The day-to-day RSD (n = 6) was 9.3% at 10 ng ml-1 and 5.1% at 100 ng ml-1. Steady-state serum levels of four schizophrenic patients were measured.

Brain regional pharmacokinetics of biperiden in rats

The pharmacokinetic profiles of biperiden (BP) in blood and in specific brain regions were investigated in rats after acute i.v. administration. The regional brain-to-blood unbound concentration ratios (Kpf) were also determined after 16 h intravenous infusion of BP. The Kpf values ranged from 30 to 75 in the different brain regions and showed decreasing concentrations in the following order: pons + medulla oblongata, basal ganglia, amygdala, hypothalamus, thalamus, mesencephalon, bulbus olfactorius + septum, hippocampus, frontal cortex, occipital cortex, cerebellum. The relationship between BP and acetylcholine (ACh) concentrations in the brain regions was examined. ACh levels in the various brain regions ranged from 8 to 44 ng g-1 tissue. There was a significant correlation between the Kpf values of BP and the levels of ACh in the brain regions except for the pons + oblongata. BP concentrations in the brain regions after BP administration were predicted based on the physiological pharmacokinetics. There was reasonable agreement between the model predictions and the observed data.

A simple but highly sensitive radioreceptor assay for the determination of scopolamine and biperiden in human plasma

Simple and sensitive methods are described for the determination of scopolamine and biperiden in human plasma. Each method consists of two steps. After extraction of scopolamine and biperiden with chloroform or n-hexane respectively and evaporation of the organic solvent, both drugs are determined by their ability to inhibit the specific binding of tritiated N-methyl-scopolamine to mouse brain homogenates. The lower limits of detection are plasma levels of about 50 pg/ml scopolamine and about 200 pg/ml biperiden.

Pharmacokinetic-dynamic study on different oral biperiden formulations in volunteers

The pharmacodynamic and kinetic profiles of two oral biperiden formulations (tablet with instant-release and sugar-coated tablet with slow-release) were studied in a total of 12 healthy subjects after the administration of a 4 mg dose and compared in part to placebo. Biperiden as slow-release formulation showed much slower absorption than the instant-release tablet; whereas plasma levels peaked as early as 1-2 h after administration of the tablet, with the slow-release formulation a first peak was seen approximately 4.5 h after intake in the majority of the subjects. In some subjects this was followed by a trough, in others by formation of a shoulder at around 0.5 ng/ml. In all subjects a peak of about 1.0 ng/ml was reached after 10-12 h. The instant-release tablet and the slow-release formulation showed comparable bioavailability. Plasma concentrations for the slow-release formulation at 24 h were around 0.5 ng/ml, about twice as high as those for the instant-release formulation. The ratio of peak to 24 h values showed the marked reduction which is characteristic for a slow-release formulation compared with the instant-release tablet. The subjective well-being of the volunteers was not influenced by the slow-release formulation, whereas after the instant-release tablet slight CNS effects (dizziness, drowsiness and fatigue) were reported up to 6 h after ingestion. Furthermore, objectively quantifiable pharmacodynamic ocular parameters (near-point accomodation, pupil size) and the secretion of saliva proved to be less affected by the slow-release formulation than by the instant-release tablet, or not affected at all.

Effects of fasting on biperiden pharmacokinetics in the rat

The effects of fasting on the pharmacokinetics of biperiden in rats were examined. Total clearance of biperiden was greater than 90% ascribable to hepatic clearance and was essentially blood-flow dependent. The number of compartments in the preferred pharmacokinetic model of biperiden changed from three (for normal rats) to two (for fasted rats). The smaller mean residence time (MRT) values found for fasted rats were attributable to decreases in distribution volume. Biperiden showed much higher lipophilicity than haloperidol, thiopental, and hexobarbital, and its tissue-to-plasma partition coefficient in adipose tissue was 20-fold higher than that in muscle. The influence of changes in volumes of adipose tissue and muscle on distribution volume (Vdss/BW) was evaluated from tissue-to-plasma partition coefficients. The value of Vdss/BW was predicted to decrease with decrease of adipose tissue, and to increase with decrease of muscle tissue. These results suggest that the observed decrease of Vdss/BW in fasted rats reflects reduced capacity to trap biperiden in the body, especially in adipose tissue. Possible clinical implications of these results are discussed.

Fundamental pharmacokinetic properties of biperiden: tissue distribution and elimination in rabbits

The pharmacokinetics of biperiden in rabbits were examined at three doses (0.2, 0.8, and 3.2 mg/kg i.v.). The data were interpreted in terms of a three-compartment open model with a linear excretion rate. The serum unbound fraction and the blood-to-plasma concentration ratio were determined as 0.39 and 1.2, respectively, over a wide concentration range (25-10000 ng/ml). Rapid and complete absorption from the injection site in muscle to the systemic circulation was observed. The bioavailability of muscular injection was unity. The hepatic extraction ratio was 0.94, and the high plasma clearance could be explained in terms of hepatic blood flow rate-limited elimination. The major tissues in which biperiden was distributed were fat and muscle. The highest tissue-to-plasma partition coefficient in the steady-state was obtained for the lung. These three tissues comprised 56% of the total distribution volume.

Pharmacokinetic and pharmacodynamic studies following the intravenous and oral administration of the antiparkinsonian drug biperiden to normal subjects

The pharmacokinetics and pharmacodynamics (changes in pupil size and salivary flow) of biperiden following a single oral and intravenous dose were investigated in six normal subjects. After the injection plasma concentrations declined biphasically, with half-times of 1.5 h for the rapid phase and 24 h for the terminal phase. Clearance and apparent volume of distribution were high (12 ml X min-1 X kg-1 and 24 l X kg-1 respectively). Absorption was rapid but the systemic availability was incomplete (33%), probably due to first-pass metabolism. Central nervous system (CNS) adverse effects and changes in pupil size were observed after both routes of administration while salivary flow was affected only by the injection.

Biperiden effects and plasma levels in volunteers

The pharmacokinetics of biperiden was studied and compared with pharmacodynamics (pupil size, accommodation, self-rating mood scale) in 6 healthy volunteers. A single-blind cross-over design was employed with placebo and biperiden (4 mg as commercially available tablets). After a lag time of 0.5 h, biperiden was rapidly absorbed with a half-life of 0.3 h, plasma peak levels of 5 ng/ml being reached after 1.5 h. Biperiden showed good tissue penetration (distribution half-life 0.6 h; ratio of total to central distribution volume 9.6), the terminal half-life time of plasma concentration was 18 h, and the oral clearance was 146 l/h. The pharmacodynamic maximum lagged behind the plasma peak concentration by 1 (self-rating) to 4 h (accommodation).