L. P. C. Delbressine

Polar Molecular Surface as a Dominating Determinant for Oral Absorption and Brain Penetration of Drugs

AbstractPurpose. To study oral absorption and brain penetration as a function of polar molecular surface area. Methods. Measured brain penetration data of 45 drug molecules were investigated. The dynamic polar surface areas were calculated and correlated with the brain penetration data. Also the static polar surface areas of 776 orally administered CNS drugs that have reached at least Phase II efficacy studies were calculated. The same was done for a series of 1590 orally administered non-CNS drugs that have reached at least Phase II efficacy studies. Results. A linear relationship between brain penetration and dynamic polar surface area (Å2) was found (n = 45, R = 0.917, F1,43 = 229). Brain penetration decreases with increasing polar surface area. A clear difference between the distribution of the polar surface area of the 776 CNS and 1590 non-CNS drugs was found. It was deduced that orally active drugs that are transported passively by the transcellular route should not exceed a polar surface area of about 120 Å2. They can be tailored to brain penetration by decreasing the polar surface to <60−70 Å2. This conclusion is supported by the inverse linear relationship between experimental brain penetration data and the dynamic polar surface area of 45 drug molecules. Conclusions. The polar molecular surface area is a dominating determinant for oral absorption and brain penetration of drugs that are transported by the transcellular route. This property should be considered in the early phase of drug screening.

Clinical pharmacokinetics of mirtazapine.

Mirtazapine is the first noradrenergic and specific serotonergic antidepressant (‘NaSSA’). It is rapidly and well absorbed from the gastrointestinal tract after single and multiple oral administration, and peak plasma concentrations are reached within 2 hours. Mirtazapine binds to plasma proteins (85%) in a nonspecific and reversible way.The absolute bioavailability is approximately 50%, mainly because of gut wall and hepatic first-pass metabolism. Mirtazapine shows linear pharmacokinetics over a dose range of 15 to 80mg. The presence of food has a minor effect on the rate, but does not affect the extent, of absorption. The pharmacokinetics of mirtazapine are dependent on gender and age: females and the elderly show higher plasma concentrations than males and young adults. The elimination half-life of mirtazapine ranges from 20 to 40 hours, which is in agreement with the time to reach steady state (4 to 6 days). Total body clearance as determined from intravenous administration to young males amounts to 31 L/h. Liver and moderate renal impairment cause an approximately 30% decrease in oral mirtazapine clearance; severe renal impairment causes a 50% decrease in clearance.There were no clinically or statistically significant differences between poor (PM) and extensive (EM) metabolisers of debrisoquine [a cytochrome P450 (CYP) 2D6 substrate] with regard to the pharmacokinetics of the racemate. The pharmacokinetics of mirtazapine appears to be enantioselective, resulting in higher plasma concentrations and longer half-life of the (R)-(−)-enantiomer (18.0±2.5h) compared with that of the (S)-(+)-enantiomer (9.9±3.1h). Genetic CYP2D6 polymorphism has different effects on the enantiomers. For the (R)-(−)-enantiomer there are no differences between EM and PM for any of the kinetic parameters; for (S)-(+)-mirtazapine the area under the concentration-time curve (AUC) is 79% larger in PM than in EM, and a corresponding longer half-life was found.Approximately 100% of the orally administered dose is excreted via urine and faeces within 4 days. Biotransformation is mainly mediated by the CYP2D6 and CYP3A4 isoenzymes. Inhibitors of these isoenzymes, such as paroxetine and fluoxetine, cause modestly increased mirtazapine plasma concentrations (17 and 32%, respectively) without leading to clinically relevant consequences. Enzyme induction by carbamazepine causes a considerable decrease (60%) in mirtazapine plasma concentrations. Mirtazapine has little inhibitory effects on CYP isoenzymes and, therefore, the pharmacokinetics of coadministered drugs are hardly affected by mirtazapine.Although no concentration-effect relationship could be established, it was found that with therapeutic dosages of mirtazapine (15 to 45 mg/day), plasma concentrations range on average from 5 to 100 μg/L.

Metabolism of three pharmacologically active drugs in isolated human and rat hepatocytes: Analysis of interspecies variability and comparison with metabolism in vivo

1. The metabolism of the three drugs (Org GB 94, Org 3770 and Org OD 14) was studied in isolated human and rat hepatocytes. The metabolic profiles in rat and human hepatocytes were compared with the available in vivo data in both species. 2. All three drugs were metabolized extensively under the conditions used, both in human and rat hepatocytes, showing both extensive phase I and II metabolism. 3. During 3-h incubation with rat hepatocytes the three compounds were metabolized completely, whereas incubation with human hepatocytes only resulted in partial metabolism, amounting for 58% (Org GB 94), 36% (Org 3770) and 94% (Org OD 14) of the dose. In addition, rat hepatocytes excreted relatively more of the formed metabolites than human hepatocytes. 4. For both species, the metabolites formed in the isolated cells were quite similar to those found in vivo. With respect to Org GB 94 and Org 3770, metabolites were detected in man in vivo and in isolated human hepatocytes that were not found in any of the animal species studied previously. 5. The reflection of interspecies differences in isolated hepatocytes, with respect to both metabolite profiles and human-specific metabolites, renders isolated human hepatocytes a very valuable tool during preclinical drug development.

A comparison of the physicochemical and biological properties of mirtazapine and mianserin

Although the chemical structures of the antidepressants mirtazapine and mianserin are closely related there are considerable differences in their biological properties. To find an explanation of this, various physicochemical properties of mirtazapine and mianserin were measured or calculated.

Fondaparinux Sodium Is Not Metabolised in Mammalian Liver Fractions and Does Not Inhibit Cytochrome P450-Mediated Metabolism of Concomitant Drugs

AbstractObjective: To investigate the in vitro metabolism of the antithrombotic agent fondaparinux sodium in mammalian liver fractions and to evaluate its potential inhibitory effect on human cytochrome P450 (CYP)-mediated metabolism of other drugs. Methods: Metabolism was evaluated by incubating radioisotope-labelled fondaparinux sodium with postmitochondrial liver fractions of rat, rabbit, monkey or human origin (three subjects). Human liver microsomal preparations and an NADPH-generating system were incubated with phenacetin, coumarin, tolbutamide, S-mephenytoin, bufuralol, chlorzoxazone or nifedipine. These are selectively metabolised by CYP isoforms: CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 or CYP3A4, respectively. Experiments were designed to determine apparent Ki (inhibitory constant) values for fondaparinux sodium against each CYP isoform, by varying concentrations of fondaparinux sodium and the selective substrate. Each experiment included control reaction mixtures containing an isoform-selective inhibitor. After incubation, the mixtures were analysed by LC-MS/MS or with fluorometric detection. Results: All liver fractions were enzymatically active, as demonstrated by degradation of [14C]testosterone. No metabolism of fondaparinux sodium was detectable in postmitochondrial liver fractions. Apparent Ki values for fondaparinux sodium against the CYP isoforms could not be determined because the oxidative metabolism of the isoform-selective CYP substrates was not significantly inhibited in pooled microsomal reaction mixtures. In the presence of selective CYP inhibitors, metabolism of each substrate was significantly reduced, confirming that inhibition could be observed in these assays. Conclusion: The demonstrated lack of mammalian hepatic metabolism of fondaparinux sodium is consistent with animal and human studies. The absence of inhibition of the human CYP isoforms commonly involved in the metabolism of drugs suggests that clinical treatment with fondaparinux sodium is unlikely to interfere with the pharmacokinetics and metabolism of a wide range of other drugs which are associated with CYP inhibition.

The clinical relevance of preclinical data: mirtazapine, a model compound.

This paper discusses how in vitro and preclinical in vivo studies might be of help for the interpretation and prediction of possible clinically relevant effects. The examples given refer to the data obtained with mirtazapine, a novel antidepressant with a dual mechanism of action, which can be best summarized as a noradrenergic and specific serotonergic antidepressant. Preclinical data on mirtazapine have shown that (i) its binding to plasma proteins is relatively low and non-specific; (ii) the contribution of its metabolites to the pharmacologic effect is negligible; (iii) it possesses high bioavailability, resulting in a low variance between individuals; (iv) it has no inducing or inhibiting effects on hepatic P450 enzymes; (v) it has a very low potential for clinically relevant pharmacokinetic interactions with other drugs; and (vi) its disposition is independent of polymorphic CYP2D6 activity. The available preclinical data on mirtazapine could be used to advise clinicians and to guide clinical practice.

Biotransformation of mianserin in laboratory animals and man

1. The biotransformation and excretion of the antidepressant mianserin were studied after oral administration of the labelled drug to rats, mice, rabbits, guinea pigs and humans. Mianserin was well absorbed and almost completely metabolized in all five species. 2. Major metabolic pathways of mianserin were p-oxidation of the N-substituted aromatic ring followed by conjugation, and oxidation and demethylation of the N-methyl moiety, followed by conjugation. Direct conjugation of the N-methyl moiety was observed as a metabolic pathway specific for man. 3. Conjugated metabolites were isolated by h.p.l.c. and identified by 1H-n.m.r. and FAB spectrometry. Novel metabolites such as an N-O-glucuronide in the guinea pig and an N-sulphonate in rat and guinea pig, were identified using these techniques. A quaternary N-glucuronide was found only in man.

Antiarrhythmic, Electrophysiological and Haemodynamic Effects of Prolonged Oral Dosing with Org 7797 in the Anaesthetized Rat

Abstract— The antiarrhythmic, electrophysiological and haemodynamic effects of chronic oral administration of Org 7797 ((16α,17β)‐17‐methylamino‐oestra‐1,3,5(10)‐triene‐3,16‐diol‐(Z)‐2‐butonedioate) were studied in rats. During dosing (10 mg kg−1 twice a day for 10 days) no effects on the electrocardiogram, monitored in conscious animals, were observed despite modest reductions (15–18%) in the maximum rate of depolarization of papillary muscle excised 1 or 6 h after completion of the dosing regime. Following anaesthesia, Org 7797 reduced the severity of arrhythmias induced by coronary artery occlusion and prevented the accompanying decrease in the ventricular fibrillation threshold (VFT) at 1 h after completion of dosing. By 6 h the effect on VFT had waned but protection against ischaemia‐induced arrhythmias was retained despite a substantial decrease in Org 7797 plasma levels. Drug treatment did not modify arterial blood pressure, heart rate or stroke volume. We conclude that Org 7797 given chronically via the oral route exerts antiarrhythmic actions which may, at least in part, be due to sodium‐channel block. In addition, our results suggest the presence of an active metabolite. The protective effects of Org 7797 were seen in the absence of electrocardiographic or haemodynamic changes suggesting that multiple oral doses of Org 7797 do not compromise normal cardiac function.

Disposition kinetics and bioavailability of the glucosidase inhibitor N-benzyl-1-deoxynojirimycin after various routes of administration in mice

AbstractPharmacokinetics, biodistribution, and excretion of the α-glucosidase inhibitor and antiviral compound N-benzyl-1-deoxynojiri-mycin (BndNM) were studied in mice, after intravenous, subcutaneous, and oral administration of a single radiolabeled dose. No metabolites were detected in plasma, urine, or feces extracts. BndNM, a lipophilic derivative of 1-deoxynojirimycin (dNM), showed a biexponential plasma decay with an initial half-life (t1/2) of 9 min and an apparent terminal t1/2 of 62 min. The mean bioavailability was 89% after subcutaneous and on the average 82% after oral dosing to fed mice. Decay curves of BndNM in plasma and total blood coincided exactly for all routes of administration, indicating equal concentrations in plasma and blood cells. Tissue concentrations of BndNM were higher than plasma concentrations at all time points (0.5-24 h) in all collected tissues (kidney, liver, large and small intestines, and stomach) but especially in kidney, where tissue/plasma concentration ratios of ...

Antiarrhythmic and Electrophysiological Effects In-vivo of the Major Metabolite of Org 7797 Found in Canine and Rodent Liver Homogenate Preparations

Org 20781, the major metabolite of Org 7797 found in in‐vitro experiments was examined for antiarrhythmic and electrophysiological effects in‐vivo.