David W. Boulton

In vitro P-glycoprotein affinity for atypical and conventional antipsychotics

The transmembrane transporter P-glycoprotein (P-gp) is an ATP-dependent efflux pump for a wide range of drugs. P-gp potentially limits access to brain tissue of psychoactive substrates, but little is known about its specificity for antipsychotics. The objective of this study was to assess the affinity of some atypical antipsychotic drugs in vitro for P-gp as indicative of their potential as P-gp substrates in vivo. The activity of P-gp towards four atypical and two conventional antipsychotics and a proven substrate, verapamil, was examined by their P-gp ATPase activity, a putative measure of P-gp affinity. The Michaelis-Menten equation was fitted to the data. The rank order of the ratio V(max) / K(m) was: verapamil (2.6) > quetiapine (1.7) > risperidone (1.4) > olanzapine (0.8) > chlorpromzaine (0.7) > haloperidol (0.3) = clozapine (0.3). The atypical antipsychotics quetiapine and risperidone were relatively good P-gp substrates, although their affinities were not as high as verapamil. Olanzapine showed intermediate affinity and clozapine showed the least affinity of the drugs studied. These results suggest that P-gp is likely to influence the access to the brain of all of the atypical antipsychotics studied to various degrees. In vivo studies are needed to confirm these findings.

Extensive binding of the bioflavonoid quercetin to human plasma proteins

Although the bioflavonoids, a large group of polyphenolic natural products, exert chemopreventive effects in cardiovascular disease and cancer, there is little information about the disposition of these dietary components in man. The objective of this study was to investigate the plasma‐protein binding of the most abundant bioflavonoid, quercetin, using 14C‐labelled quercetin.

Effect of St. John's wort (Hypericum perforatum) on cytochrome P-450 2D6 and 3A4 activity in healthy volunteers

The effects of the herb St. Johns wort (Hypericum perforatum), a purported antidepressant, on the activity of cytochrome P-450 (CYP) 2D6 and 3A4 was assessed in seven normal volunteers. Probe substrates dextromethorphan (2D6 activity) and alprazolam (3A4 activity) were administered orally with and without the co-administration of St. Johns wort. Urinary concentrations of dextromethorphan and dextrorphan were quantified and dextromethorphan metabolic ratios (DMRs) determined. Plasma samples were collected (0-60 hrs) for alprazolam pharmacokinetic analysis sufficient to estimate tmax, Cmax, t 1/2, and AUC. Validated HPLC methods were used to quantify all compounds of interest. No statistically significant differences were found in any estimated pharmacokinetic parameter for alprazolam or DMRs. These results suggest that St. Johns wort, when taken at recommended doses for depression, is unlikely to inhibit CYP 2D6 or CYP 3A4 activity.

Fate of the flavonoid quercetin in human cell lines: chemical instability and metabolism

Although cell cultures are increasingly being used as models for studying the biological actions of flavonoids, no information on the fate, such as uptake and metabolism, exists for these natural products in these models.

Pharmacokinetics of the Dipeptidyl Peptidase 4 Inhibitor Saxagliptin in Rats, Dogs, and Monkeys and Clinical Projections

Saxagliptin is a potent, selective, reversible dipeptidyl peptidase 4 (DPP4) inhibitor specifically designed for extended inhibition of the DPP4 enzyme and is currently under development for the treatment of type-2 diabetes. The pharmacokinetics of saxagliptin were evaluated in rats, dogs, and monkeys and used to predict its human pharmacokinetics. Saxagliptin was rapidly absorbed and had good bioavailability (50–75%) in the species tested. The plasma clearance of saxagliptin was higher in rats (115 ml/min/kg) than in dogs (9.3 ml/min/kg) and monkeys (14.5 ml/min/kg) and was predicted to be low to moderate in humans. The plasma elimination half-life was between 2.1 and 4.4 h in rats, dogs, and monkeys, and both metabolism and renal excretion contributed to the overall elimination. The primary metabolic clearance pathway involved the formation of a significant circulating, pharmacologically active hydroxylated metabolite, M2. The volume of distribution values observed in rats, dogs, and monkeys (1.3–5.2 l/kg) and predicted for humans (2.7 l/kg) were greater than those for total body water, indicating extravascular distribution. The in vitro serum protein binding was low (≤30%) in rats, dogs, monkeys, and humans. After intra-arterial administration of saxagliptin to Sprague-Dawley and Zucker diabetic fatty rats, higher levels of saxagliptin and M2 were observed in the intestine (a proposed major site of drug action) relative to that in plasma. Saxagliptin has prolonged pharmacodynamic properties relative to its plasma pharmacokinetic profile, presumably due to additional contributions from M2, distribution of saxagliptin and M2 to the intestinal tissue, and prolonged dissociation of both saxagliptin and M2 from DPP4.

Pharmacokinetics and pharmacodynamics of methadone enantiomers after a single oral dose of racemate

The pharmacokinetics and dynamics of methadone are characterized by high interindividual variability. This study aimed to examine a number of factors that may contribute to this variability.

Differential time course of cytochrome P450 2D6 enzyme inhibition by fluoxetine, sertraline, and paroxetine in healthy volunteers.

The selective serotonin reuptake inhibitors (SSRIs) paroxetine, sertraline, and fluoxetine have varying degrees of potency in inhibiting the hepatic cytochrome P450 (CYP) 2D6 enzyme. However, the time course for maximum inhibition to occur or for inhibition to dissipate when dosing is discontinued, requires clarification. In an open label, parallel group study of 45 healthy volunteers, the time course of CYP2D6 inhibition of the above SSRIs was evaluated. Subjects were randomized to receive paroxetine at 20 mg/day for 10 days; sertraline at 50 mg/day for 3 days, followed by sertraline at 100 mg/day for 10 days; or fluoxetine at 20 mg/day for 28 days. CYP2D6 activity was assessed using the dextromethorphan metabolic ratio (DMR) on antidepressant days 5 and 10 for sertraline and paroxetine and at weekly intervals for fluoxetine. Following SSRI discontinuation, calculation of a CYP2D6 inhibition half-life (t½inh) revealed the time course of fluoxetine inhibition (t½inh = 7.0 ± 1.5 days) to be significantly longer than either paroxetine (t½inh = 2.9 ± 1.9) or sertraline (t½inh = 3.0 ± 3.0) (p < 0.01), but the latter were not significantly different from each other (p > 0.05). Time for the extrapolated DMR versus time loglinear plots to return to baseline was significantly different between fluoxetine (63.2 ± 5.6 days) and both paroxetine (20.3 ± 6.4 days) and sertraline (25.0 ± 11.0 days) (p < 0.01), making the rank order (from longest to shortest) of time for CYP2D6 inhibition to dissipate: fluoxetine > sertraline ≥ paroxetine. Differences between mean baseline DMR values and measured values obtained after drug discontinuation for each drug group became nonsignificant on discontinuation day 5 for both paroxetine and sertraline and on discontinuation day 42 for fluoxetine. These data define the time course of a persistent effect that fluoxetine, sertraline, and paroxetine have on CYP2D6 following drug discontinuation and should be considered when initiating therapy with a CYP2D6 substrate.

The pharmacokinetics of levosalbutamol: what are the clinical implications?

Salbutamol (albuterol) is a β2-adrenoceptor agonist used as a bronchodilator for the treatment of asthma and as a uterine relaxant for the suspension of premature labour. Salbutamol has been marketed as a racemic mixture, although β2-agonist activity resides almost exclusively in the (R)-enantiomer. The enantio-selective disposition of salbutamol and the possibility that (S)-salbutamol has adverse effects have led to the development of an enantiomerically pure (R)-salbutamol formulation known as levosalbutamol (levalbuterol).Salbutamol is metabolised almost exclusively by sulphotransferase (SULT) 1 A3 to an inactive metabolite. (R)-Salbutamol is metabolised up to 12 times faster than (S)-salbutamol. This leads to relatively higher plasma concentrations of (S)-salbutamol following all routes of administration, but particularly following oral administration because of extensive metabolism by the intestine. Enantiomer concentrations are similar for the first hour following an inhaled dose, reflecting the fact that salbutamol in the lung probably undergoes little metabolism. Subsequently, (S)-salbutamol predominates due to absorption and metabolism of the swallowed portion of the inhaled dose. Following oral or inhaled administration of enantiomerically pure salbutamol, a small amount (6%) is converted to the other enantiomer, probably by acid-catalysed racemisation in the stomach.Tissue binding of salbutamol is not enantioselective and plasma protein binding is relatively low. Both enantiomers are actively excreted into the urine. Compared with healthy individuals, patients with asthma do not have substantially different pharmacokinetics of the salbutamol enantiomers, but they do appear to have less drug delivered to the lung following inhaled administration because of their narrowed airways.Levosalbutamol elicits an equal or slightly larger response than an equivalent dose of the racemic mixture. This is probably due to competitive inhibition between the enantiomers at β-adrenoceptors. Pharmacokinetic-pharmacodynamic relationships for levosalbutamol show relatively large interindividual variations. Functionally significant genetic polymorphisms have been identified for β2-adrenoceptors, SULT1 A3 and organic action transporters, all of which affect the disposition or action of levosalbutamol.Animal, in vitro and some clinical studies have reported deleterious effects of (S)-salbutamol on smooth muscle contractility or lung function. However, well-designed clinical studies in patients with asthma have failed to find evidence of significant toxicity associated with (S)-salbutamol. The clinical consequences of relatively higher plasma concentrations of (S)-salbutamol following administration of racemate remain unclear, but in the absence of clear evidence of toxicity the clinical superiority of levosalbutamol over racemic salbutamol appears to be small.

A high-performance liquid chromatography assay with ultraviolet detection for olanzapine in human plasma and urine

Olanzapine is a commonly used atypical antipsychotic medication for which therapeutic drug monitoring has been proposed as clinically useful. A sensitive method was developed for the determination of olanzapine concentrations in plasma and urine by high-performance liquid chromatography with low-wavelength ultraviolet absorption detection (214 nm). A single-step liquid-liquid extraction procedure using heptane-iso-amyl alcohol (97.5:2.5 v/v) was employed to recover olanzapine and the internal standard (a 2-ethylated olanzapine derivative) from the biological matrices which were adjusted to pH 10 with 1 M carbonate buffer. Detector response was linear from 1-5000 ng (r2>0.98). The limit of detection of the assay (signal:noise=3:1) and the lower limit of quantitation were 0.75 ng and 1 ng/ml of olanzapine, respectively. Interday variation for olanzapine 50 ng/ml in plasma and urine was 5.2% and 7.1% (n=5), respectively, and 9.5 and 12.3% at 1 ng/ml (n=5). Intraday variation for olanzapine 50 ng/ml in plasma and urine was 8.1% and 9.6% (n=15), respectively, and 14.2 and 17.1% at 1 ng/ml (n=15). The recoveries of olanzapine (50 ng/ml) and the internal standard were 83 +/- 6 and 92 +/- 6% in plasma, respectively, and 79 +/- 7 and 89 +/- 7% in urine, respectively. Accuracy was 96% and 93% at 50 and 1 ng/ml, respectively. The applicability of the assay was demonstrated by determining plasma concentrations of olanzapine in a healthy male volunteer for 48 h following a single oral dose of 5 mg olanzapine. This method is suitable for studying olanzapine disposition in single or multiple-dose pharmacokinetic studies.

Single-dose pharmacokinetics of methylphenidate in CYP2D6 extensive and poor metabolizers.

Six adults phenotyped as either extensive (N = 4) or poor (N = 2) metabolizers for cytochrome P450 (CYP) 2D6 were given a 10-mg oral dose of methylphenidate (MPH) on two separate occasions with and without quinidine, a potent CYP2D6 inhibitor. Quinidine had no significant effect on the pharmacokinetics of either MPH or ritalinic acid, its major metabolite, in either group of CYP2D6 metabolizers. These data suggest a lack of involvement of CYP2D6 in the metabolism of MPH. Drugs that are inhibitors of CYP2D6 when taken concurrently with MPH should not affect its plasma concentration.