Defence mechanisms of olfactory neuro-epithelium: mucosa regeneration, metabolising enzymes and transporters.

The olfactory neuro-epithelium is highly sensitive to chemicals and its direct microbiological environment. It also plays a role as an interface between the airways and the nervous system, and so it has developed several defence instruments for rapid regeneration or for the detoxification of the immediate environment. This review illustrates three of these defence mechanisms: regeneration of the epithelium, local production of metabolising enzymes and xenobiotic transporters. Toxicants can inflict damage by a direct toxic response. Alternatively, they may require metabolic activation to produce the proximate toxicant. In addition to detoxifying inhaled and systemically derived xenobiotics, the local olfactory metabolism may fulfil multiple functions such as the modification of inhaled odorant, the modulation of endogenous signalling molecules and the protection of other tissues such as the CNS and lungs from inhaled toxicants. Finally, the permeability of nasal and olfactory mucosa is an important efficacy parameter for some anti-allergic drugs delivered by intranasal administration or inhalation. Efflux or update transporters expressed in these tissues may therefore significantly influence the pharmacokinetics of drugs administered topically.

Stereoselective renal tubular secretion of levocetirizine and dextrocetirizine, the two enantiomers of the H1-antihistamine cetirizine.

Competition for uptake and/or efflux transporters can be responsible for drug interactions. Cetirizine is mainly eliminated unchanged in urine through both glomerular filtration and tubular secretion. The aim of this study was to investigate whether the eutomer, levocetirizine, and the distomer, dextrocetirizine, have a similar tubular secretion. The renal clearance associated with tubular secretion was calculated from the renal clearance of levocetirizine and dextrocetirizine obtained in a study in healthy volunteers. The values of the unbound fraction in plasma were obtained in an in vitro study of the binding of (14)C-cetirizine and (14)C-levocetirizine to human plasma proteins using equilibrium dialysis and chiral high-performance liquid chromatography (HPLC) with on-line liquid scintillation counting. The unbound fraction was 0.074 for levocetirizine and 0.141 for dextrocetirizine. The tubular secretion of dextrocetirizine (44.5 mL/min) is higher than that of levocetirizine (23.1 mL/min), which may have consequences for drug interactions at the renal level. The higher tubular secretion for dextrocetirizine may be due to the higher free fraction available for secretion or to a higher affinity for (a) renal transporter(s) mediating the secretion pathway.

Factors affecting the relative importance of amine oxidases and monooxygenases in the in vivo metabolism of xenobiotic amines in humans.

The monooxygenases and the amine oxidases (AOs) are the major enzyme systems involved in vivo in the oxidative metabolism of xenobiotic amines in humans. With the exception of the inhibition of the metabolism of tyramine ingested by subjects taking inhibitors of MAO-A or of both MAO-A and -B, which has been extensively investigated, the involvement of the monoamine oxidases in xenobiotic amine metabolism (drugs in particular) has been largely neglected. Furthermore, with the exception of amlodipine, there have been essentially no studies on the metabolism of drug amines by amine oxidases such as SSAOs and PAOs in humans. In contrast, monooxygenases (CYP isoenzymes, and to a lesser extent, FMOs) have been extensively investigated in terms of their involvement in xenobiotic metabolism. It is possible that the contribution of AOs to the overall metabolism of xenobiotic amines in humans has been underestimated, or erroneously estimated, as most investigations of drug metabolism have been performed using in vitro test systems optimized for CYP activity, such as liver microsomes, and most investigations of drug metabolism in vivo in humans have identified only the final, stable metabolites.

Alteration of thyroid hormone homeostasis by antiepileptic drugs in humans: involvement of glucuronosyltransferase induction.

RATIONALE: The aim of this review article is to analyse which antiepileptic drugs (AEDs) alter thyroid hormone homeostasis in humans and when this can be explained, at least partially, by the induction of the glucuronoconjugation pathways. METHODS: Electronic databases were searched which have provided more than 300 articles. These have been integrated with fundamental books and personal information by experts in the different areas examined. RESULTS: Alteration of thyroid hormone homeostasis by phenobarbital/primidone, phenytoin, and carbamazepine clearly occurs in humans. However, it is not associated with thyroid-stimulating hormone (TSH) increase and the clinical significance of altered serum concentrations of thyroid hormones by these antiepileptic drugs has remained unclear. The published information on the effect of the other antiepileptic drugs examined in this review article on thyroid hormones is lacking (felbamate, pregabalin, zonisamide) or limited. Oxcarbazepine appears to have some effects. Topiramate would need further investigations as well as gabapentin. Levetiracetam, tiagabine, vigabatrine, and lamotrigine do not alter at all, or only minimally, thyroid hormone homeostasis. CONCLUSION: Concerning the antiepileptic drugs which alter thyroid hormone homeostasis, it is highly probable that the mechanism of induction of uridine diphosphate glucuronosyltransferases (UGT) is involved, at least partially, in such an alteration. However, it is not possible to estimate the relative contribution of the UGT induction by these drugs on the total alteration observed in thyroid hormone levels, as other mechanisms not investigated, or not examined in the present article, could contribute.

Differences in absorption, distribution, metabolism and excretion of xenobiotics between the paediatric and adult populations.

In children, the therapeutic benefits and potential risks associated with drug treatment may be different from those in adults and will depend on the exposure, receptor sensitivity and relationship between effect and exposure. In this paper, key factors undergoing maturational changes accounting for differences in drug metabolism and disposition in the paediatric population compared with adults are reviewed. Gastric and duodenal pH, gastric emptying time, intestinal transit time, secretion and activity of bile and pancreatic fluid, bacterial colonisation and transporters, such as P-glycoprotein (P-gp), are important factors for drug absorption, whereas key factors explaining differences in drug distribution between the paediatric population and adults are organ size, membrane permeability, plasma protein concentration and characteristics, endogenous substances in plasma, total body and extracellular water, fat content, regional blood flow and transporters such as P-gp, which is present not only in the gut, but also in liver, kidney, brain and other tissues. As far as drug metabolism is concerned, important differences have been found in the paediatric population compared with adults both for phase I enzymes (oxidative [e.g., cytochrome P450 (CYP)1A2, and CYP3A7 versus -3A4], reductive and hydrolytic enzymes) and phase II enzymes (e.g., N-methyltransferases and glucuronosyltransferases). Generally, the major enzyme differences observed in comparison with the adult age are in newborn infants, although for some enzymes (e.g., glucuronosyltransferases and other phase II enzymes) important differences still exist between infants and toddlers and adults. Finally, key factors undergoing maturational changes accounting for differences in renal excretion in the paediatric population compared with adults are glomerular filtration and tubular secretion. The ranking of the key factors varies according to the chemical structure and physicochemical properties of the drug examined, as well as to the characteristics of its formulation. It would be important to generate additional information on the developmental aspects of renal P-gp and of other renal transporters, as has been done and is still being done with the different -isozymes involved in drug metabolism.

Comparative pharmacokinetics and metabolism of levetiracetam, a new anti-epileptic agent, in mouse, rat, rabbit and dog.

1: The pharmacokinetics and metabolism of 14C-levetiracetam, a new anti-epileptic agent, in mouse, rat, rabbit and dog after a single oral dose were investigated. Moreover, the in vitro hydrolysis of levetiracetam to its major carboxylic metabolite by rat tissue homogenates was investigated to identify tissues involved in the production of the metabolite. Data are also presented on the induction of the enzyme(s) involved in levetiracetam hydrolysis in the rat. 2: Levetiracetam was rapidly and almost completely absorbed. The unchanged drug accounted for a very high percentage of plasma radioactivity. Levetiracetam did not bind to plasma proteins. Although brain radioactivity concentrations were lower than those of whole blood at early time points, brain-to-blood ratios increased over time. The predominant route of elimination of total 14C was excretion via urine, accounting for about 81, 93, 87 and 89% of the dose in the mouse, rat, rabbit and dog, respectively. Consequently, levetiracetam was poorly metabolized. It was submitted in vivo to hydrolysis and/or oxidation. Hydrolysis of the amide function of levetiracetam produced the corresponding acid. However, levetiracetam could also be oxidized at positions 3 and 4 of the 2-oxopyrrolidine ring. Finally, the compound and the corresponding acid metabolite could be oxidized at position 5 of the 2-oxopyrrolidine ring and then hydrolysed with the opening of the ring. 3: All the investigated rat tissues (liver, kidney, lung, brain, small intestine mucosa) had the potential to produce the acid metabolite. By contrast, the acid was undetectable following incubation of levetiracetam with buffer alone or heat-denaturated liver fractions. 4: No marked species or sex differences were observed in the absorption, disposition and metabolism of levetiracetam. 5:The hydrolysis of levetiracetam is carried out by an enzymatic process characterized by a broad tissue distribution. In the rat, the enzyme system hydrolysing levetiracetam is not induced by phenobarbital, at least under the experimental conditions used herein, whereas the enzyme system(s) involved in the other metabolic pathways is induced.

In vitro inhibition of human liver drug metabolizing enzymes by second generation antihistamines.

Cetirizine, terfenadine, loratadine, astemizole and mizolastine were compared for their ability to inhibit marker activities for CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 and for some glucuronidation isoenzymes in human liver microsomes. The most pronounced effects were observed with terfenadine, astemizole and loratadine which inhibited CYP3A4-mediated testosterone 6beta-hydroxylation (IC50 of 23, 21 and 32 microM, respectively) and CYP2D6-mediated dextromethorphan O-demethylation (IC50 of 18, 36 and 15 microM, respectively). In addition, loratadine markedly inhibited the CYP2C19 marker activity, (S)-mephenytoin 4-hydroxylation (Ki of 0.17 microM). Furthermore, loratadine activated the CYP2C9-catalyzed tolbutamide hydroxylation (ca. 3-fold increase at 30 microM) and inhibited some glucuronidation enzymes. Mizolastine appeared to be a relatively weak and unspecific inhibitor of CYP2E1, CYP2C9, CYP2D6 and CYP3A4 (IC50Ss in the 100 micromolar range). Cetirizine demonstrated no effect on the investigated activities. A comparison of the inhibitory potencies of cetirizine, terfenadine, loratidine, astemizole and mizolastine with their corresponding plasma concentrations in humans suggests that these antihistamines are not likely to interfere with the metabolic clearance of coadministered drugs, with the exception of loratidine, which appears to inhibit CYP2C19 with sufficient potency to warrant additional investigation.

Inhibition of aromatase (P450Arom) by some 1-(benzofuran-2-ylmethyl)imidazoles.

Studies of a series of 1-(benzofuran-2-ylmethyl)imidazoles, 1-5, previously proposed as potential agents for prostatic cancer by their inhibition of 17beta-hydroxylase:17,20-lyase (P450 17), have been extended to their selectivity against placental microsomal aromatase (P450(Arom)) in man. The compounds were 3-7-fold more potent than aminoglutethimide and had some selectivity for P450 17 as expressed by the ratio (IC50 P450(Arom))/(IC50 P450) 17)/17.0 (2), 10.3 (3), 34.6 (4) and 42.0 (5), where IC50 is the concentration resulting in 50% inhibition. The lower potency of 1-5 towards P450(Arom) compared with the racemic alpha-phenyl-substituted compounds (6, 80-1000 x aminoglutethimide) and some racemic alpha-methyl (8.5 and 12.2 x aminoglutethimide) and alpha-ethyl (12.1 and 32.9 x aminoglutethimide) analogues has been rationalized. This work selectively extends studies of the P450 17 inhibitor 5, a potential prostatic cancer agent, towards other cytochrome P450 enzymes in the steroidogenic pathway and provides a general method for determining the relative influence of chemical manipulation of a parent inhibitor towards two enzymes in the pathway using additional literature data.

Synthesis and biological evaluation of imidazole based compounds as cytochrome P-450 inhibitors.

Several phenyl ethyl and phenyl propyl imidazole based compounds have been synthesised and their biological activity evaluated against human placental Aromatase (AR), rat testicular 17 alpha-hydroxylase/ 17,20-lyase (P450(17) alpha) and bovine cholesterol side chain cleavage (CSCC). The compounds showed good selectivity towards AR with N-[2-(4'-Nitrophenyl) ethyl] imidazole (2) (IC50 = 0.16 +/- 0.01 microM, Ki = 0.09 +/- 0.01 microM), the most potent AR inhibitor, showing some 130 times greater potency over Aminoglutethimide (AG) (IC50 = 20.0 +/- 2.6 microM, Ki = 11.0 +/- 2.0 microM). N-[3-(4'-Fluorophenyl) propyl] imidazole (10) (IC50 = 0.31 +/- 0.01 microM, Ki = 0.34 +/- 0.05 microM), N-(2-(4'-Fluorophenyl ethyl) imidazole (5) (IC50 = 0.74 +/- 0.01 microM, Ki = 0.40 +/- 0.02 microM), N-(3-(4'-Chlorophenyl propyl) imidazole (9) (IC50 = 0.82 +/- 0.02 microM) and N-[3-(4'-Nitrophenyl) propyl] imidazole (7) (IC50 = 0.84 +/- 0.02 microM, Ki = 0.10 microM) were also more potent than AG. Of the compounds tested for P450(17) alpha activity, 7 (IC50 = 25.0 +/- 2.0 microM), N-[2-(4'-Aminophenyl) ethyl] imidazole (3) (IC50 = 27.6 +/- 0.10 microM), 9 (IC50 = 29.0 +/- 4.0 microM) and 2 (IC50 = 30.2 +/- 2.0 microM) showed the highest activity, possessing approximately half the activity of Ketoconazole (IC50 = 12.1 +/- 2.9 microM). Compounds 1, 2, 3, and 7 showed 0% inhibitory activity towards CSCC at 200 microM whilst AG showed 83% inhibition under the same conditions. The compounds proved themselves to be excellent lead compounds and supported the novel models developed by Ahmed for AR and P450(17) alpha.

Synthesis and biological evaluation of novel pyrrolidine-2,5-dione inhibitors as potential anti-tumour agents.

Several novel pyrrolidine-2,5-dione based compounds have been synthesised and evaluated for their biological activity against human placental aromatase (AR), rat testicular 17 alpha-hydroxylase/17,20-lyase (P450(17) alpha) and bovine cholesterol side chain cleavage (CSCC). The compounds showed good inhibition towards AR with 1-cyclohexyl-3-[2'(4"-aminophenyl) ethyl] pyrrolidine-2,5-dione (3) (IC50 = 23.8 +/- 4.6 microM) and 1-octyl-3-[2'(4"-aminophenyl) ethyl] pyrrolidine-2,5-dione (4) (IC50 = 24.6 +/- 1.8 microM) showing equipotent activity with Aminoglutethimide (AG) (IC50 = 20.0 +/- 2.6 microM, Ki = 11.0 +/- 2.0 microM). Of the compounds tested for P450(17) alpha activity, 3 (IC50 = 18.5 +/- 1.9 microM) again showed the highest activity, being equipotent to Ketoconazole (IC50 = 12.1 +/- 2.9 microM). 3 was a poor inhibitor of CSCC with some 22% inhibitory activity at an inhibitor concentration of 200 microM, as compared to AG with 72% inhibitory activity under the same conditions. The compounds proved themselves to be excellent lead compounds and supported the novel models developed by Ahmed for AR and P450(17) alpha.

Substituted 1-[benzofuran-2-yl)-phenylmethyl]-imidazoles as potent inhibitors of aromatase in vitro and in female rats in vivo.

Substituted 1-[(benzofuran-2-yl)-phenylmethyl]-imidazoles are a new class of potent aromatase inhibitor with in vitro IC50 values < 10 nM for certain members using human placental enzyme. At a dose of 2 mg/kg in PMSG-stimulated rats, selected compounds effectively reduce the oestradiol levels by 82-98%.

1-pentyl-3-(4-aminophenyl)pyrrolidine-2,5-dione, a selective aromatase inhibitor: in vivo studies.

1-Pentyl, 1-hexyl and 1-heptyl-3-(4-aminophenyl)pyrrolidine-2,5-dione, potent inhibitors of aromatase, lower oestrogen levels in PMSG-stimulated female rats in a comparable manner to the inhibitor aminoglutethimide (AG) used clinically for the treatment of breast cancer. Pharmacokinetic studies in the rat show t 1/2 values for the 1-hexyl compound and AG of 1.8 and 5.5 h respectively. In 4 tests for CNS-depressant activity the overall order of activity was AG > 1-heptyl = 1- hexyl >> 1-pentyl. The 1-pentyl compound has less tendency than AG to depress white cell and platelet counts in mice and overall is the drug candidate for further studies.

Some 1-, and 3-substituted 3-(4'-aminophenyl)pyrrolidine-2,5-diones as selective inhibitors of aromatase.

1-Alkyl-3-(4'aminophenyl)pyrrolidine-2,5-diones (7,R = C3H7-C7H15) are potent inhibitors of aromatase in vitro, the 1-hexyl (Ki = 62nM) being about 100-fold more potent than aminoglutethimide (AG), and more selective in their ratio of aromatase:CSCC inhibitory potency. The 1-pentyl, 1-hexyl and 1-heptyl derivatives are more stable to liver microsomal metabolism in vitro than AG possibly due to inhibition of the liver cytochrome P450s. 1,3-Dialkyl-3(4'-aminophenyl)pyrrolidine-2,5-diones (9) have been synthesised by a novel method. Although the higher homologues (di-pentyl and di-hexyl) are more potent in vitro as inhibitors of aromatase than AG, they are less active than their 1-alkyl counterparts with the same alkyl substituent.

The effect of dose on the bioavailability of oral etoposide: confirmation of a clinically relevant observation.

The effect of dose on the bioavailability of oral etoposide was investigated in ten patients with malignant mesothelioma who received single-agent etoposide as part of a phase II study. Etoposide pharmacokinetics were studied in each patient at oral dose levels of 100, 200, 300, 400 and 600 mg. At doses above 200 mg, the AUC and peak concentrations of etoposide were substantially lower than predictions based on the 100-mg dose. This study confirms previous observations that etoposide absorption is dose-dependent and that a mean bioavailability of approximately 50% cannot be assumed at total oral doses greater than 200 mg.