Effects of roxithromycin, erythromycin and troleandomycin on their N-demethylation by rat and human cytochrome P450 enzymes

A Review: Effects of Macrolides on CYP450 Enzymes

As a kind of haemoglobin, cytochrome P450 enzymes (CYP450) participate in the metabolism of many substances, including endogenous substances, exogenous substances and drugs. It is estimated that 60% of common prescription drugs require bioconversion through CYP450. The influence of macrolides on CYP450 contributes to the metabolism and drug-drug interactions (DDIs) of macrolides. At present, most studies on the effects of macrolides on CYP450 are focused on CYP3A, but a few exist on other enzymes and drug combinations, such as telithromycin, which can decrease the activity of hepatic CYP1A2 and CYP3A2. This article summarizes some published applications of the influence of macrolides on CYP450 and the DDIs of macrolides caused by CYP450. And the article may subsequently guide the rational use of drugs in clinical trials. To a certain extent, poisoning caused by adverse drug interactions can be avoided. Unreasonable use of macrolide antibiotics may enable the presence of residue of macrolide antibiotics in animal-origin food. It is unhealthy for people to eat food with macrolide antibiotic residues. So it is of great significance to guarantee food safety and protect the health of consumers by the rational use of macrolides. This review gives a detailed description of the influence of macrolides on CYP450 and the DDIs of macrolides caused by CYP450. Moreover, it offers a perspective for researchers to further explore in this area.

Phenytoin - An anti-seizure drug: Overview of its chemistry, pharmacology and toxicology

Phenytoin is a long-standing, anti-seizure drug widely used in clinical practice. It has also been evaluated in the context of many other illnesses in addition to its original epilepsy indication. The narrow therapeutic index of phenytoin and its ubiquitous daily use pose a high risk of poisoning. This review article focuses on the chemistry, pharmacokinetics, and toxicology of phenytoin, with a special focus on its mutagenicity, carcinogenicity, and teratogenicity. The side effects on human health associated with phenytoin use are thoroughly described. In particular, DRESS syndrome and cerebellar atrophy are addressed. This review will help in further understanding the benefits phenytoin use in the treatment of epilepsy.

Toxicity of erythromycin to Oncorhynchus mykiss at different biochemical levels: detoxification metabolism, energetic balance, and neurological impairment

During the last decades, the presence of antibiotics in different aquatic compartments has raised increasing interest and concern, since these compounds are usually persistent and bioactive pseudo pollutants. Erythromycin (ERY) is a macrolide antibiotic, prescribed for human and veterinary medicines but also used in aquaculture and livestock production. Taking into account the recorded environmental levels of ERY, its toxicity to non-target organisms has become a still poorly studied issue, particularly in fish. In this sense, this study investigated the acute and chronic effects of realistic levels of ERY on Oncorhynchus mykiss (rainbow trout), namely, through the quantification of the activity of enzymes involved in different biochemical pathways, such as detoxification (phase I-7-ethoxyresorufin O-deethylase (EROD); phase II-glutathione S-transferases (GSTs), uridine-diphosphate-glucuronosyltransferases (UGTs)), neurotransmission (acetylcholinesterase (AChE)), and energy production (lactate dehydrogenase (LDH)). Both types of exposure caused significant increases in EROD activity in liver of O. mykiss; an increase in GST activity in gills after chronic exposure was also observed. UGT branchial activity was significantly depressed, following the long-term exposure. Thus, EROD, GST, and UGT enzymatic forms seem to be involved in the biotransformation of ERY. In terms of neurotransmission and preferential pathway of energy homeostasis, the exposed organisms appear not to have been affected, as there were no significant alterations in terms of AChE and LDH activities, respectively. The here-obtained data suggest that the observed alterations in terms of detoxification enzymes may have prevented the establishment of a set of toxic responses, namely, neurotoxic and metabolic disorders.

Functioning of drug-metabolizing microsomal cytochrome P450s: In silico probing of proteins suggests that the distal heme 'active site' pocket plays a relatively 'passive role' in some enzyme-substrate interactions

PURPOSE

The currently held mechanistic understanding of microsomal cytochrome P450s (CYPs) seeks that diverse drug molecules bind within the deep-seated distal heme pocket and subsequently react at the heme centre. To explain a bevy of experimental observations and meta-analyses, we indulge a hypothesis that involves a "diffusible radical mediated" mechanism. This new hypothesis posits that many substrates could also bind at alternate loci on/within the enzyme and be reacted without the pertinent moiety accessing a bonding proximity to the purported catalytic Fe-O enzyme intermediate.

METHODS

Through blind and heme-distal pocket centered dockings of various substrates and non-substrates (drug molecules of diverse sizes, classes, topographies etc.) of microsomal CYPs, we explored the possibility of access of substrates via the distal channels, its binding energies, docking orientations, distance of reactive moieties (or molecule per se) to/from the heme centre, etc. We investigated specific cases like- (a) large drug molecules as substrates, (b) classical marker drug substrates, (c) class of drugs as substrates (Sartans, Statins etc.), (d) substrate preferences between related and unrelated CYPs, (e) man-made site-directed mutants' and naturally occurring mutants' reactivity and metabolic disposition, (f) drug-drug interactions, (g) overall affinities of drug substrate versus oxidized product, (h) meta-analysis of in silico versus experimental binding constants and reaction/residence times etc.

RESULTS

It was found that heme-centered dockings of the substrate/modulator drug molecules with the available CYP crystal structures gave poor docking geometries and distances from Fe-heme centre. In conjunction with several other arguments, the findings discount the relevance of erstwhile hypothesis in many CYP systems. Consequently, the newly proposed hypothesis is deemed a viable alternate, as it satisfies Occam's razor.

CONCLUSIONS

The new proposal affords expanded scope for explaining the mechanism, kinetics and overall phenomenology of CYP mediated drug metabolism. It is now understood that the heme-iron and the hydrophobic distal pocket of CYPs serve primarily to stabilize the reactive intermediate (diffusible radical) and the surface or crypts of the apoprotein bind to the xenobiotic substrate (and in some cases, the heme distal pocket could also serve the latter function). Thus, CYPs enhance reaction rates and selectivity/specificity via a hitherto unrecognized modality.

Tissue distribution, bioconcentration, metabolism, and effects of erythromycin in crucian carp (Carassius auratus)

In this study, the tissue distribution, bioconcentration, metabolism and biological effects of the macrolide antibiotic erythromycin (ERY) were investigated in fish using crucian carp (Carassius auratus) as a model. Crucian carp were exposed to various concentrations of ERY (4, 20, and 100 μg/L) for 28 days. The UPLC/MS/MS analysis of both water and tissue provided the bioconcentration of ERY and its metabolites in the fish body. The results from tissue samples showed that a maximum tissue concentration occurred in the muscle and that the bioconcentration factor (BCF) of 72.2 was lower than the theoretical BCF of 90.4 calculated from the octanol-water coefficient of ERY. A significant portion of the absorbed ERY was metabolized via demethylation and dehydration and observed in the form of descladinose in fish. In addition, the relevant biomarkers, including acetylcholinesterase in the brain, as well as 7-ethoxyresorufin O-deethylase and superoxide dismutase in the liver, changed significantly during 28 days of exposure (P<0.05). These results clearly indicated that ERY accumulated in fish and that similar metabolites as those observed in mammals were produced, resulting in the biochemical disturbance of biological systems.

Effects of dietary sodium butyrate on hepatic biotransformation and pharmacokinetics of erythromycin in chickens

Butyrate, a commonly applied feed additive in poultry nutrition, can modify the expression of certain genes, including those encoding cytochrome P450 (CYP) enzymes. In comparative in vitro and in vivo experiments, the effect of butyrate on hepatic CYP genes was examined in primary cultures of chicken hepatocytes and in liver samples of chickens collected from animals that had been given butyrate as a feed additive. Moreover, the effect of butyrate on the biotransformation of erythromycin, a marker substance for the activity of enzymes of the CYP3A family, was investigated in vitro and in vivo. Butyrate increased the expression of the avian-specific CYP2H1 both in vitro and in vivo. In contrast, the avian CYP3A37 expression was decreased in hepatocytes following butyrate exposure, but not in the in vivo model. CYP1A was suppressed by butyrate in the in vitro experiments, and overexpressed in vivo in butyrate-fed animals. The concomitant incubation of hepatocytes with butyrate and erythromycin led to an increased CYP2H1 expression and a less pronounced inhibition of CYP3A37. In in vivo pharmacokinetic experiments, butyrate-fed animals given a single i.m. injection of erythromycin, a slower absorption phase (longer T(half-abs) and delayed T(max)) but a rapid elimination phase of this marker substrate was observed. Although these measurable differences were detected in the pharmacokinetics of erythromycin, it is unlikely that a concomitant application of sodium butyrate with erythromycin or other CYP substrates will cause clinically significant feed-drug interaction in chickens.

Bioconcentration, metabolism, and biomarker responses in freshwater fish Carassius auratus exposed to roxithromycin

To investigate the distribution, bioconcentration, metabolism, and biomarker responses of macrolide antibiotic roxithromycin (ROX) in fish, crucian carp (Carassius auratus) were exposed to various concentrations of ROX (4, 20, and 100μgL(-1)) for 20d. The ROX content in different tissues was quantified using UPLC/MS/MS. The liver exhibited the highest ROX concentration followed by the bile, gills, and muscle tissues. After 15d of exposure to different concentrations of ROX, the bioconcentration factors were 2.15-38.0 in the liver, 0.950-20.7 in the bile, 0.0506-19.7 in the gill, and 0.0439-13.8 in the muscle; these results were comparable to the estimated BCF values. The metabolites formed in the bile were identified based on metabolic identification in human bile. Additionally, the biomarkers, including acetylcholinesterase in the brain, as well as 7-ethoxyresorufin O-deethylase and superoxide dismutase in the liver changed significantly after 5, 10, 15, and 20d of exposure (P<0.05). Our results suggest that ROX can accumulate and be metabolized in fish; therefore, interactions between ROX or its metabolites and the biological systems may induce biochemical disturbances in fish.

Cytochrome P450 expression-induction profile and chemically mediated alterations of the WIF-B9 cell line

BACKGROUND INFORMATION

WIF-B9 is a hybrid cell line obtained by fusion of rat hepatoma cells (Fao) and human fibroblasts (WI38). It exhibits the structural and functional characteristics of differentiated hepatocytes, including active bile canaliculi. The aim of the present study was to characterize the WIF-B9 cell line as a model for analysing drug-induced hepatic effects. The drug metabolism potential of WIF-B9 cells was identified by studying the rat and human CYP (cytochrome P450) mRNA constitutive expression profile and induction potential after exposure to reference inducers. The morphological alterations provoked by chemical entities were also characterized.

RESULTS

Competitive reverse transcriptase-PCR revealed that four rat (1A1, 2B1/2, 2E1 and 4A1) and four human (1A1, 2Cs, 2D6 and 2E1) CYP mRNA isoforms were constitutively expressed in WIF-B9 cells. The rat CYP forms were expressed at levels 2-4 orders of magnitude higher than the human forms. Exposure for 20-72 h to increasing concentrations of CYP reference inducers (beta-naphthoflavone, 3-methyl cholanthrene, dexamethasone, phenobarbital, clofibrate and pregnenolone 16alpha-carbonitrile) revealed that the rat CYP 1A1, 1A2, 3A1, 3A2 and 4A1 and human CYP 1A1 and 2Cs mRNAs were inducible. Rat CYP 1A1 and 1A2 were the most inducible isoforms since they were overexpressed up to 100-fold after 20-48 h of treatment with beta-naphthoflavone. Human CYP 1A1 and 2Cs mRNAs were induced 3-fold after 48 h of treatment with phenobarbital. Other mechanisms involved in hepatotoxicity were explored using microscopy and immunofluorescence. The WIF-B9 cell line exhibited fragmentation and dilatation of bile canaliculi upon exposure to erythromycin, and to isoniazid and cytochalasins, respectively. Monensin promoted cell depolarization and cytoplasmic granulation. Ethionine promoted cytoplasmic vacuolation and dilatation of the Golgi structures.

CONCLUSIONS

These results indicate that the CYP expression and induction profiles and the morphological features of WIF-B9 cells allow prediction in vitro of the induction and hepatotoxicity profiles of chemical entities.

Cytochrome P450 CYP3A in marsupials: cloning and characterisation of the second identified CYP3A subfamily member, isoform 3A78 from koala (Phascolarctos cinereus)

Cytochromes P450 (CYPs) are critically important in the oxidative metabolism of a diverse array of xenobiotics and endogenous substrates. Previously, we cloned and characterised the CYP2C, CYP4A, and CYP4B gene subfamilies from marsupials and demonstrated important species-differences in both activity and tissue expression of these CYP enzymes. Recently, we isolated the Eastern grey kangaroo CYP3A70. Here we have cloned and characterised the second identified member of marsupial CYP3A gene subfamily, CYP3A78 from the koala (Phascolarctos cinereus). In addition, we have examined the gender-differences in microsomal erythromycin N-demethylation activity (a CYP3A marker) and CYP3A protein expression across test marsupial species. Significant differences in hepatic erythromycin N-demethylation activity were observed between male and female koalas, with the activity detected in female koalas being 2.5-fold higher compared to that in male koalas (p<0.01). No gender-differences were observed in tammar wallaby or Eastern grey kangaroo. Immunoblot analysis utilising anti-human CYP3A4 antibody detected immunoreactive proteins in liver microsomes from all test male and female marsupials including the koala, tammar wallaby, and Eastern grey kangaroo, with no gender-differences detected across test marsupials. A 1610 bp koala hepatic CYP3A complete cDNA, designated CYP3A78, was cloned by reverse transcription-polymerase chain reaction approaches. It displays 64% nucleotide and 57% amino acid sequence identity to the Eastern grey kangaroo CYP3A70. The CYP3A78 cDNA encodes a protein of 515 amino acids, shares approximately 68% nucleotide and 56% amino acid sequence identity to human CYP3A4, and displays high sequence similarity to other published mammalian CYP3As from human, monkey, cow, pig, dog, rat, rabbit, mouse, hamster, and guinea pig. Collectively, this study provides primary molecular data regarding koala hepatic CYP3A78 gene and enables further functional analyses of CYP3A enzymes in marsupials. Given the significant role that CYP3A enzymes play in the metabolism of both endogenous and exogenous compounds, the clone provides an important step in elucidating the metabolic capacity of marsupials.

WITHDRAWN: Cytochrome P450 CYP3A in marsupials: Characterisation of the first identified CYP3A subfamily member, isoform 3A70 from Eastern grey kangaroo (Macropus giganteus)

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Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Roles of human CYP2A6 and rat CYP2B1 in the oxidation of (+)-fenchol by liver microsomes

The metabolism of (+)-fenchol was investigated in vitro using liver microsomes of rats and humans and recombinant cytochrome P450 (P450 or CYP) enzymes in insect cells in which human/rat P450 and NADPH-P450 reductase cDNAs had been introduced. The biotransformation of (+)-fenchol was investigated by gas chromatography-mass spectrometry (GC-MS). (+)-Fenchol was oxidized to fenchone by human liver microsomal P450 enzymes. The formation of metabolites was determined by the relative abundance of mass fragments and retention times on GC. Several lines of evidence suggested that CYP2A6 is a major enzyme involved in the oxidation of (+)-fenchol by human liver microsomes. (+)-Fenchol oxidation activities by liver microsomes were very significantly inhibited by (+)-menthofuran, a CYP2A6 inhibitor, and anti-CYP2A6. There was a good correlation between CYP2A6 contents and (+)-fenchol oxidation activities in liver microsomes of ten human samples. Kinetic analysis showed that the Vmax/Km values for (+)-fenchol catalysed by liver microsomes of human sample HG03 were 7.25 nM-1 min-1. Human recombinant CYP2A6-catalyzed (+)-fenchol oxidation with a Vmax value of 6.96 nmol min-1 nmol-1 P450 and apparent Km value of 0.09 mM. In contrast, rat CYP2A1 did not catalyse (+)-fenchol oxidation. In the rat (+)-fenchol was oxidized to fenchone, 6-exo-hydroxyfenchol and 10-hydroxyfenchol by liver microsomes of phenobarbital-treated rats. Recombinant rat CYP2B1 catalysed (+)-fenchol oxidation. Kinetic analysis showed that the Km values for the formation of fenchone, 6-exo- hydroxyfenchol and 10-hydroxyfenchol in rats treated with phenobarbital were 0.06, 0.03 and 0.03 mM, and Vmax values were 2.94, 6.1 and 13.8 nmol min-1 nmol-1 P450, respectively. Taken collectively, the results suggest that human CYP2A6 and rat CYP2B1 are the major enzymes involved in the metabolism of (+)-fenchol by liver microsomes and that there are species-related differences in the human and rat CYP2A enzymes.

Structural identification of bitespiramycin metabolites in rat: a single oral dose study

Bitespiramycin is a macrolide antibiotic consisting of a mixture of some nine spiramycin ester derivatives. It has a similar spectrum of antibiotic activity to that of spiramycin but has superior pharmacokinetic properties. In this study, a rapid and facile LC/ESI-MSn method was applied to study the metabolism of bitespiramycin in rat following a single oral dose (80 mg kg-1). Concentrations of parent drug constituents and metabolites were determined in plasma, urine, feces and bile. Concentrations of parent drug constituents and metabolites in plasma were very low. In urine, feces and bile, parent drug constituents and 38 metabolites were identified on the basis of their chromatographic and mass spectrometric properties. The identity of 17 metabolites was confirmed by comparison with reference substances. The principal metabolites were the corresponding spiramycins formed by hydrolysis of the 4''-(3-methylbutanoate) groups. Other important metabolic pathways were: hydrolytic loss of the forosamine and mycarose sugars; aldehyde reduction; cysteine conjugation of the aldehyde group; and hydrolysis of the lactone ring. Products formed by lactone ring opening were found only in urine, and those formed by aldehyde reduction were found only in feces. Aldehyde reduction and hydrolytic loss of forosamine represent novel biotransformation pathways for spiramycin derivatives.

Selective suppression of cytochrome P450 gene expression by the medicinal herb, Thonningia sanguinea in rat liver

The effect of the administration of Thonningia sanguinea (T. S.) on the abundance of individual components of the cytochrome P450 monooxygenase enzyme was examined using Western blotting and competitive reverse-transcriptase-polymerase chain reaction (RT-PCR). We also investigated the time-course of inhibition of T. S. on drug metabolizing enzymes. A single intraperitoneal dose of T. S. extract (5 ml/kg) suppressed CYP, cytochrome b5 and NADPH-CYP reductase activity by 45%, 34% and 22% respectively 24 h after T. S. administration. While T. S. did not have any significant effect on microsomal glutathione S-transferase activity, it inhibited p-nitrophenol hydroxylase (PNPH, CYP2E1) and 7-methoxyresorufin O-demethylase (MROD, CYP 1A2) activities by 37% and 32% respectively at 12 h post-T. S. administration. PNPH, erythromycin N-demethylase (ERDM, CYP 3A1/2) and MROD activities were inhibited by 28-36% 24 h after T. S. injection. Consistent with these observations, the levels of CYP2E1, CYP1A2 and CYP3A2 proteins were also suppressed 24 h post-T. S. administration. While CYP2E1 mRNA was unaffected by T. S. administration, CYP1A2 and CYP3A2 mRNAs were decreased by T. S. Cytosolic glutathione S-transferase activity was increased by 30%, 6 h after T. S injection. These data demonstrate that administration of T. S. differentially affect CYP isoforms in the liver of rats and that T. S. selectively suppresses CYP3A2 and CYP1A2 gene expression.

Human variability in CYP3A4 metabolism and CYP3A4-related uncertainty factors for risk assessment

CYP3A4 constitutes the major liver cytochrome P450 isoenzyme and is responsible for the oxidation of more than 50% of all known drugs. Human variability in kinetics for this pathway has been quantified using a database of 15 compounds metabolised extensively (>60%) by this CYP isoform in order to develop CYP3A4-related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration-time curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was greater for the oral route (46%, 12 compounds) than for the intravenous route (32%, 14 compounds). The physiological and molecular basis for the difference between these two routes of exposure is discussed. In relation to the uncertainty factors used for risk assessment, the default kinetic factor of 3.16 would be adequate for adults, whereas a CYP3A4-related factor of 12 would be required to cover up to 99% of neonates, which have lower CYP3A4 activity.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Comparative pharmacodynamic analysis of Q-T interval prolongation induced by the macrolides clarithromycin, roxithromycin, and azithromycin in rats

In order to evaluate the arrhythmogenic potency of macrolide antibiotics in a quantitative manner, we analyzed the influence of clarithromycin (CAM), roxithromycin (RXM), and azithromycin (AZM) on Q-T intervals from pharmacokinetic and pharmacodynamic points of view and in comparison with the potency of erythromycin (EM) previously reported by us for rats. Male Sprague-Dawley rats were anesthetized, and CAM (6.6, 21.6, and 43.2 mg/kg of body weight/h), RXM (20 and 40 mg/kg/h), and AZM (40 and 100 mg/kg/h) were intravenously injected for 90 min to obtain the time courses of drug concentrations in plasma and the changes in the Q-T intervals during and after the drug injections. Distinct Q-T interval prolongation of up to 10 ms was observed with CAM at its clinical concentrations. RXM and AZM evoked Q-T interval prolongation at concentrations higher than their clinical ranges. The potencies for Q-T interval prolongation, assessed as the slope of the concentration-response relationship, were 6.09, 0.536, and 0.989 ms. ml/microg for CAM, RXM, and AZM, respectively. There was hysteresis between the change in the Q-T intervals and the time course of the plasma concentration of each drug. The rank order of clinical arrhythmogenicity was estimated to be EM > CAM > RXM > AZM, as assessed from the present results and our previous report for EM. In conclusion, RXM and AZM were estimated to be less potent at provoking arrhythmia than EM and CAM. These results should be useful for making a safer choice of an appropriate agent for patients with electrocardiographic risk factors.

Formation in vitro of an inhibitory cytochrome P450 x Fe2+-metabolite complex with roxithromycin and its decladinosyl, O-dealkyl and N-demethyl metabolites in rat liver microsomes

1. Roxithromycin and its major metabolites found in rat and human urine, namely the decladinosyl derivative (M1), O-dealkyl derivative (M2) and N-demethyl derivative (M3), were incubated with rat liver microsomes and formation of an inhibitory cytochrome P450 (CYP)-metabolite complex and of formaldehyde (measurement of N-demethylation) were determined in vitro. Troleandomycin and erythromycin were also used for comparison. 2. Dexamethasone very significantly induced the microsomal N-demethylations of these macrolide antibiotics. The order of magnitude for the Vmax/Km ratio of N-demethylations by liver microsomes from dexamethasone-treated rats was troleandomycin > erythromycin = M2 > roxithromycin > M3, M1. 3. Formation of an inhibitory P450 x Fe2+-metabolite complex was detected on incubation of these macrolide antibiotics with rat liver microsomes in the presence of an NADPH-generating system and the order of maximum complex formation was troleandomycin > erythromycin > M2 > roxithromycin > M3 > M1. 4. Troleandomycin, erythromycin and M2 inhibited CYP3A-dependent testosterone 6beta-hydroxylation catalysed by liver microsomes from the dexamethasone-treated rat by 54, 33 and 23%, respectively, but roxithromycin, M3 and M1 were very weak by comparison. In the untreated rat, only testosterone 6beta-hydroxylation, but not testosterone 16alpha- and 2alpha-hydroxylation and androstenedione formation, activities were inhibited, indicating that inhibitory actions of these antibiotics are specific for CYP3A enzymes in liver microsomes. 5. These results support the view that formation of an inhibitory P450-metabolite complex is prerequisite for the inhibition of CYP3A-dependent substrate oxidations by rat liver microsomes and that M2 (and M3, to a lesser extent) may be the active metabolite that can form an inhibitory P450-metabolite complex by CYP3A enzyme(s).

Changes in the enzymatic activities of beagle liver during maturation as assessed both in vitro and in vivo

1. We have examined changes in caffeine and trimethadione (TMO) metabolism in vivo, agents which are used as probe drugs. In this study the total body clearance (Cl) of caffeine and TMO was low 1 week after birth (week 1), increased rapidly from week 3, peaked and then decreased gradually until reaching the level for the mature, adult dog. The elimination half-life (t1/2) of caffeine and TMO was prolonged during week 1; however, it then gradually became shorter. Gradually it became longer and reached the level for the adult dog. The apparent volume of distribution (Vd) of caffeine did not change throughout the study. However, the Vd of TMO was only high during week 1. 2. The in vitro changes in a variety of typical substrates for seven different cytochrome P450 (CYP) isozymes were examined. In this study three different patterns of metabolism can be identified: (1) activity is low immediately after birth, increases, peaks and then decreases to the adult dog level (p-nitroanisole; CYP1A1, caffeine; CYP1A2, benzphetamine; CYP3A/2B(?), aniline; 2E1 and TMO; CYP2C9/2E1/3A4); (2) activity generally increases rapidly soon after birth, continues to increase, peaks and then gradually decreases to the adult level (phenytoin; CYP2C9); and (3) activity is high (about the same level as the adult) immediately after birth, decreases and then gradually increases to the adult level (erythromycin; CYP3A4/5). 3. The results of these in vivo and in vitro studies suggest that changes in enzyme activity are due to differences in P450 isoenzymes during development.

Metabolism of roxithromycin in the isolated perfused rat liver

Roxithromycin is a macrolide antibiotic with high clinical potency. N-Demethylation is considered to be one of the main pathways of roxithromycin metabolism in rats. We have studied the hepatic metabolism of roxithromycin in the isolated perfused rat liver. After addition of roxithromycin (30 microM) to the perfusion medium the parent compound and one major metabolite were detected in bile by high-performance liquid chromatography. The metabolite was identified as monodesmethylated roxithromycin by mass spectrometric analysis. Onset of biliary excretion of native roxithromycin was fast, reaching a maximum (130.52 +/- 43.88 pmol g(-1) min(-1)) after only 10 min, whereas excretion of the metabolite was delayed (maximum 75.83 +/- 11.92 pmol g(-1) min(-1) at 30 min). The cumulative excretion of roxithromycin and its metabolite into bile during the 60 min of application amounted to only 1.09 +/- 0.30 and 0.64 +/- 0.22% of the roxithromycin cleared from the perfusate during the same time. The liver content was 0.48 micromol (g liver)(-1), indicating high retention within the organ. No release of the metabolite into the perfusate was detected. In conclusion, this study has demonstrated the importance of phase-I metabolism for the biliary excretion of roxithromycin in rat liver. These findings might be predictive of roxithromycin biotransformation and biliary excretion in man.

In vitro analysis of the activity of the major human hepatic CYP enzyme (CYP3A4) using [N-methyl-14C]-erythromycin

Although the N-demethylation of erythromycin has found widespread use in a noninvasive assay with which to phenotype hepatic CYP3A function, currently, the routine in vitro analysis of erythromycin N-demethylase activity relies on the quantitation of liberated formaldehyde by relatively labor-intensive and insensitive colorimetric or fluorimetric detection. This report describes the development of a rapid, sensitive, and reproducible radioassay for human CYP3A4 using solid-phase extraction (SPE). The kinetics of erythromycin N-demethylation were best described by a one-site Michaelis-Menten model with autoinhibition and the apparent kinetic parameters for pooled human liver microsomes (HLM; Km=88 microM, Vmax=345 pmol/min/mg) and expressed CYP3A4 (Km=33 microM, Vmax=130 pmol/min/ mg) were in good agreement. Erythromycin N-demethylase activity was found to vary 14-fold in HLM and correlated with the rate of testosterone 6beta-hydroxylation (r2=0.92, p < 0.001; N=9). Ketoconazole was a potent inhibitor of the N-demethylation of erythromycin, and the estimated IC50 value (104+/-23 nM) agreed well with that obtained using testosterone as a probe for CYP3A (71+/-4 nM). The addition of this radioassay to those established for human CYP1A2, 2C9, 2D6, and 2E1 and its subsequent automation should enable the routine use of this methodology in the analysis of CYP-dependent reactions.