Selective in-vitro inhibition of hepatic oxidative metabolism by quinolones: 7-ethoxyresorufin and caffeine as model substrates

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.

Comparative studies of the effects of two novel sugar drug candidates on the CYP 1A2 and CYP 2E1 enzymes in different sexed rats using a "cocktail" approach

The sex-based differences between the effects of two novel sugar-based drug candidates, a sulfated polymannuroguluronate (SPMG-911) and an acidic oligosaccharide sugar chain compound (AOSC-971), on the enzymes CYP 1A2 and CYP 2E1 were investigated. The results showed that neither SPMG-911 nor AOSC-971 have any effect on CYP1A2, while AOSC-971 induced the CYP 2E1 in male rats. The results are useful for their safety evaluation, as well as for the prediction of inter-drug interactions associated with the two drugs.

The inhibitory effects of the fluoroquinolone antimicrobials norfloxacin and enrofloxacin on hepatic microsomal cytochrome P-450 monooxygenases in broiler chickens

The fluoroquinolone antimicrobials norfloxacin and enrofloxacin were found to inhibit hepatic microsomal cytochrome P-450 monooxygenases in the livers of broiler chickens using dosages as given in commercial flocks. Norfloxacin inhibited the process of N-demethylation of aminopyrine to a greater degree, while enrofloxacin more markedly inhibited hydroxylation of aniline.

Distinction of CYP1A1 and CYP1A2 activity by selective inhibition using fluvoxamine and isosafrole

Ethoxyresorufin O-deethylation (EROD) has been used as a specific probe for CYP1A1 and CYP1A2. Selective inhibition of one of these cytochromes P450 may differentiate their activity in human liver. Four inhibitors were chosen to examine the selective inhibition of EROD activity, using cDNA of CYP1A1 and CYP1A2. The two flavones, alpha-naphthoflavone and apigenin, while differing in potency, inhibited expressed human CYP1A1, CYP1A2, and human liver microsomes to a similar extent. Isosafrole and fluvoxamine were found to inhibit CYP1A2 selectively, with Ki values of 14 and 800 times, respectively, lower than those for CY1A1. A set of equations was developed to estimate both CYP1A1 and CYP1A2 activity. Levels of CYP1A2 in four human liver specimens ranged from 44.4 to 76.7 pmol/mg protein, which significantly correlated with phenacetin O-deethylase activity (r = 0.99; P < 0.001). Low levels of CYP1A1 activity were present in all four investigated livers, ranging from 0.4 to 2.7 pmol/mg protein.

Intermolecular interactions of antimicrobial fluoroquinolones with purified rat liver CYP1A2 studied by proton nuclear magnetic resonance spectroscopy

1. Binding and inhibition of antimicrobial fluoroquinolones towards liver CYP1A2 purified from 3-methylcholanthrene-treated rats were investigated using proton nuclear magnetic resonance (nmr) and phenacetin metabolism. 2. The proton nmr longitudinal relaxation rate study indicated that the paramagnetic effects of the haem iron of CYP1A2 were observed in protons of enoxacin with a 1,8-naphthyridine skeleton and its 4'-nitrogen atom on the 7-piperazine ring probably participated in specific binding to the haem iron. These data suggest a facile accessibility and strong binding of enoxacin to the active site of the enzyme. On the contrary, the binding region of norfloxacin with a quinoline skeleton could not be specified, and an 8-fluorinated derivative (AT-3970) had much lower paramagnetic effects and no specific binding region. 3. In a reconstituted CYP1A2 system, enoxacin exhibited the most potent inhibition of phenacetin O-deethylation. The metabolism was less inhibited by norfloxacin, and AT-3970 had a weak inhibitory activity. 4. The binding ability of the fluoroquinolones to the CYP1A2 active site is likely to determine their inhibitory activity against phenacetin metabolism.

Enoxacin is an inducer of CYP1A2 in rat liver

The induction of cytochrome P450 by enoxacin, ciprofloxacin, and ofloxacin was investigated in female Wistar rats. Animals were treated orally with daily doses ranging from 10 to 400 mg enoxacin per kg body wt, 400 mg ciprofloxacin, or 400 mg ofloxacin per kg body wt for up to 7 days. Activities of methoxyresorufin O-demethylase (MROD) and ethoxyresorufin O-deethylase (EROD) were determined fluorimetrically in hepatic microsomes. MROD activity was increased 2.6-fold after treatment with 100 mg enoxacin per kg body wt for 7 days. Lower doses of enoxacin did not induce MROD activity significantly. Antipeptide antibodies directed specifically against different rat cytochrome P450 enzymes demonstrated that CYP1A2, but not CYP1A1, was induced in rats treated with enoxacin. After ciprofloxacin or ofloxacin treatment, no induction of MROD or EROD activity was observed. Neither ciprofloxacin nor ofloxacin caused any change in CYP1A1 or CYP1A2 apoprotein levels. Further investigations with antipeptide antibodies showed that there was no induction of CYP2B1, CYP2B2, CYP2E1, CYP3A1, CYP3A2, CYP4A1, or CYP4A2 following treatment with enoxacin, ciprofloxacin, or ofloxacin. It is concluded that enoxacin, but not ciprofloxacin or ofloxacin, is an inducer of CYP1A2 in rat liver.

Metabolism of theophylline and its inhibition by fluoroquinolones in rat hepatic microsomes

1. The effects of beta-naphthoflavone, dexamethasone, phenobarbitone and isosafrole on the metabolism of theophylline by rat liver microsomes have been studied. Only beta-naphthoflavone, a known P4501A inducer, increased the rate of 1-methylxanthine formation (3-fold), whereas all the inducers studied increased the rate of 1,3-dimethyluric acid production (2.5-3-fold). 2. To study the effects of a range of fluoroquinolones on theophylline metabolism, beta-naphthoflavone-induced microsomes were used, as the ratio for metabolite production rates was similar to that of untreated microsomes (4:1,3-dimethyluric acid: 1-methylxanthine at 2 mM theophylline). High concentrations of fluoroquinolones (0.5-1.5 mM) were required to affect microsomal theophylline metabolism. 1-Methylxanthine was more sensitive to fluoroquinolone inhibition by enoxacin, ciprofloxacin, norfloxacin and pipemidic acid than 1,3-dimethyluric acid; CP67015, had a significant effect on 1,3-dimethyluric acid production only; binfloxacin had no effect on either pathway. 3. Ethoxycoumarin, a rapidly metabolized substrate, was also investigated as a surrogate for theophylline in in vitro experiments. Fluoroquinolone inhibition of ethoxycoumarin O-de-ethylation in beta-naphthoflavone-induced microsomes was quantitatively greater but qualitatively similar to theophylline metabolism (IC50s 440-870 microM at 2 microM 7-ethoxycoumarin). 4. These data are comparable with previous rat experiments in vivo, indicating that enoxacin, ciprofloxacin and norfloxacin have similar intrinsic activity in the inhibition of theophylline metabolism.

Effects of ciprofloxacin and enrofloxacin on zoxazolamine kinetics, plasma concentration and sleeping times in mice

The treatment of CD1 male mice with either ciprofloxacin (CP) or enrofloxacin (EF) prior to zoxazolamine (ZX) administration increased the mean ZX sleeping times to, respectively, 162 and 156% of the control (ZX alone). At the end of the sleeping time, the mean ZX plasma concentration in controls was 27.2 micrograms/ml and was not different in EF- or CP-treated groups (87% and 95% of controls, respectively). The animals coadministered with CP or EF and ZX eliminated the latter more slowly than the controls. The estimated zero-time drug concentration of the disposition curves of both the CP- and EF-treated groups as well as the apparent half-life of elimination and apparent overall rate of elimination of the CP-treated group were different from the control values.

Effects of fluoroquinolone antimicrobials alone and in conjunction with theophylline on seizures in amygdaloid kindled rats. Mechanistic and pharmacokinetic study

The influence of three fluoroquinolone (FQ) antimicrobial drugs (ciprofloxacin (CP), norfloxacin (NF), enrofloxacin (EF)) on seizure parameters in amygdaloid kindled rats was investigated. CP and NF (100 mg/kg i.p.) did not modify seizure parameters while EF induced a decrease in seizure activity. Since clinical data indicate a seizure enhancing interaction between FQ and theophylline (THEO) we studied the influence of concurrent FQ-THEO administration in kindled rats. CP and NF, but not EF given concurrently with a non-seizure modulating dose of THEO (10 mg/kg i.p.) caused increases in seizure activity and aggressiveness in the animals. The CP-THEO induced seizure enhancement was antagonized by 2-chloroadenosine and diazepam. Pharmacokinetic studies demonstrated that THEO serum levels and elimination were not altered by concurrent CP administration. We conclude that coadministration of FQ-THEO can aggravate amygdala kindled seizures and that this aggravation may involve centrally mediated mechanisms.