Effect of single and multiple doses of sulphinpyrazone on antipyrine metabolism and urinary excretion of 6-beta-hydroxycortisol

Candidate Proficiency Test Chemicals to Address Industrial Chemical Applicability Domains for in vitro Human Cytochrome P450 Enzyme Induction

Cytochrome P450 (CYP) enzymes play a key role in the metabolism of both xenobiotics and endogenous chemicals, and the activity of some CYP isoforms are susceptible to induction and/or inhibition by certain chemicals. As CYP induction/inhibition can bring about significant alterations in the level of in vivo exposure to CYP substrates and metabolites, CYP induction/inhibition data is needed for regulatory chemical toxicity hazard assessment. On the basis of available human in vivo pharmaceutical data, a draft Organisation for Economic Co-operation and Development Test Guideline (TG) for an in vitro CYP HepaRG test method that is capable of detecting the induction of four human CYPs (CYP1A1/1A2, 2B6, and 3A4), has been developed and validated for a set of pharmaceutical proficiency chemicals. However to support TG adoption, further validation data was requested to demonstrate the ability of the test method to also accurately detect CYP induction mediated by industrial and pesticidal chemicals, together with an indication on regulatory uses of the test method. As part of "GOLIATH", a European Union Horizon-2020 funded research project on metabolic disrupting chemical testing approaches, work is underway to generate supplemental validated data for an additional set of chemicals with sufficient diversity to allow for the approval of the guideline. Here we report on the process of proficiency chemical selection based on a targeted literature review, the selection criteria and considerations required for acceptance of proficiency chemical selection for OECD TG development (i.e. structural diversity, range of activity, relevant chemical sectors, global restrictions etc). The following 13 proposed proficiency chemicals were reviewed and selected as a suitable set for use in the additional validation experiments: tebuconazole, benfuracarb, atrazine, cypermethrin, chlorpyrifos, perfluorooctanoic acid, bisphenol A, N,N-diethyl-m-toluamide, benzo-[a]-pyrene, fludioxonil, malathion, triclosan, and caffeine. Illustrations of applications of the test method in relation to endocrine disruption and non-genotoxic carcinogenicity are provided.

Quantitative evaluation of hepatic and intestinal induction of CYP3A in clinical practice

This is the first report quantitatively evaluating the clinical induction of CYP3A in the liver and the intestine.To evaluate hepatic induction, we collected literature data on endogenous biomarkers of hepatic CYP3A induction which we then used to calculate the fold-induction (inducer-mediated change in biomarker level). Literature data on decreases in the area under the curve (AUC) of alfentanil, a CYP3A substrate, caused by CYP3A inducers were also collected. We used the hepatic intrinsic clearance of alfentanil to calculate the hepatic induction ratio (inducer-mediated change in intrinsic clearance). For intestinal induction, the intestinal bioavailability (F_g) of alfentanil was used to calculate the intestinal induction ratio. We determined in vivo maximum induction (E_max) and the average unbound plasma concentration (C_av,u) required for half the maximum induction (EC₅₀) for inducers using an E_max model analysis.In our results, fold-induction was comparable to the induction ratio at several inducer concentrations, and almost the maximum induction was achieved by a therapeutic dose. Induction ratios in the intestine were similar to the liver.Our findings suggest that, by knowing only hepatic induction ratios for common inducers, we can quantitatively predict the decreases in the AUC of substrates by CYP3A induction.

Validation of in vitro methods for human cytochrome P450 enzyme induction: Outcome of a multi-laboratory study

CYP enzyme induction is a sensitive biomarker for phenotypic metabolic competence of in vitro test systems; it is a key event associated with thyroid disruption, and a biomarker for toxicologically relevant nuclear receptor-mediated pathways. This paper summarises the results of a multi-laboratory validation study of two in vitro methods that assess the potential of chemicals to induce cytochrome P450 (CYP) enzyme activity, in particular CYP1A2, CYP2B6, and CYP3A4. The methods are based on the use of cryopreserved primary human hepatocytes (PHH) and human HepaRG cells. The validation study was coordinated by the European Union Reference Laboratory for Alternatives to Animal Testing of the European Commission's Joint Research Centre and involved a ring trial among six laboratories. The reproducibility was assessed within and between laboratories using a validation set of 13 selected chemicals (known human inducers and non-inducers) tested under blind conditions. The ability of the two methods to predict human CYP induction potential was assessed. Chemical space analysis confirmed that the selected chemicals are broadly representative of a diverse range of chemicals. The two methods were found to be reliable and relevant in vitro tools for the assessment of human CYP induction, with the HepaRG method being better suited for routine testing. Recommendations for the practical application of the two methods are proposed.

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.

Assessment of enzyme induction and enzyme inhibition in humans: toxicological implications

1. The principal methods used for the assessment of enzyme induction and enzyme inhibition are measurement of the pharmacokinetics of a model compound (probe drug), analysis of drug metabolism in vitro, and determination of changes in the disposition of, and endogenous substrate for, the enzyme of interest. 2. Probe drugs that have been used for this purpose include antipyrine, aminopyrine, tolbutamide, caffeine, theophylline, warfarin, oxazepam and paracetamol. Measurement of the excretion of metabolites of cortisol and oestradiol, which are endogenous substrates for cytochrome P450 IIIA enzymes, provides a non-invasive means of assessing enzyme induction or inhibition. 3. Combined pharmacokinetic/pharmacodynamic studies are required to assess the pharmacological relevance of either induction or inhibition of the enzymes involved in drug metabolism. 4. At present it is difficult to assess the toxicological implications of enzyme induction and inhibition in man. Safe probe drugs are required for the enzymes primarily responsible for drug detoxication, such as epoxide hydrolase and glutathione transferase, in order to identify individuals particularly at risk.

Increased 6-hydroxycortisol excretion in pregnant women: implication of drug-metabolizing enzyme induction

The effect of pregnancy on hepatic drug-metabolizing enzyme activity was investigated in nine healthy pregnant women using the ratio of 6-hydroxycortisol (6-OHF), to total 17-hydroxycorticosteroid (17-OHCS) in 24-hour urine as an index of the hepatic monooxygenase activity. The values of 6-OHF and the ratio (713 +/- 250 micrograms/d and 0.323 +/- 0.242; mean +/- SD) before delivery were significantly higher than they were during early puerperium (395 +/- 145 micrograms/d and 0.114 +/- 0.055) and approximately three months after delivery (237 +/- 67 micrograms/d and 0.066 +/- 0.034). Although the values three months after delivery were comparable to those found in the nonpregnant group (n = 10; 228 +/- 48 micrograms/d and 0.081 +/- 0.031), 6-OHF values one week after delivery were significantly higher than those observed in the control group. These observations suggest that drug-metabolizing enzyme induction may occur during pregnancy.

Interference by sulphinpyrazone with the antihypertensive effects of oxprenolol

The interfering effect of sulphinpyrazone, a uricosuric agent which reduces the activity of cyclo-oxygenase, with the antihypertensive activity of oxprenolol, a non-cardioselective beta-blocker with sympathomimetic activity, has been evaluated. Ten patients with primary arterial hypertension of mild to moderate degree entered a randomized double-blind cross-over study versus placebo. They were given oxprenolol + placebo or oxprenolol + sulphinpyrazone for 15 days, and then the treatments were crossed-over for a further 15 days. Oxprenolol significantly reduced blood pressure (161 +/- 3/101 +/- 1 vs 149 +/- 4/96 +/- 2 mmHg) and heart rate (72 +/- 3 vs 66 +/- 3 beats/min). During administration of the combination with sulphinpyrazone the blood pressure increased to its pretreatment level (156 +/- 5/101 +/- 2 mmHg). The effect of oxprenolol on heart rate was not influenced by the combined treatment (67 +/- 6 beats/min). The results may be explained by 1) sulphinpyrazone-induced inhibition of prostaglandin synthesis, which could interfere with the antihypertensive activity of oxprenolol, or 2) sulphinpyrazone-induced acceleration of the metabolism of oxprenolol.

Antipyrine metabolite kinetics in healthy human volunteers during multiple dosing of phenytoin and carbamazepine

Antipyrine total clearance and the formation clearance of its major metabolites were studied in normal, healthy male volunteers before and after multiple dosing for approximately three weeks with phenytoin (six subjects) and carbamazepine (six subjects). Total antipyrine clearance increased on average by 91% after phenytoin dosing and by 61% after carbamazepine and individual increases correlated well with mean plasma concentrations of the anti-epileptic drug. The increase in total clearance resulted largely from increased formation clearances of the 4-hydroxy and 3-hydroxymethylantipyrine metabolites with minimal effect on the norantipyrine pathway, following treatment with both enzyme-inducing drugs. It is concluded that both phenytoin and carbamazepine have similar effects on antipyrine metabolism and that these effects are mediated by induction of specific forms of cytochrome P450.

Pharmacokinetics of sulphinpyrazone and its major metabolites after a single dose and during chronic treatment

The pharmacokinetics of sulphinpyrazone and its major metabolites (sulfide, sulfone, p-hydroxysulfone and p-hydroxy-sulphinpyrazone) were investigated in 9 volunteers after a single oral dose as well as after chronic treatment for 23 days. Chronic administration of sulphinpyrazone, in comparison with a single oral dose, led to significant changes in plasma AUC (115.86 to 42.90 mg/l . h), in renal clearance (1.06 to 1.80 l/h), in hepatic intrinsic clearance (319.0 to 598.0 l/h), and in the unbound fraction in plasma 1.15 to 1.69%) and in tissue (2.73 to 1.31%). The volume of distribution changed from 20.24 to 52.04 l. The steady state concentrations predicted from the single dose were significantly higher than the values found after chronic treatment. The results suggest that sulphinpyrazone induces its own metabolism. The metabolism of the sulfone, p-hydroxysulfone and the p-hydroxy-sulphinpyrazone to further degradation products was also induced. Chronic treatment with sulphinpyrazone reduced the plasma AUC of the sulfide and caused a decrease in its elimination half-life (20.9 to 14.3 h). Since considerable amounts of the sulfide are formed in the G.I. tract, it is suggested that besides the induction of metabolism, bacteria which reduce sulphinpyrazone to the sulfide may also be responsible for the observed pharmacokinetic changes.