Induction of mixed function oxidase activity in man by rifapentine (MDL 473), a long-acting rifamycin derivative

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Evaluation of the pharmacokinetic interaction between repeated doses of rifapentine or rifampin and a single dose of bedaquiline in healthy adult subjects

This study assessed the effects of rifapentine or rifampin on the pharmacokinetics of a single dose of bedaquiline and its M2 metabolite in healthy subjects using a two-period single-sequence design. In period 1, subjects received a single dose of bedaquiline (400 mg), followed by a 28-day washout. In period 2, subjects received either rifapentine (600 mg) or rifampin (600 mg) from day 20 to day 41, as well as a single bedaquiline dose (400 mg) on day 29. The pharmacokinetic profiles of bedaquiline and M2 were compared over 336 h after the administration of bedaquiline alone and in combination with steady-state rifapentine or rifampin. Coadministration of bedaquiline with rifapentine or rifampin resulted in lower bedaquiline exposures. The geometric mean ratios (GMRs) and 90% confidence intervals (CIs) for the maximum observed concentration (Cmax), area under the concentration-time curve to the last available concentration time point (AUC0-t), and AUC extrapolated to infinity (AUC0-inf) of bedaquiline were 62.19% (53.37 to 72.47), 42.79% (37.77 to 48.49), and 44.52% (40.12 to 49.39), respectively, when coadministered with rifapentine. Similarly, the GMRs and 90% CIs for the Cmax, AUC0-t, and AUC0-inf of bedaquiline were 60.24% (51.96 to 69.84), 41.36% (37.70 to 45.36), and 47.32% (41.49 to 53.97), respectively, when coadministered with rifampin. The Cmax, AUC0-t, and AUC0-inf of M2 were also altered when bedaquiline was coadministered with rifapentine or rifampin. Single doses of bedaquiline, administered alone or with multiple doses of rifapentine or rifampin, were well tolerated, with no safety concerns related to coadministration. Daily administration of rifapentine to patients with tuberculosis presents the same drug interaction challenges as rifampin and other rifamycins. Strong inducers of the cytochrome P450 isoenzyme CYP3A4 should be avoided when considering the use of bedaquiline. (This study is registered at clinicaltrials.gov under identifier NCT02216331.).

Update on rifampin, rifabutin, and rifapentine drug interactions

BACKGROUND

Rifampin is a potent inducer of both cytochrome P-450 oxidative enzymes and the P-glycoprotein transport system. Among numerous well documented, clinically significant interactions, examples include warfarin, oral contraceptives, itraconazole, digoxin, verapamil, simvastatin, and human immunodeficiency virus-related protease inhibitors. Rifabutin reduces serum concentrations of antiretroviral agents, but less so than rifampin. Rifapentine is also an inducer of drug metabolism.

METHODS

A literature search of English language journals from 2008 to March 2012 was completed using several databases, including PubMed, EMBASE, and SCOPUS. Search terms included rifampin, rifabutin, rifapentine AND drug interactions.

FINDINGS

Examples of clinically relevant interactions with rifampin demonstrated by recent reports include posaconazole, voriconazole, oxycodone, risperidone, mirodenafil, and ebastine.

CONCLUSIONS

To avoid a reduced therapeutic response, therapeutic failure, or toxic reactions when rifampin, rifabutin, or rifapentine are added to or discontinued from medication regimens, clinicians need to be aware of these interactions. Recent studies have indicated that other transporter systems play a role in these drug interactions. As reports of rifampin drug interactions continue to grow, this review is a reminder to clinicians to be vigilant.

Safety and pharmacokinetics of escalating daily doses of the antituberculosis drug rifapentine in healthy volunteers

Rifapentine (RP T) is an antituberculosis drug that may shorten treatment duration when substituted for rifampin (RI F).The maximal tolerated daily dose of RP T and its potential for cytochrome 3A4 induction and autoinduction at clinically relevant doses are unknown. In this phase I, dose-escalation study among healthy volunteers, daily doses as high asa prespecified maximum of 20 mg/kg/day were well tolerated. Steady-state RP T concentrations increased with dose from 5 to 15 mg/kg, but area under the plasma concentration–time curve (AU C0–24) and maximum concentration (Cmax)were similar in the 15- and 20-mg/kg cohorts. Although RP T pharmacokinetics (PK) appeared to be time-dependent,accumulation occurred with daily dosing. The mean AU C0–12 of oral midazolam (MDZ), a cytochrome 3A (CYP 3A) probe drug, was reduced by 93% with the coadministration of RPT and by 74% with the coadministration of RIF (P < 0.01).Changes in the oral clearance of MDZ did not vary by RP T dose. In conclusion, RP T was tolerated at doses as high as20 mg/kg/day, its PK were less than dose-proportional, and its CYP 3A induction was robust.

Ontogeny of drug metabolizing enzymes in the neonate

Fetal exposure to xenobiotics is modulated to a considerable degree by the metabolic capabilities of the mother and the placenta. However, once liberated from the uterine environment the neonate is instantly exposed to a wide array of new macromolecules in the form of byproducts of cellular metabolism, dietary constituents, environmental toxins and pharmacologic agents. The rapid and efficient biotransformation of these compounds by Phase I and Phase II drug-metabolizing enzymes is an essential process if the infant is to avoid the accumulation of reactive compounds that could produce cellular injury or tissue dysfunction. Genetic polymorphisms and environmental factors are known to contribute dramatically to individual variation in the activity of drug-metabolizing enzymes. More recently, it has become apparent that programmed, developmental, regulatory events occur - independent of genotype - which further add to individual variation in drug metabolism. An appreciation of the impact of ontogeny on the expression and functional activity of the major drug-metabolizing enzymes enables the practicing clinician to predict the ultimate consequence of drug administration in the neonate to help guide optimal drug therapy.

Pharmacokinetic Considerations in the Treatment of Tuberculosis in Patients with Renal Failure

Tuberculosis is re-emerging in patients with altered immune status, such as those with chronic renal failure. Clinicians should thus be aware of the pharmacokinetics and dosage adjustment of antitubercular drugs in patients with renal insufficiency. Among patients with renal insufficiency, those who are dialysed should be treated with special care. Indeed, dosage should always be closely adjusted in these patients and potential removal by dialysis must be taken into account. However reliable the dosage adjustment recommendations are for these drugs in patients with renal failure, further pharmacokinetic investigations need to be performed, especially in dialysis patients in whom the influence of haemodialysis and continuous ambulatory peritoneal dialysis on drug pharmacokinetics needs to be detailed. In the meantime, it could be generally advised to administer all antitubercular drugs after the haemodialysis session, even though some drugs are known to be non-dialysable.

Cytochrome P450 3A: ontogeny and drug disposition

The maturation of organ systems during fetal life and childhood exerts a profound effect on drug disposition. The maturation of drug-metabolising enzymes is probably the predominant factor accounting for age-associated changes in non-renal drug clearance. The group of drug-metabolising enzymes most studied are the cytochrome P450 (CYP) superfamily. The CYP3A subfamily is the most abundant group of CYP enzymes in the liver and consists of at least 3 isoforms: CYP3A4, 3A5 and 3A7. Many drugs are mainly metabolised by the CYP3A subfamily. Therefore, maturational changes in CYP3A ontogeny may impact on the clinical pharmacokinetics of these drugs. CYP3A4 is the most abundantly expressed CYP and accounts for approximately 30 to 40% of the total CYPcontent in human adult liver and small intestine. CYP3A5 is 83% homologous to CYP3A4, is expressed at a much lower level than CYP3A4 in the liver, but is the main CYP3A isoform in the kidney. CYP3A7 is the major CYP isoform detected in human embryonic, fetal and newborn liver, but is also detected in adult liver, although at a much lower level than CYP3A4. Substrate specificity for the individual isoforms has not been fully elucidated. Because of large interindividual differences in CYP3A4 and 3A5 expression and activity, genetic polymorphisms have been suggested. However, although some gene mutations have been identified, the impact of these mutations on the pharmacokinetics of CYP3A substrates has to be established. Ontogeny of CYP3A activity has been studied in vitro and in vivo. CYP3A7 activity is high during embryonic and fetal life and decreases rapidly during the first week of life. Conversely, CYP3A4 is very low before birth but increases rapidly thereafter, reaching 50% of adult levels between 6 and 12 months of age. During infancy, CYP3A4 activity appears to be slightly higher than that of adults. Large interindividual variations in CYP3A5 expression and activity were observed during all stages of development, but no apparent developmental pattern of CYP3A5 activity has been identified to date. Profound changes occur in the activity of CYP3A isoforms during all stages of development. These changes have, in many instances, proven to be of clinical significance when treatment involves drugs that are substrates, inhibitors or inducers of CYP3A. Investigators and clinicians should consider the impact of ontogeny on CYP3A in both pharmacokinetic study design and data interpretation, as well as when prescribing drugs to children.

Rifapentine pharmacokinetics in adolescents

OBJECTIVE

Determination of rifapentine pharmacokinetics in healthy adolescent children.

DESIGN

Prospective Phase II clinical trial.

SETTING

Clinical research center within a university children's hospital.

PATIENTS

Twelve subjects ranging in age from 12 to 15 years, male and female.

INTERVENTIONS

A single oral dose of rifapentine was administered to healthy adolescent volunteers, 450 mg if <45 kg or 600 mg if > or =45 kg. Blood was collected at serial intervals (0, 2, 3, 4, 5, 6, 8, 12, 18, 24, 48 and 72 h postdose). Subjects were observed for adverse effects during the period of study.

MEASUREMENTS

High pressure liquid chromatography was used to measure the plasma concentration of rifapentine and 25-desacetyl rifapentine in each blood sample. For each subject a plot of mean plasma concentration vs. time data for rifapentine and its metabolite (i.e. 25-desacetyl rifapentine) were created. Subsequently model-independent methods were used to determine the pharmacokinetic profiles for each subject.

RESULTS

All subjects tolerated rifapentine without adverse effects. The 2-h postdose plasma concentrations of rifapentine (6.59 to 9.05 microg/ml) and 25-desacetyl rifapentine (0.57 to 2.64 microg/ml) far exceeded the MIC of Mycobacterium tuberculosis to rifapentine (approximately 0.12 microg/ml). The combination of a high Cmax (rifapentine, 9.95 to 18.63 microg/ml; 25-desacetyl rifapentine, 3.73 to 7.46 microg/ml) and lengthy terminal elimination phase t1/2 (rifapentine, 10 to 23 h; 25-desacetyl rifapentine, 14 to 35 h) resulted in potentially effective plasma concentrations of both compounds that persisted for at least 48 h in most subjects.

CONCLUSIONS

A well-tolerated oral rifapentine dose produced rapid and sustained plasma drug concentrations in adolescents that should effectively treat infections caused by M. tuberculosis. Rifapentine pharmacokinetics appears to be similar in adolescent and adult populations.

Rifapentine

Rifapentine is a rifamycin antibiotic with antimycobacterial activity. Rifapentine is generally more active against Mycobacterium tuberculosis than rifampicin (rifampin), although strains resistant to rifampicin are usually cross-resistant to rifapentine. Sputum culture conversion rates were slightly higher after 6 months of rifapentine- versus rifampicin-based therapy in patients with pulmonary tuberculosis in a Western study; however, relapse rates were higher in rifapentine recipients during follow-up. The excess relapses in the rifapentine group appeared to be related to poor compliance with nonrifamycin antituberculosis drugs during the intensive phase (first 2 months) of therapy. Rifapentine- and rifampicin-containing regimens produced similar sputum culture conversion rates with low rates of relapse in 2 randomised clinical trials in patients with smear-positive tuberculosis in China. In one trial, there was no difference in sputum culture conversion rates in patients treated with rifapentine once weekly or rifampicin twice weekly in combination with isoniazid and ethambutol during the continuation phase of treatment. Hyperuricaemia, which was reported only during the intensive phase, elevated ALT and AST levels and neutropenia were the most common treatment-related adverse events reported in patients receiving rifapentine- or rifampicin-containing regimens for tuberculosis in 1 Western study.

Effect of a new rifamycin derivative, rifalazil, on liver microsomal enzyme induction in rat and dog

1. The effect of a new rifamycin derivative, rifalazil (KRM-1648), on liver microsomal enzyme induction was studied in rat and dog with repeated oral administration of the compound. Relative liver weight, cytochrome b5 and P450 contents, enzyme activities of NADPH-cytochrome c reductase, aniline hydroxylase, p-nitroanisole O-demethylase, aminopyrine N-demethylase, and erythromycin N-demethylase were measured. 2. In rat, rifalazil treatment at 300 mg/kg/day for 10 days increased cytochrome b5 content but it did not affect liver weight, P450 content or enzyme activities. In contrast, rifampicin and rifabutin increased relative liver weights, cytochrome contents and enzyme activities under similar conditions. 3. In dog, rifalazil did not affect any parameters at 30 or 300 mg/kg/day for 13 weeks. 4. These findings indicate that rifalazil is not an enzyme inducer in rat and dog. This property differs from other rifamycin derivatives such as rifampicin and rifabutin.

Primary human hepatocytes as a tool for the evaluation of structure-activity relationship in cytochrome P450 induction potential of xenobiotics: evaluation of rifampin, rifapentine and rifabutin

In our laboratory, primary human hepatocytes are being investigated as an in vitro experimental system for the evaluation of pharmacokinetic drug-drug interactions. Our study here represents the first reported study that directly compares the cytochrome P450 isozyme 3A (CYP3A) induction potential of three antimicrobials derived from rifamycin B, namely, rifampin, rifapentine and rifabutin. Two endpoints of CYP3A activity, testosterone 6 beta-hydroxylation and midazolam 1-hydroxylation have been used. Results obtained with hepatocytes from four different human donors show consistently that rifampin and rifapentine are potent inducers of CYP3A, while a significantly lower induction potential is observed for rifabutin. The relative induction potency of the three antimicrobials (rifampin > rifapentine >> rifabutin) is consistent with the available human in vivo data. For CYP1A measured as ethoxyresorufin O-deethylase activity, CYP2C8/9 measured as tolbutamide 4-hydroxylation activity, CYP2D6 measured as dextromethorphan O-demethylation, and AZT glucuronidation, there is either no effect or, where induction is found to be statistically significant in these other endpoints, the maximum induction values are consistently < 100% of the control. Our results suggest that CYP3A is the major CYP induced by these rifamycin B derivatives. These studies illustrate the application of human hepatocytes in the evaluation of the structure-activity relationships in CYP induction for this class of chemicals and as an in vitro screen for drug-drug interaction potential via CYP induction.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Rifampin and rifabutin and their metabolism by human liver esterases

1. The main metabolites of rifampin and rifabutin in man are their respective 25 deacetylated derivatives, but the enzyme(s) responsible for these biotransformations are not known. 2. In experiments with human liver slices and human liver microsomes, the 25 deacetylated derivatives of these drugs were the main metabolites observed. Slices and microsomes metabolized rifabutin 3-6-fold faster than rifampin, in agreement with their relative clearance in patients. Rifabutin partitioned into slices more avidly than rifampin. 3. In microsomal incubations, deacetylation did not require NADPH, but the amount of metabolite at the end of incubation was affected by NADPH. With NADPH the amount of 25 deacetyl rifabutin decreased, whereas the amount of 25 deacetyl rifampin increased slightly. A panel of liver microsomes from seven donors showed a 3-4-fold difference in the formation of 25 deacetyl rifabutin or 25 deacetyl rifampin, with strong correlation between the production of the two metabolites (r2 = 0.94). 4. The production of 25 deacetyl rifabutin and 25 deacetyl rifampin by human liver microsomes was not significantly affected by 1 microM 4 chloromercuricbenzoic acid or bis-(4-nitrophenyl) phosphate, but was completely inhibited by 1 microM paraoxon or 1 microM diisopropylfluorophosphate. These results indicate that in man rifampin and rifabutin are deacetylated to their main metabolites by B-esterases.

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Protective effect of tulobuterol on methacholine-induced bronchospasm in asthmatic children

In a single-blind, placebo-controlled trial 19 asthmatic children aged 10-14 years were studied during a stable phase of the disease. All patients were tested with a methacholine inhalation test on two consecutive days, on the first day after oral administration of a placebo, and on the second day following an oral dose of tulobuterol HCl. The cumulative methacholine dose required to produce at least a 20% decrease in FEV1 was significantly higher after tulobuterol than after the placebo. It is concluded that tulobuterol HCl reduces methacholine-induced bronchospasm in children.