Pharmacokinetic interactions with rifampicin

Analytical Methods Practiced to Quantitation of Rifampicin: A Captious Survey

Background:

Rifampicin (RIF), also known as rifampin, a bactericidal antibiotic having broad antibacterial activity against various gram-positive and gram-negative bacteria acts by inhibiting DNA dependent RNA polymerase. RIF has been administered in different dosage forms like tablets, capsules, injections, oral suspension, powder, etc. for the treatment of several types of bacterial infections, including tuberculosis, Mycobacterium avium complex, leprosy and Legionnaires’ disease.

Introduction:

To ensure the quality, efficacy, safety and effectiveness of RIF drug product, effective and reliable analytical methods are of utmost importance. To quantify RIF for quality control or pharmacokinetic purposes, alternative analytical methods have been developed along with the official compendial methods.

Methods:

In this review paper, an extensive literature survey was conducted to gather information on various analytical instrumental methods used so far for RIF.

Results:

These methods were high-performance liquid chromatography (42%), hyphenated techniques (18%), spectroscopy (15%), high-performance thin-layer chromatography or thin-layer chromatography (7%) and miscellaneous (18%).

Conclusion:

All these methods were selective and specific for the RIF analysis.

Fabrication and Characterization of Surface Engineered Rifampicin Loaded Lipid Nanoparticulate Systems for the Potential Treatment of Tuberculosis: An In Vitro and In Vivo Evaluation

The main aim of the present investigation highlights the development of mannose appended rifampicin containing solid lipid nanoparticles (Mn-RIF-SLNs) for the management of pulmonary TB. The developed Mn-RIF-SLNs showed particle size of Mn-RIF-SLNs (479 ± 13 nm) which was found to be greater than that of unconjugated SLNs (456 ± 11 nm), with marginal reduction in percentage entrapment efficiency (79.41 ± 2.42%). The in vitro dissolution studies depicted an initial burst release followed by sustained release profile indicating biphasic release pattern, close-fitting Weibull model having least F-value. The cytotoxicity studies using J774A.1 cell line represented that the developed SLNs were non-toxic and safe as compared to free drug. Fluorescence imaging and flow cytometric (FACS) analysis depicted significant (1.79-folds) intracellular uptake of coumarin-6 (fluorescent marker) loaded Mn-C6-SLNs. The in vivo pharmacokinetic studies in sprague-dawley rats were performed and Mn-RIF-SLNs showed remarkable enhancement in terms of relative bioavailability (~17-folds) as compared to its drug solution via oral administration. The biodistribution studies revealed higher lung accumulation (1.8-folds) of Mn-RIF-SLNs as compared to the Un-RIF-SLNs. In conclusion, the developed Mn-RIF-SLNs could serve as a promising tool for delivering the drug cargo to the site of infection (lungs) in the treatment of TB.

The challenges of pharmacokinetic variability of first-line anti-TB drugs

INTRODUCTION

Inter-individual variations in the pharmacokinetics (PK) of anti-TB drugs are known to occur, which could have important therapeutic implications in patient management. Areas covered: We compiled factors responsible for PK variability of anti-TB drugs reported from different settings that would give a better understanding about the challenges of PK variability of anti-TB medications. We searched PubMed data base and Google scholar from 1976 to the present using the key words 'Pharmacokinetics', 'pharmacokinetic variability', 'first-line anti-TB therapy', 'Rifampicin', 'Isoniazid', 'Ethambutol', 'Pyrazinamide', 'food', 'nutritional status', 'HIV', 'diabetes', 'genetic polymorphisms' and 'pharmacokinetic interactions'. We also included abstracts from scientific meetings and review articles. Expert commentary: A variety of host and genetic factors can cause inter-individual variations in the PK of anti-TB drugs. PK studies conducted in various settings have adopted different designs, PK sampling time points, drug estimation methodologies. Hence comparison and interpretation of these results should be done with caution More phamacogenomic studies in different patient populations are needed for further understanding.

Interaction of an antituberculosis drug with nano-sized cationic micelle: Förster resonance energy transfer from dansyl to rifampicin in the microenvironment

In this contribution, we report studies on the interaction of an antituberculosis drug rifampicin (RF) in a macromolecular assembly of CTAB with an extrinsic fluorescent probe, dansyl chloride (DC). The absorption spectrum of the drug RF has been employed to study Förster resonance energy transfer (FRET) from DC, bound to the CTAB micelle using picosecond resolved fluorescence spectroscopy. We have applied a kinetic model developed by Tachiya to understand the kinetics of energy transfer and the distribution of acceptor (RF) molecules around the donor (DC) molecules in the micellar surface with increasing quencher concentration. The mean number of RF molecules associated with the micelle increases from 0.24 at 20 μm RF concentration to 1.5 at 190 μm RF concentration and consequently the quenching rate constant (k(q)) due to the acceptor (RF) molecules increases from 0.23 to 0.75 ns(-1) at 20 and 190 μm RF concentration, respectively. However, the mean number of the quencher molecule and the quenching rate constant does not change significantly beyond a certain RF concentration (150 μm), which is consistent with the results obtained from time resolved FRET analysis. Moreover, we have explored the diffusion controlled FRET between DC and RF, using microfluidics setup, which reveals that the reaction pathway follows one-step process.

Rifampin combination therapy for nonmycobacterial infections

The increasing emergence of antimicrobial-resistant organisms, especially methicillin-resistant Staphylococcus aureus (MRSA), has resulted in the increased use of rifampin combination therapy. The data supporting rifampin combination therapy in nonmycobacterial infections are limited by a lack of significantly controlled clinical studies. Therefore, its current use is based upon in vitro or in vivo data or retrospective case series, all with major limitations. A prominent observation from this review is that rifampin combination therapy appears to have improved treatment outcomes in cases in which there is a low organism burden, such as biofilm infections, but is less effective when effective surgery to obtain source control is not performed. The clinical data support rifampin combination therapy for the treatment of prosthetic joint infections due to methicillin-sensitive S. aureus (MSSA) after extensive debridement and for the treatment of prosthetic heart valve infections due to coagulase-negative staphylococci. Importantly, rifampin-vancomycin combination therapy has not shown any benefit over vancomycin monotherapy against MRSA infections either clinically or experimentally. Rifampin combination therapy with daptomycin, fusidic acid, and linezolid needs further exploration for these severe MRSA infections. Lastly, an assessment of the risk-benefits is needed before the addition of rifampin to other antimicrobials is considered to avoid drug interactions or other drug toxicities.

Medications for Extensively Drug-Resistant Tuberculosis: Back to the Future?

Objective:

To reexamine the existing medications for the potential treatment of extensively drug-resistant tuberculosis (XDR-TB), based on susceptibility data, and to identify potential future medications from the literature.

Data Sources:

Relevant information was identified through a search of MEDLINE (1966–November 2007), PubMed (1955–November 2007), American Search Premier (1975–November 2007), International Pharmaceutical Abstracts (1960–November 2007), Science Citation Index Expanded (1996–November 2007), Cochrane Databases (publications archived until November 2007), and various tertiary sources as listed in the references, using the terms extensively drug-resistant tuberculosis (XDR-TB), ethambutol, pyrazinamide, para-aminosalicylic acid, cycloserine, linezolid, diarylquinoline, nitroimidazopyran, fluoroquinolones, β-lactams, new treatments, and ethionamide alone or in combination regimens.

Study Selection and Data Extraction:

After identification of the relevant information, the data presented in this article were selected based on clinical relevance and value of information.

Data Synthesis:

Based on susceptibility data, pyrazinamide, ethambutol, para-aminosalicylic acid, cycloserine, and ethionamide may be used for the treatment of tuberculosis. However, due to the emergence of XDR-TB, many of these agents are no longer successful treatment regimens. We have found limited data supporting potential future use of β-lactams, clarithromycin, and linezolid in resistant TB infections. TMC207, nitroimidazopyran, and SQ109 compounds may also prove to be viable options in the near future for treatment of tuberculosis, especially in cases with resistance to mainstay medications.

Conclusions:

Extensively resistant tuberculosis appears to be a potentially catastrophic disease if allowed to spread. Due to its resistance profile, very few potentially effective agents are available, calling attention to this growing problem.

Clinically significant interactions with drugs used in the treatment of tuberculosis

Clinically significant interactions occurring during antituberculous chemotherapy principally involve rifampicin (rifampin), isoniazid and the fluoroquinolones. Such interactions between the antituberculous drugs and coadministered agents are definitely much more important than among antituberculous drugs themselves. These can be associated with consequences even amounting to therapeutic failure or toxicity. Most of the interactions are pharmacokinetic rather than pharmacodynamic in nature. The cytochrome P450 isoform enzymes are responsible for many interactions (especially those involving rifampicin and isoniazid) during drug biotransformation (metabolism) in the liver and/or intestine. Generally, rifampicin is an enzyme inducer and isoniazid acts as an inhibitor. The agents interacting significantly with rifampicin include anticoagulants, anticonvulsants, anti-infectives, cardiovascular therapeutics, contraceptives, glucocorticoids, immunosuppressants, psychotropics, sulphonylureas and theophyllines. Isoniazid interacts principally with anticonvulsants, theophylline, benzodiapines, paracetamol (acetaminophen) and some food. Fluoroquinolones can have absorption disturbance due to a variety of agents, especially the metal cations. Other important interactions of fluoroquinolones result from their enzyme inhibiting potential or pharmacodynamic mechanisms. Geriatric and immunocompromised patients are particularly at risk of drug interactions during treatment of their tuberculosis. Among the latter, patients who are HIV infected constitute the most important group. This is largely because of the advent of new antiretroviral agents such as the HIV protease inhibitors and the non-nucleoside reverse transcriptase inhibitors in the armamenterium of therapy. Compounding the complexity of drug interactions, underlying medical diseases per se may also contribute to or aggravate the scenario. It is imperative for clinicians to be on the alert when treating tuberculosis in patients with difficult co-morbidity requiring polypharmacy. With advancement of knowledge and expertise, it is hoped that therapeutic drug monitoring as a new paradigm of care can enable better management of these drug interactions.

Dapsone and sulfones in dermatology: overview and update

In their 60-year history, dapsone and the sulfones have been used as both antibacterial and anti-inflammatory agents. Dapsone has been used successfully to treat a range of dermatologic disorders, most successfully those characterized by abnormal neutrophil and eosinophil accumulation. This article reviews and updates the chemistry, pharmacokinetics, clinical application, mechanism of action, adverse effects, and drug interactions of dapsone and the sulfones in dermatology.

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.

Controversies in the management of Mycobacterium avium complex infection in AIDS patients

OBJECTIVE

To update readers on the clinical management of infections secondary to Mycobacterium avium complex (MAC) in patients with AIDS. A general description of the organism, culture and susceptibility testing, and clinical manifestations of the disease is provided. Several aspects of the treatment of the disease, including an historical perspective, current approaches, and future research opportunities, are described.

DATA SOURCES

Current medical literature, including abstracts presented at international meetings, is reviewed. References were identified through MEDLINE, Current Contents, and published meeting abstracts.

STUDY SELECTION

Data regarding the epidemiology, clinical manifestations, culture and susceptibility testing, and treatment of MAC are cited. Specific attention is given to the management of patients with MAC infection.

DATA EXTRACTION

Information contributing to the discussion of the topics selected by the author is reviewed. Data supporting and disputing specific conclusions are presented.

DATA SYNTHESIS

Disseminated MAC infection is diagnosed antemortem in approximately 30 percent of patients with AIDS; postmortem rates of isolation exceed 50 percent. The incidence of MAC may increase as attempts at isolating the organism become more aggressive. The traditional approach to the isolation, susceptibility testing, and treatment of MAC has been derived from the management of Mycobacterium tuberculosis, with disappointing results. Newer radiometric in vitro methods of susceptibility testing appear to show more promise. Current mouse models of MAC are not true AIDS models; new CD4-deficient mouse models are being developed. Clinical mycobacteriologic and pharmacokinetic laboratory support have been underused, with treatment generally proceeding empirically. New agents that may contribute to the management of disseminated MAC infection include the macrolide derivatives clarithromycin and azithromycin. Research also continues with new rifamycins (including rifabutin) and fluoroquinolones (ciprofloxacin, sparfloxacin). Preliminary results suggest a central role for macrolides in the treatment of disseminated MAC; effective companion drugs are needed to prevent the rapid emergence of macrolide-resistant MAC.

CONCLUSIONS

Treatment results for disseminated MAC infection remain poor. Therapy may be improved by selecting drugs on the basis of susceptibility data for each isolate, rather than by using empiric regimens based on susceptibility trends. Significant antimycobacterial drug malabsorption has been documented, and may contribute to poor outcomes. More-potent agents are needed to improve the clinical outcome in AIDS patients with MAC.

Metabolism of antipyrine and sulphadimidine in dwarf goats: effects of the enzyme-inducing agents phenobarbital, troleandomycin and rifampicin

1. Antipyrine (AP) and sulphadimidine (SDD) plasma elimination and metabolite formation were studied in dwarf goats before and after treatment with phenobarbital (PB), triacetyloleandomycin (TAO), and rifampicin (RIF). 2. PB treatment significantly increased AP plasma clearance in both male and female goats. With SDD, only male goats were studied, which showed a significant increase of SDD plasma clearance following PB treatment. 3. After PB treatment, partial clearance values of four AP metabolites, 3-hydroxymethylantipyrine (HMA), norantipyrine (NORA), 4-hydroxyantipyrine (OHA) and 4,4'-dihydroxyantipyrine (DOHA), were significantly increased. This induction effect was different for the individual metabolites and also showed sex-dependency. 4. In PB-induced male goats the formation of the hydroxylated SDD metabolites, 6-hydroxymethyl-SDD and 5-hydroxy-SDD, was significantly increased. 5. After TAO treatment, female goats showed a slightly reduced AP plasma clearance and a decreased partial clearance of two AP metabolites, HMA and DOHA. There was no effect on SDD plasma elimination or metabolite excretion. 6. In male goats, RIF had no effect on plasma elimination of AP and SDD. With SDD, it decreased the urinary excretion of the unchanged drug and its N4-acetylated metabolite. 7. Induction/inhibition studies of drug metabolism in food-producing animal species are desirable to gain more insight into the regulation of enzymes involved in the metabolism of xenobiotics.

Displacement of rifampicin bound to serum proteins by addition of levamisole

We have studied, by ultrafiltration, the interactions between rifampicin (15 and 30 microM) and levamisole (7 microM) since both drugs may be associated for the treatment of brucellosis. We can observe a statistically significant increase in the free plasmatic fraction of rifampicin at the studied concentration of levamisole, which indicated that levamisole reduced rifampicin bound to proteins (290 and 250%, respectively).

Interactions between chloramphenicol, acepromazine, phenylbutazone, rifampin and thiamylal in the horse

The potential for interactions between chloramphenicol, phenylbutazone, acepromazine and thiamylal and chloramphenicol, rifampin, and phenylbutazone were evaluated in two groups of experiments. In the first, five horses were given thiamylal intravenously (iv) (6.6 mg/kg) after pretreatment with acepromazine, and the time of recumbency was determined. Administration of chloramphenicol iv (25 mg/kg) 1 h prior to anaesthesia significantly lengthened the recumbency time from 21.8 +/- 4.8 mins to 36.0 +/- 8.3 mins. There was an apparent but not statistically significant decrease in recumbency time when phenylbutazone (4.4 mg/kg) was administered iv daily for 4 days prior to anaesthesia. In the second series of experiments, phenylbutazone (4.4 mg/kg), chloramphenicol (25 mg/kg) and rifampin (10 mg/kg) were administered in various sequences to five different horses. Chloramphenicol pretreatment produced a significant decrease in the elimination rate and rifampin a significant increase in the elimination rate of phenylbutazone. The half-life of elimination of phenylbutazone alone was about 4 h. Following four days pretreatment with rifampin it was approximately 2.7 h, it was approximately 5.6 h and 9.5 h, respectively, when chloramphenicol was administered in one dose 1 h before or two doses 12 h and 1 h before phenylbutazone.

Enhanced pirmenol elimination by rifampin

The potential for drug-drug interaction between pirmenol, an extensively metabolized antiarrhythmic agent, and rifampin, a potent inducer of hepatic drug-metabolizing enzymes, was evaluated in 12 healthy adults. After administration of a single 150-mg oral dose of pirmenol on day 1, pirmenol plasma and urine concentrations were determined for 72 hours postdose. On days 4 through 17, subjects received 600 mg of rifampin once daily. On day 15, subjects were given a second single 150-mg oral dose of pirmenol concomitantly with rifampin, and plasma and urine concentrations were again determined. Coadministration of rifampin with pirmenol resulted in significant (P less than .005) changes in pirmenol pharmacokinetic parameters. A sixfold decrease in pirmenol AUC and sevenfold increase in the apparent plasma clearance of pirmenol were found. Elimination half-life decreased more than twofold. Based on these findings, pirmenol dosage adjustment will be required when pirmenol is given to patients concurrently receiving rifampin. These results suggest that the administration of pirmenol with other agents that induce hepatic enzymes may result in accelerated pirmenol clearance.

Mechanisms of action of aminoglutethimide as endocrine therapy of breast cancer

During the last decade aminoglutethimide has been recognised as a valuable alternative in endocrine therapy for advanced breast cancer. Although some side effects do occur, most often these are initial effects which subside within a few weeks, and cessation of therapy is not usually indicated. Aminoglutethimide was originally introduced as an inhibitor of steroidogenesis in the adrenal cortex. It was soon recognised, however, that inhibition of the non-glandular aromatase, blocking the conversion of androgenic prohormones to oestrogens, was more important, resulting in decreased blood levels of oestrogens. In this review the role of aromatase inhibition as the only important aspect of the mechanism of action of aminoglutethimide is challenged. Evidence has accumulated during the last few years that aminoglutethimide is a most potent inducer of microsomal enzymes. In addition to the pharmacological implications this has (suggesting important interactions), it also points to the possibility that levels of oestrogens are decreased due to accelerated metabolism of these hormones. Based on new experimental data, and also clinical work with alternative aromatase inhibitors, it appears that the antitumour activity of aminoglutethimide may be due to both aromatase inhibition and accelerated metabolism of oestrogens. This seriously challenges the importance of aromatase inhibition alone as a strategy in endocrine therapy of breast cancer, and furthermore suggests that accelerated metabolism of key hormones is an alternative strategy to be explored.

Comparative effects of rifabutin and rifampicin on hepatic microsomal enzyme activity in normal subjects

The comparative enzyme inducing effects of rifabutin and the chemically related drug rifampicin have been investigated in 8 normal subjects. Rifampicin 600 mg daily for 7 days caused considerable shortening of the antipyrine half-life and a marked increase in antipyrine clearance, associated with an increased rate of conversion to norantipyrine and, to a lesser extent, 4-hydroxyantipyrine and 3-hydroxymethylantipyrine. The urinary excretion of 6-beta-hydroxycortisol was also markedly increased, while plasma GGT activity showed only a slight albeit statistically significant elevation. In the same subjects, rifabutin in the proposed therapeutic dosage (300 mg daily) for 7 days also enhanced the metabolic elimination of antipyrine, with preferential stimulation of the demethylation pathway, and increased the excretion of 6-beta-hydroxycortisol, but the magnitude of the effects was significantly less than after rifampicin. No significant change in plasma GGT was seen. The results indicate that, contrary to the findings in animals, rifabutin does have enzyme inducing properties in man, although at the dosages assessed they were considerably less than those of rifampicin.

Pharmacokinetics of antituberculosis drugs in patients

The pharmacokinetics of rifampin, isoniazid, and ethambutol were determined in 26 ambulatory male patients (aged 49.5 +/- 9.9 yr) with tuberculosis. Rifampin and isoniazid were given individually or together, with or without ethambutol; studies were done after a single dose and after chronic administration. Under the study conditions, with large variability in the extent of disease and physical status and history of alcohol and tobacco abuse and narrow age range, the pharmacokinetics of these three antituberculosis drugs were not modified significantly by patient age. Furthermore, appreciable drug-drug interactions did not occur when the three drugs were administered concurrently. Self-induction of rifampin clearance by chronic dosing with the drug may lead to subtherapeutic levels of rifampin. Administration of isoniazid and ethambutol in two divided doses resulted in peak plasma concentrations below the accepted therapeutic levels of the two drugs. Our findings indicate that at least in the middle-aged patients with tuberculosis, the current single daily dose, multiple-drug regimen is therapeutically sound pharmacokinetically, and clinicians do not have to make adjustments in dosages of these drugs for age and the extent of disease.

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

The effects of rifapentine (MDL 473) administration on hepatic mixed function oxidase activity in man have been investigated in six healthy volunteers. Administration of rifapentine (600 mg 48 h-1) for 10 days resulted in a significant reduction in antipyrine half-life (from 13.2 +/- 1.0 h to 7.7 +/- 0.4 h) and a corresponding increase in its total body clearance (from 41.8 +/- 5.5 ml min-1 to 67.4 +/- 5.6 ml min-1). Twelve days after stopping rifapentine administration, these values had largely returned to base-line. 24-Hour excretion of 6 beta-hydroxycortisol was significantly increased, by approximately three-fold, following administration of rifapentine for 10 days. Again, 12 days after stopping drug administration, 6 beta-hydroxycortisol excretion had returned to pretreatment values. Clearance of antipyrine to its three oxidative metabolites was increased by rifapentine administration, although the increase for 3-hydroxymethylantipyrine was not significant. The greatest increase (+140%) was observed for norantipyrine. Twelve days after the last dose of rifapentine, all values had returned to control levels. It is concluded that, like rifampicin, rifapentine is a potent inducer of mixed function oxidase activity in man and that the possibility of clinically significant drug interactions should be anticipated in the therapeutic use of this compound.

Influence of rifampicin and isoniazid on the kinetics of phenytoin

Clearance of phenytoin after i.v. injection of 100 mg was studied in six patients before and after 2 weeks daily treatment with 450 mg rifampicin, and in 14 patients with tuberculosis receiving standard treatment with 450 mg rifampicin, 300 mg isoniazid, and 1200 mg ethambutol daily. Acetylator status was measured by urinary acetylated sulphadimidine. Clearance of phenytoin in patients receiving only rifampicin increased from 46.7 ml min-1 +/- 20.6 ml min-1 to 97.8 ml min-1 +/- 33.4 ml min-1 (P less than 0.01), while clearance in patients on three drugs increased from 47.1 +/- 23.4 ml min-1 to 81.3 ml min-1 +/- 41.6 ml min-1 (P less than 0.01). No significant differences were observed between the six fast acetylators and the eight slow acetylators. Phenytoin kinetics were unchanged after further 3 months of combined treatment. Rifampicin is a strong inducer of the elimination of phenytoin. Combined treatment with isoniazid has no counter-acting effect in either fast or slow acetylators.

Influence of rifampicin treatment on antipyrine clearance and metabolite formation in patients with tuberculosis

The influence of an 8-day therapy with rifampicin (600 mg daily) was studied on antipyrine plasma clearance and metabolite formation in seven patients with tuberculosis (age 18-79 years), who were also treated with isoniazid and pyrazinamide. After rifampicin treatment the elimination half-life of antipyrine had decreased in all patients from 12.9 +/- 5.0 to 8.8 +/- 2.0 h (P less than 0.05). Antipyrine clearance had increased from 2.2 +/- 0.9 to 2.9 +/- 0.7 l/h (P less than 0.05), while no change in apparent volume of distribution was observed. The increase in antipyrine clearance was primarily due to a selective increase in the rate of formation of norantipyrine by 80% from 6.9 +/- 3.4 to 12.4 +/- 3.4 ml/min. Rifampicin seems to induce preferentially the cytochrome P-450 (iso-) enzyme(s) involved in the demethylation of antipyrine to norantipyrine. Other pathways of antipyrine metabolism were hardly affected. This provides further evidence for the involvement of different iso-enzymes of the cytochrome P-450 system in antipyrine metabolism in man.

Clinical pharmacokinetics of the antituberculosis drugs

The quantitative aspects of the disposition in man of 12 antituberculosis drugs [isoniazid, rifampicin, (rifampin), ethambutol, para-aminosalicylic acid, pyrazinamide, streptomycin, kanamycin, ethionamide, cycloserine, capreomycin, viomycin and thiacetazone] are reviewed. Isoniazid appears to be the only agent for which plasma concentrations and clearance are related to hereditary differences in acetylator status and for which there is an appreciable 'first-pass' effect. Recent data cast doubt on the suggestion that isoniazid may be more hepatotoxic for rapid as opposed to slow acetylators. Continuous administration of rifampicin leads to induction of enzymes in the liver with a concomitant decrease in maximum plasma concentrations, the time required to achieve this level, elimination half-life, and area under the plasma concentration-time curve (AUC). Coadministration of para-aminosalicylic acid leads to increases in the serum concentrations and elimination half-life of isoniazid. With a few exceptions, the metabolites of the antituberculosis drugs are devoid of antimicrobial activity; the exceptions are 25-desacetylrifampicin which accounts for approximately 80% of the drug's antimicrobial activity in human bile, the acetylated and glycylated metabolites of para-aminosalicylic acid, and the sulphoxide metabolites of ethionamide. The effect of renal impairment is relatively unimportant for the excretion of isoniazid, rifampicin and para-aminosalicylic acid, but the elimination half-life of streptomycin increases to 100 hours when the blood urea nitrogen level is greater than 100mg/100ml, and ototoxicity is strikingly more frequent. In states of malnutrition, such as kwashiorkor, the protein binding of para-aminosalicylic acid decreases from 15% to essentially zero and in the case of ethionamide and streptomycin binding decreases by 6% and 16% respectively. Of the data concerning age-related effects, most notable are the prolonged elimination half-life of isoniazid in neonates (up to 19.8 hours), and the lower peak serum concentrations of rifampicin in children of one-third to one-tenth those of adults following a similar dose on a weight basis. For kanamycin, the maximum plasma concentration varies inversely with age but is not influenced by birthweight; however, the clearance is directly dependent upon birthweight and postnatal age. For the elderly, age is an insignificant factor for the elimination of isoniazid when compared with young adults of similar acetylator status, and the metabolism of rifampicin may be considered globally unaltered in this age group.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of propranolol metabolism by rifampicin

The effect of rifampicin on the blood concentration-time curve of propranolol at steady-state following oral administration of 120 mg every 8 h was investigated in six normal, young, male subjects. After an initial 2 week dosing period, all individuals additionally received 600 mg rifampicin daily for 3 weeks followed by a 4 week period during which again only the propranolol was given. In four of the subjects the effects of 900 and 1200 mg rifampicin daily was also studied. Changes in disposition were assessed by estimation of propranolol's oral clearance and elimination half-life during the dosage interval. Rifampicin (600 mg/day) caused a large increase in propranolol's oral clearance (35.7 +/- 16.3 vs 96.1 +/- 26.9 ml min-1 kg-1, mean +/- s.d.), but neither the elimination half-life nor extent of plasma binding were affected. Increasing the daily dosage to 900 and 1200 mg did not cause any additional changes in oral clearance. Four weeks after discontinuing rifampicin, propranolol's oral clearance had essentially returned to its pre-induction level. The oral clearance of propranolol was significantly smaller (89.5 +/- 14.4%) during the dosage interval immediately after administration of the last rifampicin dose than the value measured 24 h later. The findings are consistent with rifampicin causing induction of the drug metabolizing enzymes responsible for propranolol's biotransformation. The marked reduction in the steady-state propranolol blood concentration that results from chronic rifampicin administration would be expected to result in a significant change in clinical effectiveness of the beta-adrenoceptor blocker when the two drugs are used concurrently.

Rifampin blood and tissue levels in patients undergoing cardiac valve surgery

Single 600-mg capsules of rifampin were given orally to 26 patients as prophylaxis during cardiac valve replacement. Antibiotic concentrations were measured in blood (serum or plasma) and tissue (excised cardiac valve). The serum or plasma levels of rifampin in 18 patients who ingested this drug 2 h before they received preoperative opiates and anticholinergics intramuscularly were not significantly different from the levels in four normal volunteers who received the drug. These levels were 15.9 +/- 6.5 micrograms/ml (mean +/- standard deviation) 2 h after drug administration, 7.1 +/- 4.3 micrograms/ml 8 h after drug administration and 2 h after a mean of 1.4 h on cardiopulmonary bypass, and 1.6 +/- 1.6 micrograms/ml 24 h after drug ingestion. The valve tissue level was 3.8 +/- 2.7 micrograms/g (mean +/- standard deviation; n = 10). This value was 65% of the simultaneous serum and plasma levels and 31% of the peak serum and plasma levels. Eight patients who were given rifampin at the same time that they received other preoperative medications had significantly lower blood levels than the 18 patients who received rifampin 2 h earlier (P less than 0.001). No rifampin was detected in valves from seven of these patients. Decreased rifampin absorption due to simultaneous administration with opiates and anticholinergics was the probable reason for the low plasma and serum levels observed. These data suggest that, if properly dosed, rifampin administered orally gives high blood and valve tissue levels, which are affected minimally by cardiopulmonary bypass in patients undergoing cardiac valve surgery.

The effect of antipyrine, phenobarbitol and rifampicin on thyroid hormone metabolism in man

The effect of three different live microsomal enzyme inducing drugs on thyroid hormone metabolism was investigated. Seven volunteers were randomly allocated in a crossover design to either antipyrine (1200 mg), phenobarbital (100 mg) or rifampicin (1200 mg) daily for 14 days. Before and after each treatment the following parameters of enzyme induction were measured: antipyrine clearance, gamma-glutamyltranspeptidase, d-glucaric acid and 6-beta-hydroxycortisol urinary excretion. In addition, thyroxine-binding globulin (TBG), T3-resin uptake (RT3U), thyroxine (T4), free thyroxine (FT4), triiodothyronine (T3), reverse T3 (rT3), and thyroid stimulating hormone were estimated. Following antipyrine and phenobarbital antipyrine clearance increased by about 45%, while with rifampicin an increase of 125% was observed. The indices of thyroid function did not change following phenobarbital and antipyrine, but after rifampicin T4, FT4 and rT3 decreased by about 14%, and T3 increased by 25%. In addition, the impact of rifampicin on the clearance of injected 125I-T4 was investigated in six additional volunteers by blocking thyroid iodine uptake. The 125I-T4 halflife decreased from 155 to 106 h and its clearance increased from 25 to 50 ml/h, while a fall in T4, FT4 and rT3 by about 40% and no rise but a decrease in T3 by 25% occurred. Therefore an increased clearance of T4 and rT3 but not of T3 seems likely following rifampicin, which might be due to enhanced hepatic metabolism and biliary excretion.

Interaction of rifampin and chlorpropamide

A 62-year-old man who had been taking 250 mg of chlorpropamide daily for several years received rifampin concomitantly and had a subsequent increased dosing requirement of chlorpropamide. When rifampin was discontinued several months later, the serum chlorpropamide concentration rose dramatically.