Variations between individuals and populations in the acetylation of isoniazid and its significance for the treatment of pulmonary tuberculosis.

Isoniazid is the most effective and widely used antituberculosis drug. The metabolism of isoniazid is noninducible and the primary metabolic route determining the rate at which isoniazid is eliminated from the body is acetylation. There are large diffrences between individuals in the rates at which isoniazid and certain other hydrazides and some sulfonamides are acetylated. The acetylation of isoniazid is inherited in a simple Mendelian fashion, individuals being either slow, heterozygous rapid, or homozygous rapid acetylators. Improved methods of discriminating between the different phenotypes have recently been described that are also more convenient to use. The proportion of slow acetylators among Japanese and Eskimos is about 10%; among Chinese about 20%; among Caucasians, Negroes, and South Indians about 60%. The results of a series of meticulously controlled clinical trials carried out in India, East Africa, Hong Kong, Singapore, Czechoslovakia, and Britain have shown the isoniazid acetylator status of tuberculosis patients treated with isoniazid-containing regimens is of no prognostic significance when treatment is given on daily basis. It may, however, be of significance when twice-weekly regimens are employed, especially in circumstances in which only a short period of initial daily chemotherapy is given, and where the companion drug employed during the twice-weekly continuation phase is relatively weak. The acetylator phenotype is, however, extremely important in once-weekly treatment, rapid acetylators always having fared considerably worse than slow acetylators on all the regimens evaluated so far. Experimental studies of the action of isoniazid against Mycobacterium tuberculosis in vitro and in vivo have enabled these findings to be understood.

The penetration of dapsone, rifampicin, isoniazid and pyrazinamide into peripheral nerves.

1 Dapsone, rifampicin, isoniazid and pyrazinamide were shown to penetrate readily into the sciatic nerves of the dog and sheep. 2 These findings suggest that the continued persistence of viable drug-sensitive leprosy bacilli in the peripheral nerves of patients treated for long periods with either dapsone or rifampicin is not due to inadequate intraneural drug penetration.

A sensitive urine-test method for monitoring the ingestion of isoniazid.

A method is described for monitoring the ingestion of isoniazid based on detecting its metabolites, isonicotinic acid, and isonicotinylglycine in the urine. The sensitivity of the method is high so that reliably positive results were obtained up to about 24 hours after the ingestion of 100 mg isoniazid. The method should facilitate monitoring the taking of isoniazid by tuberculosis patients and the use of isoniazid as a marker for assessing the regularity with which other drugs prescribed for self-administration are actually ingested.

Assay of rifampicin in serum.

Two methods for the assay of rifampicin in serum are described. The first is a conventional plate diffusion method, measuring concentrations down to 0.02 mug/ml, and the second a chemical extraction followed by measurement of the inhibition of uptake of (14)C-uridine by Staphylococcus aureus, which estimates in the range of 0.02 to 0.001 mug/ml. The methods were used to measure serum concentrations in man following doses of about 1050 mg and 75 mg rifampicin.

The evaluation of rifampicin bioavailabilities of fixed-dose combinations of anti-tuberculosis drugs: procedures for ensuring laboratory proficiency.

SETTING: The perceived need to demonstrate whether or not the rifampicin bioavailability of commercially manufactured fixed-dose combinations is satisfactory. OBJECTIVE: To establish an international laboratory network to assess rifampicin bioavailability. DESIGN: Convenient assay kits were devised to evaluate the ability of laboratories in China, India, Italy, South Africa, Thailand and the USA to determine plasma and urinary concentrations of rifampicin and desacetyl-rifampicin. RESULTS: Five laboratories, all of whom used high pressure liquid chromatographic methods, were shown to be able to accurately and precisely determine the two compounds. CONCLUSION: Such a procedure is simple, convenient and objective and could be further employed to enlarge the intended international laboratory network.

The colorimetric analysis of anti-tuberculosis fixed-dose combination tablets and capsules.

SETTING: The perceived need to demonstrate whether or not the actual amounts of rifampicin, isoniazid and pyrazinamide in fixed-dose combination tablets or capsules correspond to their stated drug contents. OBJECTIVE: To adapt specific, robust and simple colorimetric methods that have been previously applied to measuring plasma and urinary rifampicin, isoniazid, pyrazinamide and ethambutol concentrations to estimate tablet and capsule drug contents. DESIGN: The methods were applied to the analysis of 14 commercially manufactured fixed-dose combinations: two capsule and three tablet formulations containing rifampicin and isoniazid; seven tablet formulations containing rifampicin, isoniazid and pyrazinamide; and two tablet formulations containing rifampicin, isoniazid, pyrazinamide and ethambutol. RESULTS: All the combined formulations contained near to their stated drug contents. Replicate analyses confirmed the excellent precision of the drug analyses. CONCLUSION: Such methods are not only rapid to perform but should be practical in many Third World situations with relatively modest laboratory facilities.

Rifampicin bioavailability: a review of its pharmacology and the chemotherapeutic necessity for ensuring optimal absorption.

The World Health Organization encourages the use of fixed dose combinations (FDCs) of rifampicin (RMP) and isoniazid together with pyrazinamide or pyrazinamide plus ethambutol for the treatment of tuberculosis. The main advantages of such FDCs are the simplification of procurement and prescribing practices and the protection they afford against the potential selection of RMP-resistant strains of Mycobacterium tuberculosis. There is convincing evidence, however, that the rifampicin absorption from FDCs manufactured under suboptimal conditions may be significantly impaired, and this appears to be especially problematic with combined formulations of rifampicin, isoniazid and pyrazinamide. In view of the marked dose-dependence of rifampicin's bacterial sterilizing action, it is therefore essential that tuberculosis control programmes only use rifampicin-containing FDCs with proven rifampicin bioavailability. The comprehensive literature on the pharmacology of rifampicin is reviewed, together with the methods employed for determining it and its most important metabolite, desacetyl-rifampicin, in either serum or urine. By contrast, published information concerning the absorption of rifampicin from currently marketed combined formulations and on laboratory methods for precisely assessing their bioavailability is very sparse. There is therefore a crucial need to establish the quality of currently marketed rifampicin-containing FDCs in studies using adequate numbers of volunteers, precise analytical techniques and sophisticated statistical techniques.

The potential use of urinary excretion data for assessing the relative bioavailability of rifampicin in fixed dose combination anti-tuberculosis formulations.

SETTING: The perceived need for simple, non-invasive methods of assessing the relative bioavailability of rifampicin in fixed-dose combination (FDC) anti-tuberculosis formulations. OBJECTIVE: To compare the performance of methods based on urinary excretion data with those utilising plasma concentration-time profiles to assess the relative bioavailability of rifampicin in combined and single-drug formulations. DESIGN: A two-period randomised crossover bioequivalence study in healthy male volunteers with a 1 week washout period between treatments. Plasma rifampicin concentrations were measured at 0, 1, 2, 4, 6 and 8 hours after each drug administration using a high performance liquid chromatography (HPLC) method. The rifampicin and desacetylrifampicin content of complete urinary collections made from 0-4 and 4-8 hours after dosage were determined using both the HPLC and a much simpler colorimetric procedure. RESULTS: There was good agreement between the relative bioavailability of the formulations using plasma and urinary excretion data, although the precision of the urinary-based estimates was slightly less than those derived from the plasma findings. There was also good agreement between the HPLC and colorimetric estimates of the combined urinary excretion of rifampicin plus desacetylrifampicin. CONCLUSIONS: Urinary excretion data may be used for ongoing quality control to confirm that commercial combined rifampicin-containing formulations that were initially shown to be satisfactory continue to be so.

The development of a standardised screening protocol for the in vivo assessment of rifampicin bioavailability.

SETTING: The prerequisite for in vivo bioavailability testing of rifampicin in fixed-dose combination (FDC) formulations is widely accepted. However, many smaller drug regulatory authorities and drug manufacturers have difficulty implementing costly and cumbersome testing procedures. OBJECTIVE: To test whether a simplified blood sampling schedule can be used for the determination of drug bioequivalence in randomised, single dose, crossover studies of FDCs and appropriate reference formulations. METHOD: The results of three bioavailability and bioequivalence studies of different rifampicin-containing FDCs were analysed. The relationship between the number of time points employed and precision of estimated relative bioavailability was explored. The relative bioavailabilities of the drug components in the test FDCs were calculated using maximal concentration and area under the curve estimates based on an extended blood sampling schedule of up to 15 time points over 48 hours, and a contracted sampling scheme with only six blood samples over 8 hours. RESULTS: Estimates of relative bioavailability calculated using the contracted blood sampling protocol were closely similar to those derived using the extended sampling schedules. CONCLUSION: Considerable cost and convenience benefits can be gained by using the contracted sampling schedule with only a minor reduction in the precision of the estimation of relative rifampicin bioavailability.

Comparison of Chinese and Western rifapentines and improvement of bioavailability by prior taking of various meals.

Bioavailability was measured by rifapentine (RPE) serum concentrations and by the urinary ratio between RPE and creatinine, in specimens obtained 4-50 h after 600 mg RPE preceded by food. The bioavailabilities of RPEs manufactured in China and by a Western manufacturer were similar after a standard English breakfast, and serum concentrations were also similar to those obtained in an earlier Italian study following a complex meal. Although absorption of RPE was unsatisfactory after lipid-rich biscuits or shortbread, absorption after egges and toast was excellent and was nearly as good after a fast-food sandwich. The urinary measure of bioavailability at 26 h appeared as efficient as peak serum estimations at 6, 8 and 26 h. Fast-food sandwiches are being taken before RPE in a current clinical trial of Chinese RPE in Hong Kong.

Respiratory effects of lowering tar and nicotine levels of cigarettes smoked by young male middle tar smokers. II. Results of a randomised controlled trial.

STUDY OBJECTIVE: The aim was to investigate the effect on respiratory health of male middle tar smokers changing the tar and nicotine levels of the cigarettes they smoke for a six month period. DESIGN: This was a randomised controlled trial. Middle tar smokers were randomly allocated to smoke one of three different types of cigarette (low tar, middle nicotine; middle tar, middle nicotine; and low tar, low nicotine) in place of their usual cigarette for a six month period. Main outcome measures were assessment of respiratory health by documenting respiratory symptoms and peak expiratory flow rates, and of nicotine inhalation by measuring the urinary excretion of nicotine metabolites. SETTING: 21 local authority districts of England. SUBJECTS: Participants were male middle tar smokers aged 18-44 years. MAIN RESULTS: Changes in the measures of respiratory health showed little difference over the trial period between the three cigarette groups. Analyses of the urinary nicotine metabolites showed that smokers allocated to each of the three study cigarettes adjusted their smoking so that throughout the trial their nicotine inhalation differed little from their pretrial intakes when they were smoking their own cigarettes. As a result of the altered patterns of smoking to compensate for the reduced nicotine yields of the three study cigarettes, the tar intake of those allocated to smoke the middle tar, middle nicotine cigarettes remained essentially unchanged, while those allocated to smoke the low tar, low nicotine and low tar, middle nicotine cigarettes had calculated reductions in tar intakes of about 14% and 18%, respectively. CONCLUSIONS: Due to the phenomenon of compensation, tar intake can only be reduced substantially by using a cigarette with a markedly lower tar/nicotine ratio. Nevertheless reductions of up to about 18% in tar intake failed to result in any detectable effect on respiratory symptoms or peak expiratory flow rates over a six month period.

Urine test for the assessment of smoking status.

A simple, quick and inexpensive test for smoking status would be useful in a variety of settings. The non-polar barbituric acid derivative 1,3-dibutyl-2-thiobarbituric acid (DBTB) is described as a novel derivatisation reagent for nicotine and its metabolites in the König reaction to assess smoking status. The relative performance of qualitative methods for assessing smoking status using DBTB and the previously employed derivatisation reagent 1,3-diethyl-2-thiobarbituric acid (DETB), as well as quantitatively-based methods for determining 'total nicotine metabolites' (TNMs) using these two reagents, were evaluated against a cotinine-based radioimmunoassay (RIA) as the 'gold standard'. Clinical sensitivity and specificity for all the approaches studied were in excess of 94%. Simple qualitative assessment by eye was superior to quantitatively-based measures of smoking status. Correlation between estimation of nicotine metabolites using DBTB, DETB and RIA were good. The most efficient and convenient method to distinguish between smokers and non-smokers was the simple qualitative method using the more lipophilic reagent DBTB.

[The potential clinical significance of the isoniazid acetylator phenotype in the treatment of pulmonary tuberculosis]. / Signification clinique potentielle du phénotype d'acétylation de l'isoniazide dans le traitement de la tuberculose pulmonaire.

The most important factor determining the speed with which isoniazid is eliminated from the body is the rate of its acetylation in the liver. There are large differences between individuals in the rates at which isoniazid is acetylated. Over 98% of subjects can be clearly characterized as being either rapid of slow acetylators. Among the many satisfactory procedures for determining the acetylator phenotype of subjects, the simple sulphamethazine method is probably the most convenient. The proportions of rapid acetylators among different populations vary from about 40% among those of European and South Indian descent to over 85% among Japanese and Eskimos. The isoniazid acetylator phenotype of tuberculosis patients treated with isoniazid-containing regimens is without prognostic significance when treatment is given daily and only of doubtful importance when weak twice-weekly regimens are employed. However if treatment is given on a once-weekly basis, the response of rapid acetylators is generally much less satisfactory than that of slow acetylators. Since isoniazid is eliminated from the body predominantly by metabolism its clearance is not greatly diminished in the event of renal failure. As a consequence there are no grounds for reducing the dosage of isoniazid given to patients with impaired renal function. The incidence of the commonly encountered asymptomatic increases in serum transaminases associated with isoniazid treatment is similar in rapid and slow acetylators. Clinically important hepatic toxicity manifested by jaundice only occurs in 1-2% of patients treated with isoniazid-containing regimens. Despite earlier suggestions to the contrary, it is no more common in rapid than among slow acetylators. Since all patients can be treated effectively and safely with standard isoniazid dosages, determining patients' isoniazid serum/plasma levels or acetylator phenotype is hardly ever worthwhile. By contrast, since poor patient compliance is a major cause of therapeutic failure, monitoring the regularity with which patients self-administer their prescribed isoniazid treatment using simple urine-test methods can be of considerable value.