Jean M. Dickinson

Suitability of rifampicin for intermittent administration in the treatment of tuberculosis

Abstract Log-phase cultures of Mycobacterium tuberculosis were exposed to 0·2μg./ml. rifampicin for various periods, the drug was removed by filtration and viable counts were done during and after exposure. Rifampicin was more bactericidal in the first 24 hours of exposure than 1 μg./ml. isoniazid or 5μg./ml. streptomycin and there was a lag before growth restarted after exposure for only two hours. However, the lag period after exposure to rifampicin was shorter than after exposure to other bactericidal drugs. Guinea-pigs with established tuberculosis were treated for six weeks with rifampicin at various dose levels. At each mean daily dosage level, groups were given doses at intervals of one, two, four or eight days. Their response to treatment was then assessed from the amount of macroscopic disease in the organs and the viable counts on the spleen. The efficacy of treatment increased as the interval between doses was spaced out. The benefit from high doses of rifampicin given intermittently may partly arise from the disproportionately high peak serum concentrations and prolonged excretion of large doses. Rifampicin had relatively more effect on tuberculosis in the liver than in other organs, as compared to isoniazid. These findings suggest that rifampicin may be particularly suitable for intermittent administration in man, provided that the increase in the size of the individual dose given does not produce serious toxicity.

In vitro activity of new rifamycins aganst rifampicin-resistant M. tuberculosis and MAIS-complex mycobacteria

Comparisons were made of the in vitro activity of rifampicin, and the rifamycin derivatives, rifapentine, rifabutin, CGP 29861, CGP 7040 and CGP 27557, against rifampicin-sensitive and rifampicin-resistant strains of Mycobacterium tuberculosis and against the Mycobacterium avium/intracellulare/scrofulaceum (MAIS) complex. The new rifamycins had MICs four to eight times lower than those of rifampicin against sensitive M. tuberculosis strains. Of the 35 rifampicin-resistant strains of M. tuberculosis, 31% were sensitive to rifabutin but only 3-11% to the other rifamycins. The proportions of the MAIS strains found to be sensitive were 35% for rifampicin, 50-60% for CGP 27557, rifapentine and rifabutin and 85-92% for CGP 29861 and CGP 7040.

Suitability of isoniazid and ethambutol for intermittent administration in the treatment of tuberculosis

Summary After temporary exposure to isoniazid I μg./ml. or ethambutol 10 μg./ml. for periods of24 hours or more, log phase cultures of Mycobacterium tuberculosis did not begin to grow again for several days, but no lag in the subsequent growth occurred when the exposure period was less than 24 hours. Guinea-pigs with estabilished tuberculosis were treated for six weeks with isoniazid orethambutol at various dose levels. At each mean daily dosage level, groups were given the doses at intervals of one, two, four or eight days. Their response to treatment was then assessed from the amount of macroscopic disease in the organs and the viable count on the spleen. Treatment at a given mean daily dosage of isoniazid appeared most effective when the doses were spaced out at intervals of two to three days, but there was a considerable decrease in efficacy as the interval was increased from four to eight days. Serum concentrations of isoniazid in the guinea-pigs were similar to those found in human rapid inactivators of the drug and a close parallel exists between the guinea-pig and man in the relationship between rhythm of dosage and response to treatment. With ethambutol, the efficacy of treatment in the guinea-pig at a given mean daily dosageincreased with increasing intervals between doses, indicating that ethambutol is likely to be particularly suitable for intermittent chemotherapy provided that chronic toxicity to the drug is related to its mean dosage and not the size of the individual dose. The half-life of ethambutol was shorter in the guinea-pig than in man, suggesting that intermittent treatment might be even more effective in man than in the guinea-pig.

In vitro studies on the choice of drugs for intermittent chemotherapy of tuberculosis

Summary An attempt was made to study the effect of exposing cultures of Myco. tuberculosis for short periods to drugs used in the treatment of tuberculosis. Six drugs were used, the concentration of each being approximately ten times that necessary to inhibit growth. The cultures were exposed to these drugs for periods varying from two to 168 hours. Estimates of the number of viable organisms in the cultures were made before their exposure and at intervals after removal of the drug by washing with drug-free medium on a membrane filter. The effect of exposure was estimated by noting the delay before the organisms again began to grow. Streptomycin was considered most suitable for intermittent chemotherapy since it had the greatest bactericidal activity and the delay before growth began after exposure to the drug for six-96 hours and its subsequent removal, was eight-16 days; growth of organisms between doses would thus not be expected. Isoniazid and ethionamide appeared slightly less suitable as they only began to be bactericidal if the exposure period was at least one day and there was no delay before growth after exposure periods of 12 hours or less. However, after exposure periods of 24 hours or more, the delay before growth was three-17 days. With cycloserine, the delay before growth was only about one day after exposure for 24 hours but during exposure for 96 hours, this drug was more bactericidal than either isoniazid or ethionamide, and the delay before growth was then four-eight days. Thiacetazone and thiocarlide (4-4′ diisoamyloxythiocarbanilide) did not appear suitable for intermittent use since they were not bactericidal and growth started immediately after exposure.

In vitro properties of rifapentine (MDL473) relevant to its use in intermittent chemotherapy of tuberculosis

In a comparison of in vitro properties of rifapentine (RIF) and rifampicin (RMP), the minimal inhibitory concentration of RIF against Mycobacterium tuberculosis in Tween-albumin liquid medium was usually 0.02 micrograms/ml, 2-3 times lower than for RMP; the bactericidal activity against a log phase culture was slightly less than that of RMP and the recovery after pulsed exposures to 1 microgram/ml of RIF or RMP lasting 6, 24 and 96 h was identical for the two rifamycins. These findings are used to interpret published data from the chronic experimental murine tuberculosis model and support the view that in the mouse, the efficacy of RIF in widely spaced intermittent chemotherapy is the result of its long half-life.

Bactericidal activity in vitro and in the guinea-pig of isoniazid, rifampicin and ethambutol.

Serial viable counts on Mycobacterium tuberculosis exposed in vitro to isoniazid, rifampicin and ethambutol in Tween-albumin liquid medium showed no bactericidal synergism between isoniazid and rifampicin and no influence of ethambutol on the bactericidal activity of isoniazid or isoniazid plus rifampicin. Guinea-pigs with moderately advanced experimental tuberculosis were treated fro 11 weeks with either (1) ethambutol, (2) isoniazid, (3) isoniazid plus ethambutol, (4) isoniazid plus rifampicin, (5) isonaiazid plus rifampic in plus ethambutol, or (6) no chemotherapy. The amount of tuberculous disease was scored and the spleen cultured in groups killed at intervals from 0--7 1/2 months after the end of chemotherapy. The regimens containing rifampicin were no more bactericidal during treatment than the corresponding regimens without rifampicin, but the onset of relapse after chemotherapy was delayed for at least 2 months following the rifampicin-containing regimens. Ethambutol alone did not protect guinea-pigs, nor did it influence the response to isoniazid or to isoniazid plus rifampicin. It was concluded that rifampicin may act selectively on a small proportion of the bacterial population and that it may be unnecessary to prescribe it for long periods in short course chemotherapy in man. Ethambutol does not appear likely to contribute to the sterilizing activity of short course regimens though it may prevent the emergence of drug resistance.

Observations in vitro on the suitability of pyrazinamide for intermittent chemotherapy of tuberculosis

Abstract Pyrazinamide (PZA) 50 μg./ml. was added to cultures of Mycobacterium tuberculosis and removed after incubation for 6, 24 or 96 hours by filtration. Serial viable counts were done on the cultures during and after exposure to the drug. In medium with a moderately acid reaction, PZA was bactericidal to small inocula of bacilli and, after its removal, the culture did not resume growth for five to nine days. At more acid reactions the growth rate in control drug-free cultures was very slow and temporary exposure to PZA inhibited subsequent growth for at least six weeks. Experiments with larger inocula were difficult to interpret, probably because of destruction of PZA by the amidase activity of the bacilli, a phenomenon which is unlikely to be of importance in man. The lag in growth produced by temporary exposure to PZA helps to explain the value of this drug in intermittent chemotherapy.

Virulence in the Guinea-Pig and Susceptibility to Hydrogen Peroxide of Isoniazid-Sensitive Tubercle Bacilli from South Indian Patients.

Summary Tests for virulence in the guinea-pig and for susceptibility to the bactericidal activity of hydrogen peroxide were done on cultures of isoniazid-sensitive tubercle bacilli obtained from 220 South Indian patients with pulmonary tuberculosis before the start of antituberculosis chemotherapy. An association was found between the virulence of the cultures and the proportion of their organisms surviving exposure to 0·02% peroxide; 10% or more organisms survived in 11% of 152 cultures of low virulence and in 57% of 68 cultures of high virulence. There was some suggestion that the cultures could be divided into two groups, the larger, containing about 74% of the cultures, having low virulence and high peroxide susceptibility and the smaller having high virulence and low peroxide susceptibility.

In vitro observations on the suitability of new rifamycins for the intermittent chemotherapy of tuberculosis.

The bactericidal activity of six new rifamycin derivatives--rifabutin (RBU), FCE 22250 (F22), rifapentine (RPE), CGP 29861 (C29), CGP 7040 (C70) and CGP 27557 (C27) and rifampicin (RMP)--have been measured against log phase and, as a better test of sterilising activity, against stationary phase cultures of Mycobacterium tuberculosis, H37Rv. The order of activity of 1.0 and 0.2 mg/l rifamycin against log phase cultures was RMP greater than RPE & C27 greater than RBU & C29 greater than C70. The order of activity of 1.0 and 0.4 mg/l, adjusted for stability of the rifamycin, against stationary phase cultures was F22 & RMP greater than RBU greater than RPE greater than C27 & C29 greater than C70. Viable counts were done during and after pulsed exposures of 6, 24 or 96 h to C29 and RMP. The curves were similar though C29 was less bactericidal and the lag period before recovery was 1-2 days longer. F22, having high bactericidal activity against stationary organisms and a long half-life, was considered likely to be the most effective sterilising drug.

Short-term intermittent chemotherapy of experimental tuberculosis in the guinea pig.

Summary Guinea pigs with moderately severe experimental tuberculosis were treated for six weeks with thiacetazone, ethionamide or streptomycin, and the progress of the disease was assessed by scoring the extent of the lesions in the organs and by viable counts on the homogenized spleen. Each group of guinea pigs was treated with the same mean daily dosage of the drug, but was sub-divided with the doses spaced at intervals of one, two, four or eight days. As the interval between doses was increased, the efficacy of treatment greatly decreased with thiacetazone (thioacetazone), slightly decreased with ethionamide, and remained similar with streptomycin; the relationships appeared valid over the wide range of mean dosage used for each drug. These results confirm the prediction of an in vitro study of the suitability of the drugs for intermittent chemotherapy ( Dickinson & Mitchison, 1966 ). Estimations of drug concentrations in the serum of the guinea pigs showed that thiacetazone was excreted very slowly. Concentrations of ethionamide varied greatly between guinea pigs, probably due to delays in absorption. Streptomycin concentrations were proportional to the dose injected and the drug was rapidly excreted.