Serial counts of Mycobacterium tuberculosis in sputum as surrogate markers of the sterilising activity of rifampicin and pyrazinamide in treating pulmonary tuberculosis

R Brindle, J Odhiambo, D Mitchison, R Brindle, J Odhiambo, D Mitchison

Abstract

Background: Since the sterilising activity of new antituberculosis drugs is difficult to assess by conventional phase III studies, surrogate methods related to eventual relapse rates are required.

Methods: A suitable method is suggested by a retrospective analysis of viable counts of Mycobacterium tuberculosis in 12-hr sputum collections from 122 newly diagnosed patients with pulmonary tuberculosis in Nairobi, done pretreatment and at 2, 7, 14 and 28 days. Treatment was with isoniazid and streptomycin, supplemented with either thiacetazone (SHT) or rifampicin + pyrazinamide (SHRZ).

Results: During days 0-2, a large kill due to isoniazid occurred, unrelated to treatment or HIV status; thereafter it decreased exponentially. SHRZ appeared to have greater sterilising activity than SHT during days 2-7 (p = 0.044), due to rifampicin, and during days 14-28, probably due mainly to pyrazinamide. The greatest discrimination between SHRZ and SHT treatments was found between regression estimates of kill over days 2-28 (p = 0.0005) in patients who remained positive up to 28 days with homogeneous kill rates. No associations were found between regression estimates and the age, sex, and extent of disease or cavitation. An increased kill in HIV seropositive patients, unrelated to the treatment effect, was evident during days 2-28 (p = 0.007), mainly during days 2-7.

Conclusions: Surrogate marker studies should either be in small groups treated with monotherapy during days 2 to about 7 or as add-ons or replacements in isoniazid-containing standard regimens from days 2 to 28 in large groups.

Figures

Figure 1
Figure 1
Mean cfu counts during chemotherapy in patients who had complete sets of counts up to 2 days of treatment, or up to 7 days or 14 days or 28 days of treatment, but in each case no further positive sputum cultures thereafter.
Figure 2
Figure 2
Mean cfu counts during treatment with the STH and the SHRZ regimens in patients who had a complete set of sputum cfu counts up to and including 28 days.
Figure 3
Figure 3
Mean cfu counts during treatment in HIV-negative and HIV-positive patients who had a complete set of sputum cfu counts up to and including 28 days.
Figure 4
Figure 4
The bactericidal actions of pyrazinamide/isoniazid (HZ) and streptomycin, PAS and isoniazid (SPH) in the spleens of mice. Redrawn from Text-fig. 4 in reference .

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