No abstract

Colony-forming units of Mycobacterium tuberculosis in sputum were counted at 2-day intervals in 100 patients treated with 22 regimens of isoniazid, rifampin, pyrazinamide, ethambutol, or streptomycin, given alone or in combinations. The exponential fall in colony-forming units was measured by linear regression coefficients of the log counts during the initial 2-day phase of rapid, drug-determined killing and during the subsequent 12 days of much slower sterilizing activity. The regression coefficients during the first 2 days varied significantly according to the drug; the greatest effects in multiple regression analyses were due to isoniazid (p < 0.001) and rifampin (p = 0.027). The rapid kill obtained with isoniazid was unaffected by addition of other drugs, so that a change in activity after adding an unknown drug to isoniazid would not be measurable. In multiple regression analysis of the coefficients during Days 2-14, rifampin and streptomycin had significant effects (p = 0.007 and 0.006, respectively), indicating that both drugs had important sterilizing activity, streptomycin particularly early. Isoniazid and pyrazinamide had no significant effects. In analyses of combined drug regimens only, ethambutol had an effect (p = 0.01) in reverse direction to that of rifampin, suggesting it antagonized the sterilizing activity of other drugs.

PZA is a unique anti-tuberculosis drug that plays a key role in shortening the TB therapy. PZA kills non-replicating persisters that other TB drugs fail to kill, and thus making it an essential drug for inclusion in any drug combinations for treating drug susceptible and drug-resistant TB such as MDR-TB. PZA acts differently from common antibiotics by inhibiting multiple targets such as energy production, trans-translation and perhaps pantothenate /coenzyme A required for persister survival. Resistance to PZA is mostly caused by mutations in the pncA gene encoding pyrazinamidase involved in conversion of the prodrug PZA to the active form POA. Mutations in the drug target RpsA are also found in some PZA-resistant strains. The recent finding that panD mutations are found in some PZA-resistant strains without pncA or rpsA mutations may suggest a third PZA resistance gene and a potential new target of PZA. Current phenotype based PZA susceptibility testing is not reliable due to false resistance, and sequencing of the pncA gene represents a more rapid, cost-effective and more reliable molecular test for PZA susceptibility testing and should be used for guiding improved treatment of MDR/XDR-TB. Finally, the story of PZA has important implications for not only TB therapy but also chemotherapy in general. PZA serves as a model prototype persister drug and hopefully a ‘tipping point’ that inspires new efforts at developing a new type of antibiotics or drugs that target non-replicating persisters for improved treatment of not only TB but also other persistent bacterial infections.

The history of the development of modern chemotherapy for tuberculosis (TB), largely due to the British Medical Research Council, is first described. There is a current need to shorten the duration of treatment and to prevent and cure drug-resistant disease. These aims will only be achieved if the way in which multidrug treatment prevents resistance from emerging and the reasons for the very slow response to chemotherapy are understood. Consideration of mutation rates to resistance and the size of bacterial populations in lesions makes it very unlikely that resistance would emerge spontaneously, leaving irregularity in drug taking and inadequate dosage as the main reasons for its occurrence. Slow response to treatment seems due to the presence of persister populations whose natural history is only partly known. In the future, we need to explore the persister state in patients and in experimental murine TB, and to take it into account in the design of future mouse experiments. The activity of rifamycins and pyrazinamide is being increased by a rise in rifamycin dosage and the inhalation of pyrazinoic acid. New drugs are gradually being brought into use, initially TMC207 and the nitroimadazoles, PA824 and OPC67683. They will need to be tested in new combination regimens for drug-susceptible and multi- and extensively drug-resistant disease.