Modern Concepts of Tuberculosis and Its Control
Basis of Chemotherapy of Tuberculosis
By Dr. S. L. Chan
MBBS (HK), MRCP (UK), FRCP (E)
FHKCP, FHKAM (MEDICINE)
Mechanism of drug action in short course chemotherapy
Anti-tuberculosis drugs vary in their ability:
1. to kill large numbers of actively metabolising bacilli rapidly (bactericidal action).
2. to kill slowly or intermittently metabolising semidormant bacilli - the persisters (sterilising action).
3. to prevent the emergence of drug resistance.
The understanding of these mechanisms is based largely on the Pasteur Institute, Paris, group's experimental work in mice, on the in vivo and in vitro laboratory experimental work of Professor Mitchison's British Medical Research Council (BMRC) Unit and a number of controlled clinical trials.
Mitchison (1980) illustrates BMRC view of mechanisms based on the findings of the above three approaches:
1. There is a large bacterial population undergoing continuous growth, which is killed by bactericidal drugs.
2. There is evidence that pyrazinamide acts on a special bacterial population inhibited by the acid environment inside the macrophage or extracellularly in areas of acute inflammation.
3. Rifampicin too acts on a special bacterial population, being a drug whose bactericidal action starts much more rapidly than that of any other drugs.
4. There is a population of bacilli which is dormant and therefore not killed by any drug.
The practical conclusion is that the three vital drugs in short course chemotherapy are isoniazid, rifampicin and pyrazinamide.
