There are currently renewed efforts to develop drugs that could shorten the duration of antituberculosis therapy. This is best achieved by optimizing the sterilizing effect. However, the current pathway for the development of new molecules with the potential to have a sterilizing effect is inefficient. We designed an in vitro pharmacokinetic-pharmacodynamic model in which Mycobacterium tuberculosis replicating slowly at pH 5.8 was exposed to pyrazinamide by use of the concentration-time profiles encountered in patients. The sterilizing effect rates and the time to the emergence of drug resistance were examined. Daily pyrazinamide dosing for 28 days accurately achieved (i) the pyrazinamide pharmacokinetic parameters, (ii) the lack of early bactericidal activity, (iii) a sterilizing effect rate of 0.10 log 10 CFU/ml per day starting on day 6 of therapy, and (iv) a time to the emergence of resistance of the from 2 to 3 weeks of monotherapy encountered in patients with tuberculosis. Next, dose-scheduling studies were performed. Current therapy for tuberculosis (TB) consists of rifampin (rifampicin), isoniazid, and pyrazinamide (5). In order to improve therapy, several studies have examined the pharmacokinetic (PK)-pharmacodynamic (PD) relationship between isoniazid and rifampin treatment and the response of Mycobacterium tuberculosis (22,25,29,30). However, those studies were of short duration and mainly reflected the bactericidal activities of these agents, defined as the killing of bacilli in log-phase growth. No similar studies have examined the sterilizing activities of anti-TB drugs. An anti-TB drug is considered to have a sterilizing effect when it is able to kill one of two subpopulations of M. tuberculosis: slowly growing bacilli in an acidic environment (slowly replicating bacilli [SRB]) and nonreplicating persistent bacilli (5, 26). The sterilizing activity of a drug also reflects its ability to shorten anti-TB therapy duration (41). Pyrazinamide has unique sterilizing activity against SRB, which are estimated to constitute the majority of bacilli in the pulmonary cavities of patients with TB (31,35). A measure of the size of the SRB population is that the addition of pyrazinamide to isoniazid-and rifampin-containing regimens led to a one-third reduction in the duration of therapy and a two-thirds reduction in the rate of TB relapse (2, 48). Given the size of this population and the central role of pyrazinamide in eradicating it, pyrazinamide PK-PD studies could help with the further optimization of therapy.After oral administration, pyrazinamide is almost completely systematically absorbed and eventually enters the pulmonary cavities containing M. tuberculosis. Pyrazinamide works in these cavities under acidic conditions (38). There is conflicting evidence as to whether pyrazinamide enters the bacilli by simple diffusion or by active uptake (44, 56). However, once the bacilli are in the cell, bacillary nicotinamidase deaminates the pyrazinamide to pyrazinoic acid (POA), which is transported to the extrace...