The ketolides HMR 3004 and HMR 3647 (telithromycin) are a new class of macrolides that have a potential clinical efficacy against intracellular pathogens. The objectives of this study were to investigate the MIC, minimum bactericidal concentration, and time-dependent killing of two Chlamydia pneumoniae strains of the two ketolides. The killing effect was also studied with a newly developed intracellular in vitro kinetic model. Furthermore, HMR 3647 was studied for the effect of a subinhibitory concentration of 0.5 times the MIC after a preexposure of 10 times the MIC during 12 h. The MICs for both strains were 0.0039 and 0.0156 mg/liter for HMR 3004 and HMR 3647, respectively. Killing with 10 times the MIC was time dependent, increasing from a 1-log-unit decrease in the number of inclusions per well at 48 h to a maximal effect of 2.8-log-unit decrease after 96 h. A preexposure of 10 times the MIC of HMR 3647 for 12 h followed by a subinhibitory concentration of 0.5 times the MIC increased the killing effect to a 1.2-log-unit reduction in inclusions per well. An exposure for 12 h gave poor reduction of inclusions, while a static dose of 10 times the MIC for 72 h showed a 2.2-log-unit reduction in inclusions per well. In the kinetic model, a small number of inclusions were detected after 72 h by one exposure of 10 times the MIC. Regrowth could not be detected after 120 h. The ketolides HMR 3004 and HMR 3647 have bactericidal activity and show a significant sub-MIC effect on the intracellular pathogen C. pneumoniae.Chlamydia pneumoniae is an obligate intracellular bacterium with a growth cycle of 72 h. It is a widespread human pathogen causing a variety of community-acquired respiratory tract infections in adults and children. Several reports have indicated that chronic C. pneumoniae infections may be associated with coronary heart disease, atherosclerosis, and asthma (4,9,12,17). Although the acute infection is usually self-limiting, prolonged respiratory symptoms are sometimes a feature, and in such cases C. pneumoniae has been difficult to eradicate with antibiotics (6, 7). The optimal choice of antibiotic treatment and dosing regimen still needs to be defined.Pharmacodynamic studies are important tools for the determination of optimal dosing regimens of antimicrobial agents. The ketolides, a new class of macrolides characterized by a 3-keto function, which gives a strong acid stability (3), act by inhibiting bacterial protein synthesis. The ketolides have shown in vitro activity against a broad range of respiratory tract pathogens (1, 2, 15; I. Odenholt, E. Löwdin, and O. Cars, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 540, 1999). Although macrolide antibiotics show long postantibiotic effects, their antibacterial activity is not concentration dependent. The major determinant of efficacy is the time that free drug concentrations are greater than the MIC (5). The pharmacodynamics of the ketolides is less well known. However, significant postantibiotic effect and postantibiotic sub-MIC eff...