A new pharmacokinetically enhanced formulation of amoxicillin-clavulanate (2,000 mg of amoxicillin/125 mg of clavulanate twice a day; ratio 16:1) has been designed, with sustained-release technology, to allow coverage of bacterial strains with amoxicillin-clavulanic acid MICs of at least 4/2 g/ml. The bacteriological efficacy of amoxicillin-clavulanate, 2,000/125 mg twice a day, ratio 16:1, was compared in a rat model of respiratory tract infection versus four other amoxicillin-clavulanate formulations: 8:1 three times a day (1,000/125 mg), 7:1 three times a day (875/125 mg), 7:1 twice a day (875/125 mg), and 4:1 three times a day (500/125 mg); levofloxacin (500 mg once a day); and azithromycin (1,000 mg on day 1 followed thereafter by 500 mg once a day). Bacterial strains included Streptococcus pneumoniae, with amoxicillin-clavulanic acid MICs of 2/1 (one strain), 4/2, or 8/4 g/ml (three strains each), and Haemophilus influenzae, one -lactamase-positive strain and one -lactamasenegative, ampicillin-resistant strain. Animals were infected by intrabronchial instillation. Antibacterial treatment commenced 24 h postinfection, with doses delivered by computer-controlled intravenous infusion to approximate the concentrations achieved in human plasma following oral administration. Plasma concentrations in the rat corresponded closely with target human concentrations for all antimicrobials tested. Amoxicillin-clavulanate, 2,000/125 mg twice a day, ratio 16:1, was effective against all S. pneumoniae strains tested, including those with amoxicillin-clavulanic acid MICs of up to 8/4 g/ml and against -lactamase-producing and -lactamase-negative ampicillin-resistant H. influenzae. These results demonstrate the bacteriological efficacy of pharmacokinetically enhanced amoxicillin-clavulanate 2,000/125 mg twice a day (ratio 16:1) against S. pneumoniae with amoxicillin-clavulanic acid MICs of at least 4/2 g/ml and support clavulanate 125 mg twice a day as sufficient to protect against -lactamase in this rat model.Antimicrobial agent-resistant strains of the key respiratory tract pathogens are widespread, and their prevalence is increasing in many areas (12,15). For Haemophilus influenzae collected in the Alexander Project in 2001, the prevalence of -lactamase production exceeded 24% in the United States and was more than 10% in 9 of the 16 countries included within the study (27). In the same study, the prevalence of penicillin-resistant Streptococcus pneumoniae in 2001 was approximately 20% in the United States and over 30% in Spain, Japan, France, and Hong Kong. The prevalence of macrolideresistant S. pneumoniae was also high in the United States (28%) and was at least 10% in 12 of the 16 countries included within the study (27). Data from the 1998 to 2000 Alexander Project and other surveillance studies have reported similar findings (15)(16)(17)26).Amoxicillin-clavulanate is a combination of the -lactam antibiotic amoxicillin and the -lactamase inhibitor clavulanic acid in the potassium form. Clavulanic acid protect...