Overview of newer antimicrobial formulations for overcoming pneumococcal resistance.

The pharmacokinetic (PK) and pharmacodynamic (PD) profile of an antimicrobial agent provides important information that can be used to maximize bacteriologic and clinical efficacy, minimize selective pressure for the development of antimicrobial resistance, and determine an optimal dosing regimen. Judicious selection of an antimicrobial based on local susceptibility data and PK and PD parameters is imperative in this era of increasing resistance among Streptococcus pneumoniae, a leading cause of community-acquired respiratory tract infections. The beta-lactam antimicrobials display time-dependent bacterial killing with minimal to no persistent effects. Ketolides and fluoroquinolones display concentration-dependent bacterial killing, and tetracyclines and macrolides display time-dependent killing. All have prolonged persistent effects (e.g., postantibiotic effect) that retard or prevent bacterial regrowth when free drug levels fall below the minimum inhibitory concentration (MIC). New high-dose and/or extended-release formulations of traditional antimicrobials have been added to the current armamentarium for treatment of community-acquired respiratory tract infections. These formulations include amoxicillin-clavulanate potassium powder for oral suspension 90/6.4 mg/kg per day divided every 12 hours (Augmentin ES-600; GlaxoSmithKline, Research Triangle Park, NC), amoxicillin-clavulanate potassium extended-release tablets 2 x 1,000 mg/62.5 mg every 12 hours (Augmentin XR; GlaxoSmithKline), clarithromycin extended-release tablets 2 x 500 mg once daily (Biaxin XL; Abbott Laboratories, North Chicago, IL), and cefaclor extended-release tablets 375 mg or 500 mg every 12 hours (Ceclor CD; Eli Lilly Pharmaceuticals, Indianapolis, IN). Of these agents, only amoxicillin-clavulanate potassium powder for oral suspension and amoxicillin-clavulanate potassium extended-release tablets were designed to treat infections caused by penicillin-resistant pneumococci (penicillin MIC < or =2 microg/mL). Extended-release clarithromycin does not provide higher daily doses than its immediate-release counterpart; rather, it allows for once-daily dosing of this agent because of its slower absorption following oral administration. Extended-release cefaclor is considered clinically equivalent to 250 mg of immediate-release cefaclor pulvules administered 3 times daily; it cannot be used interchangeably with 500 mg 3-times-daily dosages of other cefaclor formulations. Thus, despite providing a similar or higher total daily dose than its immediate-release counterpart, extended-release cefaclor is indicated only for the treatment of patients with mild to moderate infections caused by susceptible strains of certain organisms.