β-Lactam Activity against Penicillin-Resistant Streptococcus pneumoniae Strains Exhibiting Higher Amoxicillin versus Penicillin Minimum Inhibitory Concentration Values: An in vitro Pharmacodynamic Simulation

Abstract

Background: Activity of simulated serum concentrations after oral therapy with 400 mg cefditoren pivoxil b.i.d., 500 mg cefuroxime axetil b.i.d. and 875/125 mg amoxicillin/clavulanic acid b.i.d. and t.i.d. regimens was explored over 24 h against Streptococcus pneumoniae.Methods: Computerized pharmacodynamic simulations were performed against strains with penicillin/amoxicillin/cefuroxime/cefditoren minimum inhibitory concentrations (MICs, µg/ml) and serotypes: strain 1 (0.25/0.12/1/0.12; serotype 6A), strain 2 (2/4/ 2/0.25; serotype 6B), strain 3 (4/16/4/0.5; serotype 14), and strain 4 (4/16/8/1; serotype 14). Results: Bactericidal activity (≧3 log₁₀ reduction) at 12 and 24 h was obtained against all strains with cefditoren, against strains 1 and 2 with cefuroxime and amoxicillin/clavulanic acid t.i.d., but only against strain 1 with amoxicillin/clavulanic acid b.i.d.. Bactericidal activity at 24 h was related to T > MIC of >30% dosing interval, 1.7–2.0 log₁₀ reductions with T > MIC of 20–30%, and <1 log₁₀ reduction or regrowth with T > MIC of 0%. Conclusions: It is difficult to achieve pharmacodynamic coverage and bactericidal activity by physiological concentrations of oral β-lactams against penicillin-resistant pneumococcal strains exhibiting higher amoxicillin versus penicillin MICs. Cefditoren may offer alternatives.