Relevance of serum protein binding of cefoxitin and cefazolin to their activities against Klebsiella pneumoniae pneumonia in rats

An experimental Klebsiella pneumoniae pneumonia in rats was used to study the effect of protein binding of cefoxitin and cefazolin on their therapeutic activity. Both cephalosporins were similar with respect to their antimicrobial activity against the K. pneumoniae in vitro, but they differed in their degree of protein binding, being 34% for cefoxitin and 89 to 93% for cefazolin in uninfected rats and 24 and 71 to 83%, respectively, in infected rats. Various doses of these agents were administered by continuous infusion, which started 5 h after bacterial inoculation and continued for 65 h. Antimicrobial response was evaluated with respect to the numbers of bacteria recovered from lung and blood at the end of treatment. An inhibitory effect of protein binding on the in vivo antimicrobial activity was demonstrated. Cefoxitin was therapeutically effective at a constant plasma level that reached the MIC. To obtain a similar effect with cefazolin the plasma level of that drug had to be increased to a concentration more than three times the MIC.