Bactericidal activity of cefadroxil, cephalexin, and cephradine in an in vitro pharmacokinetic model

Cefadroxil (Duricef, Mead Johnson and Company), resembles cephalexin and cephradine in spectrum of antibacterial activity but differs in human pharmacokinetic properties. Whether the latter are likely to affect activity in vivo was assessed by determining bactericidal activity against clinical isolates under conditions simulating the variation of drug concentration in the blood stream after an oral dose of 500 mg to adults. In this kinetic model, cefadroxil was more active than cephalexin or cephradine against Staphylococcus aureus, Streptococcus pneumoniae, Klebsiella pneumoniae, Proteus mirabilis, Haemophilus influenzae and one of two strains of Escherichia coli. The other strain of E. coli was virtually unaffected by the cephalosporins. S. pyogenes was equally susceptible to all three cephalosporins. Analysis of the results suggest that the pharmocokinetic properties of an antibiotic affect its activity in the blood stream, provided the susceptibility of the infecting organism is concentration-dependent within the range of drug concentration occurring in serum.