Factors affecting the in-vitro activity of roxithromycin

The effect of human serum and CO2 on the activity of roxithromycin and erythromycin was assessed. Protein binding of roxithromycin in serum from various animal sources, acid alpha 1-glycoprotein and human albumin V was determined. There was a four- to eight-fold increase in MIC and MBC of roxithromycin in the presence of 70 and 100% human serum (minimum effect seen with erythromycin) and for both compounds there was a four- to eight-fold increase in MIC for fastidious organisms in the presence of CO2. Roxithromycin appears to be selectively bound to acid alpha 1-glycoprotein, binding decreases with an increase in roxithromycin concentration (saturable at 10 mg/l) and protein binding is variable depending on animal source (86% human, 10% guinea pig) and this must be considered when data on activity from animal studies are evaluated.