Pristinamycin IA is a cyclo-peptidic macrolactone antibiotic belonging to the streptogramin family, a substrate for the P-glycoprotein. In vitro: Pristinamycin IA specifically inhibits the efflux of the P-glycoprotein substrate [3H]vinblastine, thus increasing the cellular accumulation of the drug. Pristinamycin IA also reduces by 70% the basolateral to apical secretion of [3H]vinblastine across Caco-2 cell monolayers. The cellular accumulation of [14C]pristinamycin IA is very low and is increased by P-glycoprotein inhibitors (verapamil, chlorpromazine and reserpine).