This study examined in vitro interaction between domperidone and erythromycin. Both are prescribed for refractory gastroparesis. Domperidone is metabolized via human cytochrome P4503A4. Erythromycin is a CYP3A4 inhibitor. Incubations evaluated domperidone metabolite formation in human liver microsomes and recombinant CYP3A4. Concentration- and time-dependent inhibition of 500 microM domperidone was studied with 2.5-200 microM erythromycin over 10-40 min. Domperidone metabolite (5-hydroxy domperidone, M3) formation was inhibited by erythromycin in a concentration- and time-dependent manner. The K(I) estimate was 18.4 microM in human liver microsomes and 4.1 microM in CYP3A4. Using a model incorporating CYP3A4 hepatic and gut inhibition, in vitro estimates from human liver microsomes and CYP3A4 were used to predict in vivo AUCi/AUC ratios of 2.54 and 4.95, respectively. Significant inhibition of domperidone metabolism by erythromycin occurs. This predicts greater domperidone drug exposure when used with erythromycin. This important drug-drug interaction will be evaluated in future human studies.