Real, R., Egido, E., Pérez, M., González‐Lobato, L., Barrera, B., Prieto, J. G., Álvarez, A.I., Merino, G. Involvement of breast cancer resistance protein (BCRP/ABCG2) in the secretion of danofloxacin into milk: interaction with ivermectin. J. vet. Pharmacol. Therap.34, 313–321.Danofloxacin, a veterinary fluoroquinolone antimicrobial drug, is actively secreted into milk by an as yet unknown mechanism. One of the main determinants of active drug secretion into milk is the transporter (BCRP/ABCG2). The main purpose was to determine whether danofloxacin is an in vitro substrate for Bcrp1/BCRP and to assess its involvement in danofloxacin secretion into milk. In addition, the role of potential drug–drug interactions in this process was assessed using ivermectin. Danofloxacin was transported in vitro by Bcrp1/BCRP, and ivermectin efficiently blocked this transport. Experiments with Bcrp1−/− mice showed no evidence of the involvement of Bcrp1 in plasma pharmacokinetics of danofloxacin. However, the milk concentration and milk‐to‐plasma ratio of danofloxacin were almost twofold higher in wild‐type compared with Bcrp1−/− mice. The in vivo interaction with ivermectin was studied in sheep after co‐administration of danofloxacin (1.25 mg/kg, i.m.) and ivermectin (0.2 mg/kg, s.c.). Ivermectin had no significant effect on the plasma levels of danofloxacin but significantly decreased danofloxacin concentrations in milk by almost 40%. Concomitant administration of multiple drugs, often used in veterinary therapy, may not only affect their pharmacological activity but also their secretion into milk, because of potential drug–drug interactions mediated by BCRP.