RESEARCH PAPER In Vivo Inhibition of BCRP/ABCG2 Mediated Transport of Nitrofurantoin by the Isoflavones Genistein and Daidzein: A Comparative Study in Bcrp1 -/- Mice Gracia Merino & Miriam Perez & Rebeca Real & Estefania Egido & Julio G. Prieto & Ana I. Alvarez Received: 29 January 2010 / Accepted: 28 June 2010 / Published online: 7 July 2010 # Springer Science+Business Media, LLC 2010 ABSTRACT Purpose The aim of this study was to determine in vivo inhibition by the isoflavones genistein and daidzein of nitro- furantoin (NTF), a well-known substrate of the ABC trans- porter BCRP/ABCG2. Methods MDCKII cells and their human BCRP- and murine Bcrp1-transduced subclones were used to establish inhibition in transepithelial transport assays. Bcrp1 -/- and wild-type mice were coadministered with nitrofurantoin (20 mg/kg) and a mixture of genistein (100 mg/kg) and daidzein (100 mg/kg). Results Transepithelial NFT transport was inhibited by the isoflavones. Plasma concentration of NTF at 30 min was 1.7- fold higher (p ≤ 0.05) in wild-type mice after isoflavone administration. AUC values were not significantly different. BCRP/ABCG2-mediated secretion into milk was inhibited since milk/plasma ratios were lower in wild-type mice with isoflavones (7.1±4.2 vs 4.2±1.6, p ≤ 0.05). NTF bile levels were significantly decreased by isoflavone administration in wild-type animals (8.8±3.4 μg/ml with isoflavones vs 3.7± 3.3 μg/ml without isoflavones). Conclusion Our data showed that in vivo interaction of high doses of soy isoflavones with BCRP substrates may affect plasma levels but the main effect occurs in specific target organs, in our case, liver and mammary glands. KEY WORDS BCRP/ABCG2 . Bcrp1 -/- . isoflavones . MDCKII cells . nitrofurantoin INTRODUCTION BCRP, the protein encoded by the ABCG2 gene, may play a significant role in the disposition and pharmacological activity of a broad range of compounds and in the development of multidrug resistance in cancer ( 1,2). Consistent with this notion, a plethora of studies on the tissue distribution of ABCG2, its expression and activity, as well as its pharmacokinetic interaction, have been per- formed (1–3). BCRP/ABCG2 protein is expressed mainly in the apical membrane of cells in tissues with secretory functions (liver, kidney, intestine, breast), apical membranes of capillary vessels in the blood-brain barrier and apical membrane of trophoblasts (2,4). Studies on knock-out mice have indicated that Bcrp1 plays an important role in the milk transport of drugs and xenotoxins, chemotherapeutic agents such as topotecan, cimetidine, fluoroquinolones and vitamins (5–8). Nitrofurantoin, a nitrofuran-derivate anti- bacterial agent widely used as a urinary tract antibiotic prescribed for lactating women, is actively extruded into milk by BCRP/ABCG2 ( 9 , 10 ). The relevance of nitrofurantoin-BCRP/ABCG2 interaction is evidenced through reports such as the effect of pregnancy on nitro- furantoin disposition (11), nitrofurantoin fetal distribution (12), nitrofurantoin hepatobiliary excretion and sex differ- ence (13), all of them performed on Bcrp1 -/- mice, and also as a rat chemical knock-out model (10). Genistein and daidzein are isoflavones, polyphenolic compounds belong- ing to the flavonoid class; they are abundant in soybeans, represent the major active components in soy products, although the amount of each isoflavone can vary depending on the soy supplement, and appear as a mixture in red clover. High intake of soy has been associated with a variety of beneficial effects in several common diseases (14,15). A number of studies have demonstrated inhibition of drug transporters by flavonoids (16,17). The effect of flavonoids M. Perez : R. Real : J. G. Prieto : A. I. Alvarez (*) Departamento de Ciencias Biomedicas -Fisiologia, Facultad de Veterinaria, Universidad de Leon 24071 Leon, Spain e-mail: aialvf@unileon.es G. Merino : M. Perez : E. Egido : A. I. Alvarez Instituto de Sanidad Animal y Desarrollo Ganadero Universidad de León León, Spain Pharm Res (2010) 27:2098–2105 DOI 10.1007/s11095-010-0208-5