**Quercetin Is a Flavonoid Breast Cancer Resistance Protein Inhibitor with an Impact on the Oral Pharmacokinetics of Sulfasalazine in Rats**

**Yoo-Kyung Song 1,2,**† **, Jin-Ha Yoon 3,**† **, Jong Kyu Woo <sup>3</sup> , Ju-Hee Kang <sup>3</sup> , Kyeong-Ryoon Lee <sup>2</sup> , Seung Hyun Oh <sup>3</sup> , Suk-Jae Chung 1,\* and Han-Joo Maeng 3,\***


Received: 17 March 2020; Accepted: 23 April 2020; Published: 26 April 2020

**Abstract:** The potential inhibitory effect of quercetin, a major plant flavonol, on breast cancer resistance protein (BCRP) activity was investigated in this study. The presence of quercetin significantly increased the cellular accumulation and associated cytotoxicity of the BCRP substrate mitoxantrone in human cervical cancer cells (HeLa cells) in a concentration-dependent manner. The transcellular efflux of prazosin, a stereotypical BCRP substrate, was also significantly reduced in the presence of quercetin in a bidirectional transport assay using human BCRP-overexpressing cells; further kinetic analysis revealed IC<sup>50</sup> and Ki values of 4.22 and 3.91 µM, respectively. Moreover, pretreatment with 10 mg/kg quercetin in rats led to a 1.8-fold and 1.5-fold increase in the AUC8h (i.e., 44.5 ± 11.8 min·µg/mL vs. 25.7 ± 9.98 min·µg/mL, *p* < 0.05) and Cmax (i.e., 179 ± 23.0 ng/mL vs. 122 ± 23.2 ng/mL, *p* < 0.05) of orally administered sulfasalazine, respectively. Collectively, these results provide evidence that quercetin acts as an in vivo as well as in vitro inhibitor of BCRP. Considering the high dietary intake of quercetin as well as its consumption as a dietary supplement, issuing a caution regarding its food–drug interactions should be considered.

**Keywords:** quercetin; breast cancer resistance protein; inhibitor; prazosin; sulfasalazine; kinetic analysis; pharmacokinetics; food–drug interactions
