Sulfotransferases and Breast Cancer Resistance Protein Determine the Disposition of Calycosin in Vitro and in Vivo

Sulfation is a key process of drug disposition that generally regulates drug effectiveness and toxicity. Calycosin derived from the dry root extract of Radix Astragali exhibits a variety of biological effects that easily undergo extensive phase II metabolism. However, the sulfation pathway of calycosin lacks information. We investigated the disposition mechanisms of calycosin sulfate in vitro and in vivo. We characterized the sulfation metabolism and excretion of calycosin using bidirectional transport studies. We confirmed that sulfate conjugate is breast cancer resistance protein (BCRP) substrate using the intestinal perfusion model and pharmacokinetics studies in Bcrp1–/– mice. Results showed that calycosin is rapidly and extensively metabolized to calycosin-3′-sulfate (C-3′-S) in the intestine and liver. The overexpression of BCRP led to a substantial increase (approximately 14-fold, p < 0.01) of excreted C-3′-S in the BCRP overexpressed Madin–Darby canine kidney II (MDCK II/BCRP) cells. The chemical inhibition of BCRP caused reduction (about 2-fold, p < 0.01) in C-3′-S apical excretion. Furthermore, in intestinal perfusion studies, the deletion of Bcrp1 significantly decreased C-3′-S excretion in the small intestine (82.6–90.6%, p < 0.01) and colon (97.6–98.2%, p < 0.01). In contrast, plasma level of C-3′-S was increased to 40-fold (p < 0.01) in Bcrp1–/– mice. In conclusion, calycosin undergoes an extensive sulfation metabolism and BCRP is a critical determinant to the disposition of C-3′-S.