The "Specific" P-Glycoprotein Inhibitor Tariquidar Is Also a Substrate and an Inhibitor for Breast Cancer Resistance Protein (BCRP/ABCG2)

Tariquidar was developed as a specific inhibitor of the efflux transporter ABCB1. Recent positron emission tomographic brain imaging studies using [C-11]-tariquidar to measure ABCB1 (P-gp, P-glycoprotein) density in mice indicate that the inhibitor may not be as specific as previously thought. We examined its selectivity as an inhibitor and a substrate for the human transporters P-gp, breast cancer resistance protein (BCRP, ABCG2), and multidrug resistance protein 1 (MRP1, ABCC1). Our results show that at low concentrations, tariquidar acts selectively as an inhibitor of P-gp and also as a substrate of BCRP. At much higher concentrations (>= 100 nM), tariquidar acts as an inhibitor of both P-gp and BCRP. Thus, the in vivo specificity of tariquidar depends on concentration and the relative density and capacity of P-gp vs BCRP.