Structure-activity relationships of new inhibitors of breast cancer resistance protein (ABCG2).

At the end of the last century tariquidar (XR9576) was synthesized, pharmacologically investigated, and classified as a promising 3rd generation P-glycoprotein (P-gp) modulator. Following the discovery of BCRP in 1998 an increasing number of substances were studied in relation to their potency to interact with this transporter. Recently it has been shown that XR9576 inhibits both P-gp and BCRP transport function similarly to GF120918 (elacridar). This observation prompted us to investigate 5 XR compounds and 25 structurally related derivatives synthesized in our laboratory for their BCRP inhibitory effect. The biological activity data were determined by our new Hoechst 33342 assay that has been transferred from P-gp to BCRP overexpressing cells. 3D-QSAR models (CoMFA and CoMSIA) were generated and validated by the leave-many-out method and the scrambling stability test. The best models yielded an internal predictive squared correlation coefficient higher than 0.8 and contained steric, electrostatic, hydrophobic, and hydrogen bond donor fields. To our knowledge, this is the first 3D-QSAR analysis of BCRP inhibitors. Additionally the biological activity data determined in P-gp overexpressing cells on one side and BCRP overexpressing cells on the other side were compared to identify selective and non-selective inhibitors of P-gp and BCRP. The results may help to get a better insight which structural elements are necessary to direct the interaction of these compounds with P-gp and/or BCRP.