P-glycoprotein (Pgp) is an ATP-dependent efflux transporter. It plays a significant role in anti-cancer drug resistance by pumping a chemically diverse range of cytotoxic drugs from cancerous tumors. Despite numerous studies on the transporter, the molecular features that drive anti-cancer drug efflux are poorly understood. Even subtle differences in the anti-cancer drug molecular structure can lead to dramatic differences in their transport rates. To unmask these structural differences, this study focused on the closely-related anthracycline drugs, daunorubicin (DNR), idarubicin (IDA), epirubicin (EPI), and doxorubicin (DOX). Our study suggests a conformation-gated model explains the difference in transport rate between the two drugs. Furthermore, our study also attempts to measure the efflux of Pgp using surface plasmon resonance (SPR), which is known for real-time kinetics analysis of molecular interactions. This method will overcome the drawbacks of existing methods, such as using radioactive or fluorescently labeled substrate and using Pgp overexpression cell lines that also contain multiple types of transporters. We were able to utilize SPR to detect the relative transport rate between different Pgp substrates and these values agree with the literature. SPR data with substrate transport also suggests some insights about the transport mechanism of Pgp.
