The human ABCB1 (P-glycoprotein, Pgp) protein is an active exporter expressed in the
plasma membrane of cells forming biological barriers. In accordance with its broad
substrate spectrum and tissue expression pattern, it affects the pharmacokinetics
of numerous chemotherapeutic drugs and it is involved in unwanted drug–drug interactions
leading to side effects or toxicities. When expressed in tumor tissues, it contributes
to the development of chemotherapy resistance in malignancies. Therefore, the understanding
of the molecular details of the ligand–ABCB1 interactions is of crucial importance.
In a previous study, we found that quercetin (QUR) hampers both the transport and
ATPase activity of ABCB1, while cyandin-3O-sophroside (C3S) stimulates the ATPase
activity and causes only a weak inhibition of substrate transport. In the current
study, when QUR and C3S were applied together, both a stronger ATPase inhibition and
a robust decrease in substrate transport were observed, supporting their synergistic
ABCB1 inhibitory effect. Similar to cyclosporine A, a potent ABCB1 inhibitor, co-treatment
with QUR and C3S shifted the conformational equilibrium to the “inward-facing” conformer
of ABCB1, as it was detected by the conformation-selective UIC2 mAb. To gain deeper
insight into the molecular details of ligand–ABCB1 interactions, molecular docking
experiments and MD simulations were also carried out. Our in silico studies support
that QUR and C3S can bind simultaneously to ABCB1. The most favourable ligand–ABCB1
interaction is obtained when C3S binds to the central substrate binding site and QUR
occupies the “access tunnel”. Our results also highlight that the strong ABCB1 inhibitory
effect of the combined treatment with QUR and C3S may be exploited in chemotherapy
protocols for the treatment of multidrug-resistant tumors or for improving drug delivery
through pharmacological barriers.
