A drug binding protein is a protein that directly and reversibly associates with a therapeutic agent through a molecular binding site and thereby contributes to the agent’s cellular recognition, intracellular concentration, transmembrane movement, sequestration, metabolism, or pharmacological response. Within oncology and drug transport–resistance mechanisms, the term principally encompasses membrane proteins that bind anticancer drugs as transport substrates or inhibitors, including ATP-binding cassette (ABC) efflux transporters and solute carrier (SLC) uptake transporters ^[1][2][3]. Drug binding occurs through noncovalent interactions involving hydrophobic contacts, hydrogen bonds, electrostatic forces, aromatic interactions, and conformational complementarity between the drug and the protein’s binding cavity or translocation pathway. Binding affinity, substrate specificity, binding-site accessibility, transporter conformation, expression level, and coupling to ATP hydrolysis or solute gradients govern the extent of drug movement across cellular membranes ^[2][3][4]. In tumour cells, these protein–drug interactions regulate intracellular drug exposure and can contribute to reduced drug accumulation and multidrug-resistance phenotypes ^[1][3][4].