Most patients who die of cancer have disseminated disease that has become resistant to multiple therapeutic modalities. Ample evidence suggests that the expression of ATP-binding cassette (ABC) transporters, especially the multidrug resistance protein 1 (MDR1, also known as P-glycoprotein or P-gp), which is encoded by ABC subfamily B member 1 (ABCB1), can confer resistance to cytotoxic and targeted chemotherapy. However, the development of MDR1 as a therapeutic target has been unsuccessful. At the time of its discovery, appropriate tools for the characterization and clinical development of MDR1 as a therapeutic target were lacking. Thirty years after the initial cloning and characterization of MDR1 and the implication of two additional ABC transporters, the multidrug resistance-associated protein 1 (MRP1; encoded by ABCC1)), and ABCG2, in multidrug resistance, interest in investigating these transporters as therapeutic targets has waned. However, with the emergence of new data and advanced techniques, we propose to re-evaluate whether these transporters play a clinical role in multidrug resistance. With this Opinion article, we present recent evidence indicating that it is time to revisit the investigation into the role of ABC transporters in efficient drug delivery in various cancer types and at the blood-brain barrier.

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Edited by Norma AllewellP-glycoprotein (P-gp) is a polyspecific ATP-dependent transporter linked to multidrug resistance in cancer; it plays important roles in determining the pharmacokinetics of many drugs. Understanding the structural basis of P-gp, substrate polyspecificity has been hampered by its intrinsic flexibility, which is facilitated by a 75-residue linker that connects the two halves of P-gp. Here we constructed a mutant murine P-gp with a shortened linker to facilitate structural determination. Despite dramatic reduction in rhodamine 123 and calcein-AM transport, the linker-shortened mutant P-gp possesses basal ATPase activity and binds ATP only in its N-terminal nucleotide-binding domain. Nine independently determined structures of wild type, the linker mutant, and a methylated P-gp at up to 3.3 Å resolution display significant movements of individual transmembrane domain helices, which correlated with the opening and closing motion of the two halves of P-gp. The open-andclose motion alters the surface topology of P-gp within the drugbinding pocket, providing a mechanistic explanation for the polyspecificity of P-gp in substrate interactions.The association of multidrug resistance (MDR) 3 in cancer treatment with expression of human ABC transporters on the cell surface has raised the possibility that overcoming MDR might be achieved by inhibiting these transporters (1). One transporter in particular, the human P-glycoprotein (P-gp), has been well characterized; it is able to confer MDR by transporting numerous structurally unrelated drugs at the expense of hydrolyzing ATP (2, 3). In addition, P-gp plays important roles in drug distribution in normal physiology and is an essential component of many physiological barriers (4, 5).Long-standing efforts have been devoted to understanding the mechanism of P-gp function by various experimental approaches. Among them, structural studies of P-gp from various organisms have been reported (6 -8). In particular, a number of structures for the mouse P-gp (mP-gp) were obtained (8 -10). However, the diffraction limits of "near-native," inhibitor-bound, and nanobody-associated mP-gp crystals were relatively low, mostly near 4 Å resolution. The methylated protein gave higher resolution structures (11), but the impact of reductive methylation on the structure and function of P-gp was not addressed. The low resolution diffraction of P-gp crystals has been attributed in part to the intrinsic flexibility of the molecule (12-14).Mouse P-gp is a 1276-residue polypeptide and bears 87% sequence identity to human P-gp; it consists of two homologous halves connected by a flexible linker of ϳ75 residues. Each half is made of a transmembrane domain (TMD) implicated in drug recognition and transport and a nucleotide-binding domain (NBD). Each NBD is able to bind and hydrolyze ATP (15, 16), which is, however, dependent on the other NBD being functional (17,18). The two NBDs of P-gp are highly homologous, each featuring a consensus nucleotide-binding site with full-fledged Walker...