Revisiting the role of ABC transporters in multidrug-resistant cancer

Abstract

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.

Figure 1:

Structure and mechanism of 3 ABC transporters. A, High-resolution 3D structures of ABCG2 (PDB accession 5NJ3), ABCB1 (PDB accession 5KPI) and ABCC1 (PDB accession 5UJ9). As ABCG2 forms a homodimer, the second dimer of the full protein is shown in light blue. Common substrates and inhibitors are listed. (Although the structure for ABCC1 is that of Bos Taurus, the protein identity is 91% and the structure is likely similar to human ABCC1). B, Schematic representation of the proposed pumping action of ABCB1. The substrate binds to the binding pocket and ATP binds to the two binding sites in the NBDs. This is followed by the hydrolysis of ATP that generates a conformational change, allowing the substrate to be released from the protein. The second molecule of ATP is hydrolyzed, allowing for a conformational reset, where substrate and ATP can bind again so the process can repeat.

Figure 2:

Upregulation of ABCB1 via promoter capture. Adapted from Knutsen et al.. Used by permission.

Figure 3:

Expression of ABCB1 and ABCG2 in patient tumor samples. Data from the cBioPortal website (www.cbioportal.org) showing expression of ABCB1 (A) or ABCG2 (B) in various tumor types. (C) Expression of both ABCB1 and ABCG2 in breast, thyroid, pancreas, liver, kidney and glioblastoma tumors taken from the TCGA database. The data are viewed with tcgaminer, an R Shiny-based program. The data shown in this figure are based upon data generated by the TCGA Research Network: http://cancergenome.nih.gov.

Figure 4:

Effect of transporter deletion on plasma or brain levels of drugs. Fold increase in plasma and brain drug levels in mice deficient for Abcb1a/b, Abcg2 or all three (TKO) transporters is compared to wild-type mice, which are assigned a value of 1. Grey blocks denote mice not studied. Adapted from Basseville et al and compiled from references^–,–. Used by permission.

Figure 5:

The utility and function of PET radiotracers and other probes for imaging ABC transporter function, using the CNS as a model. Combination with inhibitors of known function, or administration to knockout mice, provides insight into function. ABCB1: des-methyl loperamide (dLop) is a specific substrate of ABCB1, producing almost no signal intensity under baseline conditions. Upon inhibition of ABCB1, high brain intensity is observed, though ABCG2 inhibition has no effect. In the instance of a dual substrate of both ABCB1 and ABCG2, such as erlotinib, specifically blocking either ABCB1 or ABCG2 results in a minimal increase in brain signal, and only dual inhibition or knockout produces an effect. An alternative imaging strategy using the specific ABCG2 substrate D-luciferin, with transgenic mice expressing firefly luciferase in astrocytes. Brain bioluminescence signal was low, and specific inhibition of ABCG2 but not ABCB1 produced elevated signal. Third-generation inhibitors such as tariquidar and elacridar are considered to primarily inhibit ABCB1, while Ko143 (which has not been used in humans) acts primarily on ABCG2. No gold-standard probe for dual inhibition of ABCG2 and ABCB1 exists. These imaging tools can act as the basis for studies of multidrug resistance in tumors, and efficacy and dose-optimization of new inhibitors.