Decreasing Drug Resistance Through Modulation/Inhibition of the P-Glycoprotein

Drug resistance occurs through multiple mechanisms within a tumor cell. A well-documented mechanism of drug resistance has been identified through the expression of the P-glycoprotein (Pgp), also known as the multidrug resistance 1 (MDR1) protein [1,2]. This overexpression of the Pgp in tumor cells is responsible for the resistance of certain chemotherapy agents that are used for treating late stage cancer patients [3].

The Pgp is encoded by the ABCB1 gene, which is located on 7q21.12 with 29 exons [2,4]. The Pgp is a transmembrane protein with 1280 amino acids consisting of 12 transmembrane domains and 2 cytoplasmic domains with an ATP-binding site. This transporter is a member of the superfamily of ATP-binding cassette (ABC) transporters which is divided into seven distinct subfamilies [5]. The Pgp is expressed in the liver, the adrenal gland, intestines, kidneys, placenta, and the capillary endothelial cells of the brain and testis barriers [6]. The function of this protein is to transport substrates across the membrane of the cell using the active transport through ATP activation. In cases such as the blood-brain barrier and the intestines, the Pgp plays a protective role by using the efflux pump to excrete toxins and xenobiotic compounds from the cell before the harmful substrates are able to damage the tissues or organism [6].

In drug metabolism, the Pgp is able to transport drug substrates in or out of the cell, depending on the composition of the compound. There is a wide variety of substrates of the Pgp, these drugs include immunosuppressants, calcium blockers, beta-blockers, antihistamines, anticonvulsants, antidepressants, chemotherapy agents, and retroviral inhibitors [7]. The CYP3A4 is the CYP450 enzyme that is complimentary to the Pgp for metabolizing many of the drug therapies. The Pgp is able to transport the drug into the cell where the drug then becomes a substrate for CYP3A4 [8,9].

The Pgp has been well characterized by researchers because of the increase of the Pgp efflux with chemotherapy agents in tumor cells. The increase of the Pgp efflux leads to the tumor cells becoming resistant to the chemotherapy agents. The Pgp substrates of chemotherapy agents include taxanes (paclitaxel, docetaxel), vinca alkaloids (vincristine, vinblastine), anthracyclines (doxorubicin, daunorubicin), and epipodophyllotoxins (etoposide) [1]. These agents are cytotoxic to tumor cells and are used to treat patients with lymphoma, leukemia, lung cancer, breast cancer, colorectal cancer, or other types of solid tumors.

In tumor cells, the increase of efflux in the Pgp occurs through over expression of the protein which can happen through somatic or epigenetic alterations in the tumor cells. These genetic changes can occur before or after the initiation of chemotherapy. One type of alteration associated with increased Pgp expression is the somatic mutations of either oncogenes, such as Ras and raf kinase, or the tumor suppressor gene, TP53. Somatic mutations in these cancer driver genes have been implicated with regulating the expression of the ABCB1 gene [10-12]. Another reason for increased Pgp expression in tumor cells is through the epigenetic changes of demethylation in the promoter of the ABCB1 gene or histone deacetylation. Several in vitro studies with leukemia, breast cancer, and bladder cancer have demonstrated that the loss of methylation in the ABCB1 promoter is associated with the activation of Pgp expression in drug resistant cells [13-15]. The acetylation status of histones was also associated with the increased expression of Pgp in colorectal and sarcoma cells through in vitro experiments. Researchers identified that the increased acetylation of histone H3 influenced the expression of several ABC transporters such as Pgp [16,17]. Identifying these somatic or epigenetic changes in the multi-drug resistant tumor cells may play an important role in the future for the use of targeted therapy for regulating the Pgp expression.

Several approaches have been developed to reverse the effects of drug resistance in the tumor cells. This reversal of drug resistance can be achieved through competition or inhibiting efflux pump of the Pgp. Decreasing the Pgp activity can be accomplished through the use of either modulators or inhibitors for the Pgp. These modulators and inhibitors for Pgp are drugs that are currently being used for treatment. Thirty years ago, researchers identified the first generation of Pgp modulators [18]. These drugs are able to compete with the chemotherapy agents for the Pgp efflux pump. The drugs included the immunosuppressant, cyclosporin A, and calcium channel blocker, verapamil [19,20]. Through in vitro and in vivo studies, the modulators were unsuccessful for decreasing drug resistance due to toxicity issues and unpredictable drug interactions [21,22]. A second generation Pgp modulators were identified to increase potency of binding to the efflux pump and decreasing the issues of toxicity from the lesson learned in the first generation modulators. One of the most studied second generation modulators was the Valspodar (PSC833), which is a derivative of cyclosporine D [18,23]. Using in vitro studies, the experimental Valspodar in conjunction with chemotherapy agents resulted with a higher effect of binding to the efflux pump than the first generation Pgp modulators, but the second generation modulator and chemotherapy agents were discovered to be both substrates for CYP3A4 [24,25]. This discovery showed that the competition for the CYP3A4 caused a decrease in the metabolism of the chemotherapy agents. This competition for CYP3A4 led to less of the active chemotherapy agent to kill the tumor cells [26]. The third generation of Pgp modulators/inhibitors such as Tariquidar, Zosuquidar, Sorafenib, and Lapatinib are being studied as potential drugs for lowering the efflux activity of the Pgp and may be successful for decreasing drug resistance [27-31]. Some of these third generation drugs are currently being used in clinical trials as an option for decreasing drug resistance in several cancers including lung, colorectal, and breast.

In future oncology treatment, personalized therapy will be available for cancer patients. A wide variety of chemotherapy agents and targeted cancer therapies will be able to remove the issues of multi-drug resistance in tumor cells. With the identification of increase Pgp in primary tumors, patients will be placed on drugs that are able to either suppress the Pgp or the patient will be given another drug that is not a Pgp substrate. The increase of cancer therapy options will lead to better outcomes for late cancer patients.

References

1. Gottesman MM, Fojo T, Bates SE. Multidrug resistance in cancer: role of ATP-dependent transporters. Nat Rev Cancer. 2002; 2: 48-58.
2. Gottesman MM, Ling V. The molecular basis of multidrug resistance in cancer: the early years of P-glycoprotein research. FEBS Lett. 2006; 580: 998-1009.
3. O'Connor R. A review of mechanisms of circumvention and modulation of chemotherapeutic drug resistance. Curr Cancer Drug Targets. 2009; 9: 273-280.
4. Sikic BI. Modulation of multidrug resistance: a paradigm for translational clinical research. Oncology (Williston Park). 1999; 13: 183-187.
5. Dean M, Annilo T. Evolution of the ATP-binding cassette (ABC) transporter superfamily in vertebrates. Annu Rev Genomics Hum Genet. 2005; 6: 123-142.
6. Gottesman MM, Ambudkar SV. Overview: ABC transporters and human disease. J Bioenerg Biomembr. 2001; 33: 453-458.
7. Ferry DR, Traunecker H, Kerr DJ. Clinical trials of P-glycoprotein reversal in solid tumours. Eur J Cancer. 1996; 32A: 1070-1081.
8. Lum BL, Gosland MP. MDR expression in normal tissues. Pharmacologic implications for the clinical use of P-glycoprotein inhibitors. Hematol Oncol Clin North Am. 1995; 9: 319-336.
9. Chan LM, Cooper AE, Dudley AL, Ford D, Hirst BH. P-glycoprotein potentiates CYP3A4-mediated drug disappearance during Caco-2 intestinal secretory detoxification. J Drug Target. 2004; 12: 405-413.
10. Chin KV, Ueda K, Pastan I, Gottesman MM. Modulation of activity of the promoter of the human MDR1 gene by Ras and p53. Science. 1992; 255: 459-462.
11. Cornwell MM, Smith DE. A signal transduction pathway for activation of the mdr1 promoter involves the proto-oncogene c-raf kinase. J Biol Chem. 1993; 268: 15347-15350.
12. Thottassery JV, Zambetti GP, Arimori K, Schuetz EG, Schuetz JD. p53-dependent regulation of MDR1 gene expression causes selective resistance to chemotherapeutic agents. Proc Natl Acad Sci U S A. 1997; 94: 11037-11042.
13. Reed K, Hembruff SL, Laberge ML, Villeneuve DJ, Côté GB, Parissenti AM, et al. Hypermethylation of the ABCB1 downstream gene promoter accompanies ABCB1 gene amplification and increased expression in docetaxel-resistant MCF-7 breast tumor cells. Epigenetics. 2008; 3: 270-280.
14. Desiderato L, Davey MW, Piper AA. Demethylation of the human MDR1 5' region accompanies activation of P-glycoprotein expression in a HL60 multidrug resistant subline. Somat Cell Mol Genet. 1997; 23: 391-400.
15. Kantharidis P, El-Osta A, deSilva M, Wall DM, Hu XF, Slater A, et al. Altered methylation of the human MDR1 promoter is associated with acquired multidrug resistance. Clin Cancer Res. 1997; 3: 2025-2032.
16. Jin S, Scotto KW. Transcriptional regulation of the MDR1 gene by histone acetyltransferase and deacetylase is mediated by NF-Y. Mol Cell Biol. 1998; 18: 4377-4384.
17. Baker EK, Johnstone RW, Zalcberg JR, El-Osta A. Epigenetic changes to the MDR1 locus in response to chemotherapeutic drugs. Oncogene. 2005; 24: 8061-8075.
18. Krishna R, Mayer LD. Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs. Eur J Pharm Sci. 2000; 11: 265-283.
19. Cornwell MM, Pastan I, Gottesman MM. Certain calcium channel blockers bind specifically to multidrug-resistant human KB carcinoma membrane vesicles and inhibit drug binding to P-glycoprotein. J Biol Chem. 1987; 262: 2166-2170.
20. Goldberg H, Ling V, Wong PY, Skorecki K. Reduced cyclosporin accumulation in multidrug-resistant cells. Biochem Biophys Res Commun. 1988; 152: 552-558.
21. Ferry DR, Traunecker H, Kerr DJ. Clinical trials of P-glycoprotein reversal in solid tumours. Eur J Cancer. 1996; 32A: 1070-1081.
22. Ambudkar SV, Dey S, Hrycyna CA, Ramachandra M, Pastan I, Gottesman MM, et al. Biochemical, cellular, and pharmacological aspects of the multidrug transporter. Annu Rev Pharmacol Toxicol. 1999; 39: 361-398.
23. Twentyman PR, Bleehen NM. Resistance modification by PSC-833, a novel non-immunosuppressive cyclosporin [corrected]. Eur J Cancer. 1991; 27: 1639-1642.
24. Lum BL, Gosland MP. MDR expression in normal tissues. Pharmacologic implications for the clinical use of P-glycoprotein inhibitors. Hematol Oncol Clin North Am. 1995; 9: 319-336.
25. Wandel C, Kim RB, Kajiji S, Guengerich P, Wilkinson GR, Wood AJ, et al. P-glycoprotein and cytochrome P-450 3A inhibition: dissociation of inhibitory potencies. Cancer Res. 1999; 59: 3944-3948.
26. Fischer V, Rodríguez-Gascón A, Heitz F, Tynes R, Hauck C, Cohen D, et al. The multidrug resistance modulator valspodar (PSC 833) is metabolized by human cytochrome P450 3A. Implications for drug-drug interactions and pharmacological activity of the main metabolite. Drug Metab Dispos. 1998; 26: 802-811.
27. Martin C, Berridge G, Mistry P, Higgins C, Charlton P, Callaghan R, et al. The molecular interaction of the high affinity reversal agent XR9576 with P-glycoprotein. Br J Pharmacol. 1999; 128: 403-411.
28. Fox E, Bates SE. Tariquidar (XR9576): a P-glycoprotein drug efflux pump inhibitor. Expert Rev Anticancer Ther. 2007; 7: 447-459.
29. Cripe LD, Tallman M, Karanes C. A phase II trial of zosuquidar (LY335979), a modulator of P-glycoprotein (P-gp) activity, plusdaunorubicin and high-dose cytarabine in patients with newly-diagnosed secondary acute myeloid leukemia (AML), refractory anemia with excess blasts in transformation (RAEB-t), or relapsed/refractory AML. Blood. 2001; 98: 59.
30. Hoffmann Katrin. "Sorafenib modulates the gene expression of multi-drug resistance mediating ATP-binding cassette proteins in experimental hepatocellular carcinoma." Anticancer research 2010; 30: 4503-4508.
31. Polli JW, Humphreys JE, Harmon KA, Castellino S, O'Mara MJ, Olson KL, et al. The role of efflux and uptake transporters in [N-{3-chloro-4-[(3-fluorobenzyl)oxy]phenyl}-6-[5-({[2-(methylsulfonyl)ethyl] amino}methyl)-2-furyl]-4-quinazolinamine (GW572016, lapatinib) disposition and drug interactions. Drug Metab Dispos. 2008; 36: 695-701.