Broad based inhibition of multidrug resistance proteins for improved cancer treatment

A variety of chemotherapies are available to oncologists and these generally reduce the rate of tumor progression. However intrinsic or acquired tumor-mediated drug resistance is a major clinical obstacle that can result in the lack of tumor responsiveness in patients undergoing treatment. Multidrug resistance (MDR) is in part due to active efflux transporters belonging to the ATP-binding cassette (ABC) superfamily, such as p-glycoprotein (Pgp), multidrug resistance protein (MRP-1) and breast cancer resistance protein (BCRP). For example, Pgp is over-expressed in about 30–40%of primary and more than 50% of metastatic breast cancer patient samples (click here for an analysis of emerging breast cancer therapeutics). More recent data have suggested that Pgp is also involved in the passage of molecules across the blood brain barrier, the intestinal wall, and in inducing apoptosis in peripheral blood mononuclear cells. The development of substrates or inhibitors of this protein therefore represents an active area of the pharmaceutical industry. Indeed over 15 molecules are in development, over half of which are in preclinical phases of evaluation.

Clinical trials have been conducted to evaluate the efficacy of MDR-reversing agents such as verapamil, quinidine and cyclosporin A. Results have been mixed, however, some small-scale studies showed that incorporation of verapamil or cyclosporin in chemotherapy significantly improved overall survival. New, more potent p-glycoprotein inhibitors such as PSC388, GF120918, dexverapamil and XR9576 are now being evaluated in clinical trials, and preliminary results to date indicate that at minimum it is possible to obtain serum levels of reversing agents sufficient to block p-glycoprotein.

Paclitaxel, isolated from the bark of the pacific yew tree in the 1970’s, is an antitumor drug that binds to beta-tubulin and inhibits its depolymerization. Significant antitumor efficacy is seen in ovarian, breast, lung, head and neck, bladder and esophageal cancers. Docetaxel is a more potent analog of paclitaxel, and is effective in breast, ovarian, lung, gastric and prostate cancers. Both paclitaxel and docetaxel are substrates for Pgp- and MRP-1-mediated efflux, and their efficacy is thus compromised in cells that overexpress Pgp or MRP-1. Optimization of paclitaxel with the aim of increasing potency and reducing susceptibility to Pgp-mediated efflux has gained significant attention. Of note Ralph Bernacki’s laboratory at the Roswell Park Cancer Institute, has identified orataxel (formerly IDN-5109, BAY 59-8862) as a potent paclitaxel analog that modulates efflux mediated by Pgp and MRP-1 and BCRP. This same group has also developed non-cytotoxic taxane analogs that modulate all three of the ABC transporters that mediate MDR.

Non-cytotoxic synthetic taxane-based reversal agents (tRAs) have the taxane baccatin backbone, but lack the C-13 side chain of paclitaxel that binds beta-tubulin and mediates cytotoxicity. More than 100 non-cytotoxic taxane-based reversal agents (tRAs) have been synthesized to date, with diverse side chains. In their most recently reported study, Dr Bernacki and colleagues searched their library of synthetic tRAs in order to identify non-cytotoxic modulators of Pgp, MRP-1 and BCRP that might be developed as broad spectrum clinical MDR modulators.

In an initial screen for activity against Pgp, 26 compounds were identified that, at sub-micromolar concentrations, reduced the IC50 of paclitaxel in a breast cancer cell line resistant to this cytoxic agent by over 75%. The most potent molecules were then tested in a secondary assay of cell lines that express Pgp, MRP-1 and BCRP, for their ability to limit the efflux of another chemotherapeutic agents, mitoxantrone. Following uptake of mitoxantrone, 8226/Dox6, HL60/ADR and 8226/MR20 cells, which overexpress Pgp, MRP-1 and BCRP-R482, respectively, effluxed 40-50% of their intracellular mitoxantrone content during 90-minute incubation in medium alone. Of the 26 molecules selected from the primary screen, 4 reduced the efflux of mitoxantrone in each of the three cell lines studied. These molecules inhibited efflux in the three cell lines with minimally active concentrations of 0.1-10 micromolar, and mirrored the increased sensitivity of each cell line to the cytotoxic effects of mitoxantrone. The lead molecule, tRA 98006, reduced the IC50 by between 2- and 20-fold depending on the cell line investigated. Similarly this molecule also enhanced the cytotoxicity of other anthracyclines including doxorubicin and daunorubicin in other chemo-resistant cell lines.

Most MDR modulation clinical trials have, to date, been disappointing and may be due in part to the focus on single MDR proteins even though many cancers are associated with the overexpression of multiple MDR proteins. The most successful trial to date involved the use of cyclosporin A which does have moderate activity against MDR proteins other than Pgp. The molecules in the present study, notably tRA 98006, have been rationally selected so as to optimize the degree of broad-based activity. As an added benefit the expression of MDR proteins in the intestinal tract and the blood brain barrier may allow for improved oral absorption and also brain penetration. Clinical trials are therefore eagerly awaited on tRA 98006 or related molecules to determine whether efficacy is indeed optimized. Before such trials are conducted careful attention must be placed on toxicity, pharmacokinetics and drug interactions as effective modulation of drug efflux proteins modifies each of these.

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Entry date Tuesday, January 27, 2004

Adapted from Brooks et al, Mol Cancer Ther. 2003 Nov; 2(11): 1195-205