Conventional cancer chemotherapy is seriously limited by the multidrug resistance (MDR) commonly exhibited by tumour cells. One mechanism by which a living cell can achieve multiple resistances is via the active efflux of a broad range of anticancer drugs through the cellular membrane by MDR proteins. Such drugs are exported in both ATP-dependent and -independent manners, and can occur despite considerable concentration gradients. To the ATP-dependent group belongs the ATP-binding cassette (ABC) transporter family, which includes P-gp, MRP, BCRP, etc. Another protein related to MDR, though not belonging to the ABC transporter family, is lung resistance-related protein (LRP). All of these proteins are involved in diverse physiological processes, and are responsible for the uptake and efflux of a multitude of substances from cancer cells. Many inhibitors of MDR transporters have been identified over the years. Firstly, MDR drugs were not specifically developed for inhibiting MDR; in fact, they had other pharmacological properties, as well as a relatively low affinity for MDR transporters. They included compounds of diverse structure and function, such as verapamil and cyclosporine, and caused side effects. Secondly, the new drugs were more inhibitor-specific, in terms of MDR transport, and were designed to reduce such side effects (e.g., R-verapamil, dexniguldipine, etc.). Unfortunately, they displayed poor response in clinical studies. Recently, new compounds obtained from drug development programs conducted by the pharmaceutical industry are characterized by a high affinity to MDR transporters and are efficient at nanomolar concentrations. Some of these compounds (e.g., MS-209) are currently under clinical trials for specific forms of advanced cancers. We aim to provide an overview of the properties associated with those mammalian MDR transporters known to mediate significant transport of relevant drugs in cancer treatments. We also summarize recent advances concerning resistance to cancer drug therapies with respect to the function and overexpression of ABC and LRP multidrug transporters.

A series of easy-to-make fluorinated tripodal anion transporters containing urea and thiourea groups have been prepared and their anion transport properties studied. Vesicle anion transport assays using ion-selective electrodes show that this class of compound is capable of transporting chloride through a lipid bilayer via a variety of mechanisms, including chloride/H+ cotransport and chloride/nitrate, chloride/bicarbonate, and to a lesser extent an unusual chloride/sulfate antiport process. Calculations indicate that increasing the degree of fluorination of the tripodal transmembrane transporters increases the lipophilicity of the transporter and this is shown to be the major contributing factor in the superior transport activity of the fluorinated compounds, with a maximum transport rate achieved for clog P = 8. The most active transporter 5 contained a urea functionality appended with a 3,5-bis(trifluoromethyl)phenyl group and was able to mediate transmembrane chloride transport at receptor to lipid ratios as low as 1:250000. Proton NMR titration and single crystal X-ray diffraction revealed the ability of the tripodal receptors to bind different anions with varying affinities in a 1:1 or 2:1 stoichiometry in solution and in the solid state. We also provide evidence that the most potent anion transporters are able to induce apoptosis in human cancer cells by using a selection of in vitro viability and fluorescence assays.

Synthetic anion transporters can facilitate H + transport via deprotonation, or OH À transport via hydrogen bonding to OH À , thus allowing dissipation of transmembrane pH gradients, an undesired side-effect for biomedical applications as Cl À ionophores. To address this limitation, Gale and colleagues have developed two anionophores that show high Cl À > H + /OH À selectivity. Preliminary cellular studies support the biological relevance of the selectivity.

In this Account, we discuss the development of new lipid bilayer anion transporters based on the structure of anionophoric natural products (the prodigiosins) and purely synthetic supramolecular systems. We have studied the interaction of these compounds with human cancer cell lines, and, in general, the most active anion transporter compounds possess the greatest anti-cancer properties. Initially, we describe the anion transport properties of synthetic molecules that are based on the structure of the family of natural products known as the prodiginines. Obatoclax, for example, is a prodiginine derivative with an indole ring that is currently in clinical trials for use as an anti-cancer drug. The anion transport properties of the compounds were correlated with their toxicity toward small cell human lung cancer GLC4 cells. We studied related compounds with enamine moieties, tambjamines, that serve as active transporters. These molecules and others in this series could depolarize acidic compartments within GLC4 cells and trigger apoptosis. In a study of the variation of lipophilicity of a series of these compounds, we observed that, as log P increases, the anion transport efficiency reaches a peak and then decreases. In addition, we discuss the anion transport properties of series of synthetic supramolecular anion receptor species. We synthesized trisureas and thioureas based on the tren backbone, and found that the thiourea compounds effectively transport anions. Fluorination of the pendant phenyl groups in this series of compounds greatly enhances the transport properties. Similar to our earlier results, the most active anion transporters reduced the viability of human cancer cell lines by depolarizing acidic compartments in GLC4 cells and triggering apoptosis. In an attempt to produce simpler transporters that obey Lipinski's Rule of Five, we synthesized simpler systems containing a single urea or thiourea group. Once again the thiourea systems, and in particular a thiourea with a pendant indole group, transported anions efficiently. A series of related compounds containing a pendant trifluoromethyl group showed enhanced transport and significant anticancer properties. Researchers still need to determine of the exact mechanism of how these compounds depolarize acidic organelles within cancer cells. However, this work shows that these transporters based upon both natural products and purely synthetic supramolecular systems transport anions, depolarize acidic compartments within cancer cells and trigger apoptosis.

1 The eects of supernatant from the bacterial strain Serratia marcescens 2170 (CS-2170) on the viability of dierent haematopoietic cancer cell lines (Jurkat, NSO, HL-60 and Ramos) and nonmalignant cells (NIH-3T3 and MDCK) was studied. We examined whether this cytotoxic eect was due to apoptosis, and we puri®ed the molecule responsible for this eect and determined its chemical structure.2 Using an MTT assay we showed a rapid (4 h) decrease in the number of viable cells. This cytotoxic eect was due to apoptosis, according to the fragmentation pattern of DNA, Hoechst 33342 staining and FACS analysis of the phosphatidylserine externalization. This apoptosis was blocked by using the caspase inhibitor Z-VAD.fmk, indicating the involvement of caspases. 3 Prodigiosin is a red pigment produced by various bacteria including S. marcescens. Using mutants of S. marcescens (OF, WF and 933) that do not synthesize prodigiosin, we further showed that prodigiosin is involved in this apoptosis. This evidence was corroborated by spectroscopic analysis of prodigiosin isolated from S. marcescens. 4 These results indicate that prodigiosin, an immunosuppressor, induces apoptosis in haematopoietic cancer cells with no marked toxicity in nonmalignant cells, raising the possibility of its therapeutic use as an antineoplastic drug.