Effect of a Novel Vacuolar-H+-ATPase Inhibitor on Cell and Tumor Response to Camptothecins

Transforming the Chemical Structure and Bio-Nano Activity of Doxorubicin by Ultrasound for Selective Killing of Cancer Cells

Reconfiguring the structure and selectivity of existing chemotherapeutics represents an opportunity for developing novel tumor-selective drugs. Here, as a proof-of-concept, the use of high-frequency sound waves is demonstrated to transform the nonselective anthracycline doxorubicin into a tumor selective drug molecule. The transformed drug self-aggregates in water to form ≈200 nm nanodrugs without requiring organic solvents, chemical agents, or surfactants. The nanodrugs preferentially interact with lipid rafts in the mitochondria of cancer cells. The mitochondrial localization of the nanodrugs plays a key role in inducing reactive oxygen species mediated selective death of breast cancer, colorectal carcinoma, ovarian carcinoma, and drug-resistant cell lines. Only marginal cytotoxicity (80-100% cell viability) toward fibroblasts and cardiomyocytes is observed, even after administration of high doses of the nanodrug (25-40 µg mL^-1 ). Penetration, cytotoxicity, and selectivity of the nanodrugs in tumor-mimicking tissues are validated by using a 3D coculture of cancer and healthy cells and 3D cell-collagen constructs in a perfusion bioreactor. The nanodrugs exhibit tropism for lung and limited accumulation in the liver and spleen, as suggested by in vivo biodistribution studies. The results highlight the potential of this approach to transform the structure and bioactivity of anticancer drugs and antibiotics bearing sono-active moieties.

Exploring the Therapeutic Potential of Membrane Transport Proteins: Focus on Cancer and Chemoresistance

Improving the therapeutic efficacy of conventional anticancer drugs represents the best hope for cancer treatment. However, the shortage of druggable targets and the increasing development of anticancer drug resistance remain significant problems. Recently, membrane transport proteins have emerged as novel therapeutic targets for cancer treatment. These proteins are essential for a plethora of cell functions ranging from cell homeostasis to clinical drug toxicity. Furthermore, their association with carcinogenesis and chemoresistance has opened new vistas for pharmacology-based cancer research. This review provides a comprehensive update of our current knowledge on the functional expression profile of membrane transport proteins in cancer and chemoresistant tumours that may form the basis for new cancer treatment strategies.

Identification of Novel Bisbenzimidazole Derivatives as Anticancer Vacuolar (H⁺)-ATPase Inhibitors

The vacuolar (H⁺)-ATPases (V-ATPases) are a family of ATP-driven proton pumps and they have been associated with cancer invasion, metastasis, and drug resistance. Despite the clear involvement of V-ATPases in cancer, the therapeutic use of V-ATPase-targeting small molecules has not reached human clinical trials to date. Thus, V-ATPases are emerging as important targets for the identification of potential novel therapeutic agents. We identified a bisbenzimidazole derivative (V) as an initial hit from a similarity search using four known V-ATPase inhibitors (I–IV). Based on the initial hit (V), we designed and synthesized a focused set of novel bisbenzimidazole analogs (2a–e). All newly prepared compounds have been screened for selected human breast cancer (MDA-MB-468, MDA-MB-231, and MCF7) and ovarian cancer (A2780, Cis-A2780, and PA-1) cell lines, along with the normal breast epithelial cell line, MCF10A. The bisbenzimidazole derivative (2e) is active against all cell lines tested. Remarkably, it demonstrated high cytotoxicity against the triple-negative breast cancer (TNBC) cell line, MDA-MB-468 (IC₅₀ = 0.04 ± 0.02 μM). Additionally, it has been shown to inhibit the V-ATPase pump that is mainly responsible for acidification. To the best of our knowledge the bisbenzimidazole pharmacophore has been identified as the first V-ATPase inhibitor in its class. These results strongly suggest that the compound 2e could be further developed as a potential anticancer V-ATPase inhibitor for breast cancer treatment.

Covalent Modulators of the Vacuolar ATPase

The vacuolar H⁺ ATPase (V-ATPase) is a complex multisubunit machine that regulates important cellular processes through controlling acidity of intracellular compartments in eukaryotes. Existing small-molecule modulators of V-ATPase either are restricted to targeting one membranous subunit of V-ATPase or have poorly understood mechanisms of action. Small molecules with novel and defined mechanisms of inhibition are thus needed to functionally characterize V-ATPase and to fully evaluate the therapeutic relevance of V-ATPase in human diseases. We have discovered electrophilic quinazolines that covalently modify a soluble catalytic subunit of V-ATPase with high potency and exquisite proteomic selectivity as revealed by fluorescence imaging and chemical proteomic activity-based profiling. The site of covalent modification was mapped to a cysteine residue located in a region of V-ATPase subunit A that is thought to regulate the dissociation of V-ATPase. We further demonstrate that a previously reported V-ATPase inhibitor, 3-bromopyruvate, also targets the same cysteine residue and that our electrophilic quinazolines modulate the function of V-ATPase in cells. With their well-defined mechanism of action and high proteomic specificity, the described quinazolines offer a powerful set of chemical probes to investigate the physiological and pathological roles of V-ATPase.

Selective inhibition of tumor cell associated Vacuolar-ATPase 'a2' isoform overcomes cisplatin resistance in ovarian cancer cells

Development of resistance to platinum compounds significantly hinders successful ovarian cancer (OVCA) treatment. In tumor cells, dysregulated pH gradient across cell membranes is a key physiological mechanism of metastasis/chemo-resistance. These pH alterations are mediated by aberrant activation of key multi-subunit proton pumps, Vacuolar-ATPases (V-ATPases). In tumor cells, its 'a2' isoform (V-ATPase-V0a2) is a component of functional plasma-membrane complex and promotes tumor invasion through tumor-acidification and immuno-modulation. Its involvement in chemo-resistance has not been studied. Here, we show that V-ATPase-V0a2 is over-expressed in acquired-cisplatin resistant OVCA cells (cis-A2780/cis-TOV112D). Of all the 'a' subunit isoforms, V-ATPase-V0a2 exhibited an elevated expression on plasma membrane of cisplatin-resistant cells compared to sensitive counterparts. Immuno-histochemistry revealed V-ATPase-V0a2 expression in both low grade (highly drug-resistant) and high grade (highly recurrent) human OVCA tissues indicating its role in a centralized mechanism of tumor resistance. In cisplatin resistant cells, shRNA mediated inhibition of V-ATPase-V0a2 enhanced sensitivity towards both cisplatin and carboplatin. This improved cytotoxicity was mediated by enhanced cisplatin-DNA-adduct formation and suppressed DNA-repair pathway, leading to enhanced apoptosis. Suppression of V0a2 activity strongly reduced cytosolic pH in resistant tumor cells, which is known to enhance platinum-associated DNA-damage. As an indicator of reduced metastasis and chemo-resistance, in contrast to plasma membrane localization, a diffused cytoplasmic localization of acidic vacuoles was observed in V0a2-knockdown resistant cells. Interestingly, pre-treatment with monoclonal V0a2-inhibitory antibody enhanced cisplatin cytotoxicity in resistant cells. Taken together, our findings suggest that the isoform specific inhibition of V-ATPase-V0a2 could serve as a therapeutic strategy for chemo-resistant ovarian carcinoma and improve efficacy of platinum drugs.

Chemoresistance to concanamycin A1 in human oral squamous cell carcinoma is attenuated by an HDAC inhibitor partly via suppression of Bcl-2 expression

V-ATPase is involved in the acidification of the microenvironment around/in solid tumors, such as oral squamous cell carcinoma (OSCC). V-ATPase is thought to induce tumor invasion and multi-drug resistance in several malignant tumors, and it also contributes to maintaining the intracellular pH under an acidic microenvironment by inducing proton extrusion into the extracellular medium. However, there is little information regarding the effects of V-ATPase inhibitors on OSCCs. In this study, the effects of a V-ATPase inhibitor, concanamycin A1 (CMA), on the proliferation and apoptosis of OSCC were investigated in vitro. We used four OSCC cell lines, MISK81-5, SAS, HSC-4 and SQUU-B. Acridine orange staining revealed that the red fluorescence was reduced in all of the low concentration CMA-treated OSCC cells, indicating that the acidification of vesicular organelles in the OSCCs was prevented by the treatment with low-concentration of CMA. CMA treatment induced apoptosis in MISK81-5, SAS and HSC-4 cells, but not in SQUU-B cells. The p-p38 expression was not altered in CMA-treated SQUU-B cells, but their levels were increased in the other cells. The Bax/Bcl-2 ratio in CMA-treated SQUU-B cells was dramatically decreased in comparison with that in the other cell lines treated with CMA. However, when the SQUU-B cells were treated with CMA and a histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA), the SQUU-B cells became more susceptible to the CMA-induced apoptosis. SAHA treatment led to a significantly decrease in the Bcl-2 expression in CMA-treated SQUU-B cells, resulting in a dramatically increased Bax/Bcl-2 ratio in comparison with that observed in the SQUU-B cells treated with CMA alone. These findings suggest that CMA could have an anti-tumor effect on OSCCs. In addition, combination of CMA with other agents, such as SAHA, could help improve the pro-apoptotic effects of CMA even in CMA-resistant OSCC cells.

Effect of pantoprazole on human gastric adenocarcinoma SGC7901 cells through regulation of phospho‑LRP6 expression in Wnt/β-catenin signaling

Recent studies have found that an acidic tumor microenvironment is the key to managing cancer progression and metastasis. Our previous study found that proton pump inhibitors (PPIs) inhibit the expression of vacuolar-ATPases (V-ATPases) and reverse the transmembrane pH gradient. The present study was conducted to explore the effect of pantoprazole on gastric adenocarcinoma through the regulation of Wnt/β-catenin signaling. We used SGC7901 human gastric cancer cells as an in vitro model to study the effect of pantoprazole. The antiproliferative, pro-apoptotic and anti‑invasive effects of pantoprazole were examined. The effects of pantoprazole on the expression of the Wnt/β-catenin signaling pathway were also studied by western blotting. Our study found that pantoprazole inhibited the proliferation and induced the apoptosis of SGC7901 human gastric cancer cells. The expression of V-ATPases was decreased following treatment with pantoprazole. Further study found that pantoprazole treatment caused a decrease in phospho-LRP6, but not in LRP6. β-catenin in Wnt/β-catenin signaling and its target genes c-Myc and cyclin D1 were also decreased upon the inhibition of V-ATPases. Therefore, pantoprazole could be characterized as a V-ATPase inhibitor for treating gastric cancer by inhibiting the phosphorylation of LRP6 in Wnt/β-catenin signaling.

Antitumor activity of dichloroacetate on C6 glioma cell: in vitro and in vivo evaluation

Dichloroacetate (DCA), a small molecule mitochondria-targeting agent, can penetrate the blood-brain barrier, showing potential therapeutic effects on brain tumors. Considering the effects of DCA on tumor cellular metabolism, penetrating across the blood-brain barrier, as well as having potential antitumor activity on brain tumors, the purpose of this study is to investigate the antitumor activity of DCA on C6 glioma cells in vitro and in vivo. DCA inhibited C6 glioma cell proliferation, induced C6 cell apoptosis, and arrested C6 cells in S phase. DCA can inhibit the expression of heat shock proteins 70 (Hsp70) in a dose-dependent and time-dependent manner (P < 0.01). Our in vivo antitumor effect results indicated that DCA markedly inhibited the growth of C6 glioma tumors in both C6 brain tumor-bearing rats and C6 tumor-bearing nude mice (P < 0.01). DCA significantly induced the ROS production and decreased the mitochondrial membrane potential in tumor tissues. Our in vivo antitumor effect results also indicated that DCA has potential antiangiogenic effects. In conclusion, DCA may be a viable therapeutic agent in the treatment of gliomas.

Antitumor efficacy of a novel CLA-PTX microemulsion against brain tumors: in vitro and in vivo findings

BACKGROUND

Considering the observations that linoleic acid conjugated with paclitaxel (CLA-PTX) possesses antitumor activity against brain tumors, is able to cross the blood-brain barrier, but has poor water solubility, the purpose of this study was to prepare a novel CLA-PTX microemulsion and evaluate its activity against brain tumors in vitro and in vivo.

METHODS

The in vitro cytotoxicity of a CLA-PTX microemulsion was investigated in C6 glioma cells. The in vivo antitumor activity of the CLA-PTX microemulsion was evaluated in tumor-bearing nude mice and rats. The pharmacokinetics of the CLA-PTX microemulsion were investigated in rats, and its safety was also evaluated in mice.

RESULTS

The average droplet size of the CLA-PTX microemulsion was approximately 176.3 ± 0.8 nm and the polydispersity index was 0.294 ± 0.024. In vitro cytotoxicity results showed that the IC(50) of the CLA-PTX microemulsion was 1.61 ± 0.83 μM for a C6 glioma cell line, which was similar to that of free paclitaxel and CLA-PTX solution (P > 0.05). The antitumor activity of the CLA-PTX microemulsion against brain tumors was confirmed in our in vivo C6 glioma tumor-bearing nude mice as well as in a rat model. In contrast, Taxol(®) had almost no significant antitumor effect in C6 glioma tumor-bearing rats, but could markedly inhibit growth of C6 tumors in C6 glioma tumor-bearing nude mice. The pharmacokinetic results indicated that CLA-PTX in solution has a much longer circulation time and produces higher drug plasma concentrations compared with the CLA-PTX microemulsion. The results of the acute toxicity study showed that the LD(50) of CLA-PTX solution was 103.9 mg/kg. In contrast, the CLA-PTX microemulsion was well tolerated in mice when administered at doses up to 200 mg/kg.

CONCLUSION

CLA-PTX microemulsion is a novel formulation with significant antitumor efficacy in the treatment of brain tumors, and is safer than CLA-PTX solution.

Integrating constitutive gene expression and chemoactivity: mining the NCI60 anticancer screen

Studies into the genetic origins of tumor cell chemoactivity pose significant challenges to bioinformatic mining efforts. Connections between measures of gene expression and chemoactivity have the potential to identify clinical biomarkers of compound response, cellular pathways important to efficacy and potential toxicities; all vital to anticancer drug development. An investigation has been conducted that jointly explores tumor-cell constitutive NCI60 gene expression profiles and small-molecule NCI60 growth inhibition chemoactivity profiles, viewed from novel applications of self-organizing maps (SOMs) and pathway-centric analyses of gene expressions, to identify subsets of over- and under-expressed pathway genes that discriminate chemo-sensitive and chemo-insensitive tumor cell types. Linear Discriminant Analysis (LDA) is used to quantify the accuracy of discriminating genes to predict tumor cell chemoactivity. LDA results find 15% higher prediction accuracies, using ∼30% fewer genes, for pathway-derived discriminating genes when compared to genes derived using conventional gene expression-chemoactivity correlations. The proposed pathway-centric data mining procedure was used to derive discriminating genes for ten well-known compounds. Discriminating genes were further evaluated using gene set enrichment analysis (GSEA) to reveal a cellular genetic landscape, comprised of small numbers of key over and under expressed on- and off-target pathway genes, as important for a compound’s tumor cell chemoactivity. Literature-based validations are provided as support for chemo-important pathways derived from this procedure. Qualitatively similar results are found when using gene expression measurements derived from different microarray platforms. The data used in this analysis is available at http://pubchem.ncbi.nlm.nih.gov/andhttp://www.ncbi.nlm.nih.gov/projects/geo (GPL96, GSE32474).

LASS2 enhances chemosensitivity of breast cancer by counteracting acidic tumor microenvironment through inhibiting activity of V-ATPase proton pump

A main obstacle to overcome during the treatment of tumors is drug resistance to chemotherapy; emerging studies indicate that a key factor contributing to this problem is the acidic tumor microenvironment. Here, we found that LASS2 expression was significantly lower in drug-resistant Michigan Cancer Foundation-7/adriamycin (MCF-7/ADR) human breast cancer cells than the drug-sensitive MCF-7 cells, and low expression of LASS2 was associated with poor prognosis in patients with breast cancer. Our results showed that the overexpression of LASS2 in MCF-7/ADR cells increased the chemosensitivity to multiple chemotherapeutic agents, including doxorubicin (Dox), whereas LASS2 knockdown in MCF-7 cells decreased the chemosensitivity. Cell-cycle analysis revealed a corresponding increase in apoptosis in the LASS2-overexpressing cells following Dox exposure, showing that the overexpression of LASS2 increased the susceptibility to Dox cytotoxicity. This effect was mediated by a significant increase in pHe (extracellular pH) and lysosomal pH, and more Dox entered the cells and stayed in the nuclei of cells. In nude mice, the combination of LASS2 overexpression and Dox significantly inhibited the growth of xenografts. Our findings suggest that LASS2 is involved in chemotherapeutic outcomes and low LASS2 expression may predict chemoresistance.

Effects of diphyllin as a novel V-ATPase inhibitor on gastric adenocarcinoma

The natural compound diphyllin, a cytostatic lignan isolated from Cleistanthus collinus, can dramatically inhibit the proliferation and induce the apoptosis of human gastric cancer cells, SGC7901. Our study found that diphyllin can inhibit the expression of V-ATPases in a dose-dependent manner, decrease the internal pH (pHi) and reverse the transmembrane pH gradient in SGC7901 cells. Changes of the pH gradient were positively correlated with diphyllin concentration. Further study found that diphyllin treatment caused a decrease in phospho-LRP6, but not in LRP6. β-catenin in Wnt/β-catenin signaling and its target genes, c-myc and cyclin-D1, were also decreased with the inhibition of V-ATPases. Therefore, diphyllin could be characterized as a new V-ATPase inhibitor in treating gastric cancer and inhibiting the phosphorylation of LRP6 in Wnt/β-catenin signaling.

The therapeutic efficacy of conjugated linoleic acid - paclitaxel on glioma in the rat

Considering the effects of conjugated linoleic acid (CLA) on anti-tumor and anti-angiogenic in brain tumor, synergistic anti-tumor activity with taxane as well as potential activity for transporting chemotherapeutic agents across the blood-brain barrier (BBB), the purpose of this study was to synthesize CLA-paclitaxel (CLA-PTX) conjugate which could reach to the brain tissue and target brain tumor. The CLA was covalently linked to PTX. The conjugate was stable in PBS and rat plasma in vitro and had no microtubule assembly activity in solution and slight effect of arresting cell cycle progression at the G(2)-M phase. The in vitro cytotoxicity of conjugate was lower than that of PTX (p < 0.05). The conjugate showed higher cellular uptake efficiency on C6 glioma cells. The entire pharmacokinetic index revealed the significant enhancement of the conjugate pharmacokinetics compared with that in PTX (p < 0.01). The conjugate, unlike PTX, could distribute in brain tissue and retained higher concentrations throughout 360 h. The anti-tumor efficacy in brain tumor-bearing rats after administering conjugate was significantly higher than that after giving Taxol (p < 0.01). In conclusion, this CLA-PTX conjugate showed great potential to become a new prodrug of PTX and the methodology can be applied to other anticancer drugs.

Efficacy of ST1968 (namitecan) on a topotecan-resistant squamous cell carcinoma

ST1968 (namitecan), a novel 7-modified hydrophilic camptothecin, was found to be effective against tumor models relatively resistant to topotecan and irinotecan. Based on this observation, this study was designed to investigate the cellular and antitumor effects of ST1968 in a subline of A431, squamous cell carcinoma, selected for resistance to topotecan (A431/TPT). This model was characterized by a slow growth rate, associated with downregulation of EGFR and topoisomerase I. In contrast to other camptothecins (SN38 and gimatecan), ST1968 was able to overcome almost completely the resistance at cellular level. The cellular pharmacokinetics indicated a comparable accumulation and retention of ST1968 in sensitive and resistant cells, in spite of expression of the efflux transporter, P-glycoprotein, in resistant cells. The uptake and retention of topotecan were dramatically reduced in both tumor cell lines, but more evident in the resistant one. In contrast to topotecan, ST1968 retained an outstanding efficacy in vivo against the resistant tumor (A431/TPT). The results are consistent with the interpretation that ST1968 was able to overcome the most relevant mechanisms associated with the development of topotecan resistance (i.e., slow proliferation and target downregulation) owing to its peculiar pharmacokinetic behaviour.

V-ATPase inhibitors and implication in cancer treatment

Acidity is one of the main features of the tumors. The V-ATPase is the primary responsible for the control of tumor microenvironment by proton extrusion to the extracellular medium. The acid environment favors tissue damage, activation of destructive enzymes in the extracellular matrix, the acquisition of metastatic cell phenotypes as well as increasing the destructive capacity. The application of specific inhibitors of V-ATPases, can decrease the acidity of tumor and may allow the reduction of tumor metastasis, acting on the survival of tumor cells and prevent the phenomena of chemoresistance. Among the most important inhibitors can be distinguished benzolactone enamides (salicylihalamide), lobatamide A and B, apicularen, indolyls, oximidine, macrolactone archazolid, lobatamide C, and cruentaren. The latest generation of inhibitors includes NiK12192, FR202126, and PPI SB 242784. The purpose of this paper is to describe the latest advances in the field of V-ATPase inhibitors, describe further developments related to the classic inhibitors, and discuss new potential applications of these drugs in cancer treatment.

Small interfering RNA targeting the subunit ATP6L of proton pump V-ATPase overcomes chemoresistance of breast cancer cells

One of the mechanisms of multiple drug resistance (MDR) is inappropriate sequestration of basic chemotherapeutic agents in acidic endo-lysosomes of cells. The protonation, sequestration, and secretion (PSS) model indicates that drug distribution can be affected by intracellular pH such as lysosomal pH. The vacuolar-H(+)-ATPase (V-ATPase) plays an important role in regulation of intracellular pH by pumping protons into acidic endosomes via an ATP-driven process. In this study, ATP6L, the 16kDa subunit of V-ATPase, was knocked-down by anti-ATP6L small interfering RNA (siRNA) to study the effect on chemosensitivity in the human drug-resistant breast cancer cells MCF-7/ADR. Introduction of anti-ATP6L small interfering RNA duplex into drug-resistant cancer cells significantly inhibited the expression of ATP6L mRNA and protein, as detected by qRT-PCR and Western blot. Inhibition of ATP6L expression by siRNA in MCF-7/ADR sensitized the cells to the cytotoxicity of basic chemotherapeutic agents like doxorobicin, 5-fluorourocil and vincristine. This effect was mediated by a significant increase in lysosomal pH and retention of anticancer drugs into nuclei of cells. These results support the role of tumor acidity in resistance to chemotherapy and provide a rationale for the use of tumor pH modifier agents as coadjuvants in novel anticancer therapies.

Targeting topoisomerase I: molecular mechanisms and cellular determinants of response to topoisomerase I inhibitors

BACKGROUND

Topoisomerase I is required for DNA relaxation during critical cellular functions. The identification of camptothecins as specific enzyme inhibitors and their clinical efficacy have stimulated extensive efforts to exploit topoisomerase I as a tumor target and explain the putative mechanisms of antitumor-specific action.

OBJECTIVE

This review provides an overview of the recent achievements in the development of topoisomerase I inhibitors and in the explanation of the biological pathways involved in tumor response.

RESULTS/CONCLUSION

In spite of the difficulty to identify novel topoisomerase I inhibitors with improved pharmacological properties, a growing body of evidence supports the possibility of optimizing the therapeutic profile of available agents. The explanation of defense mechanisms and the molecular determinants of tumor cell response is expected to provide a basis for the design of combination approaches for optimization of topoisomerase I inhibitors-based therapy.

Development of inhibitors of ATP-binding cassette drug transporters: present status and challenges

BACKGROUND

Multi-drug resistance (MDR) of cancer cells is an obstacle to effective chemotherapy of cancer. The ATP-binding cassette (ABC) transporters, including P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2, play an important role in the development of this resistance. An attractive approach to overcoming MDR is the inhibition of the pumping action of these transporters. Several inhibitors/modulators of ABC transporters have been developed, but cytotoxic effects and adverse pharmacokinetics have prohibited their use. The ongoing search for such inhibitors/modulators that can be applied in the clinic has led to three generations of compounds. The most recent inhibitors are more potent and less toxic than first-generation compounds, yet some are still prone to adverse effects, poor solubility and unfavorable changes in the pharmacokinetics of the anticancer drugs.

OBJECTIVE

This review provides an update of the published work on the development of potent modulators to overcome MDR in cancer cells, their present status in clinical studies and suggestions for further improvement to obtain better inhibitors.

METHODS

This review summarizes recent advances in the development of less toxic modulators, including small molecules and natural products. In addition, a brief overview of other novel approaches that can be used to inhibit ABC drug transporters mediating MDR has also been provided.

CONCLUSION

The multifactorial nature of MDR indicates that it may be important to develop modulators that can simultaneously inhibit both the function of the drug transporters and key signaling pathways, which are responsible for development of this phenomenon.

ABCG2: structure, function and role in drug response

ABCG2 was discovered in multi-drug-resistant cancer cells, with the identification of chemotherapeutic agents, such as mitoxantrone, flavopiridol, methotrexate and irinotecan as substrates. Later, drugs from other therapeutic groups were also described as substrates, including antibiotics, antivirals, HMG-CoA reductase inhibitors and flavonoids. An expanding list of compounds inhibiting ABCG2 has also been generated. The wide variety of drugs transported by ABCG2 and its normal tissue distribution with highest levels in the placenta, intestine and liver, suggest a role in protection against xenobiotics. ABCG2 also has an important role in the pharmacokinetics of its substrates. Single nucleotide polymorphisms of the gene were shown to alter either plasma concentrations of substrate drugs or levels of resistance against chemotherapeutic agents in cell lines. ABCG2 was also described as the determinant of the side population of stem cells. All these aspects of the transporter warrant further research aimed at understanding ABCG2's structure, function and regulation of expression.

Antimetastatic Effect of a Small-Molecule Vacuolar H+-ATPase Inhibitor in in Vitro and in Vivo Preclinical Studies

On the basis of the evidence that vacuolar H(+)-ATPase is implicated in the development of the metastatic phenotype, we have explored the possibility to target the enzyme function in an attempt to control the metastatic behavior of tumor cells. In this study, we used an indole derivative, NiK-12192 [4-(5,6-dichloro-1H-indol-2-yl)-3-ethoxy-N-(2,2,6,6-tetramethyl-piperidin-4-yl)-benzamide], recently identified as an effective inhibitor of vacuolar H(+)-ATPase, as a potential antimetastatic agent in the treatment of NSCLC H460 xenograft, which is able to induce lung metastases in mice. Oral administration of NiK-12192 caused a significant inhibition of formation of spontaneous metastases. In contrast, the drug exhibited a negligible effect on the development of artificial metastases (i.e., after i.v. injection of tumor cells), thus supporting that the drug affects the early events of the metastatic process (e.g., migration and invasion). Cellular effects are consistent with this interpretation. In conclusion, the available results show for the first time that a vacuolar H(+)-ATPase inhibitor is effective in modulation of the metastatic behavior of a lung carcinoma, supporting its potential therapeutic interest as a novel treatment approach.

Serial In vivo Spectroscopic Nuclear Magnetic Resonance Imaging of Lactate and Extracellular pH in Rat Gliomas Shows Redistribution of Protons Away from Sites of Glycolysis

The acidity of the tumor microenvironment aids tumor growth, and mechanisms causing it are targets for potential therapies. We have imaged extracellular pH (pHe) in C6 cell gliomas in rat brain using 1H magnetic resonance spectroscopy in vivo. We used a new probe molecule, ISUCA [(±)2-(imidazol-1-yl)succinic acid], and fast imaging techniques, with spiral acquisition in k-space. We obtained a map of metabolites [136 ms echo time (TE)] and then infused ISUCA in a femoral vein (25 mmol/kg body weight over 110 min) and obtained two consecutive images of pHe within the tumor (40 ms TE, each acquisition taking 25 min). pHe (where ISUCA was present) ranged from 6.5 to 7.5 in voxels of 0.75 μL and did not change detectably when [ISUCA] increased. Infusion of glucose (0.2 mmol/kg·min) decreased tumor pHe by, on average, 0.150 (SE, 0.007; P &lt; 0.0001, 524 voxels in four rats) and increased the mean area of measurable lactate peaks by 54.4 ± 3.4% (P &lt; 0.0001, 287 voxels). However, voxel-by-voxel analysis showed that, both before and during glucose infusion, the distributions of lactate and extracellular acidity were very different. In tumor voxels where both could be measured, the glucose-induced increase in lactate showed no spatial correlation with the decrease in pHe. We suggest that, although glycolysis is the main source of protons, distributed sites of proton influx and efflux cause pHe to be acidic at sites remote from lactate production. [Cancer Res 2007;67(16):7638–45]

ClC-3 expression enhances etoposide resistance by increasing acidification of the late endocytic compartment

Resistance to anticancer drugs and consequent failure of chemotherapy is a complex problem severely limiting therapeutic options in metastatic cancer. Many studies have shown a role for drug efflux pumps of the ATP-binding cassette transporters family in the development of drug resistance. ClC-3, a member of the CLC family of chloride channels and transporters, is expressed in intracellular compartments of neuronal cells and involved in vesicular acidification. It has previously been suggested that acidification of intracellular organelles can promote drug resistance by increasing drug sequestration. Therefore, we hypothesized a role for ClC-3 in drug resistance. Here, we show that ClC-3 is expressed in neuroendocrine tumor cell lines, such as BON, LCC-18, and QGP-1, and localized in intracellular vesicles co-labeled with the late endosomal/lysosomal marker LAMP-1. ClC-3 overexpression increased the acidity of intracellular vesicles, as assessed by acridine orange staining, and enhanced resistance to the chemotherapeutic drug etoposide by almost doubling the IC(50) in either BON or HEK293 cell lines. Prevention of organellar acidification, by inhibition of the vacuolar H(+)-ATPase, reduced etoposide resistance. No expression of common multidrug resistance transporters, such as P-glycoprotein or multidrug-related protein-1, was detected in either the BON parental cell line or the derivative clone overexpressing ClC-3. The probable mechanism of enhanced etoposide resistance can be attributed to the increase of vesicular acidification as consequence of ClC-3 overexpression. This study therefore provides first evidence for a role of intracellular CLC proteins in the modulation of cancer drug resistance.

Relevance of extracellular and intracellular interactions of camptothecins as determinants of antitumor activity

Camptothecins are potent antitumor agents that stabilize the covalent binding of topoisomerase I to DNA forming a reversible ternary complex which, following collision with the replication forks, converts the single-strand breaks into lethal double-strand breaks. This cytotoxic mechanism has been originally ascribed to the closed lactone form, because opening of the lactone ring resulted in loss of antitumor activity. Since the lipophilic lactone favours passive diffusion into the cancer cells, the stability of the closed form is expected to be predictive for activity. Thus, the in vivo pharmacological behavior of camptothecins, which is dependent on the pH-dependent dynamics, is likely a critical determinant of their antitumor efficacy and therapeutic index. The physicochemical properties could influence a number of cellular and in vivo interactions, including stability of the ternary DNA-enzyme-drug complex, binding to serum proteins, recognition by transport systems. These interactions are also implicated in the processes responsible of toxic side effects and drug resistance which are major limitations of the efficacy of camptothecin-based therapy. A number of strategies have been developed to overcome the limitations associated with the peculiar in vivo reactivity and the reversibility of drug-target interaction. Modifications with hydrophilic or lipophilic substituents at specific positions may have a variable (and somewhat opposite) influence on interaction with the intracellular target and plasma proteins and on recognition by membrane transporters. Here, we highlight the interactions of camptothecins which could be exploited to optimize therapeutic efficacy.