Role of pH in tumor-trapping of the anticancer drug 5-fluorouracil

MCT4 Promotes Tumor Malignancy in F98 Glioma Cells

Monocarboxylate transporter 4 (MCT4, SLC16A3) is elevated under hypoxic conditions in many malignant tumors including gliomas. Moreover, MCT4 expression is associated with shorter overall survival. However, the functional consequences of MCT4 expression on the distinct hallmarks of cancer have not yet been explored at the cellular level. Here, we investigated the impact of MCT4 overexpression on proliferation, survival, cell death, migration, invasion, and angiogenesis in F98 glioma cells. Stable F98 glioma cell lines with MCT4 overexpression, normal expression, and knockdown were generated. Distinct hallmarks of cancer were examined using in silico analysis, various in vitro cell culture assays, and ex vivo organotypic rat brain slice culture model. Consistent with its function as lactate and proton exporter, MCT4 expression levels correlated inversely with extracellular pH and proportionally with extracellular lactate concentrations. Our results further indicate that MCT4 promotes proliferation and survival by altered cell cycle regulation and cell death mechanisms. Moreover, MCT4 overexpression enhances cell migration and invasiveness via reorganization of the actin cytoskeleton. Finally, MCT4 inhibition mitigates the induction of angiogenesis, suggesting that MCT4 also plays a crucial role in tumor-related angiogenesis. In summary, our data highlight MCT4/SLC16A3 as a key gene for distinct hallmarks of tumor malignancy in glioma cells.

Mapping Extracellular pH of Gliomas in Presence of Superparamagnetic Nanoparticles: Towards Imaging the Distribution of Drug-Containing Nanoparticles and Their Curative Effect on the Tumor Microenvironment

Since brain's microvasculature is compromised in gliomas, intravenous injection of tumor-targeting nanoparticles containing drugs (D-NPs) and superparamagnetic iron oxide (SPIO-NPs) can deliver high payloads of drugs while allowing MRI to track drug distribution. However, therapeutic effect of D-NPs remains poorly investigated because superparamagnetic fields generated by SPIO-NPs perturb conventional MRI readouts. Because extracellular pH (pH_e) is a tumor hallmark, mapping pH_e is critical. Brain pH_e is measured by biosensor imaging of redundant deviation in shifts (BIRDS) with lanthanide agents, by detecting paramagnetically shifted resonances of nonexchangeable protons on the agent. To test the hypothesis that BIRDS-based pH_e readout remains uncompromised by presence of SPIO-NPs, we mapped pH_e in glioma-bearing rats before and after SPIO-NPs infusion. While SPIO-NPs accumulation in the tumor enhanced MRI contrast, the pH_e inside and outside the MRI-defined tumor boundary remained unchanged after SPIO-NPs infusion, regardless of the tumor type (9L versus RG2) or agent injection method (renal ligation versus coinfusion with probenecid). These results demonstrate that we can simultaneously and noninvasively image the specific location and the healing efficacy of D-NPs, where MRI contrast from SPIO-NPs can track their distribution and BIRDS-based pH_e can map their therapeutic impact.

The Valley of Death in anticancer drug development: a reassessment

The past decade has seen an explosion in our understanding of cancer biology and with it many new potential disease targets. Nonetheless, our ability to translate these advances into therapies is poor, with a failure rate approaching 90%. Much discussion has been devoted to this so-called 'Valley of Death' in anticancer drug development, but the problem persists. Could we have overlooked some straightforward explanations to this highly complex problem? Important aspects of tumor physiology, drug pharmacokinetics, preclinical models, drug delivery, and clinical translation are not often emphasized, but could be crucial. This perspective summarizes current views on the problem and suggests feasible alternatives.

MR evaluation of response to targeted treatment in cancer cells

The development of molecular technologies, together with progressive sophistication of molecular imaging methods, has allowed the further elucidation of the multiple mutations and dysregulatory effects of pathways leading to oncogenesis. Acting against these pathways by specifically targeted agents represents a major challenge for current research efforts in oncology. As conventional anatomically based pharmacological endpoints may be inadequate to monitor the tumor response to these targeted treatments, the identification and use of more appropriate, noninvasive pharmacodynamic biomarkers appear to be crucial to optimize the design, dosage and schedule of these novel therapeutic approaches. An aberrant choline phospholipid metabolism and enhanced flux of glucose derivatives through glycolysis, which sustain the redirection of mitochondrial ATP to glucose phosphorylation, are two major hallmarks of cancer cells. This review focuses on the changes detected in these pathways by MRS in response to targeted treatments. The progress and limitations of our present understanding of the mechanisms underlying MRS-detected phosphocholine accumulation in cancer cells are discussed in the light of gene and protein expression and the activation of different enzymes involved in phosphatidylcholine biosynthesis and catabolism. Examples of alterations induced in the MRS choline profile of cells exposed to different agents or to tumor environmental factors are presented. Current studies aimed at the identification in cancer cells of MRS-detected pharmacodynamic markers of therapies targeted against specific conditional or constitutive cell receptor stimulation are then reviewed. Finally, the perspectives of present efforts addressed to identify enzymes of the phosphatidylcholine cycle as possible novel targets for anticancer therapy are summarized.

Tumor imaging using technetium-99m bound to pH-sensitive peptides

Solid tumors often display metabolic abnormalities that consistently produce low pH in the extracellular space of poorly perfused tissue. These acidic regions may provide a mechanism for drug targeting. Peptides have been designed in such a manner that they exist in an anionic hydrophilic form at the pH of normal tissues, but then undergo a sharp transition to a non-ionic lipophilic form at reduced pH. Peptides were labeled with fluorescein or technetium-99m (99mTc) and evaluated in vitro and in two murine models of cancer. Our studies suggest that PAP-1, an 18 amino acid pH activated peptide with a pH of transition between hydrophilic and lipophilic forms (pT) of 6.4, will deliver fluorescein and 99mTc to tumors. Activation of PAP-1 by low pH and penetration into the plasma membrane of cells and tumors were confirmed using flow cytometry, fluorescence microscopy, and gamma scintigraphy. These results support our central hypothesis that PAP-1 may enable the selective delivery of macromolecules to tumors. This technology has potential for exploiting a common property of tumors to achieve highly specific medical intervention.

Carbogen and nicotinamide increase blood flow and 5-fluorouracil delivery but not 5-fluorouracil retention in colorectal cancer metastases in patients

PURPOSE

To examine whether carbogen and nicotinamide increases 5-fluorouracil (5-FU) delivery to colorectal cancer metastases.

EXPERIMENTAL DESIGN

Six patients were scanned using positron emission tomography. Two scans were done to coincide with the start of separate chemotherapy cycles. At the second positron emission tomography session, 60 mg/kg nicotinamide was given orally 2 to 3 hours before 10-minute carbogen inhalation. In the middle of carbogen treatment, [15O]H2O (to measure regional tissue perfusion) and then [18F]5-FU (to measure 5-FU tissue pharmacokinetics) were administered.

RESULTS

Regions of interest were drawn in 12 liver metastases, 6 spleens, 6 livers, and 12 kidneys. Nicotinamide and carbogen administration increased mean blood pO2 from 93 mm Hg (95% confidence interval, 79-198) to 278 mm Hg (95% confidence interval, 241-316; P = 0.031). Regional perfusion (mL(blood)/min/mL(tissue)) increased in metastases (mean change = 52%, range -32% to +261%, P = 0.024), but decreased in kidney (mean change = -42%, range -82% to -11%, P = 0.0005) and liver (mean change = -34%, range -43% to -26%, P = 0.031). 5-FU uptake at 3.75 minutes (m(2)/mL) increased in tumor (mean change = 40%, range -39% to +196%, P = 0.06) and decreased in kidney (mean change = -25%, range -71% to 12%, P = 0.043). 5-FU delivery measured as K1 increased in tumor (mean change = 74%, range -23% to +293%, P = 0.0039). No differences were seen in [18F]5-FU tumor exposure (net area under curve) and retention.

CONCLUSION

Nicotinamide and carbogen administration can increase 5-FU delivery to colorectal cancer liver metastases. Despite an increase in perfusion and 5-FU delivery, the effects were not directly related and did not increase 5-FU retention or tissue exposure.

Evaluation of hydrogen ion concentrations in prostates from rats and dogs using fluorescent confocal microscopy

The knowledge of intracellular spatial distribution of pH in prostates in animal models reflective of human prostate may have implications for drug development upon pH dependent drug delivery and activity. Freshly dissected prostate tissues (in vitro) or the entire prostate gland (in vivo) were loaded with fluorescent dyes and viewed using confocal microscopy. Images were initially taken in tissues perfused with RPMI-1640 medium. Calibration in situ was performed with high potassium buffers of known pH containing nigericin. Acetoxymethyl ester carboxy-SNARF-1 was visible in epithelial cells (but not stroma) in rat and dog prostates. The pH of lysosomes in prostate epithelial cells was 5.2 as determined by fluorescence of Lyso Sensor Green DND-189. A method of in situ confirmation of tissue viability was developed by a secondary loading and visualization of the BCECF fluorescent dye. Besides the direct measurement of the pH in rat and dog tissues (pH approximately 7.0), a method of pH measurement in prostate tissue (rather than in cell culture) was developed.

Investigations in vivo of the effects of carbogen breathing on 5-fluorouracil pharmacokinetics and physiology of solid rodent tumours

PURPOSE

We have shown previously that carbogen (95% 0(2), 5% CO(2)) breathing by rodents can increase uptake of anticancer drugs into tumours. The aim of this study was to extend these observations to other rodent models using the anticancer drug 5-fluorouracil (5FU). 5FU pharmacokinetics in tumour and plasma and physiological effects on the tumour by carbogen were investigated to determine the locus of carbogen action on augmenting tumour uptake of 5FU.

METHODS

Two different tumour models were used, rat GH3 prolactinomas xenografted s.c. into nude mice and rat H9618a hepatomas grown s.c. in syngeneic Buffalo rats. Uptake and metabolism of 5FU in both tumour models with or without host carbogen breathing was studied non-invasively using fluorine-19 magnetic resonance spectroscopy ((19)F-MRS), while plasma samples from Buffalo rats were used to construct a NONMEM pharmacokinetic model. Physiological effects of carbogen on tumours were studied using (31)P-MRS for energy status (NTP/Pi) and pH, and gradient-recalled echo magnetic resonance imaging (GRE-MRI) for blood flow and oxygenation.

RESULTS

In both tumour models, carbogan-induced GRE-MRI signal intensity increases of approximately 60% consistent with an increase in tumour blood oxygenation and/or flow. In GH3 xenografts, (19)F-MRS showed that carbogen had no significant effect on 5FU uptake and metabolism by the tumours, and (31)P-MRS showed there was no change in the NTP/Pi ratio. In H9618a hepatomas, (19)F-MRS showed that carbogen had no effect on tumour 5FU uptake but significantly ( p=0.0003) increased 5FU elimination from the tumour (i.e. decreased the t(1/2)) and significantly ( p=0.029) increased (53%) the rate of metabolism to cytotoxic fluoronucleotides (FNuct). The pharmacokinetic analysis showed that carbogen increased the rate of tumour uptake of 5FU from the plasma but also increased the rate of removal. (31)P-MRS showed there were significant ( p<or=0.02) increases in the hepatoma NTP/Pi ratio of 49% and transmembrane pH gradient of 0.11 units.

CONCLUSIONS

We suggest that carbogen can transiently increase tumour blood flow, but this effect alone may not increase uptake of anticancer drugs without a secondary mechanism operating. In the case of the hepatoma, the increase in tumour energy status and pH gradient may be sufficient to augment 5FU metabolism to cytotoxic FNuct, while in the GH3 xenografts this was not the case. Thus carbogen breathing does not universally lead to increased uptake of anticancer drugs.

Tumor acidity, ion trapping and chemotherapeutics. I. Acid pH affects the distribution of chemotherapeutic agents in vitro

Resistance to anti-cancer chemotherapies often leads to regional failure, and can be caused by biochemical and/or physiological mechanisms. Biochemical mechanisms include the overexpression of resistance-conferring proteins. In contrast, physiological resistance involves the tumor microenvironment, and can be caused by poor perfusion, hypoxia and/or acidity. This communication investigates the role of tumor acidity in resistance to a panel of chemotherapeutic agents commonly used against breast cancer, such as anthracyclines, taxanes, anti-metabolites and alkylating agents. The effects of pH on the cytotoxicity of these agents were determined, and ion trapping was confirmed by monitoring the effect of pH on the cellular uptake of radiolabeled anthracyclines. Furthermore, pH-dependent cytotoxicity and uptake were compared between parental drug sensitive MCF-7 cells and variants overexpressing p-glycoprotein (MDR-1) and Breast Cancer Resistance Protein. These data indicate that the magnitude of physiological resistance from pH-dependent ion trapping is comparable to biochemical resistance caused by overexpression of drug efflux pumps. Hence, microenvironment-based ion trapping is a significant barrier to anthracycline-based chemotherapy and can itself be a therapeutic target to enhance the efficacy of existing chemotherapies.