A mathematical model of tumour and blood pHe regulation: The HCO3-/CO2 buffering system

Modulating Nonlinear Acoustic Response of Phospholipid-Coated Microbubbles with pH for Ultrasound Imaging

Activatable microbubble contrast agents for contrast-enhanced ultrasound have a potential role for measuring physiologic and pathologic states in deep tissues, including tumor acidosis. In this study, we describe a novel observation of increased harmonic oscillation of phosphatidylcholine microbubbles (PC-MBs) in response to lower ambient pH using a clinical ultrasound scanner. MB echogenicity and nonlinear echoes were monitored at neutral and acidic pH using B-mode and Cadence contrast pulse sequencing (CPS), a harmonic imaging technique at 7.0 and 1.5 MHz. A 3-fold increase in harmonic signal intensity was observed when the pH of PC-MB suspensions was decreased from 7.4 to 5.5 to mimic normal and pathophysiological levels that can be encountered in vivo. This pH-mediated activation is tunable based on the chemical structure and length of phospholipids composing the MB shell. It is also reliant on the presence of phosphate groups, as the use of lipids without phosphate instead of phospholipids completely abrogated this phenomenon. The increased harmonic signal likely is the result of increased MB oscillation caused by a decrease of the interfacial tension induced at a lower pH, altering the lipid conformation. While relative signal changes are interpreted clinically as mostly related to blood flow, pH effects could be significant contributors, particularly when imaging tumors. While our observation can be used clinically, it requires further research to isolate the effect of pH from other variables. These findings could pave the way toward for the development of new smart ultrasound contrast agents that expand the clinical utility of contrast-enhanced ultrasound.

Targeted hyperalkalization with NaOH-loaded starch implants enhances doxorubicin efficacy in tumor treatment

High-alkali treatment using sodium hydroxide (NaOH) injection can be a therapeutic approach for killing tumor cells. Alkalization can damage cellular structures and lead to cell death. Increased alkalinity can also enhance the efficacy of certain chemotherapeutic drugs such as doxorubicin (DOX). In this study, NaOH-loaded starch implants (NST implants) were used to induce hyperalkalization (increase pH) in the tumor environment, thereby inducing necrosis and enhancing the effects of DOX. NaOH is a strongly alkaline substance that can increase the pH when injected into a tumor. However, the administration of NaOH can have toxic side effects because it increases the pH of the entire body, not just at the tumor site. To overcome this problem, we developed an injectable NST implant, in which NaOH can be delivered directly into the tumor. This study showed that NST implants could be easily administered intratumorally in mice bearing 4T1 tumors and that most of the NaOH released from the NST implants was delivered to the tumors. Although some NaOH from NST implants can be systemically absorbed, it is neutralized by the body's buffering effect, thereby reducing the risk of toxicity. This study also confirmed both in vitro and in vivo that DOX is more effective at killing 4T1 cells when alkalized. It has been shown that administration of DOX after injection of an NST implant can kill most tumors. Systemic absorption and side effects can be reduced using an NST implant to deliver NaOH to the tumor. In addition, alkalinization induced by NST implants not only exerts anticancer effects but can also enhance the effect of DOX in killing cancer cells. Therefore, the combination of NaOH-loaded starch implants and DOX treatment has the potential to be a novel therapy for tumors.

Alkalization of cellular pH leads to cancer cell death by disrupting autophagy and mitochondrial function

We previously found that lactic acidosis in the tumor environment was permissive to cancer cell surviving under glucose deprivation and demonstrated that neutralizing lactic acidosis restored cancer cell susceptibility to glucose deprivation. We then reported that alternate infusion of bicarbonate and anticancer agent into tumors via tumor feeding artery markedly enhanced the efficacy of transarterial chemoembolization (TACE) in the local control of hepatocellular carcinoma (HCC). Here we sought to further investigate the mechanism by which bicarbonate enhances the anticancer activity of TACE. We propose that interfering cellular pH by bicarbonate could induce a cascade of molecular events leading to cancer cell death. Alkalizing cellular pH by bicarbonate decreased pH gradient (ΔpH), membrane potential (ΔΨ_m), and proton motive force (Δp) across the inner membrane of mitochondria; disruption of oxidative phosphorylation (OXPHOS) due to collapsed Δp led to a significant increase in adenosine monophosphate (AMP), which activated the classical AMPK-mediated autophagy. Meanwhile, the autophagic flux was ultimately blocked by increased cellular pH, reduced OXPHOS, and inhibition of lysosomal proton pump in alkalized lysosome. Bicarbonate also induced persistent mitochondrial permeability (MPT) and damaged mitochondria. Collectively, this study reveals that interfering cellular pH may provide a valuable approach to treat cancer.

Polyamines Counteract Carbonate-Driven Proteasome Stalling in Alkaline Conditions

Cancer cells tend to increase intracellular pH and, at the same time, are known to intensively produce and uptake polyamines such as spermine. Here, we show that various amines, including biogenic polyamines, boost the activity of proteasomes in a dose-dependent manner. Proteasome activity in the classical amine-containing buffers, such as 2-(N-morpholino)ethanesulfonic acid (MES), Tris, (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), glycylglycine, bis-Tris propane, and bicine, has a skewed distribution with a maximum at pH of 7.0–8.0. The activity of proteasomes in buffers containing imidazole and bis-Tris is maintained almost on the same level, in the pH range of 6.5–8.5. The third type of activation is observed in buffers based on the amino acids arginine and ornithine, as well as the natural polyamines spermine and spermidine. Proteasome activity in these buffers is dramatically increased at pH values greater than 7.5. Anionic buffers such as phosphate or carbonate, in contrast, inhibit proteasome activity during alkalization. Importantly, supplementation of a carbonate–phosphate buffer with spermine counteracts carbonate-driven proteasome stalling in alkaline conditions, predicting an additional physiological role of polyamines in maintaining the metabolism and survival of cancer cells.

The control of acidity in tumor cells: a biophysical model

Acidosis of the tumor microenvironment leads to cancer invasion, progression and resistance to therapies. We present a biophysical model that describes how tumor cells regulate intracellular and extracellular acidity while they grow in a microenvironment characterized by increasing acidity and hypoxia. The model takes into account the dynamic interplay between glucose and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_2$$\end{document}O2 consumption with lactate and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_2$$\end{document}CO2 production and connects these processes to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {H}^+$$\end{document}H+ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HCO}_3^-$$\end{document}HCO3- fluxes inside and outside cells. We have validated the model with independent experimental data and used it to investigate how and to which extent tumor cells can survive in adverse micro-environments characterized by acidity and hypoxia. The simulations show a dominance of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {H}^+$$\end{document}H+ exchanges in well-oxygenated regions, and of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HCO}_3^-$$\end{document}HCO3- exchanges in the inner hypoxic regions where tumor cells are known to acquire malignant phenotypes. The model also includes the activity of the enzyme Carbonic Anhydrase 9 (CA9), a known marker of tumor aggressiveness, and the simulations demonstrate that CA9 acts as a nonlinear \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {pH}_i$$\end{document}pHi equalizer at any \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_2$$\end{document}O2 level in cells that grow in acidic extracellular environments.

Targeting the Acidic Tumor Microenvironment: Unexpected Pro-Neoplastic Effects of Oral NaHCO3 Therapy in Murine Breast Tissue

The acidic tumor microenvironment modifies malignant cell behavior. Here, we study consequences of the microenvironment in breast carcinomas. Beginning at carcinogen-based breast cancer induction, we supply either regular or NaHCO₃-containing drinking water to female C57BL/6j mice. We evaluate urine and blood acid-base status, tumor metabolism (microdialysis sampling), and tumor pH (pH-sensitive microelectrodes) in vivo. Based on freshly isolated epithelial organoids from breast carcinomas and normal breast tissue, we assess protein expression (immunoblotting, mass spectrometry), intracellular pH (fluorescence microscopy), and cell proliferation (bromodeoxyuridine incorporation). Oral NaHCO₃ therapy increases breast tumor pH in vivo from 6.68 ± 0.04 to 7.04 ± 0.09 and intracellular pH in breast epithelial organoids by ~0.15. Breast tumors develop with median latency of 85.5 ± 8.2 days in NaHCO₃-treated mice vs. 82 ± 7.5 days in control mice. Oral NaHCO₃ therapy does not affect tumor growth, histopathology or glycolytic metabolism. The capacity for cellular net acid extrusion is increased in NaHCO₃-treated mice and correlates negatively with breast tumor latency. Oral NaHCO₃ therapy elevates proliferative activity in organoids from breast carcinomas. Changes in protein expression patterns-observed by high-throughput proteomics analyses-between cancer and normal breast tissue and in response to oral NaHCO₃ therapy reveal complex influences on metabolism, cytoskeleton, cell-cell and cell-matrix interaction, and cell signaling pathways. We conclude that oral NaHCO₃ therapy neutralizes the microenvironment of breast carcinomas, elevates the cellular net acid extrusion capacity, and accelerates proliferation without net effect on breast cancer development or tumor growth. We demonstrate unexpected pro-neoplastic consequences of oral NaHCO₃ therapy that in breast tissue cancel out previously reported anti-neoplastic effects.

Explosible nanocapsules excited by pulsed microwaves for efficient thermoacoustic-chemo combination therapy

Microwave irradiation is a powerful non-invasive approach for treating deep-seated diseases in a clinical setting. Pulsed microwave-induced thermoacoustic cavitation allows precise cancer treatment with microwave-absorbing materials. This differs from the traditional continuous microwave-induced thermotherapy which may be harmful to off-target tissues. Here we first report the integration of thermocavitation and cytoplasmic drug release into highly explosible cell-penetrating nanocapsules for effective tumor inhibition under pulsed microwave irradiation. The nanocapsules were formulated from arginine-tethered reduction-responsive copolymers, P(ArgMA-co-DMA)-b-PPOPMA, microwave-absorbing AB and chemotherapeutic DOX using a double-emulsion method. The nanocapsules were internalized by cancer cells rapidly via major energy-independent pathways. Upon pulsed microwave irradiation, AB absorbed energy to generate a giant thermoacoustic shockwave, simultaneously decomposing into carbon dioxide and ammonia which enforced the cavitation damage effect. The thermoacoustic shockwave and gas burst also mechanically disrupted the intracellular organelles resulting in high-ratio cell necrosis and promoted the cytosolic release of DOX into the nucleus to initiate cell death. Importantly, in vivo results demonstrated significantly suppressed tumor growth by the pulsed microwave-triggered thermocavitation and drug release, and minimal systemic toxicity from the microwave treatment. Therefore, our study provides a new strategy for effectively engineering pulsed microwave-responsive nanomaterials for smart cancer therapy.

Quantification of lactate from various metabolic pathways and quantification issues of lactate isotopologues and isotopmers

¹³C-labeled glucose combined with chromatography and mass spectrometry enables us to decipher the percentage of lactate generated from various metabolic pathways. We showed that lactate derived from glycolysis, pentose phosphate pathway, Krebs cycle, and other sources accounted for 82-90%, 6.0-11%, 0.67-1.8% and 1.5-7.9%, respectively, depending on different types of cells. When using glucose isotopomers ([1-¹³C]-, [3-¹³C]-, [4-¹³C]-, and [6-¹³C]glucose) or isotopologues ([1,2-¹³C₂]- and [1,2,3-¹³C₃]glucose) for tracing, the ratio of lactate derived from glucose carbon 1, 2, 3 over 4, 5, 6 via glycolysis varied significantly, ranging from 1.6 (traced with [1,2-¹³C₂]glucose) to 0.85 (traced with [6-¹³C]glucose), but the theoretical ratio should be 1. The odd results might be caused by intramolecular ¹³C, which may significantly affect lactate fragmentation under tandem mass spectrometry condition, leading to erroneous quantification. Indeed, the fragmentation efficiency of [U-¹³C]lactate, [2,3-¹³C]lactate, and [3-¹³C]lactate were 1.4, 1.5 and 1.2 folds higher than lactate, respectively, but [1-¹³C]lactate was similar to lactate, suggesting that carbon-13 at different positions could differentially influence lactate fragmentation. This observed phenomenon was inconsistent with the data based on theoretical calculation, according to which activation energies for all lactate isotopomers and isotopologues are nearly identical. The inconsistency suggested a need for further investigation. Our study suggests that calibration is required for quantifying metabolite isotopolugues and isotopomers.

Central role of lactate and proton in cancer cell resistance to glucose deprivation and its clinical translation

Targeting common weaknesses of cancer is an important strategy for cancer therapy. Glucose is a nutrient that maintains essential cellular metabolism, supporting cancer cell survival, growth and proliferation. Depriving glucose rapidly kills cancer cells. Most cancer cells possess a feature called Warburg effect, which refers to that cancer cells even with ample oxygen exhibit an exceptionally high glycolysis rate and convert most incoming glucose to lactate. Although it is recognized that Warburg effect confers growth advantage to cancer cells when glucose supply is sufficient, this feature could be considered as a fatal weakness of cancer cells when glucose supply is a problem. As glucose supply in many solid tumors is poor, and as most cancer cells have exceptionally high glycolytic capacity, maximizing cancer cell glycolysis rate would possibly exhaust intratumoral glucose, leading cancer cell to death. Lactate and proton are two common factors in solid tumors, they jointly protect cancer cells against glucose deprivation, and they are also powerful regulators dictating glucose metabolic phenotypes of cancer cells. Disrupting the joint action of lactate and proton, for example, by means of bicarbonate infusion into tumor, could maximize cancer cell glycolytic rate to rapidly use up glucose, expose their vulnerability to glucose deprivation and ultimately kill cancer cells. A pilot clinical study demonstrated that this approach achieved a remarkable improvement in local control of large and huge hepatocellular carcinoma.

NaHCO3 enhances the antitumor activities of cytokine-induced killer cells against hepatocellular carcinoma HepG2 cells

The extracellular pH is lower inside solid tumors than in normal tissue. The acidic environment inhibits the cytotoxicity of lymphocytes in vitro and promotes tumor cell invasion. In the present study, both in vitro and in vivo experiments were conducted to investigate how NaHCO₃ would affect the antitumor activities of cytokine-induced killer (CIK) cells against hepatocellular carcinoma (HCC) cells. For the in vitro experiments, HepG2 cells were cultured at pH 6.5 and 7.4 in the presence of CIK cells or CIK cell-conditioned medium (CM_CIK). For the in vivo experiments, nude mice were xenografted with HepG2-luc cells and divided into four groups: i) CIK cells injection plus NaHCO₃ feeding; ii) CIK cells injection plus drinking water feeding; iii) normal saline injection plus NaHCO₃ feeding; and iv) normal saline injection plus drinking water feeding. The results indicated that the viability and growth rate of HepG2 cells were remarkably suppressed when co-cultured with CIK cells or CM_CIK at pH 7.4 compared with those of HepG2 cells cultured under the same conditions but at pH 6.5. In the xenograft study, a marked synergistic antitumor effect of the combined therapy was observed. NaHCO₃ feeding augmented the infiltration of cluster of differentiation 3-positive T lymphocytes into the tumor mass. Taken together, these data strongly suggest that the antitumor activities of CIK cells against HepG2 cells were negatively affected by the acidic environment inside the tumors, and neutralizing the pH (for example, via NaHCO₃ administration), could therefore reduce or eliminate this influence. In addition, it should be recommended that oncologists routinely prescribe soda water to their patients, particularly during CIK cell therapy.

A nonrandomized cohort and a randomized study of local control of large hepatocarcinoma by targeting intratumoral lactic acidosis

Background:

Previous works suggested that neutralizing intratumoral lactic acidosis combined with glucose deprivation may deliver an effective approach to control tumor. We did a pilot clinical investigation, including a nonrandomized (57 patients with large HCC) and a randomized controlled (20 patients with large HCC) study.

Methods:

The patients were treated with transarterial chemoembolization (TACE) with or without bicarbonate local infusion into tumor.

Results:

In the nonrandomized controlled study, geometric mean of viable tumor residues (VTR) in TACE with bicarbonate was 6.4-fold lower than that in TACE without bicarbonate (7.1% [95% CI: 4.6%–10.9%] vs 45.6% [28.9%–72.0%]; p<0.0001). This difference was recapitulated by a subsequent randomized controlled study. TACE combined with bicarbonate yielded a 100% objective response rate (ORR), whereas the ORR treated with TACE alone was 44.4% (nonrandomized) and 63.6% (randomized). The survival data suggested that bicarbonate may bring survival benefit.

Conclusions:

Bicarbonate markedly enhances the anticancer activity of TACE.

Funding:

Funded by National Natural Science Foundation of China.

Clinical trial number:

ChiCTR-IOR-14005319.

Surgery is the main treatment for liver cancer, but the most common liver cancer – called hepatocellular carcinoma – can sometimes become too large to remove safely. An alternative option to kill the tumor is to block its blood supply via a process called embolization. This procedure deprives the tumor cells of oxygen and nutrients such as glucose. However, embolization also prevents a chemical called lactic acid – which is commonly found around tumors – from being removed. Lactic acid actually helps to protect cancer cells and also aids the growth of new blood vessels, and so the “trapped” lactic acid may reduce the anticancer activity of embolization.

Previous works suggested that neutralizing the acidic environment in a tumor while depriving it of glucose via embolization could become a new treatment option for cancer patients. Chao et al. now report a small clinical trial that tested this idea and involved patients with large hepatocellular carcinomas. First, a group of thirty patients received the embolization treatment together with an injection of bicarbonate – a basic compound used to neutralize the lactic acid – that was delivered directly to the tumor. The neutralization killed these large tumors more effectively than what is typically seen in patients who just undergo embolization

Chao et al. then recruited another twenty patients and randomly assigned them to receive either just the embolization or the embolization with bicarbonate treatment. This randomized trial showed that the tumors died more and patients survived for longer if they received the bicarbonate together with the embolization treatment compared to those patients that were only embolized. In fact, four patients initially assigned to, and treated in, the embolization-only group subsequently asked to cross over to, and indeed received, the bicarbonate treatment as well.

These data indicate that this bicarbonate therapy may indeed be effective for patients with large tumors that are not amenable to surgery. In future, larger clinical trials will need to be carried out to verify these initial findings.

Extracellular pH Modulates Neuroendocrine Prostate Cancer Cell Metabolism and Susceptibility to the Mitochondrial Inhibitor Niclosamide

Neuroendocrine prostate cancer is a lethal variant of prostate cancer that is associated with castrate-resistant growth, metastasis, and mortality. The tumor environment of neuroendocrine prostate cancer is heterogeneous and characterized by hypoxia, necrosis, and numerous mitoses. Although acidic extracellular pH has been implicated in aggressive cancer features including metastasis and therapeutic resistance, its role in neuroendocrine prostate cancer physiology and metabolism has not yet been explored. We used the well-characterized PNEC cell line as a model to establish the effects of extracellular pH (pH 6.5, 7.4, and 8.5) on neuroendocrine prostate cancer cell metabolism. We discovered that alkalinization of extracellular pH converted cellular metabolism to a nutrient consumption-dependent state that was susceptible to glucose deprivation, glutamine deprivation, and 2-deoxyglucose (2-DG) mediated inhibition of glycolysis. Conversely, acidic pH shifted cellular metabolism toward an oxidative phosphorylation (OXPHOS)-dependent state that was susceptible to OXPHOS inhibition. Based upon this mechanistic knowledge of pH-dependent metabolism, we identified that the FDA-approved anti-helminthic niclosamide depolarized mitochondrial potential and depleted ATP levels in PNEC cells whose effects were enhanced in acidic pH. To further establish relevance of these findings, we tested the effects of extracellular pH on susceptibility to nutrient deprivation and OXPHOS inhibition in a cohort of castrate-resistant prostate cancer cell lines C4-2B, PC-3, and PC-3M. We discovered similar pH-dependent toxicity profiles among all cell lines with these treatments. These findings underscore a potential importance to acidic extracellular pH in the modulation of cell metabolism in tumors and development of an emerging paradigm that exploits the synergy of environment and therapeutic efficacy in cancer.

Heterogeneity and proliferation of invasive cancer subclones in game theory models of the Warburg effect

OBJECTIVES

The Warburg effect, a switch from aerobic energy production to anaerobic glycolysis, promotes tumour proliferation and motility by inducing acidification of the tumour microenvironment. Therapies that reduce acidity could impair tumour growth and invasiveness. I analysed the dynamics of cell proliferation and of resistance to therapies that target acidity, in a population of cells, under the Warburg effect.

MATERIALS AND METHODS

The dynamics of mutant cells with increased glycolysis and motility has been assessed in a multi-player game with collective interactions in the framework of evolutionary game theory. Perturbations of the level of acidity in the microenvironment have been used to simulate the effect of therapies that target glycolysis.

RESULTS

The non-linear effects of glycolysis induce frequency-dependent clonal selection leading to coexistence of glycolytic and non-glycolytic cells within a tumour. Mutants with increased motility can invade such a polymorphic population and spread within the tumour. While reducing acidity may produce a sudden reduction in tumour cell proliferation, frequency-dependent selection enables it to adapt to the new conditions and can enable the tumour to restore its original levels of growth and invasiveness.

CONCLUSIONS

The acidity produced by glycolysis acts as a non-linear public good that leads to coexistence of cells with high and low glycolysis within the tumour. Such a heterogeneous population can easily adapt to changes in acidity. Therapies that target acidity can only be effective in the long term if the cost of glycolysis is high, that is, under non-limiting oxygen concentrations. Their efficacy, therefore, is reduced when combined with therapies that impair angiogenesis.

Evaluations of extracellular pH within in vivo tumors using acidoCEST MRI

PURPOSE

A practical, noninvasive method is needed to measure the extracellular pH (pHe) within in vivo tumors to longitudinally monitor tumor acidosis. We have optimized a biomedical imaging method, termed acidoCEST MRI, to provide noninvasive assessments of tumor pHe in preclinical models of mammary carcinoma.

METHODS

A CEST-FISP MRI method was optimized to detect the chemical exchange saturation transfer (CEST) of two amide protons of a clinically approved CT contrast agent, iopromide. The ratio of the two CEST effects was used to measure pH. Routes of administration of iopromide were evaluated to ensure sufficient delivery of the agent to the tumor. The optimized acidoCEST MRI method was then used to evaluate the change in tumor pHe following alkalinizing bicarbonate treatment.

RESULTS

The acidoCEST MRI protocol measured pH between 6.2 and 7.2 pH units. Greater delivery of iopromide was shown to improve the precision of the measurement of tumor pHe, but the agent did not influence the tumor pHe. AcidoCEST MRI was used to longitudinally monitor the effect of bicarbonate treatment on the pHe of tumors and bladders.

CONCLUSION

This study demonstrates that an optimized acidoCEST MRI method is a practical, noninvasive method for assessing changes in tumor acidosis.

Quantified pH imaging with hyperpolarized (13) C-bicarbonate

PURPOSE

Because pH plays a crucial role in several diseases, it is desirable to measure pH in vivo noninvasively and in a spatially localized manner. Spatial maps of pH were quantified in vitro, with a focus on method-based errors, and applied in vivo.

METHODS

In vitro and in vivo (13) C mapping were performed for various flip angles for bicarbonate (BiC) and CO2 with spectral-spatial excitation and spiral readout in healthy Lewis rats in five slices. Acute subcutaneous sterile inflammation was induced with Concanavalin A in the right leg of Buffalo rats. pH and proton images were measured 2 h after induction.

RESULTS

After optimizing the signal to noise ratio of the hyperpolarized (13) C-bicarbonate, error estimation of the spectral-spatial excited spectrum reveals that the method covers the biologically relevant pH range of 6 to 8 with low pH error (< 0.2). Quantification of pH maps shows negligible impact of the residual bicarbonate signal. pH maps reflect the induction of acute metabolic alkalosis. Inflamed, infected regions exhibit lower pH.

CONCLUSION

Hyperpolarized (13) C-bicarbonate pH mapping was shown to be sensitive in the biologically relevant pH range. The mapping of pH was applied to healthy in vivo organs and interpreted within inflammation and acute metabolic alkalosis models.

Hyperthermia-mediated local drug delivery by a bubble-generating liposomal system for tumor-specific chemotherapy

As is widely suspected, lysolipid dissociation from liposomes contributes to the intravenous instability of ThermoDox (lysolipid liposomes), thereby impeding its antitumor efficacy. This work evaluates the feasibility of a thermoresponsive bubble-generating liposomal system without lysolipids for tumor-specific chemotherapy. The key component in this liposomal formulation is its encapsulated ammonium bicarbonate (ABC), which is used to actively load doxorubicin (DOX) into liposomes and trigger a drug release when heated locally. Incubating ABC liposomes with whole blood results in a significantly smaller decrease in the retention of encapsulated DOX than that by lysolipid liposomes, indicating superior plasma stability. Biodistribution analysis results indicate that the ABC formulation circulates longer than its lysolipid counterpart. Following the injection of ABC liposome suspension into mice with tumors heated locally, decomposition of the ABC encapsulated in liposomes facilitates the immediate thermal activation of CO2 bubble generation, subsequently increasing the intratumoral DOX accumulation. Consequently, the antitumor efficacy of the ABC liposomes is superior to that of their lysolipid counterparts. Results of this study demonstrate that this thermoresponsive bubble-generating liposomal system is a highly promising carrier for tumor-specific chemotherapy, especially for local drug delivery mediated at hyperthermic temperatures.

A general reaction-diffusion model of acidity in cancer invasion

We model the metabolism and behaviour of a developing cancer tumour in the context of its microenvironment, with the aim of elucidating the consequences of altered energy metabolism. Of particular interest is the Warburg Effect, a widespread preference in tumours for cytosolic glycolysis rather than oxidative phosphorylation for glucose breakdown, as yet incompletely understood. We examine a candidate explanation for the prevalence of the Warburg Effect in tumours, the acid-mediated invasion hypothesis, by generalising a canonical non-linear reaction-diffusion model of acid-mediated tumour invasion to consider additional biological features of potential importance. We apply both numerical methods and a non-standard asymptotic analysis in a travelling wave framework to obtain an explicit understanding of the range of tumour behaviours produced by the model and how fundamental parameters govern the speed and shape of invading tumour waves. Comparison with conclusions drawn under the original system--a special case of our generalised system--allows us to comment on the structural stability and predictive power of the modelling framework.

Multiscale imaging and computational modeling of blood flow in the tumor vasculature

The evolution in our understanding of tumor angiogenesis has been the result of pioneering imaging and computational modeling studies spanning the endothelial cell, microvasculature and tissue levels. Many of these primary data on the tumor vasculature are in the form of images from pre-clinical tumor models that provide a wealth of qualitative and quantitative information in many dimensions and across different spatial scales. However, until recently, the visualization of changes in the tumor vasculature across spatial scales remained a challenge due to a lack of techniques for integrating micro- and macroscopic imaging data. Furthermore, the paucity of three-dimensional (3-D) tumor vascular data in conjunction with the challenges in obtaining such data from patients presents a serious hurdle for the development and validation of predictive, multiscale computational models of tumor angiogenesis. In this review, we discuss the development of multiscale models of tumor angiogenesis, new imaging techniques capable of reproducing the 3-D tumor vascular architecture with high fidelity, and the emergence of "image-based models" of tumor blood flow and molecular transport. Collectively, these developments are helping us gain a fundamental understanding of the cellular and molecular regulation of tumor angiogenesis that will benefit the development of new cancer therapies. Eventually, we expect this exciting integration of multiscale imaging and mathematical modeling to have widespread application beyond the tumor vasculature to other diseases involving a pathological vasculature, such as stroke and spinal cord injury.

Predicting the safety and efficacy of buffer therapy to raise tumour pHe: an integrative modelling study

BACKGROUND

Clinical positron emission tomography imaging has demonstrated the vast majority of human cancers exhibit significantly increased glucose metabolism when compared with adjacent normal tissue, resulting in an acidic tumour microenvironment. Recent studies demonstrated reducing this acidity through systemic buffers significantly inhibits development and growth of metastases in mouse xenografts.

METHODS

We apply and extend a previously developed mathematical model of blood and tumour buffering to examine the impact of oral administration of bicarbonate buffer in mice, and the potential impact in humans. We recapitulate the experimentally observed tumour pHe effect of buffer therapy, testing a model prediction in vivo in mice. We parameterise the model to humans to determine the translational safety and efficacy, and predict patient subgroups who could have enhanced treatment response, and the most promising combination or alternative buffer therapies.

RESULTS

The model predicts a previously unseen potentially dangerous elevation in blood pHe resulting from bicarbonate therapy in mice, which is confirmed by our in vivo experiments. Simulations predict limited efficacy of bicarbonate, especially in humans with more aggressive cancers. We predict buffer therapy would be most effectual: in elderly patients or individuals with renal impairments; in combination with proton production inhibitors (such as dichloroacetate), renal glomular filtration rate inhibitors (such as non-steroidal anti-inflammatory drugs and angiotensin-converting enzyme inhibitors), or with an alternative buffer reagent possessing an optimal pK of 7.1-7.2.

CONCLUSION

Our mathematical model confirms bicarbonate acts as an effective agent to raise tumour pHe, but potentially induces metabolic alkalosis at the high doses necessary for tumour pHe normalisation. We predict use in elderly patients or in combination with proton production inhibitors or buffers with a pK of 7.1-7.2 is most promising.

Drug resistance and cellular adaptation to tumor acidic pH microenvironment

Despite advances in developing novel therapeutic strategies, a major factor underlying cancer related death remains resistance to therapy. In addition to biochemical resistance, mediated by xenobiotic transporters or binding site mutations, resistance can be physiological, emerging as a consequence of the tumor's physical microenvironment. This review focuses on extracellular acidosis, an end result of high glycolytic flux and poor vascular perfusion. Low extracellular pH, pHe, forms a physiological drug barrier described by an "ion trapping" phenomenon. We describe how the acid-outside plasmalemmal pH gradient negatively impacts drug efficacy of weak base chemotherapies but is better suited for weakly acidic therapeutics. We will also explore the physiologic changes tumor cells undergo in response to extracellular acidosis which contribute to drug resistance including reduced apoptotic potential, genetic alterations, and elevated activity of a multidrug transporter, p-glycoprotein, pGP. Since low pHe is a hallmark of solid tumors, therapeutic strategies designed to overcome or exploit this condition can be developed.

Measuring in vivo tumor pHe with CEST-FISP MRI

Paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents have been developed that can measure pH in solution studies, but these agents have not previously been detected in vivo. To use the PARACEST agent Yb-DO3A-oAA to measure the extracellular pH (pHe) in tumor tissue, a chemical exchange saturation transfer fast imaging with steady state precession MRI protocol was developed, the saturation period was optimized for sensitive chemical exchange saturation transfer (CEST) detection, and median filtering was used to remove artifacts in CEST spectra. These improvements were used to correlate pH with a ratio of two CEST effects of Yb-DO3A-oAA at a 7 T magnetic field strength (R(2) = 0.99, standard deviation of precision = 0.011 pH units). The PARACEST agent could not be detected in tumor tissue following i.v. injection due to the low sensitivity of in vivo CEST MRI. Yb-DO3A-oAA was detected in tumor tissue and leg muscle after directly injecting the PARACEST agent into these tissues. The measured CEST effects were used to measure a tumor pH of 6.82 ± 0.21 and a leg muscle pH of 7.26 ± 0.14, and parametric pH maps were also generated from these tissue regions. These results demonstrated that tumor pHe can be measured with a PARACEST agent and a rapid CEST-MRI protocol.