Acidity generated by the tumor microenvironment drives local invasion

Targeted imaging of urothelium carcinoma in human bladders by an ICG pHLIP peptide ex vivo

Bladder cancer is the fifth most common in incidence and one of the most expensive cancers to treat. Early detection greatly improves the chances of survival and bladder preservation. The pH low insertion peptide (pHLIP) conjugated with a near-infrared fluorescent dye [indocyanine green (ICG)] targets low extracellular pH, allowing visualization of malignant lesions in human bladder carcinoma ex vivo. Cystectomy specimens obtained after radical surgery were immediately irrigated with nonbuffered saline and instilled with a solution of the ICG pHLIP construct, incubated, and rinsed. Bladders were subsequently opened and imaged, the fluorescent spots were marked, and a standard pathological analysis was carried out to establish the correlation between ICG pHLIP imaging and white light pathological assessment. Accurate targeting of bladder lesions was achieved with a sensitivity of 97%. Specificity is 100%, but reduced to 80% if targeting of necrotic tissue from previous transurethral resections or chemotherapy are considered as false positives. The ICG pHLIP imaging agent marked high-grade urothelial carcinomas, both muscle invasive and nonmuscle invasive. Carcinoma in situ was accurately diagnosed in 11 cases, whereas only four cases were seen using white light, so imaging with the ICG pHLIP peptide offers improved early diagnosis of bladder cancers and may also enable new treatment alternatives.

Phenotypic changes of acid-adapted cancer cells push them toward aggressiveness in their evolution in the tumor microenvironment

The inter- and intra-tumoral metabolic phenotypes of tumors are heterogeneous, and this is related to microenvironments that select for increased glycolysis. Increased glycolysis leads to decreased pH, and these local microenvironment effects lead to further selection. Hence, heterogeneity of phenotypes is an indirect consequence of altering microenvironments during carcinogenesis. In early stages of growth, tumors are stratified, with the most aggressive cells developing within the acidic interior of the tumor. However, these cells eventually find themselves at the tumor edge, where they invade into the normal tissue via acid-mediated invasion. We believe acid adaptation during the evolution of cancer cells in their niche is a Rubicon that, once crossed, allows cells to invade into and outcompete normal stromal tissue. In this study, we illustrate some acid-induced phenotypic changes due to acidosis resulting in more aggressiveness and invasiveness of cancer cells.

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.

Plasticity of Cancer Cell Invasion-Mechanisms and Implications for Therapy

Cancer cell migration is a plastic and adaptive process integrating cytoskeletal dynamics, cell-extracellular matrix and cell-cell adhesion, as well as tissue remodeling. In response to molecular and physical microenvironmental cues during metastatic dissemination, cancer cells exploit a versatile repertoire of invasion and dissemination strategies, including collective and single-cell migration programs. This diversity generates molecular and physical heterogeneity of migration mechanisms and metastatic routes, and provides a basis for adaptation in response to microenvironmental and therapeutic challenge. We here summarize how cytoskeletal dynamics, protease systems, cell-matrix and cell-cell adhesion pathways control cancer cell invasion programs, and how reciprocal interaction of tumor cells with the microenvironment contributes to plasticity of invasion and dissemination strategies. We discuss the potential and future implications of predicted "antimigration" therapies that target cytoskeletal dynamics, adhesion, and protease systems to interfere with metastatic dissemination, and the options for integrating antimigration therapy into the spectrum of targeted molecular therapies.

Regulatory circuits of T cell function in cancer

Recent progress in cancer immunotherapy emphasizes the importance of understanding immune-regulatory pathways in tumours. Dysfunction of antitumour T cells may be due to mechanisms that are evolutionarily conserved or acquired by somatic mutations. The dysfunctional state of T cells has been termed 'exhaustion', on the basis of similarities to dysfunctional T cells in chronic infections. However, despite shared properties, recent studies have identified marked differences between T cell dysfunction in cancer and chronic infection. In this Review, we discuss T cell-intrinsic molecular alterations and metabolic communication in the tumour microenvironment. Identification of the underlying molecular drivers of T cell dysfunction is essential for the continued progress of cancer research and therapy.

Tunable Design of Gold(III)-Doxorubicin Complex-PEGylated Nanocarrier. The Golden Doxorubicin for Oncological Applications

To date, the translation of Au (III) complexes into chemotherapeutic agents has been hindered by their low stability under physiological conditions, a crucial parameter in drug development. In this study, we report an innovative four-step synthesis of a stable Au (III)-doxorubicin (DOX) complex, acting as a key constitutive component of doxorubicin-loaded PEG-coated nanoparticles (DOX IN-PEG-AuNPs). For therapeutic purposes, such AuNPs were then functionalized with the anti-Kv11.1 polyclonal antibody (pAb), which specifically recognizes the hERG1 channel that is overexpressed on the membrane of human pancreatic cancer cells. The nature of the interactions between DOX and Au (III) ions was probed by various analytical techniques (Raman spectroscopy, UV-vis, and (1)H NMR), which enabled studying the Au (III)-DOX interactions during AuNPs formation. The theoretical characterization of the vibrational bands and the electronic transitions of the Au (III)-DOX complex calculated through computational studies showed significant qualitative agreement with the experimental observations on AuNPs samples. Stability in physiological conditions and efficient drug loading (up to to 85 w/w %) were achieved, while drug release was strongly dependent on the structure of DOX IN-PEG-AuNPs and on the pH. Furthermore, the interactions among DOX, PEG, and Au (III) ions in DOX IN-PEG-AuNPs differed significantly from those found in polymer-modified AuNPs loaded with DOX by covalent linkage, referred to as DOX ON-PEG-AuNPs. In vitro experiments indeed demonstrated that such differences strongly influenced the therapeutic potential of AuNPs in pancreatic cancer treatment, with a significant increase of the DOX therapeutic index when complexed to Au (III) ions. Collectively, our study demonstrated that Au (III)-DOX complexes as building blocks of PEGylated AuNPs constitutes a promising approach to transform promising Au (III) complexes into real chemotherapeutic drugs for the treatment of pancreatic cancer.

PEGylation of paclitaxel largely improves its safety and anti-tumor efficacy following pulmonary delivery in a mouse model of lung carcinoma

Pulmonary delivery offers an attractive route of administration for chemotherapeutic agents, with the advantages of high drug concentrations locally and low side effects systemically. However, fast clearance mechanisms result in short residence time of small molecule drugs in the lungs. Moreover, the local toxicity induced by antineoplastic drugs is considered a major obstacle for the clinical application of inhaled chemotherapy. In this study, we explored the utility of 6kDa and 20kDa polyethylene glycol-paclitaxel (PEG-PTX) conjugates to retain paclitaxel within the lungs, achieve its sustained release locally, and thereby, improve its efficacy and reduce its pulmonary toxicity. The conjugates increased the maximum tolerated dose of paclitaxel by up to 100-fold following intratracheal instillation in healthy mice. PEG-PTX conjugates induced lung inflammation. However, the inflammation was lower than that induced by an equivalent dose of the free drug and it was reversible. Conjugation of paclitaxel to both PEG sizes significantly enhanced its anti-tumor efficacy following intratracheal instillation of a single dose in a Lewis lung carcinoma model in mice. PEG-PTX 20k showed equivalent efficacy as PEG-PTX 6k delivered at a 2.5-fold higher dose, suggesting that the molecular weight of the conjugate plays a role in anti-cancer activity. PEG-PTX 20k conjugate presented a prolonged residency and a sustained paclitaxel release within the lungs. This study showed that PEGylation of paclitaxel offers a potential delivery system for inhalation with improved anti-cancer efficacy, prolonged exposure of lung-resident tumors to the antineoplastic drug and reduced local toxicity.

Regulation of breast tumorigenesis through acid sensors

The low extracellular pH in the microenvironment has been shown to promote tumor growth and metastasis; however, the underlying mechanism is poorly understood. Particularly, little is known how the tumor cell senses the acidic signal to activate the acidosis-mediated signaling. In this study, we show that breast cancer cells express acid-sensing ion channel 1 (ASIC1), a proton-gated cation channel primarily expressed in the nervous system. RNA interference, knockout and rescue experiments demonstrate a critical role for ASIC1 in acidosis-induced reactive oxidative species and NF-κB activation, two key events for tumorigenesis. Mechanistically, ASIC1 is required for acidosis-mediated signaling through calcium influx. We show that as a cytoplasmic membrane protein, ASIC1 is also associated with mitochondria, suggesting that ASIC1 may regulate mitochondrial calcium influx. Importantly, interrogation of the Cancer Genome Atlas breast invasive carcinoma data set indicates that alterations of ASIC1 alone or combined with other 4 ASIC genes are significantly correlated with poor patient survival. Furthermore, ASIC1 inhibitors cause a significant reduction of tumor growth and tumor load. Together, these results suggest that ASIC1 contributes to breast cancer pathogenesis in response to acidic tumor microenvironments, and ASIC1 may serve as a prognostic marker and a therapeutic target for breast cancer.

Acidosis Acts through HSP90 in a PHD/VHL-Independent Manner to Promote HIF Function and Stem Cell Maintenance in Glioma

Hypoxia is a common feature of solid tumors, which controls multiple aspects of cancer progression. One important function of hypoxia and the hypoxia-inducible factors (HIF) is the maintenance of cancer stem-like cells (CSC), a population of tumor cells that possess stem cell-like properties and drives tumor growth. Among the changes promoted by hypoxia is a metabolic shift resulting in acidification of the tumor microenvironment. Here, we show that glioma hypoxia and acidosis functionally cooperate in inducing HIF transcription factors and CSC maintenance. We found that these effects did not involve the classical PHD/VHL pathway for HIF upregulation, but instead involved the stress-induced chaperone protein HSP90. Genetic or pharmacologic inactivation of HSP90 inhibited the increase in HIF levels and abolished the self-renewal and tumorigenic properties of CSCs induced by acidosis. In clinical specimens of glioma, HSP90 was upregulated in the hypoxic niche and was correlated with a CSC phenotype. Our findings highlight the role of tumor acidification within the hypoxic niche in the regulation of HIF and CSC function through HSP90, with implications for therapeutic strategies to target CSC in gliomas and other hypoxic tumors. Cancer Res; 76(19); 5845-56. ©2016 AACR.

Oleyl group-functionalized insulating gate transistors for measuring extracellular pH of floating cells

The extracellular ionic microenvironment has a close relationship to biological activities such as by cellular respiration, cancer development, and immune response. A system composed of ion-sensitive field-effect transistors (ISFET), cells, and program-controlled fluidics has enabled the acquisition of real-time information about the integrity of the cell membrane via pH measurement. Here we aimed to extend this system toward floating cells such as T lymphocytes for investigating complement activation and pharmacokinetics through alternations in the plasma membrane integrity. We functionalized the surface of tantalum oxide gate insulator of ISFET with oleyl-tethered phosphonic acid for interacting with the plasma membranes of floating cells without affecting the cell signaling. The surface modification was characterized by X-ray photoelectron spectroscopy and water contact angle measurements. The Nernst response of -37.8 mV/pH was obtained for the surface-modified ISFET at 37 °C. The oleyl group-functionalized gate insulator successfully captured Jurkat T cells in a fluidic condition without acute cytotoxicity. The system was able to record the time course of pH changes at the cells/ISFET interface during the process of instant addition and withdrawal of ammonium chloride. Further, the plasma membrane injury of floating cells after exposure by detergent Triton™ X-100 was successfully determined using the modified ISFET with enhanced sensitivity as compared with conventional hemolysis assays.

Effect of Modified Alkaline Supplementation on Syngenic Melanoma Growth in CB57/BL Mice

Tumor extracellular acidity is a hallmark of malignant cancers. Thus, in this study we evaluated the effects of the oral administration of a commercially available water alkalizer (Basenpulver®) (BP) on tumor growth in a syngenic melanoma mouse model. The alkalizer was administered daily by oral gavage starting one week after tumor implantation in CB57/BL mice. Tumors were calipered and their acidity measured by in vivo MRI guided 31P MRS. Furthermore, urine pH was monitored for potential metabolic alkalosis. BP administration significantly reduced melanoma growth in mice; the optimal dose in terms of tolerability and efficacy was 8 g/l (p< 0.05). The in vivo results were supported by in vitro experiments, wherein BP-treated human and murine melanoma cell cultures exhibited a dose-dependent inhibition of tumor cell growth. This investigation provides the first proof of concept that systemic buffering can improve tumor control by itself and that this approach may represent a new strategy in prevention and/or treatment of cancers.

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.

Identification of V-ATPase as a molecular sensor of SOX11-levels and potential therapeutic target for mantle cell lymphoma

Background

Mantle cell lymphoma (MCL) is an aggressive disease with short median survival. Molecularly, MCL is defined by the t(11;14) translocation leading to overexpression of the CCND1 gene. However, recent data show that the neural transcription factor SOX11 is a disease defining antigen and several involved signaling pathways have been pin-pointed, among others the Wnt/β-catenin pathway that is of importance for proliferation in MCL. Therefore, we evaluated a compound library focused on the Wnt pathway with the aim of identifying Wnt-related targets that regulate growth and survival in MCL, with particular focus on SOX11-dependent growth regulation.

Methods

An inducible SOX11 knock-down system was used to functionally screen a library of compounds (n = 75) targeting the Wnt signaling pathway. A functionally interesting target, vacuolar-type H⁺-ATPase (V-ATPase), was further evaluated by western blot, siRNA-mediated gene silencing, immunofluorescence, and flow cytometry.

Results

We show that 15 out of 75 compounds targeting the Wnt pathway reduce proliferation in all three MCL cell lines tested. Furthermore, three substances targeting two different targets (V-ATPase and Dkk1) showed SOX11-dependent activity. Further validation analyses were focused on V-ATPase and showed that two independent V-ATPase inhibitors (bafilomycin A1 and concanamycin A) are sensitive to SOX11 levels, causing reduced anti-proliferative response in SOX11 low cells. We further show, using fluorescence imaging and flow cytometry, that V-ATPase is mainly localized to the plasma membrane in primary and MCL cell lines.

Conclusions

We show that SOX11 status affect V-ATPase dependent pathways, and thus may be involved in regulating pH in intracellular and extracellular compartments. The plasma membrane localization of V-ATPase indicates that pH regulation of the immediate extracellular compartment may be of importance for receptor functionality and potentially invasiveness in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2550-4) contains supplementary material, which is available to authorized users.

Mechanotransduction pulls the strings of matrix degradation at invadosome

Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.

Acidic tumor microenvironment in human melanoma

One characteristic of solid tumors such as malignant melanoma is the acidification of the tumor microenvironment. The deregulation of cancer cell metabolism is considered a main cause of extracellular acidosis. Here, cancer cells utilize aerobic glycolysis instead of oxidative phosphorylation even under normoxic conditions, as originally described by Otto Warburg. These metabolic alterations cause enhanced acid production, especially of lactate and carbon dioxide (CO2 ). The extensive production of acidic metabolites and the enhanced acid export to the extracellular space cause a consistent acidification of the tumor microenvironment, thus promoting the formation of an acid-resistant tumor cell population with increased invasive and metastatic potential. As melanoma is one of the deadliest and most metastatic forms of cancer, understanding the effects of this extracellular acidosis on human melanoma cells with distinct metastatic properties is important. The aim of this review was to summarize recent studies of the acidification of the tumor microenvironment, focusing on the specific effects of the acidic milieu on melanoma cells and to give a short overview of therapeutic approaches.

Multi-functionalized hyaluronic acid nanogels crosslinked with carbon dots as dual receptor-mediated targeting tumor theranostics

Hyaluronic acid (HA)-based theranostic nanogels were designed for the tumor diagnosis and chemotherapy, by crosslinking the folate-terminated poly(ethylene glycol) modified hyaluronic acid (FA-PEG-HA) with carbon dots (CDs) for the first time. Due to the extraordinary fluorescence property of the integrated CDs, the theranostic nanogels could be used for the real-time and noninvasive location tracking to cancer cells. HA could load Doxorubicin (DOX) via electrostatic interaction with a drug-loading capacity (DLC) of 32.5%. The nanogels possessed an ideal release of DOX in the weak acid environment, while it was restrained in the neutral media, demonstrating the pH-responsive controlled release behavior. The cytotoxicity and cellular uptake results clearly illustrated that most DOX was released and accumulated in the cell nuclei and killed the cancer cells efficaciously, due to their dual receptor-mediated targeting characteristics.

MUC16-mediated activation of mTOR and c-Myc reprograms pancreatic cancer metabolism

MUC16, a transmembrane mucin, facilitates pancreatic adenocarcinoma progression and metastasis. In the current studies, we observed that MUC16 knockdown pancreatic cancer cells exhibit reduced glucose uptake and lactate secretion along with reduced migration and invasion potential, which can be restored by supplementing the culture media with lactate, an end product of aerobic glycolysis. MUC16 knockdown leads to inhibition of mTOR activity and reduced expression of its downstream target c-MYC, a key player in cellular growth, proliferation and metabolism. Ectopic expression of c-MYC in MUC16 knockdown pancreatic cancer cells restores the altered cellular physiology. Our LC-MS/MS based metabolomics studies indicate global metabolic alterations in MUC16 knockdown pancreatic cancer cells, as compared to the controls. Specifically, glycolytic and nucleotide metabolite pools were significantly decreased. We observed similar metabolic alterations that correlated with MUC16 expression in primary tumor tissue specimens from human pancreatic adenocarcinoma cancer patients. Overall, our results demonstrate that MUC16 plays an important role in metabolic reprogramming of pancreatic cancer cells by increasing glycolysis and enhancing motility and invasiveness.

Exosomes: novel implications in diagnosis and treatment of gastrointestinal cancer

BACKGROUND

Amongst all cancer subtypes, gastrointestinal tumours are responsible for most cancer-related deaths. In most of the cases, the limitation of the prognosis of patients with malignant gastrointestinal tumours can be attributed to delayed diagnosis of the disease. In the last decade, secondary prevention strategies, in particular tumour screenings, have been identified to significantly improve the identification of patients with early-stage disease, leading to more effective therapeutic interventions. Therefore, new screening methods and further innovative treatment approaches may lead to an increase in progression-free and overall survival rates.

PURPOSE

Exosomes are small microvesicles with a size of 50-150 nm. They are formed in the endosomal system of many different cell types, where they are packed with nucleotides and proteins from the parental cell. After their release into the extracellular space, exosomes can deliver their cargo into recipient cells. By this mechanism, tumour cells can recruit and manipulate the adjacent and systemic microenvironment in order to support invasion and dissemination. Cancer-derived exosomes in the blood may provide detailed information about the tumour biology of each individual patient. Moreover, tumour-derived exosomes can be used as targetable factors and drug delivery agents in clinical practice.

CONCLUSION

In this review, we summarise new aspects about novel implications in the diagnosis and treatment of gastrointestinal cancer and show how circulating exosomes have come into the spotlight of research as a high potential source of 'liquid biopsies'.

Acidic extracellular pH of tumors induces octamer-binding transcription factor 4 expression in murine fibroblasts in vitro and in vivo

Octamer-binding transcription factor 4 (OCT-4) is an important marker of cellular de-differentiation that can be induced by environmental stressors, such as acidity. Here we demonstrate that chronic acidic stress in solid tumors induced OCT-4 expression in fibroblasts and other stromal cells in four tumor models. The results have implications for how tumors utilize pH modulation to recruit associated stromal cells, induce partial reprogramming of tumor-associated stromal cells, and respond to therapy.

Acidic microenvironments induce lymphangiogenesis and IL-8 production via TRPV1 activation in human lymphatic endothelial cells

Local acidosis is one of the characteristic features of the cancer microenvironment. Many reports indicate that acidosis accelerates the proliferation and invasiveness of cancer cells. However, whether acidic conditions affect lymphatic metastasis is currently unknown. In the present study, we focused on the effects of acidosis on lymphatic endothelial cells (LECs) to assess the relationship between acidic microenvironments and lymph node metastasis. We demonstrated that normal human LECs express various acid receptors by immunohistochemistry and reverse transcriptase-polymerase chain reaction (PCR). Acidic stimulation with low pH medium induced morphological changes in LECs to a spindle shape, and significantly promoted cellular growth and tube formation. Moreover, real-time PCR revealed that acidic conditions increased the mRNA expression of interleukin (IL)-8. Acidic stimulation increased IL-8 production in LECs, whereas a selective transient receptor potential vanilloid subtype 1 (TRPV1) antagonist, 5'-iodoresiniferatoxin, decreased IL-8 production. IL-8 accelerated the proliferation of LECs, and inhibition of IL-8 diminished tube formation and cell migration. In addition, phosphorylation of nuclear factor (NF)-κB was induced by acidic conditions, and inhibition of NF-κB activation reduced acid-induced IL-8 expression. These results suggest that acidic microenvironments in tumors induce lymphangiogenesis via TRPV1 activation in LECs, which in turn may promote lymphatic metastasis.

Application of Good's buffers to pH imaging using hyperpolarized (13)C MRI

N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES), one of Good's buffers, was applied to pH imaging using hyperpolarized (13)C magnetic resonance spectroscopy. Rapid NMR- and MRI-based pH measurements were obtained by exploiting the sensitive pH-dependence of its (13)C chemical shift within the physiologic range.

Tissue-specific conditional PKCε knockout mice: a model to precisely reveal PKCε functional role in initiation, promotion and progression of cancer

PKCε is a transforming oncogene and a predictive biomarker of various human cancers. However, a precise in vivo link of PKCε to cancer induction, progression and metastasis remain undefined. To achieve these goals, we generated tissue specific conditional PKCε knockout mice (PKCε-CKO) using cre-lox technology. Homozygous PKCε^LoxP/LoxP mice have normal body weight and phenotype. To determine what effect loss of PKCε would have on the prostate, the PKCε^LoxP/LoxP mice were bred to probasin cre (PB-Cre4⁺) mice which express cre specifically in the prostate epithelium of postnatal mice. Western blot and immunohistochemical analyses showed reduced levels of PKCε specifically in the prostate of PKCε-CKO mice. Histopathological analyses of prostate from both PKCε^LoxP/LoxP and prostate PKCε-CKO mice showed normal pathology. To determine the functional impact of prostate specific deletion of PKCε on prostate tumor growth, we performed an orthotopic xenograft study. Transgenic adenocarcinoma of the mouse prostate (TRAMP) cells (TRAMPC1, 2×10⁶) were implanted in the prostate of PKCε-CKO mice. Mice were sacrificed at 6th week post-implantation. Results demonstrated a significant (P<0.05) decrease in the growth of TRAMPC1 cells-derived xenograft tumors in PKCε-CKO mice compared to wild type. To determine a link of PKCε to ultraviolet radiation (UVR) exposure-induced epidermal Stat3 phosphorylation, PKCε^LoxP/LoxP mice were bred to tamoxifen-inducible K14 Cre mice. PKCε deletion in the epidermis resulted in inhibition of UVR-induced Stat3 phosphorylation. In summary, our novel PKCε^LoxP/LoxP mice will be useful for defining the link of PKCε to various cancers in specific organ, tissue, or cells.

Metastasis Stimulation by Hypoxia and Acidosis-Induced Extracellular Lipid Uptake Is Mediated by Proteoglycan-Dependent Endocytosis

Hypoxia and acidosis are inherent stress factors of the tumor microenvironment and have been linked to increased tumor aggressiveness and treatment resistance. Molecules involved in the adaptive mechanisms that drive stress-induced disease progression constitute interesting candidates of therapeutic intervention. Here, we provide evidence of a novel role of heparan sulfate proteoglycans (HSPG) in the adaptive response of tumor cells to hypoxia and acidosis through increased internalization of lipoproteins, resulting in a lipid-storing phenotype and enhanced tumor-forming capacity. Patient glioblastoma tumors and cells under hypoxic and acidic stress acquired a lipid droplet (LD)-loaded phenotype, and showed an increased recruitment of all major lipoproteins, HDL, LDL, and VLDL. Stress-induced LD accumulation was associated with increased spheroid-forming capacity during reoxygenation in vitro and lung metastatic potential in vivo On a mechanistic level, we found no apparent effect of hypoxia on HSPGs, whereas lipoprotein receptors (VLDLR and SR-B1) were transiently upregulated by hypoxia. Importantly, however, using pharmacologic and genetic approaches, we show that stress-mediated lipoprotein uptake is highly dependent on intact HSPG expression. The functional relevance of HSPG in the context of tumor cell stress was evidenced by HSPG-dependent lipoprotein cell signaling activation through the ERK/MAPK pathway and by reversal of the LD-loaded phenotype by targeting of HSPGs. We conclude that HSPGs may have an important role in the adaptive response to major stress factors of the tumor microenvironment, with functional consequences on tumor cell signaling and metastatic potential. Cancer Res; 76(16); 4828-40. ©2016 AACR.

Current status and perspectives in atomic force microscopy-based identification of cellular transformation

Understanding the complex interplay between cells and their biomechanics and how the interplay is influenced by the extracellular microenvironment, as well as how the transforming potential of a tissue from a benign to a cancerous one is related to the dynamics of both the cell and its surroundings, holds promise for the development of targeted translational therapies. This review provides a comprehensive overview of atomic force microscopy-based technology and its applications for identification of cellular progression to a cancerous phenotype. The review also offers insights into the advancements that are required for the next user-controlled tool to allow for the identification of early cell transformation and thus potentially lead to improved therapeutic outcomes.

Ratiometric fluorescent pH-sensitive polymers for high-throughput monitoring of extracellular pH

Extracellular pH has a strong effect on cell metabolism and growth. Precisely detecting extracellular pH with high throughput is critical for cell metabolism research and fermentation applications. In this research, a series of ratiometric fluorescent pH sensitive polymers are developed and the ps-pH-neutral is characterized as the best one for exculsive detection of extracellular pH. Poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) is used as the host polymer to increase the water solubility of the pH sensitive polymer without introducing cell toxicity. The fluorescent emission spectra from the polymeric sensor under excitation at the isosbestic point 455 nm possess two fluorescence peaks at 475 nm and 505 nm, which have different responding trends to pH. This enables the polymer to detect pH using fluorescent maxima at 475 nm and 505 nm (I475nm /I505nm ) ratiometrically. The cell impermeability ensures the sensor can solely detect the environmental pH. The sensor is tested to detect the extracellular pH of bacteria or eukaryotic cells in high throughput assays using a microplate reader. Results showed that the pH sensor can be used for high throughput detection of extracellular pH with high repeatability and low photobleaching effect.

Assessing Metabolic Changes in Response to mTOR Inhibition in a Mantle Cell Lymphoma Xenograft Model Using AcidoCEST MRI

AcidoCEST magnetic resonance imaging (MRI) has previously been shown to measure tumor extracellular pH (pHe) with excellent accuracy and precision. This study investigated the ability of acidoCEST MRI to monitor changes in tumor pHe in response to therapy. To perform this study, we used the Granta 519 human mantle cell lymphoma cell line, which is an aggressive B-cell malignancy that demonstrates activation of the phosphatidylinositol-3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway. We performed in vitro and in vivo studies using the Granta 519 cell line to investigate the efficacy and associated changes induced by the mTOR inhibitor, everolimus (RAD001). AcidoCEST MRI studies showed a statistically significant increase in tumor pHe of 0.10 pH unit within 1 day of initiating treatment, which foreshadowed a decrease in tumor growth of the Granta 519 xenograft model. AcidoCEST MRI then measured a decrease in tumor pHe 7 days after initiating treatment, which foreshadowed a return to normal tumor growth rate. Therefore, this study is a strong example that acidoCEST MRI can be used to measure tumor pHe that may serve as a marker for therapeutic efficacy of anticancer therapies.

Metabolic Symbiosis Enables Adaptive Resistance to Anti-angiogenic Therapy that Is Dependent on mTOR Signaling

Therapeutic targeting of tumor angiogenesis with VEGF inhibitors results in demonstrable, but transitory efficacy in certain human tumors and mouse models of cancer, limited by unconventional forms of adaptive/evasive resistance. In one such mouse model, potent angiogenesis inhibitors elicit compartmental reorganization of cancer cells around remaining blood vessels. The glucose and lactate transporters GLUT1 and MCT4 are induced in distal hypoxic cells in a HIF1α-dependent fashion, indicative of glycolysis. Tumor cells proximal to blood vessels instead express the lactate transporter MCT1, and p-S6, the latter reflecting mTOR signaling. Normoxic cancer cells import and metabolize lactate, resulting in upregulation of mTOR signaling via glutamine metabolism enhanced by lactate catabolism. Thus, metabolic symbiosis is established in the face of angiogenesis inhibition, whereby hypoxic cancer cells import glucose and export lactate, while normoxic cells import and catabolize lactate. mTOR signaling inhibition disrupts this metabolic symbiosis, associated with upregulation of the glucose transporter GLUT2.

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Angiogenesis inhibitors causing acute hypoxia elicit metabolic compartmentalization

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Hypoxic cancer cells import and metabolize glucose, secreting lactate

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Normoxic vessel-proximal cancer cells import and metabolize lactate, involving mTOR

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Co-inhibiting mTOR disrupts the metabolic symbiosis

Sirtuins and the Metabolic Hurdles in Cancer

The nutrient demands of cancer cannot be met by normal cell metabolism. Cancer cells undergo dramatic alteration of metabolic pathways in a process called reprogramming, characterized by increased nutrient uptake and re-purposing of these fuels for biosynthetic, bioenergetic or signaling pathways. Partitioning carbon sources toward growth and away from ATP production necessitates other means of generating energy for biosynthetic reactions. Additionally, cancer cell adaptations frequently lead to increased production of reactive oxygen species and lactic acid, which can be beneficial to cancer growth but also are potentially toxic and must be appropriately cleared. Sirtuins are a family of deacylases and ADP-ribosyltransferases with clear links to regulation of cancer metabolism. Through their unique ability to integrate cellular stress and nutrient status with coordination of metabolic outputs, sirtuins are well poised to play pivotal roles in tumor progression and survival. Here, we review the multi-faceted duties of sirtuins in tackling the metabolic hurdles in cancer. We focus on both beneficial and adverse effects of sirtuins in the regulation of energetic, biosynthetic and toxicity barriers faced by cancer cells.

Overcoming Hypoxia-Mediated Tumor Progression: Combinatorial Approaches Targeting pH Regulation, Angiogenesis and Immune Dysfunction

Hypoxia is an important contributor to the heterogeneity of the microenvironment of solid tumors and is a significant environmental stressor that drives adaptations which are essential for the survival and metastatic capabilities of tumor cells. Critical adaptive mechanisms include altered metabolism, pH regulation, epithelial-mesenchymal transition, angiogenesis, migration/invasion, diminished response to immune cells and resistance to chemotherapy and radiation therapy. In particular, pH regulation by hypoxic tumor cells, through the modulation of cell surface molecules such as extracellular carbonic anhydrases (CAIX and CAXII) and monocarboxylate transporters (MCT-1 and MCT-4) functions to increase cancer cell survival and enhance cell invasion while also contributing to immune evasion. Indeed, CAIX is a vital regulator of hypoxia mediated tumor progression, and targeted inhibition of its function results in reduced tumor growth, metastasis, and cancer stem cell function. However, the integrated contributions of the repertoire of hypoxia-induced effectors of pH regulation for tumor survival and invasion remain to be fully explored and exploited as therapeutic avenues. For example, the clinical use of anti-angiogenic agents has identified a conundrum whereby this treatment increases hypoxia and cancer stem cell components of tumors, and accelerates metastasis. Furthermore, hypoxia results in the infiltration of myeloid-derived suppressor cells (MDSCs), regulatory T cells (Treg) and Tumor Associated Macrophages (TAMs), and also stimulates the expression of PD-L1 on tumor cells, which collectively suppress T-cell mediated tumor cell killing. Therefore, combinatorial targeting of angiogenesis, the immune system and pH regulation in the context of hypoxia may lead to more effective strategies for curbing tumor progression and therapeutic resistance, thereby increasing therapeutic efficacy and leading to more effective strategies for the treatment of patients with aggressive cancer.

Respiration gating and Bloch fitting improve pH measurements with acidoCEST MRI in an ovarian orthotopic tumor model

We have developed a MRI method that can measure extracellular pH in tumor tissues, known as acidoCEST MRI. This method relies on the detection of Chemical Exchange Saturation Transfer (CEST) of iopamidol, an FDA-approved CT contrast agent that has two CEST signals. A log₁₀ ratio of the two CEST signals is linearly correlated with pH, but independent of agent concentration, endogenous T₁ relaxation time, and B₁ inhomogeneity. Therefore, detecting both CEST effects of iopamidol during in vivo studies can be used to accurately measure the extracellular pH in tumor tissues. Past in vivo studies using acidoCEST MRI have suffered from respiration artifacts in orthotopic and lung tumor models that have corrupted pH measurements. In addition, the non-linear fitting method used to analyze results is unreliable as it is subject to over-fitting especially with noisy CEST spectra. To improve the technique, we have recently developed a respiration gated CEST MRI pulse sequence that has greatly reduced motion artifacts, and we have included both a prescan and post scan to remove endogenous CEST effects. In addition, we fit the results by parameterizing the contrast of the exogenous agent with respect to pH via the Bloch equations modified for chemical exchange, which is less subject to over-fitting than the non-linear method. These advances in the acidoCEST MRI technique and analysis methods have made pH measurements more reliable, especially in areas of the body subject to respiratory motion.

Imaging the intratumoral-peritumoral extracellular pH gradient of gliomas

Solid tumors have an acidic extracellular pH (pHe ) but near neutral intracellular pH (pHi ). Because acidic pHe milieu is conducive to tumor growth and builds resistance to therapy, simultaneous mapping of pHe inside and outside the tumor (i.e., intratumoral-peritumoral pHe gradient) fulfills an important need in cancer imaging. We used Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), which utilizes shifts of non-exchangeable protons from macrocyclic chelates (e.g., 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate) or DOTP(8-) ) complexed with paramagnetic thulium (Tm(3) (+) ) ion, to generate in vivo pHe maps in rat brains bearing 9L and RG2 tumors. Upon TmDOTP(5-) infusion, MRI identified the tumor boundary by enhanced water transverse relaxation and BIRDS allowed imaging of intratumoral-peritumoral pHe gradients. The pHe measured by BIRDS was compared with pHi measured with (31) P-MRS. In normal tissue, pHe was similar to pHi , but inside the tumor pHe was lower than pHi . While the intratumoral pHe was acidic for both tumor types, peritumoral pHe varied with tumor type. The intratumoral-peritumoral pHe gradient was much larger for 9L than RG2 tumors because in RG2 tumors acidic pHe was found in distal peritumoral regions. The increased presence of Ki-67 positive cells beyond the RG2 tumor border suggested that RG2 was more invasive than the 9L tumor. These results indicate that extensive acidic pHe beyond the tumor boundary correlates with tumor cell invasion. In summary, BIRDS has sensitivity to map the in vivo intratumoral-peritumoral pHe gradient, thereby creating preclinical applications in monitoring cancer therapeutic responses (e.g., with pHe -altering drugs). Copyright © 2016 John Wiley & Sons, Ltd.

Glucose is a key driver for GLUT1-mediated nanoparticles internalization in breast cancer cells

The mesenchymal state in cancer is usually associated with poor prognosis due to the metastatic predisposition and the hyper-activated metabolism. Exploiting cell glucose metabolism we propose a new method to detect mesenchymal-like cancer cells. We demonstrate that the uptake of glucose-coated magnetic nanoparticles (MNPs) by mesenchymal-like cells remains constant when the glucose in the medium is increased from low (5.5 mM) to high (25 mM) concentration, while the MNPs uptake by epithelial-like cells is significantly reduced. These findings reveal that the glucose-shell of MNPs plays a major role in recognition of cells with high-metabolic activity. By selectively blocking the glucose transporter 1 channels we showed its involvement in the internalization process of glucose-coated MNPs. Our results suggest that glucose-coated MNPs can be used for metabolic-based assays aimed at detecting cancer cells and that can be used to selectively target cancer cells taking advantage, for instance, of the magnetic-thermotherapy.

A collagen-binding EGFR antibody fragment targeting tumors with a collagen-rich extracellular matrix

Many tumors over-express collagen, which constitutes the physical scaffold of tumor microenvironment. Collagen has been considered to be a target for cancer therapy. The collagen-binding domain (CBD) is a short peptide, which could bind to collagen and achieve the sustained release of CBD-fused proteins in collagen scaffold. Here, a collagen-binding EGFR antibody fragment was designed and expressed for targeting the collagen-rich extracellular matrix in tumors. The antibody fragment (Fab) of cetuximab was fused with CBD (CBD-Fab) and expressed in Pichia pastoris. CBD-Fab maintained antigen binding and anti-tumor activity of cetuximab and obtained a collagen-binding ability in vitro. The results also showed CBD-Fab was mainly enriched in tumors and had longer retention time in tumors in A431 s.c. xenografts. Furthermore, CBD-Fab showed a similar therapeutic efficacy as cetuximab in A431 xenografts. Although CBD-Fab hasn’t showed better therapeutic effects than cetuximab, its smaller molecular and special target may be applicable as antibody–drug conjugates (ADC) or immunotoxins.

Mass Spectrometry Uncovers Molecular Reactivities of Coordination and Organometallic Gold(III) Drug Candidates in Competitive Experiments That Correlate with Their Biological Effects

The reactivity of three cytotoxic organometallic gold(III) complexes with cyclometalated C,N,N and C,N ligands (either six- or five-membered metallacycles), as well as that of two representative gold(III) complexes with N-donor ligands, with biological nucleophiles has been studied by ESI-MS on ion trap and time-of-flight instruments. Specifically, the gold compounds were reacted with mixtures of nucleophiles containing l-histidine (imine), l-methionine (thioether), l-cysteine (thiol), l-glutamic acid (carboxylic acid), methylseleno-l-cysteine (selenoether), and in situ generated seleno-l-cysteine (selenol) to judge the preference of the gold compounds for binding to selenium-containing amino acid residues. Moreover, the gold compounds' reactivity was studied with proteins and nucleic acid building blocks. These experiments revealed profound differences between the coordination and organometallic families and even within the family of organometallics, which allowed insights to be gained into the compounds mechanisms of action. In particular, interactions with seleno-l-cysteine appear to reflect well the compounds' inhibition properties of the seleno-enzyme thioredoxin reductase and to a certain extent their antiproliferative effects in vitro. Therefore, mass spectrometry is successfully applied for linking the molecular reactivity and target preferences of metal-based drug candidates to their biological effects. Finally, this experimental setup is applicable to any other metallodrug that undergoes ligand substitution reactions and/or redox changes as part of its mechanism of action.

Tumor Targeting and Pharmacokinetics of a Near-Infrared Fluorescent-Labeled δ-Opioid Receptor Antagonist Agent, Dmt-Tic-Cy5

Fluorescence molecular imaging can be employed for the development of novel cancer targeting agents. Herein, we investigated the pharmacokinetics (PK) and cellular uptake of Dmt-Tic-Cy5, a delta-opioid receptor (δOR) antagonist-fluorescent dye conjugate, as a tumor-targeting molecular imaging agent. δOR expression is observed normally in the CNS, and pathologically in some tumors, including lung liver and breast cancers. In vitro, in vivo, and ex vivo experiments were conducted to image and quantify the fluorescence signal associated with Dmt-Tic-Cy5 over time using in vitro and intravital fluorescence microscopy and small animal fluorescence imaging of tumor-bearing mice. We observed specific retention of Dmt-Tic-Cy5 in tumors with maximum uptake in δOR-expressing positive tumors at 3 h and observable persistence for >96 h; clearance from δOR nonexpressing negative tumors by 6 h; and systemic clearance from normal organs by 24 h. Live-cell and intravital fluorescence microscopy demonstrated that Dmt-Tic-Cy5 had sustained cell-surface binding lasting at least 24 h with gradual internalization over the initial 6 h following administration. Dmt-Tic-Cy5 is a δOR-targeted agent that exhibits long-lasting and specific signal in δOR-expressing tumors, is rapidly cleared from systemic circulation, and is not retained in non-δOR-expressing tissues. Hence, Dmt-Tic-Cy5 has potential as a fluorescent tumor imaging agent.

Mechanisms and biomaterials in pH-responsive tumour targeted drug delivery: A review

As the mainstay in the treatment of various cancers, chemotherapy plays a vital role, but still faces many challenges, such as poor tumour selectivity and multidrug resistance (MDR). Targeted drug delivery using nanotechnology has provided a new strategy for addressing the limitations of the conventional chemotherapy. In the last decade, the volume of research published in this area has increased tremendously, especially with functional nano drug delivery systems (nanocarriers). Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for improving therapeutic outcomes. Among the various stimuli, pH triggered delivery is regarded as the most general strategy, targeting the acidic extracellular microenvironment and intracellular organelles of solid tumours. In this review, we discuss recent advances in the development of pH-sensitive nanocarriers for tumour-targeted drug delivery. The review focuses on the chemical design of pH-sensitive biomaterials, which are used to fabricate nanocarriers for extracellular and/or intracellular tumour site-specific drug release. The pH-responsive biomaterials bring forth conformational changes in these nanocarriers through various mechanisms such as protonation, charge reversal or cleavage of a chemical bond, facilitating tumour specific cell uptake or drug release. A greater understanding of these mechanisms will help to design more efficient drug delivery systems to address the challenges encountered in conventional chemotherapy.

The Warburg Effect: How Does it Benefit Cancer Cells?

Cancer cells rewire their metabolism to promote growth, survival, proliferation, and long-term maintenance. The common feature of this altered metabolism is the increased glucose uptake and fermentation of glucose to lactate. This phenomenon is observed even in the presence of completely functioning mitochondria and, together, is known as the 'Warburg Effect'. The Warburg Effect has been documented for over 90 years and extensively studied over the past 10 years, with thousands of papers reporting to have established either its causes or its functions. Despite this intense interest, the function of the Warburg Effect remains unclear. Here, we analyze several proposed explanations for the function of Warburg Effect, emphasize their rationale, and discuss their controversies.

Intraarterial gelation of injectable cationic pH/temperature-sensitive radiopaque embolic hydrogels in a rabbit hepatic tumor model and their potential application for liver cancer treatment

Radiopaque embolic solutions employing cationic poly(amino ester urethane)-based copolymers are injected into a rabbit hepatic tumor artery and form hydrogels in response to local temperature and pH for chemoembolization.

Multisensitive drug-loaded polyurethane/polyurea nanocapsules with pH-synchronized shell cationization and redox-triggered release

A one-pot versatile method for the preparation of sub-30 nm multisensitive polyurethane/polyurea nanocapsules with pH-synchronized shell cationization is presented. The nanocapsules have been loaded with different drugs which are released through a redox-triggered mechanism.

An affinity triggered MRI nanoprobe for pH-dependent cell labeling

The pH-sensitive affinity pair composed by neutravidin and iminobiotin was used to develop a multilayered Magnetic Resonance Imaging (MRI) nanoprobe responsive to the acidic pH of tumor microenvironment.

Neutralization of Tumor Acidity Improves Antitumor Responses to Immunotherapy

Cancer immunotherapies, such as immune checkpoint blockade or adoptive T-cell transfer, can lead to durable responses in the clinic, but response rates remain low due to undefined suppression mechanisms. Solid tumors are characterized by a highly acidic microenvironment that might blunt the effectiveness of antitumor immunity. In this study, we directly investigated the effects of tumor acidity on the efficacy of immunotherapy. An acidic pH environment blocked T-cell activation and limited glycolysis in vitro. IFNγ release blocked by acidic pH did not occur at the level of steady-state mRNA, implying that the effect of acidity was posttranslational. Acidification did not affect cytoplasmic pH, suggesting that signals transduced by external acidity were likely mediated by specific acid-sensing receptors, four of which are expressed by T cells. Notably, neutralizing tumor acidity with bicarbonate monotherapy impaired the growth of some cancer types in mice where it was associated with increased T-cell infiltration. Furthermore, combining bicarbonate therapy with anti-CTLA-4, anti-PD1, or adoptive T-cell transfer improved antitumor responses in multiple models, including cures in some subjects. Overall, our findings show how raising intratumoral pH through oral buffers therapy can improve responses to immunotherapy, with the potential for immediate clinical translation.

Ratiometric Organic Fibers for Localized and Reversible Ion Sensing with Micrometer-Scale Spatial Resolution

A fundamental issue in biomedical and environmental sciences is the development of sensitive and robust sensors able to probe the analyte of interest, under physiological and pathological conditions or in environmental samples, and with very high spatial resolution. In this work, novel hybrid organic fibers that can effectively report the analyte concentration within the local microenvironment are reported. The nanostructured and flexible wires are prepared by embedding fluorescent pH sensors based on seminaphtho-rhodafluor-1-dextran conjugate. By adjusting capsule/polymer ratio and spinning conditions, the diameter of the fibers and the alignment of the reporting capsules are both tuned. The hybrid wires display excellent stability, high sensitivity, as well as reversible response, and their operation relies on effective diffusional kinetic coupling of the sensing regions and the embedding polymer matrix. These devices are believed to be a powerful new sensing platform for clinical diagnostics, bioassays and environmental monitoring.

Caged [(18)F]FDG Glycosylamines for Imaging Acidic Tumor Microenvironments Using Positron Emission Tomography

Solid tumors are hypoxic with altered metabolism, resulting in secretion of acids into the extracellular matrix and lower relative pH, a feature associated with local invasion and metastasis. Therapeutic and diagnostic agents responsive to this microenvironment may improve tumor-specific delivery. Therefore, we pursued a general strategy whereby caged small-molecule drugs or imaging agents liberate their parent compounds in regions of low interstitial pH. In this manuscript, we present a new acid-labile prodrug method based on the glycosylamine linkage, and its application to a class of positron emission tomography (PET) imaging tracers, termed [(18)F]FDG amines. [(18)F]FDG amines operate via a proposed two-step mechanism, in which an acid-labile precursor decomposes to form the common radiotracer 2-deoxy-2-[(18)F]fluoro-d-glucose, which is subsequently accumulated by glucose avid cells. The rate of decomposition of [(18)F]FDG amines is tunable in a systematic fashion, tracking the pKa of the parent amine. In vivo, a 4-phenylbenzylamine [(18)F]FDG amine congener showed greater relative accumulation in tumors over benign tissue, which could be attenuated upon tumor alkalinization using previously validated models, including sodium bicarbonate treatment, or overexpression of carbonic anhydrase. This new class of PET tracer represents a viable approach for imaging acidic interstitial pH with potential for clinical translation.