Iopamidol (Isovue®—Bracco Diagnostic Inc.) is a clinically approved X‐Ray contrast agent used in the last 30 years for a wide variety of diagnostic applications with a very good clinical acceptance. Iopamidol contains two types of amide functionalities that can be exploited for the generation of chemical exchange saturation transfer effect. The exchange rate of the two amide proton pools is markedly pH‐dependent. Thus, a ratiometric method for pH assessment has been set‐up based on the comparison of the saturation transfer effects induced by selective irradiation of the two resonances. This ratiometric approach allows to rule out the concentration effect of the contrast agent and provides accurate pH measurements in the 5.5–7.4 range. Upon injection of Iopamidol into healthy mice, it has been possible to acquire pH maps of kidney regions. Furthermore, it has been also shown that the proposed method is able to report about pH‐changes induced in control mice fed with acidified or basified water for a period of a week before image acquisition. Magn Reson Med, 2010.

In the liver, insulin-mediated activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway is at the core of metabolic control. Multiple PI3K and Akt isoenzymes are found in hepatocytes and whether isoform-selective interplays exist is currently unclear. Here we report that insulin signalling triggers the association of the liver-specific class II PI3K isoform γ (PI3K-C2γ) with Rab5-GTP, and its recruitment to Rab5-positive early endosomes. In these vesicles, PI3K-C2γ produces a phosphatidylinositol-3,4-bisphosphate pool specifically required for delayed and sustained endosomal Akt2 stimulation. Accordingly, loss of PI3K-C2γ does not affect insulin-dependent Akt1 activation as well as S6K and FoxO1-3 phosphorylation, but selectively reduces Akt2 activation, which specifically inhibits glycogen synthase activity. As a consequence, PI3K-C2γ-deficient mice display severely reduced liver accumulation of glycogen and develop hyperlipidemia, adiposity as well as insulin resistance with age or after consumption of a high-fat diet. Our data indicate PI3K-C2γ supports an isoenzyme-specific forking of insulin-mediated signal transduction to an endosomal pool of Akt2, required for glucose homeostasis.

The acidic pH of tumors profoundly inhibits effector functions of activated CD8 + T-cells. We hypothesize that this is a physiological process in immune regulation, and that it occurs within lymph nodes (LNs), which are likely acidic because of low convective flow and high glucose metabolism. Here we show by in vivo fluorescence and MR imaging, that LN paracortical zones are profoundly acidic. These acidic niches are absent in athymic Nu/Nu and lymphodepleted mice, implicating T-cells in the acidifying process. T-cell glycolysis is inhibited at the low pH observed in LNs. We show that this is due to acid inhibition of monocarboxylate transporters (MCTs), resulting in a negative feedback on glycolytic rate. Importantly, we demonstrate that this acid pH does not hinder initial activation of naïve T-cells by dendritic cells. Thus, we describe an acidic niche within the immune system, and demonstrate its physiological role in regulating T-cell activation.

Cancer cells are characterized by a metabolic shift in cellular energy production, orchestrated by the transcription factor HIF-1α, from mitochondrial oxidative phosphorylation to increased glycolysis, regardless of oxygen availability (Warburg effect). The constitutive upregulation of glycolysis leads to an overproduction of acidic metabolic products, resulting in enhanced acidification of the extracellular pH (pHe ~ 6.5), which is a salient feature of the tumor microenvironment. Despite the importance of pH and tumor acidosis, there is currently no established clinical tool available to image the spatial distribution of tumor pHe. The purpose of this review is to describe various imaging modalities for measuring intracellular and extracellular tumor pH. For each technique, we will discuss main advantages and limitations, pH accuracy and sensitivity of the applied pH-responsive probes and potential translatability to the clinic. Particular attention is devoted to methods that can provide pH measurements at high spatial resolution useful to address the task of tumor heterogeneity and to studies that explored tumor pH imaging for assessing treatment response to anticancer therapies.

Chemical exchange saturation transfer (CEST) is a novel contrast mechanism for magnetic resonance imaging (MRI). CEST MRI selectively saturates exchangeable protons that are transferred to MRI-detectable bulk water signal. MRI-CEST (pH)-responsive agents are probes able to map pH in the microenvironment in which they distribute. To minimize the confounding effects of contrast agent concentration, researchers have developed ratiometric CEST imaging, which investigates contrast agents containing multiple magnetically non-equivalent proton groups, whose prototropic exchange have different pH responses. However, conventional ratiometric CEST MRI imposes stringent requirements on the selection of CEST contrasts agents. In this study, a novel ratiometric pH MRI method based on the analysis of CEST effects under different radio frequency irradiation power levels was developed. The proposed method has been demonstrated using iobitridol, an X-ray contrast agent analog of iopamidol but containing a single set of amide protons, both in vitro and in vivo.

The vast majority of cancers exhibit increased glucose uptake and glycolysis regardless of oxygen availability. This metabolic shift leads to an enhanced production of lactic acid that decreases extracellular pH (pHe), a hallmark of the tumor microenvironment. In this way, dysregulated tumor pHe and upregulated glucose metabolism are linked tightly and their relative assessment may be useful to gain understanding of the underlying biology. Here we investigated noninvasively the in vivo correlation between tumor ¹⁸F-FDG uptake and extracellular pH values in a murine model of HER2⁺ breast cancer. Tumor extracellular pH and perfusion were assessed by acquiring MRI-CEST (chemical exchange saturation transfer) images on a 3T scanner after intravenous administration of a pH-responsive contrast agent (iopamidol). Static PET images were recorded immediately after MRI acquisitions to quantify the extent of ¹⁸F-FDG uptake. We demonstrated the occurrence of tumor pHe changes that report on acidification of the interstitial fluid caused by an accelerated glycolysis. Combined PET and MRI-CEST images reported complementary spatial information of the altered glucose metabolism. Notably, a significant inverse correlation was found between extracellular tumor pH and ¹⁸F-FDG uptake, as a high ¹⁸F-FDG uptake corresponds to lower extracellular pH values. These results show how merging the information from ¹⁸F-FDG-uptake and extracellular pH measurements can improve characterization of the tumor microenvironment. Cancer Res; 76(22); 6463–70. ©2016 AACR.