Glioblastoma is an aggressive brain cancer that is very difficult to treat. Clinically, it is important to be able to distinguish aggressive from non-aggressive brain tumors. Previous studies have shown that some drugs can induce a rapid change in intracellular pH that could help to identify aggressive cancer. The sodium proton exchanger (NHE1) plays a significant role in maintaining pH balance in the tumor microenvironment. Cariporide is a sodium proton exchange inhibitor that is well tolerated by humans in cardiac applications. We hypothesized that cariporide could selectively acidify brain tumors. The purpose of this study was to determine whether amine/amide concentration-independent detection (AACID) chemical exchange saturation transfer (CEST) MRI measurement of tumor pH could detect acidification after cariporide injection. Using a 9.4T MRI scanner, CEST spectra were acquired in six mice approximately 14 days after implanting 10 U87 human glioblastoma multiforme cells in the brain, before and after administration of cariporide (dose: 6 mg/kg) by intraperitoneal injection. Three additional mice were studied as controls and received only vehicle injection (DMSO + PBS). Repeated measures t test was used to examine changes in tumor and contralateral tissue regions of interest. Two hours after cariporide injection, there was a significant 0.12 ± 0.03 increase in tumor AACID value corresponding to a 0.48 decrease in pH and no change in AACID value in contralateral tissue. A small but significant increase of 0.04 ± 0.017 in tumor AACID value was also observed following vehicle injection. This study demonstrates that acute CEST MRI contrast changes, indicative of intracellular acidification, after administration of cariporide could help localize glioblastoma.

Intracellular pH (pH) plays an important role in the maintenance of normal cell function, and is maintained within a narrow range by the activity of transporters located at the plasma membrane. Modulation of tumor pH may influence proliferation, apoptosis, chemotherapy resistance, and thermosensitivity. Chemical exchange saturation transfer (CEST) is a novel MRI contrast mechanism that is dependent on cellular pH. Amine and amide concentration-independent detection (AACID) is a recently developed CEST contrast method that is intracellular pH (pH) weighted. Dichloroacetate (DCA) can alter tumor pH by inhibiting the enzyme pyruvate dehydrogenase kinase causing reduced lactate (increasing pH), or by decreasing the expression of monocarboxylate transporters and vacuolar ATPase leading to reduced pH. Since the net in vivo effect of DCA on pH is difficult to predict, the purpose of this study was to quantify the magnitude of acute pH change in glioblastoma after a single DCA injection using AACID CEST MRI. Using a 9.4T MRI scanner, CEST spectra were acquired in six mice approximately 14 days after implanting 10 U87 human glioblastoma multiforme (GBM) cells in the brain, before and after intravenous injection of DCA (dose: 200 mg/kg). Three additional mice received only phosphate buffered saline (PBS) injection and were studied as controls. Repeated measures t test was used to compare AACID changes in tumor and contralateral tissue regions of interest. One hour after DCA injection there was a significant increase in tumor AACID level by 0.04 ± 0.01 corresponding to a 0.16 decrease in pH, and no change in AACID in contralateral tissue. Inspection of AACID maps following PBS injection showed no differences. The use of DCA to induce a tumor specific pH change detectable by AACID CEST MRI is consistent with previous studies that have shown similar effects for lonidamine and topiramate. This study demonstrates that a single dose of DCA can be used as a pharmacological challenge to induced rapid tumor intracellular acidification.

The response of tumor intracellular pH to a pharmacological challenge could help identify aggressive cancer. Chemical exchange saturation transfer (CEST) is an MRI contrast mechanism that is dependent on intracellular pH (pH). pH is important in the maintenance of normal cell function and is normally maintained within a narrow range by the activity of transporters located at the plasma membrane. In cancer, changes in pH have been correlated with both cell proliferation and cell death. Quercetin is a bioflavonoid and monocarboxylate transporter (MCT) inhibitor. Since MCTs plays a significant role in maintaining pH balance in the tumor microenvironment, we hypothesized that systemically administered quercetin could selectively acidify brain tumors. The goals of the current study were to determine whether CEST MRI measurements sensitive to tumor pH could detect acidification after quercetin injection and to measure the magnitude of the pH change (ΔpH). Using a 9.4 T MRI, amine and amide concentration independent detection (AACID) CEST spectra were acquired in six mice approximately 15 ± 1 days after implanting 10 U87 human glioblastoma multiforme cells in the brain, before and after administration of quercetin (dose: 200 mg/kg) by intraperitoneal injection. Three additional mice were studied as controls and received only vehicle dimethyl sulfoxide (DMSO) injection. Repeated measures t-test was used to compare AACID changes in tumor and contralateral tissue regions of interest. Two hours after quercetin injection there was a significant increase in tumor AACID by 0.07 ± 0.03 corresponding to a 0.27 decrease in pH, and no change in AACID in contralateral tissue. There was also a small average increase in AACID in tumors within the three mice injected with DMSO only. The use of the natural compound quercetin in combination with pH weighted MRI represents a unique approach to cancer detection that does not require injection of an imaging contrast agent.

Introduction Non-invasively distinguishing aggressive from non-aggressive brain tumors is an important clinical challenge. Intracellular pH (pH i ) regulation is essential for normal cell function and is normally maintained within a narrow range. Cancer cells are characterized by a reversed intracellular to extracellular pH gradient, compared to healthy cells, that is maintained by several distinct mechanisms. Previous studies have demonstrated acute pH modulation in glioblastoma detectable by chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) after blocking individual pH regulatory mechanisms. The purpose of the current study was to simultaneously block five pH regulatory mechanisms while also providing glucose as an energy substrate. We hypothesized that this approach would increase the acute pH modulation effect allowing the identification of aggressive cancer. Methods Using a 9.4 T MRI scanner, CEST spectra were acquired sensitive to pH i using amine/amide concentration independent detection (AACID). Twelve mice were scanned approximately 11 ± 1 days after implanting 10 5 U87 human glioblastoma multiforme cells in the brain, before and after intraperitoneal injection of a combination of five drugs (quercetin, cariporide, dichloroacetate, acetazolamide, and pantoprazole) with and without glucose. Results Two hours after combination drug injection there was a significant 0.1 ± 0.03 increase in tumor AACID value corresponding to a 0.4 decrease in pH i . After injecting the drug combination with glucose the AACID value increased by 0.18 ± 0.03 corresponding to a 0.72 decrease in pH i . AACID values were also slightly increased in contralateral tissue. Conclusions The combined drug treatment with glucose produced a large acute CEST MRI contrast indicating tumor acidification, which could be used to help localize brain cancer and monitor tumor response to chemotherapy.

This study presents the first longitudinal measurement of the intracellular/extracellular pH gradient in a rat glioma model using noninvasive magnetic resonance imaging. The acid–base balance in the brain is tightly controlled by endogenous buffers. Tumors often express a positive pH gradient (pHi – pHe) compared with normal tissue that expresses a negative gradient. Alkaline pHi in tumor cells increases activity of several enzymes that drive cellular proliferation. In contrast, acidic pHe is established because of increased lactic acid production and subsequent active transport of protons out of the cell. pHi was mapped using chemical exchange saturation transfer, whereas regional pHe was determined using hyperpolarized 13C bicarbonate magnetic resonance spectroscopic imaging. pHi and pHe were measured at days 8, 12, and 15 postimplantation of C6 glioma cells into rat brains. Measurements were made in tumors and compared to brain tissue without tumor. Overall, average pH gradient in the tumor changed from −0.02 ± 0.12 to 0.10 ± 0.21 and then 0.19 ± 0.16. Conversely, the pH gradient of contralateral brain tissue changed from −0.45 ± 0.16 to −0.25 ± 0.21 and then −0.34 ± 0.25 (average pH ± 1 SD) Spatial heterogeneity of tumor pH gradient was apparent at later time points and may be useful to predict local areas of treatment resistance. Overall, the intracellular/extracellular pH gradients in this rat glioma model were noninvasively measured to a precision of ∼0.1 pH units at 3 time points. Because most therapeutic agents are weak acids or bases, a priori knowledge of the pH gradient may help guide choice of therapeutic agent for precision medicine.