Brain tumor acidification using drugs simultaneously targeting multiple pH regulatory mechanisms

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.