Dichloroacetate induced intracellular acidification in glioblastoma: in vivo detection using AACID-CEST MRI at 9.4 Tesla

Intracellular pH (pH_i) 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_i 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_i) weighted. Dichloroacetate (DCA) can alter tumor pH_i by inhibiting the enzyme pyruvate dehydrogenase kinase causing reduced lactate (increasing pH_i), or by decreasing the expression of monocarboxylate transporters and vacuolar ATPase leading to reduced pH_i. Since the net in vivo effect of DCA on pH_i is difficult to predict, the purpose of this study was to quantify the magnitude of acute pH_i 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 10⁵ 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_i, 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.