Correlation of hyperpolarized ¹³ C-MRI data with tissue extract measurements

Hyperpolarized (HP) $$ MRI provides the means to monitor lactate metabolism noninvasively in tumours. Since $$ -lactate signal levels obtained from HP $$ imaging depend on multiple factors, such as the rate of $$ substrate delivery via the vasculature, the expression level of monocarboxylate transporters (MCTs) and lactate dehydrogenase (LDH), and the local lactate pool size, the interpretation of HP $$ metabolic images remains challenging. In this study, ex vivo tissue extract measurements (i.e., NMR isotopomer analysis, western blot analysis) derived from an MDA-MB-231 xenograft model in nude rats were used to test for correlations between the in vivo $$ data and the ex vivo measures. The lactate-to-pyruvate ratio from HP $$ MRI was strongly correlated with [1- $$ ]lactate concentration measured from the extracts using NMR (R = 0.69, p $$ 0.05), as well as negatively correlated with tumour wet weight (R = $$  0.60, p $$ 0.05). In this tumour model, both MCT1 and MCT4 expressions were positively correlated with wet weight ( $$ = 0.78 and 0.93, respectively, p $$ 0.01). Lactate pool size and the lactate-to-pyruvate ratio were not significantly correlated.