UCSF

Abstract Today, there is no reliable noninvasive imaging method available to monitor glioblastoma (GBM) response to therapy and predict survival prior to tumor shrinkage. Dissolution Dynamic Nuclear Polarization (DNP) combined with hyperpolarized 13C Magnetic Resonance Spectroscopic Imaging (MRSI) is a novel imaging method that allows probing real-time tumor metabolism. Recent studies using 13C MRSI have shown decreased lactate production from pyruvate in GBM responsive to a dual PI3K/mTOR inhibitor and/or Temozolomide, mediated by lower expression of LDHA or PKM2 enzymes, respectively. SAHA, the histone deacetylase (HDAC) inhibitor, is a novel drug that inhibits cell proliferation by inducing cell cycle arrest followed by apoptosis. The goal of this study was to detect early HDAC inhibition using advanced 13C MRSI. Analysis of dynamic real-time cellular metabolic changes in SAHA-treated U87 live cells in bioreactors demonstrated a significant 37.7% decrease in hyperpolarized [1-13C]-lactate production. SAHA-treated cells also showed a 29.6% decrease of steady-state lactate level in cell extracts. Furthermore, we demonstrated a significant 30.3% decrease in hyperpolarized lactate-to-pyruvate ratio in SAHA-treated U87-bearing mice, which occurred prior to MRI-detectable changes in tumor size that was associated with enhanced animal survival. In order to mechanistically validate our findings, we tested the levels of expression of LDHA, MCT1 and MCT4, the main players in pyruvate-to-lactate interconversion. While expression of LDHA, the enzyme that catalyzes pyruvate-to-lactate conversion was independent of SAHA treatment, expression of both MCT1 and MCT4 transporters, that shuttle pyruvate and lactate in and out of the cell, are increased. We thus propose that increased MCT1/4 led to a decrease in lactate in response to HDAC inhibition and resulted in a reduced pyruvate-to-lactate conversion. Our findings confirm the potential translational value of the hyperpolarized lactate-to-pyruvate ratio as a biomarker for noninvasively assessing the early effects of emerging therapies for patients with GBM.