Hyperpolarized 13C MR spectroscopic imaging can be used to monitor Everolimus treatment in vivo in an orthotopic rodent model of glioblastoma

Abstract

Glioblastoma (GBM) is the most common and lethal primary malignant brain tumor in humans. Because the phosphatidylinositol-3-kinase (PI3K) signaling pathway is activated in more than 88% of GBM, new drugs which target this pathway, such as the mTOR inhibitor Everolimus, are currently in clinical trials. Early tumor response to molecularly targeted treatments remains challenging to assess non-invasively, because it is often associated with tumor stasis or slower tumor growth. Innovative neuroimaging methods are therefore critically needed to provide metabolic or functional information that is indicative of targeted therapeutic action at early time points during the course of treatment. In this study, we demonstrated for the first time that hyperpolarized (HP) 13C magnetic resonance spectroscopic imaging (MRSI) can be used on a clinical MR system to monitor early metabolic response of orthotopic GBM tumors to Everolimus treatment through measurement of the HP lactate-to-pyruvate ratios. The study was performed on a highly invasive non-enhancing orthotopic GBM tumor model in rats (GS-2 tumors), which replicates many fundamental features of human GBM tumors. Seven days after initiation of treatment there was a significant drop in the HP lactate-to-pyruvate ratio from the tumor tissue in treated animals relative to day 0 (67%±27% decrease). In the control group, no significant changes in the HP lactate-to-pyruvate ratios were observed. Importantly, at the 7 day time point, conventional MR imaging (MRI) was unable to detect a significant difference in tumor size between control and treated groups. Inhibition of tumor growth by conventional MRI was observed from day 15 of treatment. This implies that the decrease in the HP lactate-to-pyruvate ratio could be detected before any treatment-induced inhibition of tumor growth. Using immunohistochemical staining to further examine tumor response to treatment, we found that the decrease in the HP lactate-to-pyruvate ratio was associated with a drop in expression of lactate dehydrogenase, the enzyme that catalyzes pyruvate to lactate conversion. Also evident was decreased staining for carbonic anhydrase IX (CA-IX), an indicator of hypoxia-inducible factor 1α (HIF-1α) activity, which, in turn, regulates expression of lactate dehydrogenase. To our knowledge, this study is the first report of the use of HP 13C MRSI at a clinical field strength to monitor GBM response to molecularly targeted treatments. It highlights the potential of HP lactate-to-pyruvate ratio as an early biomarker of response, thereby supporting further investigation of this non-invasive imaging approach for eventual clinical application.

Hyperpolarized 13 C MR metabolic imaging can detect neuroinflammation in vivo in a multiple sclerosis murine model

“…Through increase in the MRdetectable signal of 13 C compounds by up to 50,000 [so-called hyperpolarized (HP) probes] this methodology has enabled the noninvasive monitoring of several metabolic reactions in vivo (27)(28)(29)(30)(31)(32). In particular, in the context of cancer, HP 13 C MRSI has been shown to improve the noninvasive detection of Significance Cells from the innate immune system, namely microglia and macrophages (mononuclear phagocytes, MPs), play a central role in the progression of neurological disorders such as multiple sclerosis.…”

Integration of imaging into clinical practice to assess the delivery and performance of macromolecular and nanotechnology-based oncology therapies

“…Imaging of changes in activity of specific molecular pathways to targeted treatments Hyperpolarized 13 C MRS imaging has the potential for imaging response to targeted therapeutics, due to the ability to functionally visualize the breakdown of 13 C-labeled substrates as a means to follow the activity of individual pathways. For example, preclinical studies showed that changes in the pyruvate-lactate ratio in a glioblastoma multiforme model may be assessed as indications of PI3K/mTOR pathway inhibition following treatment with the mTOR inhibitor everolimus [153]. Importantly, this study showed that treatment-specific changes in tumors could be observed as early as 7 days post-treatment initiation.…”

“…Importantly, this study showed that treatment-specific changes in tumors could be observed as early as 7 days post-treatment initiation. At this time-point, conventional MRI imaging was incapable of detecting changes in tumor size between treated and untreated groups, which was only observed at 15 days post-treatment initiation [153]. The ratio of 13 C-pyruvate to 13 C-lactate has also been used as a surrogate for assessment of platelet-derived growth factor receptor (PDGFR) signaling following targeted treatment with the inhibitor imatinib [154].…”

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

“…This approach was extended to study the response of chemotherapy (Temozolomide) and it was found that a decrease in lactate-to-pyruvate ratio (day 1) preceded a decrease in tumor size (day 5 or 6), suggesting that this technique can be used to monitor tumor response to therapy [246]. This was further corroborated by a study using a PI3K signaling inhibitor (Everolimus) to treat implanted GBM that showed changes in HP lactate-to-pyruvate ratio about 7 days before any changes in tumor size or growth were detected [247]. HP [1- 13 C]pyruvate imaging has also been used to study C6 glioma, showing a higher lactate-to-bicarbonate ratio, higher lactate, and lower bicarbonate than healthy rat brains [248].…”