Cancer metabolism in a snapshot: MRS(I)

Abstract

The contribution of MRS(I) to the in vivo evaluation of cancer-metabolism-derived metrics, mostly since 2016, is reviewed here. Increased carbon consumption by tumour cells, which are highly glycolytic, is now being sampled by 13 C magnetic resonance spectroscopic imaging (MRSI) following the injection of hyperpolarized [1-13 C] pyruvate (Pyr). Hot-spots of, mostly, increased lactate dehydrogenase activity or flow between Pyr and lactate (Lac) have been seen with cancer progression in prostate (preclinical and in humans), brain and pancreas (both preclinical) tumours. Therapy response is usually signalled by decreased Lac/Pyr 13 C-labelled ratio with respect to untreated or nonresponding tumour. For therapeutic agents inducing tumour hypoxia, the 13 C-labelled Lac/bicarbonate ratio may be a better metric than the Lac/Pyr ratio. 31 P MRSI may sample intracellular pH changes from brain tumours (acidification upon antiangiogenic treatment, basification at fast proliferation and relapse). The steady state tumour metabolome pattern is still in use for cancer evaluation. Metrics used for this range from quantification of single oncometabolites (such as 2-hydroxyglutarate in mutant IDH1 glial brain tumours) to selected metabolite ratios (such as total choline to N-acetylaspartate (plain ratio or CNI index)) or the whole 1 H MRSI(I) pattern through pattern recognition analysis. These approaches have been applied to address different questions such as tumour subtype definition, following/predicting the response to therapy or defining better resection or radiosurgery limits. KEYWORDSanimal model study, cancer, cellular and molecular cancer imaging, hyperpolarized C-13, MRS and MRSI methods, phosphorus MRS/MRSI | TEMPORAL AND THEMATIC BOUNDARIES OF THE TOPICS COVEREDThe authors of this review declare at the outset that there is no intention of being exhaustive or systematic. The temporal boundaries reviewed will be modest: only relevant literature since 2016, chosen according to our bias. Earlier work or reviews will only be referred to in order to put recent findings in due perspective. Authors hope that this summary will give readers a picture of how far cancer metabolism analysis with MRS(I) has advanced and, perhaps, why it is not advancing further.For quite a long time MRS, rather than MRI, has been the only approach able to evaluate cancer metabolism. Nevertheless, the MRI-derived information related to cancer metabolism should not be forgotten: consider for example the chemical exchange saturation transfer (CEST) effect caused by exchangeable protons in water-derived images of glucose uptake and metabolism in tumours. 1,2 However, apart from these CESTrelated studies, we should acknowledge that most information about cancer metabolism using MR still refers to MRS(I) acquisitions, which are the ones dealt with in our text.Supplementary Figure 1 shows a PUBMED search illustrating the different proportions of articles investigating several cancer types through MRS(I) approaches. The retrieved information can help unders...

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