Metabolism of hyperpolarised [1-13 C]pyruvate in awake and anaesthetised rat brains.

The use of hyperpolarised 13 C pyruvate for nononcological neurological applications has not been widespread so far, possibly due to delivery issues limiting the visibility of metabolites. First proof-of-concept results have indicated that metabolism can be detected in human brain, and this may supersede the results obtained in preclinical settings. One major difference between the experimental setups is that preclinical MRI/MRS routinely uses anaesthesia, which alters both haemodynamics and metabolism. Here, we used hyperpolarised [1-13 C]pyruvate to compare brain metabolism in awake rats and under isoflurane, urethane or medetomidine anaesthesia. Spectroscopic [1-13 C]pyruvate time courses measured sequentially showed that pyruvate-to-bicarbonate and pyruvate-to-lactate labelling rates were lower in isoflurane animals than awake animals. An increased bicarbonate-to-lactate ratio was observed in the medetomidine group compared with other groups. The study shows that hyperpolarised [1-13 C]pyruvate experiments can be performed in awake rats, thus avoiding anaesthesia-related issues. The results suggest that haemodynamics probably dominate the observed pyruvate-to-metabolite labelling rates and area-under-time course ratios of referenced to pyruvate. On the other hand, the results obtained with medetomidine suggest that the ratios are also modulated by the underlying cerebral metabolism. However, the ratios between intracellular metabolites were unchanged in awake compared with isoflurane-anaesthetised rats.

Detection of lentiviral suicide gene therapy in C6 rat glioma using hyperpolarised [1-13 C]pyruvate.

Hyperpolarised [1-13 C]pyruvate MRI has shown promise in monitoring therapeutic efficacy in a number of cancers including glioma. In this study, we assessed the pyruvate response to the lentiviral suicide gene therapy of herpes simplex virus-1 thymidine kinase with the prodrug ganciclovir (HSV-TK/GCV) in C6 rat glioma and compared it with traditional MR therapy markers. Female Wistar rats were inoculated with 106 C6 glioma cells. Treated animals received intratumoural lentiviral HSV-TK gene transfers on days 7 and 8 followed by 2-week GCV therapy starting on day 10. Animals were repeatedly imaged during therapy using volumetric MRI, diffusion and relaxation mapping, as well as metabolic [1-13 C]pyruvate MRS imaging. Survival (measured as time before animals reached a humane endpoint and were euthanised) was assessed up to day 30 posttherapy. HSV-TK/GCV gene therapy lengthened the median survival time from 12 to 25 days. This was accompanied by an apparent tumour growth arrest, but no changes in diffusion or relaxation parameters in treated animals. The metabolic response was more evident in the case-by-case analysis than in the group-level analysis. Treated animals also showed a 37 ± 15% decrease (P < 0.05, n = 5) in lactate-to-pyruvate ratio between therapy weeks, whereas a 44 ± 18% increase (P < 0.05, n = 6) was observed in control animals. Hyperpolarised [1-13 C]pyruvate MRI can offer complementary metabolic information to traditional MR methods to give a more comprehensive picture of the slowly developing gene therapy response. This may benefit the detection of the successful therapy response in patients.

Cyclodextrin-Based Organic Radical Contrast Agents for in†vivo Imaging of Gliomas.

Cyclodextrins (CDs), a class of cyclic oligosaccharides formed by α-(1,4) linked glucopyranose units, were functionalized with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radicals to prepare water soluble supramolecular organic radical contrast agents (ORCAs) for the in vivo detection of glioma tumor in animal models. A first set of molecules (CDn1, n=6,7,8 is the number of both TEMPO and glucopyranose units) was studied by superconducting quantum interference devices (SQUID) magnetometry in order to define the role of the CD macrocycle on the effective magnetic moment (µeff ). The µeff value increased from 3.982 µB (CD61) to 4.522 µB (CD81) but was limited by intramolecular antiferromagnetic (AF) interactions. A set of water-soluble ORCAs (CDn8, n=6,7,8) was prepared by a sequence of thiol-ene and Cu(I)-catalyzed alkyne-azide "click" reactions. Their 1 H water relaxivities r1 of these ORCAs were between 0.739 mM-1 s-1 (CD68) to 1.047 mM-1 s-1 (CD88) in D2 O/H2 O 9 : 1 (v : v) at 300 K. One of them (CD78) was tested on glioma-bearing rats with reduced side effects and good relaxivity in vivo.

Assessment of the Relaxation-Enhancing Properties of a Nitroxide-Based Contrast Agent TEEPO-Glc with In Vivo Magnetic Resonance Imaging.

Magnetic resonance imaging examinations are frequently carried out using contrast agents to improve the image quality. Practically all clinically used contrast agents are based on paramagnetic metals and lack in selectivity and specificity. A group of stable organic radicals, nitroxides, has raised interest as new metal-free contrast agents for MRI. Their structures can easily be modified to incorporate different functionalities. In the present study, a stable nitroxide TEEPO (2,2,6,6-tetraethylpiperidin-1-oxyl) was linked to a glucose moiety (Glc) to construct a water-soluble, potentially tumor-targeting compound with contrast-enhancing ability. The ability was assessed with in vivo MRI experiments. The constructed TEEPO-Glc agent proved to shorten the T 1 relaxation time in tumor, while the T 1 time in healthy brain tissue remained the same. The results indicate the potential of TEEPO-Glc as a valuable addition to the growing field of metal-free contrast enhancement in MRI-based diagnostics.