Overhauser Enhanced Magnetic Resonance Imaging (OMRI)

Division Of Basic Sciences - Nci

Investigators

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Abstract

Project Summary: Imaging Aminiopeptidase activity in vivo metabolic 13C MRI. Aminopeptidase N (APN), also known as CD13, is an essential enzyme involved in tumor angiogenesis and endothelial cell migration. Elevated APN expression is associated with more aggressive cancer phenotypes, making it a promising biomarker for tumor activity and a potential target for therapeutic intervention. Therefore, we rationally designed an Ala-[1- 13C]Gly-d-NMe 2 2 probe as known as AGN probe for dissolution dynamic nuclear polarization (dDNP). An AGN probe allows real-time monitoring of enzymatic activity by measuring the conversion of the area under the curve (AUC) ratio of probe-to-product with hyperpolarized magnetic resonance spectroscopy (MRS). This quantitative approach provides valuable insights into the APN enzymatic state of tumors in vivo. Despite its promising applications, the role of the AGN probe in cancer metabolism remains unclear. To better understand the mechanisms of the AGN probe, in this study, we performed multimodal imaging assessments with an anti-cancer drug. Sunitinib is widely used as an anti-angiogenic drug in tumors as a multi-targeted tyrosine kinase inhibitor. Hence, this drug is suitable to validate the physiological characteristics of AGN probes in vivo for angiogenesis. Dynamic contrast-enhanced MRI (DCE-MRI) assesses blood perfusion, diffusion-weighted imaging (DWI) evaluates tumor cellularity, and electron paramagnetic resonance (EPR) imaging measures tissue oxygenation. Our primary goal is to confirm the relationship between APN activity and conversion rate of the AGN probe-to-product across tumor types with varying APN expression and to verify physiological characteristics, and spatial distributions of metabolisms in vivo with other imaging modalities. We used hyperpolarized 13C MRS and EPSI to measure APN activity in tumors with varying APN expression. DCE-MRI, DWI, and EPR imaging to evaluate several tumor physiological parameters. A positive correlation was observed between APN protein levels and the AUC ratio of AGN probe-to-product. Hyperpolarized 13C MRS detected distinct probe and product peaks in both HT-29 and HT-1080 tumors. Following sunitinib treatment, Echo Planar Spectroscopic Imaging (EPSI) of hyperpolarized 13C AGN on xenograft tumors indicated drastic changes in the distributions of AGN product/probe on sunitinib treatment. The conversion of AGN product/probe was transferred to the rim region of the tumor from the center region of the tumor following the treatment of sunitinib. The product peaks were significantly decreased with the treatment of sunitinib on quantitative assessments. DCE-MRI revealed the differences in permeability of Gd contrast with the K trans map before and after treatment of sunitinib. Quantitative assessment showed a significant reduction in mean K trans trans, indicating decreased blood perfusion, while median K remained unchanged with the treatment of sunitinib . ADC derived from DWI showed low signal intensity that matched the tumor region. Quantitative assessment of ADC showed no significant changes in either mean or median ADC values . Quantitative assessment of EPRI demonstrated no significant changes in mean tumor pO2 and hypoxic fraction. Our results highlight the hyperpolarized AGN probe as a promising non-invasive tool to monitor APN activity in tumors. The probe successfully distinguished between high APN-expressing HT-1080 tumors and low APN-expressing HT-29 tumors, demonstrating its potential to evaluate tumor aggressiveness and guide treatment decisions. DCE-MRI and EPRI data showed HT-1080 tumors with low permeability and low pO2 lesions. Directly Monitoring the Dynamic In Vivo Metabolisms of Hyperpolarized 13C Glutathione with Higher Molecular Weights by Breaking Intrinsic 13C T1 Boundaries. Hyperpolarized MRI is a cutting-edge MR methodology to interrogate in vivo metabolism non-invasively. This technology has been intrinsically limited for molecules with smaller molecular weights(M.W.~ 200) due to the limitation related to T1 spin-lattice relaxation times. Therefore, clinically promising peptide-based in vivo hyperpolarized probes with larger molecular weights have not been possible due to their short T1s. Here, we demonstrate our recent rationally designed structural framework successfully enables us to directly observe in vivo metabolic activities of a tripeptide, glutathione (M.W. 300~), which plays a central role in cellular metabolism and redox status. The newly developed hyperpolarized 13C-GSH can report site-specific enzymatic activities in vivo and monitor treatment responses on tumor xenografts successfully. 13C-GSH enabled us to monitor in vivo enzymatic activities of g-glutamyl transpeptidase (GGT), and dipeptidases. The optimized sample conditions of HP13C-GSH shows excellent polarization and T1 relaxation time of 58±3 seconds in H2O on 3T MRI. In addition to its central roles in various metabolic activities, the biocompatibility of the 13C-GSH is exceptional as the intracellular GSH in mammalian cells is in the millimolar range5. Above mentioned MR properties of the newly developed 13C-GSH where in vivo and hyperpolarized 13C-GSH MRI can successfully monitor real-time GGT activities in various parts of the healthy mice bodies, and provide us site-specific enzymatic activities on MR spectroscopic imaging (n=3). The obtained results are consistent with the reported information that GGT is abundantly expressed in tissues with secretory or absorptive functions, including kidneys and liver regions5. The MRSI results indicate that a highly selective GGT inhibitor, GGsTop (25mg/kg, i.v.), specifically inhibits its enzymatic activities effectively. Novel catalysts for hyperpolarization of tracers for metabolic imaging: Hyperpolarization of 13C labeled metabolic tracers enables imaging by significantly enhancing the sensitivity of 13C MRI and allow mapping enzymatic fluxes relevant to study the biochemical profiles of tissue/tumor. While dissolution dynamic nuclear polarization (dDNP) has allowed preclinical studies which were translated clinically, the cost associated with the methodology has not allowed widespread dissemination. Recent studies with parahydrogen induced polarization (PHIP) to enhance sensitivity has made this approach promising given the low cost and simpler approach. The one aspect which needs to be resolved is to remove the Iiridium (Ir) metal catalyst to less than parts per billion (ppb) level. To mitigate Ir contamination, we introduce a novel iteration of the SABRE catalyst, incorporating bis(polyfluoroalkylated) imidazolium salts. This novel perfluorinated SABRE catalyst retained polarization properties while exhibiting an enhanced hydrophobicity. This modification allows the easy removal of the perfluorinated SABRE catalyst from HP[1-13C]-pyruvate after polarization in an aqueous solution. The residual Ir content after removal was measured via ICP-MS at 177 ppb, which is the lowest reported to date for Pyruvate and is sufficiently safe for use in clinical investigations.