The Development of Contrast Agents for PET/MRI and CatalyCEST MRI

Abstract

A class of MRI contrast agent, CEST MRI agents, has offered a new paradigm to image physiochemical functions and molecular biomarkers. 125 CEST MRI agents were evaluated regarding concentration, toxicity, saturation power and time, and chemical shift for clinical translation. Small molecule and macromolecule PARACEST agents generate CEST signal using lanthanide metals with low concentrations but high saturation powers and times. LipoCEST agents have lower concentration in vivo with low saturation powers and times. However, the chemical shifts of lipoCEST agents are lower that are similar to DIACEST agents and have toxicity similar to PARACEST agents. Transition metals have lower concentration and higher chemical shifts but these agents have not been assessed in vivo. DIACEST MRI agents using amino acids and metabolites avoid toxicity and concentration issues. These agents overlap with the bulk water signal and have variable saturation powers and times. Glucose, iopamidol, and iopromide have consistency with saturation powers and time with low toxicity but only have been assessed for uptake/metabolism and pH mapping studies. Salicylic acid based agents offer an option for clinical development due to the adaptability to enzyme response with low saturation powers and times. The unknown toxicity and high concentration may slow development for the clinic. We have developed an innovative imaging method, catalyCEST MRI, to detect the activity of reductase enzymes within in vivo mouse tumor models. This methodology creates CEST by selectively saturating the magnetic resonance of a proton on the agent which transfers the saturation effect to water through a chemical exchange. With catalyCEST MRI, the enzyme catalyzes a change in the chemical structure of the agent and the change in CEST is monitored. Additionally, a control CEST signal unresponsive to enzymatic catalysis is also monitored to improve quantitative imaging. We have synthesized a carbamate catalyCEST agent designed to detect nitroreductase activitites. Derived from salicylic acid, this agent has the potential to be translated for clinical studies. This report will present the catalyCEST MRI mechanism for the detection of reductase enzyme activity and progression towards in vivo studies with mouse tumor models. Several Gd3+-based pH-sensitive contrast agents that change T1-weighted MRI contrast have been synthesized but have been limited to in vitro studies. A previously described pH sensitive 4AMP-Gd3+-DOTA contrast agent was utilized to map the extracellular tissue pH in a rat glioma model. Additionally, the 4AMP-Gd3+-DOTA contrast agent has been conjugated with an 18F-fluorine analogue and demonstrated to quantitatively measure pH in phantom models as a bimodal PET/MRI contrast agent. In this study, we describe the synthesis of a 4AMP-Gd3+-DOTA derivative conjugated with FDG through functionalization of an α-amino-DOTA and subsequent glycosylation with commercially-available 18F-FDG. The efficacy of the synthesis route may provide an alternative to the design and development of a bimodal pH quantitative PET/MRI contrast agent. The availability and half-life of 18F analogues may provide synthesis challenges to offer suitable yields for a bimodal contrast agents for quantitative pH studies. Our alternative synthesis described in this study provides a possible solution to the synthesis limitations utilizing a facile functionalization of α-amino-DOTA and commercially-available 18F-FDG. We envision the synthesis route will directly contribute towards the design and development of a potential PET/MRI bimodal contrast agent that accurately measures pH for tumor detection and diagnostics. This synthesis method can be used to label other biomolecules with 18F-FDG, facilitating radiochemistry at locations that do not have a cyclotron but have access to 18F-FDG. The reduction of a nitroaromatic catalyCEST agent for monitoring NTR activity was used to evaluate hypoxic conditions in the tumor microenvironment. Several nitroaromatics are proposed to be conjugated to 4-ASA as alternatives to the agent presented. Quinone-based agents are also discussed as compounds to monitor DT-diaphorase activity and CEST agents for theranostics are suggested as compounds for dual chemotherapy and hypoxia imaging. In addition to catalyCEST agents, several compounds and reactions are discussed for further developing the PET/MRI agents. Alternative peripheal groups are considered for measuring tumor pHe. T2ex and PARACEST MRI agents are reviewed as possible agents for bimodal agents with PET. Finally, Click chemistry, Suzuki coupling, and nucleophilic substitution are addressed as alternate methods for radiolabeling with an 18F-fluorine moiety with higher efficiency than glycosylation with 18F-FDG.