New Concepts in Molecular Imaging: Non-Invasive MRI Spotting of Proteolysis Using an Overhauser Effect Switch

Abstract

BackgroundProteolysis, involved in many processes in living organisms, is tightly regulated in space and time under physiological conditions. However deregulation can occur with local persistent proteolytic activities, e.g. in inflammation, cystic fibrosis, tumors, or pancreatitis. Furthermore, little is known about the role of many proteases, hence there is a need of new imaging methods to visualize specifically normal or disease-related proteolysis in intact bodies.Methodology/Principal FindingsIn this paper, a new concept for non invasive proteolysis imaging is proposed. Overhauser-enhanced Magnetic Resonance Imaging (OMRI) at 0.2 Tesla was used to monitor the enzymatic hydrolysis of a nitroxide-labeled protein. In vitro, image intensity switched from 1 to 25 upon proteolysis due to the associated decrease in the motional correlation time of the substrate. The OMRI experimental device used in this study is consistent with protease imaging in mice at 0.2 T without significant heating. Simulations show that this enzymatic-driven OMRI signal switch can be obtained at lower frequencies suitable for larger animals or humans.Conclusions/SignificanceThe method is highly sensitive and makes possible proteolysis imaging in three dimensions with a good spatial resolution. Any protease could be targeted specifically through the use of taylor-made cleavable macromolecules. At short term OMRI of proteolysis may be applied to basic research as well as to evaluate therapeutic treatments in small animal models of experimental diseases.

Overhauser-Enhanced MRI of Elastase Activity from In Vitro Human Neutrophil Degranulation

“…Gradients maximum intensity was 15 mT m −1 . Electron spin excitation was induced by a resonant hyperfrequency (HF) cylindrical cavity (Bruker, Wissembourg, France) running in TE011 mode, positioned at the center of the MRI magnet bore [4]. Its geometry (240 mm diameter, 29 mm width) was designed to reduce the electric component of the HF Field in its center, thus minimizing sample heating upon microwave emission.…”

“…Thus the trend has turned towards the molecular imaging approach to localize such processes. This branch potentially gives access to antigens and receptors targeting or to biochemical activities like the redox status [1], [2] and enzyme activity[3], [4].…”

“…The concept of taking advantage of the proteolytic activity for magnetic resonance imaging (MRI) has been explored by several authors either using gadolinium derivatives [12], [13] or Overhauser magnetic resonance imaging (OMRI) [4]. OMRI is a double resonance experiment establishing a polarization transfer from the free electron of a stable free radical to the surrounding water protons.…”

“…Furthermore the involvement of a free radical is favorable to molecular imaging since the free electron is very sensitive to various changes in its environment: redox status [15], pH, molecular rotational correlation time [4]. In a previous paper we showed that the proteolytic hydrolysis of nitroxide-labeled bovine serum albumin could be followed in vitro by OMRI with the generation of high contrast[4].…”

Alkoxyamines: a new family of pro-drugs against cancer. Concept for theranostics

“…18 Applied to neutrophile degranulation, this method proved capable of detecting faint elastolytic activity ex vivo. 18 In vivo detection of proteolysis based upon OMRI detection of nitroxides would rely on the generation of nitroxide faster than its elimination processes (bio-reduction, physiological clearance, etc.) to ensure a local nitroxide concentration high enough to afford an efficient MRI acquisition of this biological process.…”