Positive imaging of venous thrombi and thromboemboli with Ga-67 DFO-DAS-fibrinogen

Siderophores for molecular imaging applications

This review covers publications on siderophores applied for molecular imaging applications, mainly for radionuclide-based imaging. Siderophores are low molecular weight chelators produced by bacteria and fungi to scavenge essential iron. Research on these molecules has a continuing history over the past 50 years. Many biomedical applications have been developed, most prominently the use of the siderophore desferrioxamine (DFO) to tackle iron overload related diseases. Recent research described the upregulation of siderophore production and transport systems during infection. Replacing iron in siderophores by radionuclides, the most prominent Ga-68 for PET, opens approaches for targeted imaging of infection; the proof of principle has been reported for fungal infections using ⁶⁸Ga-triacetylfusarinine C (TAFC). Additionally, fluorescent siderophores and therapeutic conjugates have been described and may be translated to optical imaging and theranostic applications. Siderophores have also been applied as bifunctional chelators, initially DFO as chelator for Ga-67 and more recently for Zr-89 where it has become the standard chelator in Immuno-PET. Improved DFO constructs and bifunctional chelators based on cyclic siderophores have recently been developed for Ga-68 and Zr-89 and show promising properties for radiopharmaceutical development in PET. A huge potential from basic biomedical research on siderophores still awaits to be utilized for clinical and translational imaging.

Radiopharmaceuticals for thrombus detection

Most of the components of the thrombotic and fibrinolytic systems have at some time been evaluated as a means of carrying a radiolabel specifically to thrombi, although very few have been promising enough to emerge from investigational status to routine clinical use. New approaches are being explored, including improved methods of labeling platelets, chemically modified forms of previously tested plasma proteins, and new biomolecules, including monoclonal antibodies specific for fibrin and platelets. The current goal is to find one or more radiotracers that bind specifically and rapidly to thrombi, and that also have a rapid blood disappearance rate, permitting a clear diagnosis within a few hours after injection. Because this test may be needed to assess the course of therapy in an anticoagulated patient, the ideal radiopharmaceutical should be able to locate thrombi without interference by anticoagulants. Until a suitable thrombus-specific radiopharmaceutical becomes generally available, many hospitals will continue to attempt to make a diagnosis with nonspecific radiopharmaceuticals that can at best provide blood pool images to indicate filling defects. Several of the new approaches seem likely to provide the radiopharmaceutical sought, although clinical trials are at an early stage.