Harnessing the Maltodextrin Transport Mechanism for Targeted Bacterial Imaging: Structural Requirements for Improved invivo Stability in Tracer Design

Diagnosis and localization of bacterial infections remains a significant clinical challenge. Harnessing bacteria-specific metabolic pathways, such as the maltodextrin transport mechanism, may allow specific localization and imaging of small or hidden colonies. This requires that the intrabacterial tracer accumulation provided by the transporter is matched by high serum stability of the tracer molecule. Herein, radiolabeled maltodextrins of varying chain lengths and with free nonreducing/reducing ends are reported and their behavior against starch-degrading enzymes in the blood, which compromise their serum stability, is evaluated. Successful single-photon emission computed tomography (SPECT) imaging is shown in a footpad infection model invivo by using the newly developed model tracer, [Tc-99m]MB1143, and the signal is compared with that of F-18-fluorodeoxyglucose positron emission tomography ([F-18]FDG-PET) as a nonbacterial specific marker for inflammation. Although the [Tc-99m]MB1143 imaging signal is highly specific, it is low, most probably due to insufficient serum stability of the tracer. A series of stability tests with different F-18-labeled maltodextrins finally yielded clear structural guidelines regarding substitution patterns and chain lengths of maltodextrin-based tracers for nuclear imaging of bacterial infections.