In vitro evaluation of radiolabeled amphotericin B for molecular imaging of mold infections

Invasive pulmonary aspergillosis and mucormycosis are life-threatening complications in immunocompromised patients. A rapid diagnosis followed by early antifungal treatment is essential for patient survival. Given the limited spectrum of biomarkers for invasive mold infections, recent studies have proposed radiolabeled siderophores or antibodies as molecular probes to increase the specificity of radiological findings by nuclear imaging modalities. While holding enormous diagnostic potential, most of the currently available molecular probes are tailored to the detection of Aspergillus species and their cost-intensive and sophisticated implementation restrict the accessibility at less specialized centers. In order to develop cost-efficient and broadly applicable tracers for pulmonary mold infections, this study established streamlined and high-yielding protocols to radiolabel amphotericin B (AMB) with the gamma-emitter technetium-99m (^99mTc-AMB) and the positron-emitter gallium-68 (⁶⁸Ga-AMB). Radiochemical purity of the resulting tracers consistently exceeded 99 % and both probes displayed excellent stability in human serum (> 98 % after 60-240 min at 37 °C). The uptake kinetics by representative mold pathogens were assessed in an in vitro Transwell^® assay using infected endothelial cell layers. Both tracers accumulated intensively and specifically in Transwell^® inserts infected with Aspergillus fumigatus, Rhizopus arrhizus, and other clinically relevant mold pathogens as compared with uninfected and bacterial controls. Inoculum-dependent enrichment was confirmed by gamma-counting and autoradiographic imaging. Taken together, this pilot in vitro study proposes ^99mTc-AMB and ⁶⁸Ga-AMB as facile, stable, and specific probes meriting further pre-clinical in vivo evaluation of radiolabeled amphotericin B for molecular imaging in invasive mycoses.