The diagnosis of invasive pulmonary aspergillosis (IPA) is difficult and lacks specificity and sensitivity. In the pathophysiology of Aspergillus fumigatus, iron plays an essential role as a nutrient during infection. A. fumigatus uses a specific and highly efficient iron uptake mechanism based on iron-complexing ferric ion Fe(III) siderophores, which are a requirement for A. fumigatus virulence. We aimed to evaluate the potential of siderophores radiolabeled with 68Ga, a positron emitter with complexing properties comparable to those of Fe(III), as a radiopharmaceutical for imaging IPA.
68Ga radiolabeling of the A. fumigatus siderophores desferri-triacetylfusarinine C (TAFC) and desferri-ferricrocin (FC) was performed at high specific activity. Stability, protein binding, and log P values were determined. In vitro uptake in A. fumigatus cultures was tested under varying conditions. Biodistribution was studied in healthy noninfected BALB/c mice, and uptake was studied in a model of A. fumigatus infection using immunosuppressed Lewis rats.
High-specific-activity 68Ga labeling could be achieved, and resulting complexes were stable in serum, toward diethylenetriaminepentaacetic acid and Fe(III) challenge. Both siderophores showed hydrophilic properties (68Ga-TAFC, log P = −2.59; 68Ga-FC, log P = −3.17) with low values of protein binding for 68Ga-TAFC (<2%). Uptake of both siderophores was highly dependent on the mycelial iron load and could be blocked with an excess (10 μM) of siderophore or NaN3, indicating specific, energy-dependent uptake. In noninfected mice, 68Ga-TAFC showed rapid renal excretion and low blood values (1.6 ± 0.37 percentage injected dose per gram [%ID/g] at 30 min); in urine only intact 68Ga-TAFC was detected. In contrast, 68Ga-FC revealed high retention in blood (16.1 ± 1.07 %ID/g at 90 min) and rapid metabolism. In the rat IPA model, lung uptake of 68Ga-TAFC was dependent on the severity of infection, with less than 0.04 %ID/g in control rats (n = 5) and 0.29 ± 0.11 %ID/g in mildly infected (n = 3) and 0.95 ± 0.37 %ID/g in severely infected (n = 4) rats. PET showed focal accumulation in infected lung tissue.
Both siderophores bound 68Ga with high affinity, and 68Ga-TAFC, especially, showed high stability. 68Ga-TAFC displayed highly selective accumulation by A. fumigatus subspecies in vitro and in vivo. The high and specific uptake by A. fumigatus proves the potential of 68Ga-labeled siderophores for the specific detection of A. fumigatus during infection. They hold promise as new PET agents for IPA.