68Ga-siderophores for PET imaging of invasive pulmonary aspergillosis: proof of principle.
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 (68)Ga, a positron emitter with complexing properties comparable to those of Fe(III), as a radiopharmaceutical for imaging IPA.


METHODS
(68)Ga 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.


RESULTS
High-specific-activity (68)Ga labeling could be achieved, and resulting complexes were stable in serum, toward diethylenetriaminepentaacetic acid and Fe(III) challenge. Both siderophores showed hydrophilic properties ((68)Ga-TAFC, log P = -2.59; (68)Ga-FC, log P = -3.17) with low values of protein binding for (68)Ga-TAFC (<2%). Uptake of both siderophores was highly dependent on the mycelial iron load and could be blocked with an excess (10 microM) of siderophore or NaN(3), indicating specific, energy-dependent uptake. In noninfected mice, (68)Ga-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 (68)Ga-TAFC was detected. In contrast, (68)Ga-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 (68)Ga-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.


CONCLUSIONS
Both siderophores bound (68)Ga with high affinity, and (68)Ga-TAFC, especially, showed high stability. (68)Ga-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 (68)Ga-labeled siderophores for the specific detection of A. fumigatus during infection. They hold promise as new PET agents for IPA.
Petrik
Milos
M
Clinical Department of Nuclear Medicine, Innsbruck Medical University, Innsbruck, Austria.
Haas
Hubertus
H
Dobrozemsky
Georg
G
Lass-Flörl
Cornelia
C
Helbok
Anna
A
Blatzer
Michael
M
Dietrich
Hermann
H
Decristoforo
Clemens
C
D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D001692:Biological Transport; D019317:Evidence-Based Medicine; D005260:Female; D005710:Gallium Radioisotopes / Q000737:chemistry; D055744:Invasive Pulmonary Aspergillosis / Q000000981:diagnostic imaging* / Q000378:metabolism; D007553:Isotope Labeling; D051379:Mice; D049268:Positron-Emission Tomography / Q000379:methods*; D051381:Rats; D017262:Siderophores / Q000737:chemistry* / Q000378:metabolism / Q000493:pharmacokinetics; D014018:Tissue Distribution
D023181:Antimicrobial Cationic Peptides; D005710:Gallium Radioisotopes; D017262:Siderophores; C015790:desferri-ferricrocin