PET imaging of Aspergillus infection using Zirconium-89 labeled anti-ß-glucan antibody fragments.

PURPOSE: Invasive fungal diseases, such as pulmonary aspergillosis, are common life-threatening infections in immunocompromised patients and effective treatment is often hampered by delays in timely and specific diagnosis. Fungal-specific molecular imaging ligands can provide non-invasive readouts of deep-seated fungal pathologies. In this study, the utility of antibodies and antibody fragments (Fab) targeting ß-glucans in the fungal cell wall to detect Aspergillus infections was evaluated both in vitro and in preclinical mouse models. METHODS: The binding characteristics of two commercially available ß-glucan antibody clones and their respective antigen-binding Fabs were tested using biolayer interferometry (BLI) assays and immunofluorescence staining. In vivo binding of the Zirconium-89 labeled antibodies/Fabs to fungal pathogens was then evaluated using PET/CT imaging in mouse models of fungal infection, bacterial infection and sterile inflammation. RESULTS: One of the evaluated antibodies (HA-ßG-Ab) and its Fab (HA-ßG-Fab) bound to ß-glucans with high affinity (KD = 0.056 & 21.5 nM respectively). Binding to the fungal cell wall was validated by immunofluorescence staining and in vitro binding assays. ImmunoPET imaging with intact antibodies however showed slow clearance and high background signal as well as nonspecific accumulation in sites of infection/inflammation. Conversely, specific binding of [89Zr]Zr-DFO-HA-ßG-Fab to sites of fungal infection was observed when compared to the isotype control Fab and was significantly higher in fungal infection than in bacterial infection or sterile inflammation. CONCLUSIONS: [89Zr]Zr-DFO-HA-ßG-Fab can be used to detect fungal infections in vivo. Targeting distinct components of the fungal cell wall is a viable approach to developing fungal-specific PET tracers.

Evaluation of 2-[18F]-Fluorodeoxysorbitol PET Imaging in Preclinical Models of Aspergillus Infection.

Despite increasing associated mortality and morbidity, the diagnosis of fungal infections, especially with Aspergillus fumigatus (A. fumigatus), remains challenging. Based on known ability of Aspergillus species to utilize sorbitol, we evaluated 2-[18F]-fluorodeoxysorbitol (FDS), a recently described Enterobacterales imaging ligand, in animal models of A. fumigatus infection, in comparison with 2-[18F]-fluorodeoxyglucose (FDG). In vitro assays showed slightly higher 3H-sorbitol uptake by live compared with heat-killed A. fumigatus. However, this was 10.6-fold lower than E. coli uptake. FDS positron emission tomography (PET) imaging of A. fumigatus pneumonia showed low uptake in infected lungs compared with FDG (0.290 ± 0.030 vs. 8.416 ± 0.964 %ID/mL). This uptake was higher than controls (0.098 ± 0.008 %ID/mL) and minimally higher than lung inflammation (0.167 ± 0.007 %ID/mL). In the myositis models, FDS uptake was highest in live E. coli infections. Uptake was low in A. fumigatus myositis model and only slightly higher in live compared with the heat-killed side. In conclusion, we found low uptake of 3H-sorbitol and FDS by A. fumigatus cultures and infection models compared with E. coli, likely due to the need for induction of sorbitol dehydrogenase by sorbitol. Our findings do not support FDS as an Aspergillus imaging agent. At this point, FDS remains more selective for imaging Gram-negative Enterobacterales.