Synthesis and evaluation of a 99mTechnetium labeled chitin-binding protein as potential specific radioligand for the detection of fungal infections in mice.

AIM: Radiopharmaceuticals can be used to exploit differences between microorganisms in order to distinguish fungal from bacterial infection. Chitin, abundant in the cell wall of fungi, is not present in mammalian or bacterial cells and therefore represents a highly specific target to localize fungal infection. In this study, we have examined the potential of chitin-binding protein (CBP21) from Serratia marcescens as a specific radiotracer for the detection of invasive fungal infections. METHODS: CBP21 was labeled with 99mTc via hydrazinonicotinamide (HYNIC) and its characteristics were analyzed. In vitro binding studies with polymorphic chitin forms and microorganisms (fungi as well as bacteria) were performed. In vivo biodistribution of the compound was studied in immunocompromised mice with bacterial and fungal infections in the left and right thigh muscle, respectively, using 99mTc-HYNIC-myoglobin as size-matched control and 67Ga-citrate as positive control. Scintigraphic images were acquired at 1 and 7 h postinjection of the tracer. RESULTS: 99mTc-HYNIC-CBP21 was labeled with a radiochemical yield of 61% and a specific activity of 22.3 MBq/nmol. Highest in vitro binding percentages were found with beta-chitin (86.8+/-2.4%). Binding interactions to fungi were higher than to bacteria (P<0.05). In vivo, best ratios of fungal infection versus bacterial infection were seen at 5 and 7 h (3.6+/-1.2 and 2.9+/-1.4, respectively) postinjection of the tracer. Maximum uptake of the tracer in fungal infections (0.63+/-0.11%ID/g) at 7 h was significantly (P<0.05) higher than uptake seen in bacterial infections (0.34+/-0.11%ID/g) or the uptake of 99mTc-HYNIC-myoglobin (P<0.05) in the same infections (0.35+/-0.11%ID/g, respectively, 0.3+/-0.01%ID/g). CONCLUSIONS: This study shows that 99mTc-HYNIC-CBP21 is able to specifically interact with chitin in vitro. Scintigraphy and postmortem in vivo data indicate that 99mTc-HYNIC-CBP21 is able to distinguish fungal infection from bacterial infection probably due to a specific interaction of the protein with the chitin in the fungal cell wall