68Ga-siderophores for diagnosis of aspergillosis with PET

Introduction: Invasive Aspergillosis (IA) is a major cause of death in immunocompromised patients, that have increased tremendously over the last years. Early diagnosis could be live saving, but is difficult to achieve and lacks specificity and sensitivity, even though considerably research efforts have been made to develop suitable diagnostic agents. In the pathophysiology of Aspergillus fumigatus (A.f.) iron plays an essential role during infection. A.f. employs a specific and highly efficient iron transporter mechanism based on iron complexing siderophores, mainly Triacetylfusarenine (TAFC) and Ferricrocin (FC). Recent findings have revealed that this siderophore system is upregulated in A.f. during infection and is essential for A.f. virulence. It is known that Fe(III) can be replaced by gallium having similar binding affinity for a variety of siderophores. 68Ga is a generator based radio-nuclide with daily availibility that can be utilized for Positron Emission-Tomography (PET). Preliminary studies: In first preliminary studies we have chosen TAFC and FC as model peptide siderophores for 68Ga labelling. Both siderophores could be labelled at high specific activities and high purity as shown in HPLC studies, the 68Ga complexes showed high hydrophilicity and good solution stability. Uptake by A.f. was highly dependent on iron load, under iron deficient conditions high uptake was observed in a specific, energy dependent manner for both 68Ga labelled siderophores. In normal mice especially 68Ga TAFC showed showed rapid renal excretion and low blood values even short time after injection (1,6%ID/g 30min), in urine only intact 68Ga-TAFC was detected. In a rat aspergillosis model higher lung uptake in infected vs non infected rats was found. Overall these studies showed that 68Ga can be used to label siderophores with high stability and they show very promising properties in terms of target uptake and pharmacokinetics. Aim and workplan: The aim of the current project is to translate the initial findings into a potential agent for early and specific diagnosis of IA using PET. Within two years of work 6 working packages will adress the follwing issues: Selection and characterisation of the ideal 68Ga siderophore by 68Ga labelling and stability studies, uptake and relase assays in A.f. and evaluation of pharmacokinetics in normal mice. Targeting will be evaluated by first optimization of a rat aspergillosis model followed by evaluation of selected 68Ga-siderophores concering target uptake and time after infection. Selectivity and specificity will be tested in uptake studies with other microorganisms and challenging studies with other (radio)metals. Outcome: This project aims to answer whether any 68Ga siderophore can be used for early diagnosis of IA with sufficient sensitivity and specificity. Further strategies for dissemination of results, to ensure intellectual property rights and to enter clinical trials are envisaged.