68Ga-labeled phage-display selected peptides as tracers for positron emission tomography imaging of Staphylococcus aureus biofilm-associated infections: Selection, radiolabelling and preliminary biological evaluation

Abstract

IntroductionStaphylococcus aureus is a major cause of skin and deep-sited infections, often associated with the formation of biofilms. Early diagnosis and initiated therapy is essential to prevent disease progression and to reduce complications that can be serious. Imaging techniques are helpful combining anatomical with functional data in order to describe and characterize site, extent and activity of the disease. The purpose of the study was to identify and 68Ga-label peptides with affinity for S. aureus biofilm and evaluate their potential as bacteria-specific positron emission tomography (PET) imaging agents. MethodsPhage-displayed dodecapeptides were selected using an in vitro grown S. aureus biofilm as target. One cyclic (A8) and two linear (A9, A11) dodecapeptides were custom synthesized with 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA) conjugated via a lysine linker (K), and for A11 also a glycine–serine–glycine spacer (GSG). The 68Ga-labeling of A8-K-DOTA, A9-K-DOTA, and A11-GSGK-DOTA were optimized and in vitro bacterial binding was evaluated for 68Ga-A9-K-DOTA and 68Ga-A11-GSGK-DOTA. Stability of 68Ga-A9-K-DOTA was studied in vitro in human serum, while the in vivo plasma stability was analyzed in mice and pigs. Additionally, the whole-body distribution kinetics of 68Ga-A9-K-DOTA was measured in vivo by PET imaging of pigs and ex vivo in excised mice tissues. ResultsThe 68Ga-A9-K-DOTA and 68Ga-A11-GSGK-DOTA remained stable in product formulation, whereas 68Ga-A8-K-DOTA was unstable. The S. aureus binding of 68Ga-A11-GSGK-DOTA and 68Ga-A9-K-DOTA was observed in vitro, though blocking of the binding was not possible by excess of cold peptide. The 68Ga-A9-K-DOTA was degraded slowly in vitro, while the combined in vivo evaluation in pigs and mice showed a rapid blood clearance and renal excretion of the 68Ga-A9-K-DOTA. ConclusionThe preliminary in vitro and in vivo studies of the phage-display S. aureus biofilm-selected 68Ga-A9-K-DOTA showed desirable features for a novel bacteria-specific imaging agent, despite of relative fast blood degradation in vivo.