In vivo characterisation of a therapeutically relevant self-assembling 18 F-labelled β-sheet forming peptide and its hydrogel using positron emission tomography.

Positron emission tomography (PET) and fluorescence labelling have been used to assess the pharmacokinetics, biodistribution and eventual fate of a hydrogel-forming nonapeptide, FEFKFEFKK (F9), in healthy mice, using F-18-labelled and fluorescein isothiocyanate (FITC)-labelled F9 analogues. F9 was site-specifically radiolabelled with 2-[F-18]fluoro-3-pyridinecarboxaldehyde ([F-18]FPCA) via oxime bond formation. [F-18]FPCA-F9 in vivo fate was evaluated both as a solution, following intravenous administration, and as a hydrogel when subcutaneously injected. The behaviour of FITC-F9 hydrogel was assessed following subcutaneous injection. [F-18]FPCA-F9 demonstrated high plasma stability and primarily renal excretion; [F-18]FPCA-F9 when in solution and injected into the bloodstream displayed prompt bladder uptake (53.4 +/- 16.6 SUV at 20minutes postinjection) and rapid renal excretion, whereas [F-18]FPCA-F9 hydrogel, formed by co-assembly of [F-18]FPCA-F9 monomer with unfunctionalised F9 peptide and injected subcutaneously, showed gradual bladder accumulation of hydrogel fragments (3.8 +/- 0.4 SUV at 20minutes postinjection), resulting in slower renal excretion. Gradual disaggregation of the F9 hydrogel from the site of injection was monitored using FITC-F9 hydrogel in healthy mice (60 +/- 3 over 96hours), indicating a biological half-life between 1 and 4days. The in vivo characterisation of F9, both as a gel and a solution, highlights its potential as a biomaterial.