In vivo characterisation of a therapeutically relevant self-assembling ¹⁸F-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 ¹⁸F-labelled and fluoresceinisothiocyanate (FITC) - labelled F9 analogues. F9 was site-specifically radiolabelled with 2-[18F]fluoro-3-pyridinecarboxaldehyde ([¹⁸F]FPCA) via oxime bond formation. [¹⁸F]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. [¹⁸F]FPCA-F9 demonstrated high plasma stability and primarily renal excretion; [¹⁸F]FPCA-F9 when in solution and injected into the bloodstream displayed prompt bladder uptake (53.4 ± 16.6 SUV at 20 minutes post injection) and rapid renal excretion, whereas [18F]FPCA-F9 hydrogel, formed by co-assembly of [¹⁸F]FPCA-F9 monomer with unfunctionalised F9 peptide and injected subcutaneously, showed gradual bladder accumulation of hydrogel fragments (3.8 ± 0.4 SUV at 20 minutes post injection), 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 96 hours), indicating a biological half-life between 1-4 days. The in vivo characterisation of F9, both as a gel and a solution highlights its potential as a biomaterial.