Efficient bioconjugation of 5-fluoro-5-deoxy-ribose (FDR) to RGD peptides for positron emission tomography (PET) imaging of alpha(v)beta(3) integrin receptor

The utility of 5-fluoro-5-deoxyribose (FDR) as an efficient bioconjugation agent for radiolabelling of the RGD peptides c(RGDfK) and c(RGDfC) is demonstrated. The bioconjugation is significantly superior to that achieved with 2-fluoro-2-deoxyglucose (FDG) and benefits from the location of the fluorine at C-5, and that ribose is a 5-membered ring sugar rather than a 6-membered ring. Both features favour ring opening to the aldehydic form of the sugar to promote smooth oxime ligation with aminooxy ether functionalised peptides. [F-18]FDR was prepared in this study by synthesis from fluoride-18 using an automated synthesis protocol adapting that used routinely for [F-18]FDG. c(RGDfK) was functionalised with an aminooxyacetyl group (Aoa) via its lysine terminus, while c(RGDfC) was functionalised with an aminooxyhexylmaleimide (Ahm) through a cysteine maleimide conjugation. Bioconjugation of [F-18]FDR to c(RGDfC)-Ahm proved to be more efficient than c(RGDfK)-Aoa (92% versus 65%). The unlabelled (F-19) bioconjugates c(RGDfK)-Aoa-FDR and c(RGDfC)-Ahm-FDR were prepared and their in vitro affinity to purified integrin alpha(v)beta(3) was determined. c(RGDfK)-Aoa-FDR showed the greater affinity. Purified "hot" bioconjugates c(RGDfK)-Aoa-[F-18]FDR and c(RGDfC)-Ahm-[F-18]FDR were assayed by incubation with MCF7, LNCaP and PC3 cell lines. In both cases the conjugated RGD peptides showed selectivity for PC3 cells, which express alpha(v)beta(3) integrin, with the c(RGDfK)-Aoa-[F-18]FDR demonstrating better binding, consistent with its higher in vitro affinity. The study demonstrates that [F-18]FDR is an efficient bioconjugation ligand for RGD bioactive peptides.