Improved targeting of the αvβ3 integrin by multimerisation of RGD peptides

Purpose: The integrin αvβ3 is expressed on sprouting endothelial cells and on various tumour cell types. Due to the restricted expression of αvβ3 in tumours, αvβ3 is considered a suitable receptor for tumour targeting. In this study the αvβ3 binding characteristics of an 111In-labelled monomeric, dimeric and tetrameric RGD analogue were compared. Methods: A monomeric (E-c(RGDfK)), dimeric (E-[c(RGDfK)]2), and tetrameric (E{E[c(RGDfK)]2}2) RGD peptide were synthesised, conjugated with DOTA and radiolabelled with 111In. In vitro αvβ3 binding characteristics were determined in a competitive binding assay. In vivo αvβ3 targeting characteristics of the compounds were assessed in mice with SK-RC-52 xenografts. Results: The IC50 values for DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)]2, and DOTA-E{E[c(RGDfK)]2}2were 120 nM, 69.9 nM and 19.6 nM, respectively. At all time points, the tumour uptake of the dimer was significantly higher as compared to that of the monomer. At 8 h p.i., tumour uptake of the tetramer (7.40±1.12%ID/g) was significantly higher than that of the monomer (2.30±0.34%ID/g), p<0.001, and the dimer (5.17±1.22%ID/g), p<0.05. At 24 h p.i., the tumour uptake was significantly higher for the tetramer (6.82±1.41%ID/g) than for the dimer (4.22±0.96%ID/g), p<0.01, and the monomer (1.90±0.29%ID/g), p<0.001. Conclusion Multimerisation of c(RGDfK) resulted in enhanced affinity for αvβ3 as determined in vitro. Tumour uptake of a tetrameric RGD peptide was significantly higher than that of the monomeric and dimeric analogues, presumably owing to the enhanced statistical likelihood for rebinding to αvβ3