Improved targeting of the αvβ3 integrin by multimerisation of RGD peptides

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. 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. 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. 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. (orig.)