title: Improved targeting of the alpha(v)beta (3) integrin by multimerisation of RGD peptides.
abstract: OBJECTIVE
  The integrin alpha(v)beta(3) is expressed on sprouting endothelial cells and on various tumour cell types. Due to the restricted expression of alpha(v)beta(3) in tumours, alpha(v)beta(3) is considered a suitable receptor for tumour targeting. In this study the alpha(v)beta(3) binding characteristics of an (111)In-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 (111)In. In vitro alpha(v)beta(3) binding characteristics were determined in a competitive binding assay. In vivo alpha(v)beta(3) targeting characteristics of the compounds were assessed in mice with SK-RC-52 xenografts.
  
  
  RESULTS
  The IC(50) values for DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)](2), and DOTA-E{E[c(RGDfK)](2)}(2)were 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.
  
  
  CONCLUSIONS
  Multimerisation of c(RGDfK) resulted in enhanced affinity for alpha(v)beta(3) 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 alpha(v)beta(3).
authors.affiliation: Department of Nuclear Medicine, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands. I.Dijkgraaf@nucmed.umcn.nl
authors.forename: Ingrid
authors.initials: I
authors.lastname: Dijkgraaf
authors.forename: John A W
authors.initials: JA
authors.lastname: Kruijtzer
authors.forename: Shuang
authors.initials: S
authors.lastname: Liu
authors.forename: Annemieke C
authors.initials: AC
authors.lastname: Soede
authors.forename: Wim J G
authors.initials: WJ
authors.lastname: Oyen
authors.forename: Frans H M
authors.initials: FH
authors.lastname: Corstens
authors.forename: Rob M J
authors.initials: RM
authors.lastname: Liskamp
authors.forename: Otto C
authors.initials: OC
authors.lastname: Boerman
mesh_terms: D000818:Animals; D019281:Dimerization; D016503:Drug Delivery Systems / Q000379:methods*; D005260:Female; D007205:Indium Radioisotopes / Q000493:pharmacokinetics; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D051379:Mice; D008819:Mice, Nude; D009374:Neoplasms, Experimental / Q000000981:diagnostic imaging* / Q000378:metabolism*; D009842:Oligopeptides / Q000493:pharmacokinetics*; D009928:Organ Specificity; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D014018:Tissue Distribution
chemical_list: D007205:Indium Radioisotopes; D039302:Integrin alphaVbeta3; D009842:Oligopeptides; D019275:Radiopharmaceuticals; C047981:arginyl-glycyl-aspartic acid