title: Effects of linker variation on the in vitro and in vivo characteristics of an 111In-labeled RGD peptide.
abstract: BACKGROUND
  Due to the selective expression of the alpha(v)beta3 integrin in tumors, radiolabeled arginine-glycine-aspartic acid (RGD) peptides are attractive candidates for tumor targeting. Minor modifications of these peptides could have a major impact on in vivo characteristics. In this study, we systematically investigated the effects of linker modification between two cyclic RGD sequences and DOTA (1,4,7,10-tetraazadodecane-N,N',N",N'''-tetraacetic acid) on the in vitro and in vivo characteristics of the tracer.
  
  
  METHODS
  A dimeric RGD peptide was synthesized and conjugated either directly with DOTA or via different linkers: PEG4 (polyethylene glycol), glutamic acid or lysine. The RGD peptides were radiolabeled with 111In, and their in vitro and in vivo alpha(v)beta3-binding characteristics were determined.
  
  
  RESULTS
  LogP values varied between -2.82+/-0.06 and -3.95+/-0.33. The IC50 values for DOTA-E-[c(RGDfK)]2, DOTA-PEG4-E-[c(RGDfK)]2, DOTA-E-E-[c(RGDfK)]2 and DOTA-K-E-[c(RGDfK)]2 were comparable. Two hours after injection, the tumor uptakes of the 111In-labeled compounds were not significantly different. The kidney accumulation of [111In]-DOTA-K-E-[c(RGDfK)]2 [4.05+/-0.20% of the injected dose per gram (ID/g)] was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (2.63+/-0.19% ID/g; P<.05) as well as that of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.16+/-0.21% ID/g; P<.01). The liver uptake of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.12+/-0.09% ID/g) was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (1.64+/-0.1% ID/g; P<.05) as well as that of [111In]-DOTA-K-E-[c(RGDfK)]2 (1.52+/-0.04% ID/g; P<.01).
  
  
  CONCLUSIONS
  Linker variation did not affect affinity for alpha(v)beta3 and tumor uptake. Insertion of lysine caused enhanced kidney retention; that of glutamic acid also resulted in enhanced retention in the kidneys. PEG4 appeared to be the most suitable linker as compared with glutamic acid and lysine because it has the highest tumor-to-blood ratio and the lowest uptake in the kidney and liver.
authors.affiliation: Department of Nuclear Medicine, Radboud University Nijmegen Medical Center, PO Box 9101, HB Nijmegen 6500, The Netherlands. i.dijkgraaf@nucmed.umcn.nl
authors.forename: Ingrid
authors.initials: I
authors.lastname: Dijkgraaf
authors.forename: Shuang
authors.initials: S
authors.lastname: Liu
authors.forename: John A W
authors.initials: JA
authors.lastname: Kruijtzer
authors.forename: Annemieke C
authors.initials: AC
authors.lastname: Soede
authors.forename: Wim J G
authors.initials: WJ
authors.lastname: Oyen
authors.forename: Rob M J
authors.initials: RM
authors.lastname: Liskamp
authors.forename: Frans H M
authors.initials: FH
authors.lastname: Corstens
authors.forename: Otto C
authors.initials: OC
authors.lastname: Boerman
mesh_terms: D000818:Animals; D002292:Carcinoma, Renal Cell / Q000000981:diagnostic imaging* / Q000378:metabolism*; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000737:chemistry; D003432:Cross-Linking Reagents / Q000737:chemistry; D005260:Female; D006573:Heterocyclic Compounds, 1-Ring / Q000493:pharmacokinetics; D006801:Humans; D007205:Indium Radioisotopes / Q000493:pharmacokinetics*; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D009842:Oligopeptides / Q000737:chemistry / Q000493:pharmacokinetics*; D009928:Organ Specificity; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D014018:Tissue Distribution
chemical_list: D002614:Chelating Agents; D003432:Cross-Linking Reagents; D006573:Heterocyclic Compounds, 1-Ring; D007205:Indium Radioisotopes; D039302:Integrin alphaVbeta3; D009842:Oligopeptides; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; C047981:arginyl-glycyl-aspartic acid