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Radiolabeled cyclic peptides containing the (Arg-Gly-Asp) RGD sequence for use in positron emission tomography (PET) imaging, single-photon emission computed tomography (SPECT) imaging, and targeted radionuclide therapy of cancer have been reported. In this study, RGD was used as a model carrier peptide for diagnosis and therapy of cancer. To evaluate the characteristics of radiohalogen-labeled peptides, several kinds of labeled RGD peptides [ 125 I-c(RGDyK), 77 Br-c(RGDyK), [ 125 I]SIB-c(RGDfK), [ 77 Br]SBrB-c(RGDfK), [ 125 I]SIB-EG 2 -c(RGDfK), and [ 77 Br]SBrB-EG 2 -c(RGDfK)] were designed, prepared, and evaluated. In these initial studies, 77 Br ( 1/2 =57.0 h) and 125 I ( 1/2 =59.4 d) were used because of their longer half-lives. Precursor peptides were synthesized using a standard 9-fluorenylmethyloxycarbonyl (Fmoc)-based solid-phase methodology. Radiolabeled peptides were prepared by chloramine-T method or conjugation of RGD peptides with [ 125 I]N-succinimidyl 3-iodobenzoate ([ 125 I]SIB) or [ 77 Br]N-succinimidyl 3-bromobenzoate ([ 77 Br]SBrB). Measurement of the partition coefficients, integrin binding assay, and biodistribution experiments in tumor-bearing mice were performed. 125 I and 77 Br labeling were successfully performed using similar methods, and in vitro characteristics and biodistributions were similar between the 125 I-labeled and corresponding 77 Br-labeled peptides. [ 125 I]SIB- and [ 77 Br]SBrB-conjugated RGD peptides showed higher partition coefficients, lower tumor uptakes, and higher intestinal uptake than 125 I-c(RGDyK) and 77 Br-c(RGDyK). [ 125 I]SIB-EG 2 -c(RGDfK) and [ 77 Br]SBrB-EG 2 -c(RGDfK), which possess an ethylene glycol linker, decreased lipophilicity and uptake in intestine compared with [ 125 I]SIB-c(RGDfK) and [ 77 Br]SBrB-c(RGDfK), which possess no linker. However, the improvement in biodistribution of [ 125 I]SIB-EG 2 -c(RGDfK) and [ 77 Br]SBrB-EG 2 -c(RGDfK)] was insufficient. In conclusion, directly radiohalogenated c(RGDyK) peptides are potentially more useful for tumor imaging and therapy than indirectly radiohalogenated ones.

Comparison of Radioiodine- or Radiobromine-Labeled RGD Peptides between Direct and Indirect Labeling Methods