Convenient solid-phase synthesis of diethylenetriaminepenta-acetic acid (DTPA)-conjugated cyclic RGD peptide analogues

Solid-phase synthesis of radiometal chelator-conjugated peptides can facilitate the creation of radioactive peptide libraries to be utilized in high throughput in vivo screening of targeted nuclear-imaging agents. In this study, a new diethylenetriaminepentaacetic acid (DTPA) derivative, 1-(p- succinamidobenzyl)-DTPA penta-t-butyl ester [DTPA(But)₅-Bz-NH-SA], and its precursor molecule, 1-(p-aminobenzyl)-DTPA penta-t-butyl ester (DTPA(But)₅-Bz-NH₂), were applied to the solid-phase synthesis of DTPA-conjugated cyclic peptides containing the Arg-Gly-Asp (RGD) motif with high efficiency. The resulting conjugates, DTPA-Bz-NH-SA-c(Lys-Arg- Gly-Asp-phe) [DTPA-Bz-NH-SA-c(KRGDf)] and DTPA-Bz-NH-c(Glu-Arg-Gly-Asp-phe) [DTPA-Bz-NH-c(KRGDf)], demonstrated similar in vitro biologic activities as their corresponding parent peptides. ¹¹¹In-labeled, DTP Á-conjugated RGD peptides showed selective binding to integrin ανβ3 in human melanoma M21 tumors grown in nude mice. Furthermore, ¹¹¹In-DTPA-Bz-NH-c(ERGDf) showed lower retention in the liver and the kidney than ¹¹¹In-DTPA-Bz-NH-SA-c(KRGDf) did, which contributed to higher target to nontarget ratio for ¹¹¹In-DTPA-Bz-NH-C(ERGDf). The method reported here can be extended to the construction of peptide libraries containing DTPA for high throughput in vitro and in vivo screening of molecularly targeted imaging agents.