Conventional and high-yield synthesis of DTPA-conjugated peptides: Application of a monoreactive DTPA to DTPA-D-Phe(1)-octreotide synthesis

Successful imaging of somatostatin receptor-positive tumors with In-111-DTPA-D-Phe(1)-octreotide has stimulated development of peptide radiopharmaceuticals using DTPA as the chelating agent. However, use of cyclic DTPA dianhydride (cDTPA) resulted in low synthetic yields of DTPA-peptide by either solution or solid-phase syntheses. This paper reports a novel high-yield synthetic procedure for DTPA-D-Phe(1)-octreotide that is applicable to other peptides of interest using a monoreactive DTPA derivative. A monoreactive DTPA that possesses one free terminal carboxylic acid along with four carboxylates protected with tert-butyl ester (mDTPA) was synthesized. Fmoc-Thr(Bu-t)-ol, prepared from Fmoc-Thr(Bu-t)-OH, was loaded onto 2-chlorotrityl chloride resin. After construction of the peptide chains by Fmoc chemistry, mDTPA was coupled to the ct amine group of the peptide on the resin in the presence of 1,3-diisopropylcarbodiimide and 1-hydroxybenzotriazole. Treatment of the mDTPA-peptide-resin with trifluoroacetic acid-thioanisole removed the protecting groups and liberated [Cys(Acm)(2,7)]-octreotide-D-Phe(1)-DTPA from the resin. Iodine oxidation of the DTPA-peptide, followed by the reversed-phase HPLC purification, produced DTPA-D-Phe(1)-octreotide in overall 31.8% yield based on the starting Fmoc-Thr(Bu-t)-ol-resin. The final product gave a single peak on analytical HPLC, and amino acid analysis and mass spectrometry confirmed the integrity of the product. radiolabeling of the product provided In-111-DTPA-D-Phe(1)-octreotide with >95% radiochemical yield, as confirmed by analytical reversed-phase HPLC, TLC, and CAE. These findings indicated that use of mDTPA during solid-phase peptide synthesis greatly increased the synthetic yield of DTPA-D-Phe(1)-octreotide, due to the absence of nonselective reactions that are unavoidable when cDTPA is used. These results also suggested that mDTPA would be a versatile reagent to introduce DTPA with high yield into peptides of interest.