Conventional and High-Yield Synthesis of DTPA-Conjugated Peptides: Application of a Monoreactive DTPA to DTPA-d-Phe1-octreotide Synthesis

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Abstract
Successful imaging of somatostatin receptor-positive tumors with 111In-DTPA-d-Phe1-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-Phe1-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(tBu)-ol, prepared from Fmoc-Thr(tBu)-OH, was loaded onto 2-chlorotrityl chloride resin. After construction of the peptide chains by Fmoc chemistry, mDTPA was coupled to the α 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-Phe1-DTPA from the resin. Iodine oxidation of the DTPA-peptide, followed by the reversed-phase HPLC purification, produced DTPA-d-Phe1-octreotide in overall 31.8% yield based on the starting Fmoc-Thr(tBu)-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. 111In radiolabeling of the product provided 111In-DTPA-d-Phe1-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-Phe1-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.