Preparation via coligand exchange and characterization of [99mTc‐EDDA‐HYNIC‐D‐Phe¹,Tyr³]Octreotide (99mTc–EDDA/HYNIC–TOC)

Abstract [ 99m Tc‐EDDA–HYNIC‐D‐Phe 1 ,Tyr 3 ]octreotide ( 99m Tc‐EDDA/HYNIC–TOC) is a promising new agent with the potential to replace [ 111 In‐DTPA‐D‐Phe 1 ]‐octreotide in somatostatin receptor scintigraphy. This hydrazinonicotinic acid derivatized somatostatin complex contains ethylenediamine N,N′ diacetic acid (EDDA) as a coligand resulting in a high in vitro and in vivo stability. Since direct 99m Tc‐labelling of HYNIC–TOC with EDDA results in low labelling yields, in this study we describe the preparation of 99m Tc‐EDDA/HYNIC‐TOC via coligand exchange from Tricine for EDDA. Exchange of coligands is achieved at elevated temperature and under optimized conditions of pH, EDDA concentration and stannous ion. High labelling yields (mean 95.9%) were achieved at high specific activities (>37GBq/µmol). Characterization via HPLC, receptor binding and LC–MS of the resulting complex is described. The formulation developed enables rapid and simple labelling of 99m Tc‐EDDA/HYNIC–TOC in a manner suitable for a clinical setting.