Optimised labeling, preclinical and initial clinical aspects of CCK-2 receptor-targeting with 3 radiolabeled peptides

Abstract 

Medullary thyroid carcinoma (MTC) expresses CCK-2 receptors. ¹¹¹In-labeled DOTA-DGlu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ (DOTA-MG11), DOTA-DAsp-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH₂ (DOTA-CCK), and ^99mTc-labeled N₄-Gly-DGlu-(Glu)₅-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ (^99mTc-Demogastrin 2) are analogs developed for CCK-2 receptor-targeted scintigraphy. All 3 radiolabeled analogs were selected on the basis of their high CCK-2 receptor affinity and their good in vitro serum stability, with in vitro serum t_1/2 values of several hours. Radiolabeling of DOTA-peptides with ¹¹¹In requires a heating procedure, typically in the range of 80°-100°C up to 30 min. Following this procedure with DOTA-MG11 resulted in a >98 % incorporation of ¹¹¹In, however, with a radiochemical purity (RCP) of <50 %. The decrease in RCP was found to be due to oxidation of the methionine residue in the molecule. Moreover, this oxidized compound lost its CCK-2 receptor affinity. Therefore, conditions during radiolabeling were optimised: labeling of DOTA-MG11 and DOTA-CCK with ¹¹¹In involved 5 min heating at 80°C and led to an incorporation of ¹¹¹In of >98 %. In addition, all analogs were radiolabeled in the presence of quenchers to prevent radiolysis and oxidation resulting in a RCP of >90 %.

All 3 radiolabeled analogs were i.v. administered to 6 MTC patients: radioactivity cleared rapidly by the kidneys, with no significant differences in the excretion pattern of the 3 radiotracers. All 3 radiolabeled analogs exhibited a low in vivo stability in patients, as revealed during analysis of blood samples, with the respective t_1/2 found in the order of minutes. In patient blood, the rank of radiopeptide in vivo stability was: ^99mTc-Demogastrin 2 (t_1/2 10-15 min)>¹¹¹In-DOTA-CCK (t_1/2≈5-10 min)>¹¹¹In-DOTA-MG11 (t_1/2<5 min).

Keywords: Medullary thyroid carcinoma, Kidney uptake, Receptor scintigraphy, CCK, DOTA, Minigastrin