A comparative study of copper‐67 radiolabeling and kinetic stabilities of antibody‐macrocycle chelate conjugates

Abstract

Background. The development of new chelating agents and radiolabeling protocols is essential to progress in radioimmunotherapy with antibody‐chelate conjugates. Methods. Immunoconjugates of four polyazamacrocycles with N‐bonded acetate groups were prepared by conjugation via 2‐iminothiolane to Lym‐1, a murine antilymphoma immunoglobulin G2a MoAb. To optimize 67Cu radiolabeling, complexation conditions were explored. The kinetic stabilities in vitro in human serum of four 67Cu labeled immunoconjugates were investigated. Results. Lym‐1‐2IT‐6‐BAT‐67Cu, the chelate conjugate of 6‐[p‐(bromoacetamido)benzyl]‐1,4,8,11‐tetraaza‐cyclotetradecane‐N,N′,N″N″‐tetraacetic acid, exhibited excellent kinetic stability in human serum, while Lym‐1‐2IT‐2‐BAT‐67Cu, prepared from the structural isomer 2‐[p‐(bromoacetamido)benzyl]‐1,4,8,11‐tetraazacyclotetradecane‐N,N′,N″,N″‐tetraacetic acid, exhibited a markedly higher rate of loss of radiometal. It was observed that the radiolabeling ratio of Lym‐1‐2IT‐6‐BAT‐67Cu, in mCi per mg immunoconjugate, was limited solely by the specific activity of the radiometal, which varied significantly from lot to lot. This ratio for a given lot of 67Cu can be predicted by a preliminary titration. Conclusions. The preparation of 67Cu labeled immunoconjugates of therapeutic quality has been improved by the determination of optimum radiolabeling conditions, and by development of a titration protocol which rapidly and accurately predicts the radiolabeling ratio in mCi per mg immunoconjugate. The surprising difference in the properties of 6‐BAT and 2‐BAT shows the exquisite dependence of kinetic stability on structure. Cancer 1994; 73:779–86. Copyright © 1994 American Cancer Society