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

Abstract

The development of new chelating agents and radiolabeling protocols is essential to progress in radioimmunotherapy with antibody-chelate conjugates. 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. Lym-1-2IT-6-BAT-67Cu, the chelate conjugate of 6-[p-(bromoacetamido)benzyl]-1,4,8,11-tetraazacyclotetradecane- 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. 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.