A COMPARATIVE-STUDY OF COPPER-67 RADIOLABELING AND KINETIC STABILITIES OF ANTIBODY-MACROCYCLE CHELATE CONJUGATES

Background. The development of new chelating agents and radiolabeling protocols is essential to progress in radioimmunotherapy with antibody -chelate conjugates. Methods. Immunoconjugates of four polyazamacrocyc les with N-bonded acetate groups were prepared by conjugation via 2-im inothiolane to Lym-1, a murine antilymphoma immunoglobulin G(2a) MoAb. To optimize Cu-67 radiolabeling, complexation conditions were explore d. The kinetic stabilities in vitro in human serum of four Cu-67 label ed immunoconjugates were investigated. Results. Lym-1-2IT-6-BAT-Cu-67, the chelate conjugate of etamido)benzyl]-1,4,8,11-tetraazacyclotetrad ecane- N,N',N''N'''-tetraacetic acid, exhibited excellent kinetic stab ility in human serum, while Lym-1-2IT-2-BAT-Cu-67, prepared from the s tructural isomer etamido)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-Cu-67, in mCi per mg immunoconjugate, was limited solely by the specific activity of the radiometal, which varied significantly fr om lot to lot. This ratio for a given lot of Cu-67 can be predicted by a preliminary titration. Conclusions. The preparation of Cu-67 labele d immunoconjugates of therapeutic quality has been improved by the det ermination of optimum radiolabeling conditions, and by development of a titration protocol which rapidly and accurately predicts the radiola beling 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.