Immunogenicity in rabbits and mice of an antibody-chelate conjugate: Comparison of (S) and (R) macrocyclic enantiomers and an acyclic chelating agent

The macrocyclic bifunctional chelating agent 2-(p-bromoacetamidobenzyl)- 1,4,7,10-tetraazacyclododecanetetraacetic acid (BAD), forms inert metal complexes ideal for radioimmunotherapy. Kosmas et al. (Cancer Res., 52: 904- 911, 1992) found 2-imminothiolane linker-(S)-BAD monoclonal antibody HMFG1 highly immunogenic in patients. We studied the immunogenicity of (S) and (R) enantiomers of 2-imminothiolane linker-BAD rabbit IgG, monoclonal antibody Lym-1, and Lym-1 2-imminothiolane linker-(S)-bromoacetamidobenzyl-EDTA in 15 rabbits. Five groups of three each were given 0.1, 1.0, or 10 mg of ¹¹¹In conjugate i.v., blood samples were taken daily for 14 days and biweekly for 70 days, and the plasma T( 1/2 ) was calculated. A drop in plasma ¹¹¹In at 6- 8 days coincided with the appearance of antibody on enzyme-linked immunosorbent assay. Specific anti-(S)-BAD, anti-(R)-BAD, anti-(S)- bromoacetamidobenzyl-EDTA, and anti-mouse IgG were measured. Rabbit IgG conjugates did not elicit an immune response. Mouse IgG conjugates were immunogenic on the first exposure, with both anti-1,4,7,10- tetraazacylododecane N,N',N'',N'''-tetraacetic acid and anti-mouse responses. Anti-1,4,7,10-tetraazacylododecane N,N',N'',N'''-tetraacetic acid was specific for the (S) or (R) enantiomer, but cross-reaction appeared with reboosting. A second injection of the opposite enantiomer gave a response to that enantiomer. Lym-1 bromoacetamidobenzyl-EDTA produced anti- bromoacetamidobenzyl-EDTA and anti-mouse response.