Effect of chemical modification strategy on the characteristics of copper-67-Labeled immunoconjugates, Part I: Immunoreactivity

The synthetic porphyrins N-benzyl-5,10,15,20-tetrakis(4-carboxyphenyl)porphine (N-bzHTCPP) and N-4-nitrobenzyl-5-(4-carboxyphenyl)-10,15,20-tris(4-sulfophenyl)porphine (NbzHCS₃P) represent excellent radiocopper chelating agents that may find utility in antibody-mediated diagnosis and/or therapy. Detailed analyses were performed to explore the effect of the chemical modification strategy on the characteristics of immunoconjugates prepared from the anti—renal cell carcinoma monoclonal antibody A6H and N-bzHCS₃P. The parameters included in the study were antibody type [intact A6H and two of its fragments, half A6H and A6H-F(ab')₂], chemical linkage type and site, the presence or absence of intermediate linker molecules, and the nature of the chemical modification steps employed. The immunoconjugate synthesized by directly coupling N-bzHCS₃P to whole antibody retained 75-85% of the immunoreactivity of the unmodified antibody. In general, however, immunoconjugates prepared using the fragments or the intermediate linkers lacked immunoreactivity. This deficiency appeared to be a function of the multistep processes employed and not a function of the linker molecule. The results emphasize the critical importance of the type and number of antibody modification steps and the caution that must be exercised in the design of immunoconjugates for drug delivery purposes.     

Cell Culture and Animal Studies for Intracerebral Delivery of Borocaptate in Liposomal Formulation

The efficacy of a liposomal formulation for intracerebral delivery of borocaptate (BSH) to brain tumor cells has been investigated using cell culture to study BSH uptake and persistence and using tumor-bearing rats to determine BSH distribution in the brain. During a 16-hr incubation, cellular uptake of BSH solution or BSH liposomal formulation was similar. However, the cellular persistence of BSH greatly increased when BSH was present in liposome. The differences in cellular persistence for BSH solution and BSH-loaded liposomes were significant both in 12-hr and 24-hr incubation experiments (p < 0.05 and p < 0.01, respectively). For the studies involving tumor-bearing rats, BSH level in tumor tissue was significantly higher than that in normal brain tissue at 2 hr and 6 hr after intracerebral injection of BSH-loaded liposomes (p < 0.01). Our study indicated that the liposomal formulation enhanced cellular persistence of BSH in tumor cells and therefore favored the boron accumulation in the cells. With the prolonged physical retention of liposomes at the local injection site and the cellular retention of BSH enhanced by the liposomes, the intracerebral delivery of BSH using liposomal formulation may provide an effective boron delivery approach for boron neutron capture therapy of brain tumors.