In vitro and in vivo evaluation of streptavidin immunoconjugates of the second generation TAG-72 monoclonal antibody CC49.

Streptavidin was conjugated to the second-generation TAG-72 monoclonal antibody CC49 at lysine amino acids, oxidized carbohydrates or reduced disulfides on the immunoglobulin. The streptavidin immunoconjugates were radiolabelled with 111In-DTPA-biocytin and their immunological characteristics evaluated in vitro and in vivo. FPLC analysis showed a single peak (mol. wt > 350 kDa) for the lysine conjugate and sulfhydryl conjugate (mol. wt approximately 210 kDa) but multiple peaks (approximately 210 to > 350 kDa) for the carbohydrate conjugate. There were only small differences in immunoreactivity against bovine submaxillary mucin in vitro. However, in mice bearing subcutaneous LS174T tumours, the lysine conjugate exhibited significantly lower tumour uptake (< 2% i.d./g) compared to the other streptavidin-CC49 conjugates (10-40% i.d./g) or DTPA-CC49 (14-18% i.d./g). Due to the monomeric nature and smaller molecular size of the sulphhydryl conjugate, and its similar in vitro and in vivo characteristics compared with DTPA-CC49, this conjugate has been selected for future pretargeting studies with 111In and 90Y biotin.

Immunoscintigraphy of human colon cancer xenografts in nude mice using a second-generation TAG-72 monoclonal antibody labelled with 99Tcm.

Monoclonal antibody CC83 is a second-generation high-affinity antibody directed against the TAG-72 antigen in colorectal cancer. Our objectives were to evaluate the biodistribution, pharmacokinetics and imaging properties of CC83 labelled with 99Tcm via a modified Schwartz technique. The immunological integrity of 99Tcm-CC83 was evaluated by size-exclusion FPLC and by determining the immunoreactive fraction in vitro against bovine submaxillary mucin. The biodistribution of 99Tcm-CC83 up to 24 h postinjection was evaluated in nude mice bearing subcutaneous LS174T human colon cancer xenografts. Blood radioactivity data was fitted to a one-compartment pharmacokinetic model. Images of tumour-bearing mice were obtained at 17-24 h postinjection with 99Tcm-CC83. 99Tcm-CC83 was eluted as intact immunoglobulin by FPLC analysis and the mean immunoreactive fraction was 0.49 +/- 0.15. Tumour uptake at 24 h postinjection was 11.2 +/- 4.1% i.d.g-1. Radioactivity in the blood was eliminated rapidly with a half-life of 8 h and tumour:blood ratios were > 2:1 at 24 h postinjection. LS174T tumours were successfully imaged in 3/3 mice. In vitro studies showed instability of 99Tcm-CC83 when challenged with cysteine and glutathione but not metallothionein, suggesting a metabolic route for the 99Tcm antibody in vivo. We conclude that CC83 labelled directly with 99Tcm retains its immunological integrity and capability specifically to target subcutaneous LS174T human colon cancer tumours hosted in nude mice. These results further suggest that 99Tcm-CC83 may have potential for imaging colorectal cancer in humans.

A simple method to determine the immunoreactivity of radiolabelled monoclonal antibodies to the TAG-72 antigen.

A simple method has been developed for determining the immunoreactivity of radiolabelled monoclonal antibodies to the TAG-72 antigen. The method involves binding of a constant small amount of the antibody to increasing concentrations of bovine submaxillary mucin. The immunoreactive fraction (IRF) is then determined by linear extrapolation of binding to infinite antigen excess. Using this assay, the IRF of radioiodinated anti-TAG-72 antibodies ranged from 0.22-0.48.