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.