Pharmacokinetic analysis of antibody localization in human colon cancer: comparison with immunoscintigraphy

The biodistribution and imaging characteristics of the 111In-labeled anti CEA monoclonal antibody ZCE-025 were studied in five patients with suspicion of colorectal carcinoma. Evaluation included antibody pharmacokinetics and assessment of antibody distribution in surgical specimen, making a comparison with whole-body imaging with a gamma camera. ZCE-025 localization in tumors was demonstrated by gamma-camera imaging in 4 of the 5 patients, corresponding to surgical findings. Persistent accumulation of 111In in the lymph nodes was observed in one patient, whereas surgical exploration of these lymph nodes showed no gross or microscopic evidence of metastases of colon carcinoma. Analysis of individual plasma by size exclusion HPLC showed two radioactivity peaks, labeled antibody and free DTPA. No transchelation of 111In to circulating transferrin was observed. The blood clearance was fitted to a two-compartment equation and its half-lives were found to be 10.8 +/- 8.7 h and 69.5 +/- 21.8 h for t1/2 alpha and t1/2 beta, respectively. Total urinary excretion averaged 0.3% of the injected dose/h with a small patient to patient variation. At 24 hrs postadministration the predominant radiolabeled species in urine was free DTPA. Thereafter, radioactivity in urine was partly present as a low molecular weight catabolic product. No apparent correlation between CEA content and uptake of 111In-ZCE-025 in tumors resected by surgery could be found. How 111In-labeled antibody is accumulated into tumors as well as into some nontumor tissues needs further study.