The effect of serum CEA on the distribution and clearance of anti-CEA antibody in a pancreatic tumour xenograft model

A human pancreatic adenocarcinoma was used to develop two histologically distinct xenograft lines, one associated with high levels (180-2000 ng ml-1) and one with low levels (greater than 2.0 less than 8.0 ng ml-1) of serum carcinoembryonic antigen (CEA). A strong correlation was found between tumour size and both circulating and tumour CEA levels in the former group, and also correlation at the 5% level between tumour size and serum CEA in the latter. Administration of either monoclonal or polyclonal 125I-anti-CEA antibody led to the formation of intravascular antigen-antibody immune complexes in mice with high CEA levels, and these were rapidly cleared by the liver, deiodination commencing within the first hour. Blood activity was reduced to 20% of the injected dose by 15 min, and by 24 h the radioactivity in all tissues except muscle was significantly below that found in either the low CEA group or in mice without tumours. No difference in radio-antibody clearance pattern was found between mice without tumours and the group with low levels of serum CEA. In spite of higher levels of CEA within the tumour in mice with elevated serum CEA, the rapid clearance of antigen-antibody complexes reduced tumour localisation to one quarter of that seen in mice with low serum, and correspondingly low tumour, CEA levels. Gamma-camera imaging confirmed these results. Possible implications to patient selection and treatment are discussed.