LOCALIZATION OF MONOCLONAL-ANTIBODIES REACTING WITH DIFFERENT EPITOPES ON CARCINOEMBRYONIC ANTIGEN (CEA) - IMPLICATIONS FOR TARGETED THERAPY

Antibody targeting has potential for selective delivery of cancer ther apy. However, there is a wide Variation in the degree of antibody loca lisation in individual patients with colorectal adenocarcinoma. Colore ctal adenocarcinomas are composed of glandular structures separated fr om fibrovascular stroma by a basal lamina which may represent a signif icant barrier to extravasated antibody. Basement membrane-associated C EA epitopes may be more accessible to antibodies than those which are cytoplasmic or lumenal. We have investigated, by immunohistochemistry and in vivo localisation, the extent to which distribution of antigen epitopes influences targeting. Two monoclonal antibodies (A5B7 and EA7 7) recognising nonoverlapping CEA epitopes were reacted immunohistoche mically with samples of 39 rumours. Intensity and site of reaction wer e assessed for basement membrane, cytoplasmic or lumenal surface assoc iation. I-125-labelled antibodies were injected into nude mice bearing LS174T tumour. Per cent injected activity per gram was measured in tu mour and normal tissues, 24, 72 and 168 h later. Tissues reacted immun ohistochemically for CEA were autoradiographed to assess the relations hip of injected antibody to target antigen. Immunohistochemistry showe d that A5B7 antibody favours basement membrane aspects of malignant gl ands; in contrast, EA77 concentrated generally on lumenal surfaces. In vivo localisation showed that per cent inj.act g(-1) in tumour for A5 B7 reached 36.5% at 24 h. EA77 localised to a lesser extent (9.1% at 2 4 h), despite a longer circulatory half-life. Autoradiography combined with immunohistochemistry showed A5B7 reacting with antigen close to vasculature after 24 h, slowly penetrating deeper parts of the tumour by 72 h. In contrast, EA77 was confined mainly to fibrovascular stroma , showing little labelling of antigen-positive tumour cells. Localisat ion differences between A5B7 and EA77 may partly be due to accessibili ty of epitopes on tumour cells.