MONOCLONAL-ANTIBODY SEN7 RECOGNIZES A NEW EPITOPE ON THE NEURAL CELL-ADHESION MOLECULE PRESENT ON SMALL-CELL LUNG-CANCER BUT NOT ON LYMPHOCYTES

In our continuing attempt to select monoclonal antibodies for immunota rgeting of small cell lung carcinoma (SCLC) we have developed the IgG1 murine antibody SEN7 which by immunofluorescence stained all SCLC cel l lines tested. On frozen tumor sections six of seven SCLCs were posit ively stained. The reactivity of this antibody in a series of lung tum ors and on normal tissues has some similarities with cluster 1 antibod ies and cluster w4 antibodies, as defined by the First and Second Inte rnational Workshop on Lung Cancer Antigens [P. C. L. Beverley, Y. Olab rian, J. A. Ledermann, L. G. Bobrow, and R. L. Souhami, Br. J. Cancer, 63 (Suppl): 10-19, 1991], particularly with regard to staining of neu roendocrine tissues. The similarities in staining of neuroendocrine ti ssues between antibody SEN7 and cluster 1 and cluster w4 antibodies pr ompted us to examine the binding of SEN7 with transfectants expressing the respective antigens. On the murine lymphoma cells B-9, stably tra nsfected with a complementary DNA, clone coding for an M(r) 140,000 is oform of human SCLC neural cell adhesion molecule (NCAM), antibody SEN 7 reacted positively whereas the cluster w4 antibody was negative. The reaction of antibody SEN7 with the NCAM transfected murine lymphoma c ells was unexpected in view of its lack of binding to peripheral blood mononuclear cells which regularly stain positive with NCAM antibodies . Western blotting of a renatured SCLC extract revealed a strong band around M(r) 180,000, in contrast to other cluster 1 antibodies which r ecognized a broad polydisperse band with a molecular weight of 140,000 to 210,000. Antibody binding was sensitive to tunicamycin treatment, suggesting the epitope to reside on an N-linked carbohydrate structure . No significant competition for SEN7 binding on SCLC cells was seen w ith other NCAM antibodies against the three distinct epitopes describe d on SCLC. This finding together with the lack of staining of peripher al blood mononuclear cells and the selected reactivity with the M(r) 1 80,000 band of NCAM indicate the antibody SEN7 recognizes an epitope o n NCAM which has not been described previously. Biodistribution studie s with radiolabeled SEN7 in nude mice bearing s.c. SCLC xenografts dem onstrated the selective localization of more than 30% of the total inj ected dose per g tissue at day 4 following i.v. injection. The homogen eous binding to SCLC, the lack of binding to peripheral blood mononucl ear cells, and the favorable tumor localization in a xenograft model i ndicates that SEN7 is a good antibody for immunotargeting of SCLC.