Single-cell cytotoxicity with radiolabeled antibodies

Previous studies demonstrated the effective, antigen-specific killing of Raji B-lymphoma cells in vitro by radiolabeled anti-CD74, attributable largely to the high level of uptake, of approximately 10(7) antibody (Ab) molecules/cell/ day. This Ab is rapidly delivered to lysosomes for catabolism, so the radionuclide delivered accumulates primarily in lysosomes. In this study, we have tested Abs that bind to the same target cells in similar amounts, but remain primarily on the cell surface, to compare the potency of radioactivity delivered to the cell surface versus the cytoplasm. The Abs tested were anti-major histocompatibility complex class II and anti-CD20. 111In-labeled conjugates made with these two Abs killed cells very effectively and specifically, with 100% kill of sample of 5 x 10(5) cells. Because these Abs remain primarily on the cell surface, it would be predicted that residualizing radiolabels, which are trapped in lysosomes after Ab catabolism, would not be required, and this was observed, i.e., these two Abs were effective when labeled with either 125I or 131I, using conventional iodination, as well as with the residualizing label 111In-labeled DTPA. These results are in contrast to results obtained with anti-CD74, which required a residualizing radiolabel for effectiveness. The uptake of these radionuclides, in cpm/cell, was monitored, and this allowed estimation of the radiation dose delivered; the cytotoxicity observed was consistent with the estimated radiation dose delivered. To establish the generality of the results, we also demonstrated that 111In-labeled anti-CD74 effectively killed three other B-lymphoma cell lines, in addition to Raji and the adherent melanoma cell line SK-MEL-37. By using more potent radionuclides or conjugates of higher specific activity, this approach might be effective with other, lower density antigens.