Single-cell cytotoxicity with radiolabeled antibodies

Previous studies demonstrated the effective, antigen-specific killing of Ra ji B-lymphoma cells in vitro by radio-labeled anti-CD74, attributable large ly to the high level of uptake, of approximately 10(7) antibody (Ab) molecu les/cell/day. This Ab is rapidly delivered to lysosomes for catabolism, so the radionuclide delivered accumulates primarily in lysosomes, In this stud y, 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 radio activity delivered to the cell surface versus the cytoplasm, The Abs tested were anti-major histocompatibility complex class II and anti-CD20, In-111- labeled conjugates made with these two Abs killed cells very effectively an d 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 residu alizing radiolabels, which are trapped in lysosomes after Ab catabolism, wo uld not be required, and this was observed, ie., these two Abs were effecti ve when labeled with either I-125 Or I-131, using conventional iodination, as well as with the residualizing label In-111-labeled DTPA, These results are in contrast to results obtained with anti-CD74, which required a residu alizing 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 rad iation dose delivered. To establish the generality of the results, we also demonstrated that In-111-labeled anti-CD74 effectively killed three other B -lymphoma cell lines, in addition to Raji and the adherent melanoma cell li ne SK-MEL-37, By using more potent radionuclides or conjugates of higher sp ecific activity, this approach might be effective with other, lower density antigens.