DELIVERY OF THE RIBOSOME-INACTIVATING PROTEIN, GELONIN, TO LYMPHOMA-CELLS VIA CD22 AND CD38 USING BISPECIFIC ANTIBODIES

It is well established that bispecific antibodies (BsAbs) can be used effectively in targeting the ribosome-inactivating protein (RIP), sapo rin, against neoplastic B cells. We have now extended this delivery sy stem for use with gelonin. By measuring antigen-binding characteristic s and epitope mapping a panel of anti-gelonin MAbs using the IAsys res onant mirror biosensor, we were able to rapidly select the most suitab le for making BaAbs. The Fab' fragments from these MAbs were chemicall y conjugated with Fab' from either anti-CD22 or anti-CD38. Cytotoxicit y assays showed that BsAbs were highly efficient at delivering gelonin to cultured Daudi cells and achieved levels of toxicity which correla ted closely with the affinity of the BsAbs. Using pairs of anti-CD22 B sAbs we were able to generate bivalent BsAb-gelonin complexes which ac hieved IC50 values of 2 x 10(-11) M gelonin, a potency which is equiva lent to that reached by saporin in this targeting system. However, bec ause gelonin is 5-10 times less toxic than saporin, the therapeutic ra tio for gelonin is superior, making it potentially a more useful agent for human treatment. Cytotoxicity assays and kinetic analysis showed that targeting gelonin via CD38 was 2-5 times less effective than deli very through CD22. However, with a pair of BsAbs designed to co-target gelonin via CD22 and CD38, the cytotoxicity achieved equalled that ob tained with a pair of anti-CD22 BsAbs (IC50 = 1 x 10(-11) M). This imp ortant result suggests that the anti-CD38 helps bind the gelonin to th e cell and is then 'dragged' or 'piggy-backed' into the cell by the an ti-CD22 BsAb. The implication of these findings for cancer therapy is discussed.