Evaluation of four CD22 antibodies as ricin a chain‐containing immunotoxins for the in vivo therapy of human B‐cell leukemias and lymphomas

Abstract Ricin A chain‐containing immunotoxins (IT‐As) specific for the human B‐cell antigen, CD22, were prepared from 4 monoclonal antibodies (MAbs) or their Fab′ fragments: RFB4, HD6, UV22‐1 and UV22‐2. The ITs were tested for their ability to kill cells from the Burkitt lymphoma line, Daudi, the pre‐B‐cell leukemia line, NALM‐6, and the myeloma cell line, ARH‐77. Daudi expresses high levels of CD22, whereas NALM‐6 and ARH‐77 express low levels of CD22. The IgG‐RFB4‐A was highly toxic to all 3 cell lines; it killed 50% of the Daudi cells at a concentration of 1.2 × 10‐ 12 M and 50% of NALM‐6 and ARH‐77 cells at concentrations of 1.5 to 2.1 × 10 −11 M. IgG‐RFB4‐A was 10–30 times more toxic to Daudi cells than were the IgG‐As constructed from the other 3 CD22 MAbs and 10 times more toxic than ricin itself. IT‐As constructed from the Fab′ fragments of the 4 CD22 antibodies were 2 to 5 times less toxic to Daudi cells than their IgG‐A counterparts. Fab′‐RFB4‐A was twice as toxic to Daudi cells as ricin, whereas the other Fab′‐As were about 7 times less toxic than ricin. Scatchard analyses of the binding of the radio‐iodinated antibodies to Daudi cells showed that the intact RFB4 antibody bound 3–10 times more strongly than the other antibodies, whereas the Fab′‐RFB4 bound 1.2 to 3.5 times more strongly than the Fab′ fragments prepared from the other antibodies. Thus, the potent cytotoxic activity of the RFB4‐As appears to derive, in part, from their superior binding affinity. Prior studies have shown that UV22‐1 and HD6 cross‐react with certain normal human tissues lacking cells of B‐cell lineage, whereas UV22‐2 and RFB4 are B‐cell‐specific. This fact, together with its superior potency as an IT‐A, suggests that RFB4 is the antibody of choice for preparing Fab′‐As or IgG‐As for in vivo therapy of human B‐cell leukemias and lymphomas.