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

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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.