Immunotoxins constructed with anti‐CD25 monoclonal antibodies and deglycosylated ricin a‐chain have potent anti‐tumour effects against human hodgkin cells in vitro and solid hodgkin tumours in mice

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Abstract
Twenty‐three monoclonal antibodies (MAbs) against the IL‐2 receptor α‐chain (CD25) were evaluated as ricin A‐chain immunotoxins for the treatment of Hodgkin's disease. Primary screening used an indirect assay in which the cells were treated with the test antibody followed by a Fab immunotoxin against mouse immunoglobulin. This screening identified 5 MAbs which inhibited protein synthesis in L540 Hodgkin cells by 50% at a concentration (IC50) of 6 × 10−11 M or less: RFT5γ1, RFT5γ2a, B‐B10, B‐F2 and B‐G3. These MAbs were then linked directly to deglycosylated ricin A‐chain (dgA) and were confirmed to have potent and specific toxicity for L540 cells. The immunotoxins had the following potency order: RFT5γ1 > RFT5γ2a > B‐B10 > B‐F2 > B‐G3. The most effective immunotoxin, RFT5γ1.dgA, had an IC50 value of 7 × 10−12 M, which is the same as that of whole ricin. In vivo, a single intravenous injection of 48 μg of RFT5γ1.dgA, RFT5γ2a.dgA, B‐B10.dgA or B‐F2 induced lasting complete remissions in 78, 66, 50 and 44%. respectively, of nude mice bearing subcutaneous solid L540 tumours of 0.7 cm diameter. Two tumours which regrew after B‐B10.dgA treatment were re‐established in tissue culture. Both had reduced sensitivity to B‐B10.dgA in vitro but not to immunotoxins recognizing different antigens on Hodgkin cells. The MAbs that produced the most potent immunotoxins, RFT5γ1, RFT5γ2a and B‐B10, had no significant cross‐reactivity with normal human tissues outside the lymphoid system as judged from indirect immunoperoxidase staining of frozen sections. By contrast, B‐F2 strongly stained normal human renal tubules.

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