The antitumor effects of two anti-CD22 ricin A chain-containing immunotoxin (IT) constructs were compared in mice with severe combined immunodeficiency disease with human Daudi cell tumors (SCID-Daudi mice). SCID-Daudi mice develop disseminated lymphoma that clinically resembles African Burkitt's lymphoma, i.e., extranodal disease including infiltration of the vertebral column and spinal canal. In the absence of treatment, the mean survival time of SCID-Daudi mice was 45.9 ± 4.3 days. The mice were given injections of a dose of IT equal to 40% of the 50% lethal dose. The ITs consisted of either IgG or Fab' fragments of mouse anti-CD22 antibody coupled to deglycosylated ricin A chain (dgA). Both ITs were potent and specific and inhibited protein synthesis in Daudi cells in vitro by 50% at concentrations of 1.2 × 10−12 (IgG-dgA) and 1.3 × 10−11m (Fab′-dgA). When administered to mice beginning 1 day after inoculation with tumor cells, both ITs extended the mean survival time, to 87.2 ± 18.9 days (IgG-dgA) or 57.9 ± 3.8 days (Fab′-dgA). The latter represented the killing of 2 logs of Daudi cells, and the former 4 logs. IgG antibody alone killed 1 log of tumor cells. The IgG-dgA had an antitumor effect even when administered 20–23 days after tumor inoculation. Gross and histological examinations of IT-treated tumor-bearing mice showed a marked decrease in the number and size of neoplastic foci in both lymphoid organs and extranodal sites.

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Supported by NIH Grants CA42228, CA28149, and RR00890, Texas Technology Grant 18512, a grant from the Welch Foundation I-947, and a grant from the Overton Foundation.

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