Complete regression of human B-cell lymphoma xenografts in mice treated with recombinant anti-CD22 immunotoxin RFB4(dsFv)-PE38 at doses tolerated by cynomolgus monkeys.

RFB4(dsFv)-PE38 is a recombinant immunotoxin in which the variable light domain (V(L)) is disulfide bonded via cysteine residues to the variable heavy domain (V(H)), which in turn is fused to PE38, a mutant form of Pseudomonas exotoxin A. RFB4 binds to CD22, which is a differentiation antigen expressed on the majority of B-cell leukemias and lymphomas. To examine the potential efficacy of RFB4(dsFv)-PE38 when administered at a dose schedule appropriate for phase I testing, mice bearing CA46 human CD22+ Burkitt's lymphoma xenografts were treated on alternate days i.v. for 3 doses (QOD x 3). Complete regressions were observed in 80% and 100% of mice treated with 200 and 275 microg/kg QOD x 3, respectively. The higher dose was 27% of the LD50 and 34% of the LD10 in mice. Because RFB4(dsFv)-PE38 is stable at 37 degrees C, it could also be given by continuous infusion using pumps placed in the peritoneal cavity; complete regressions also resulted from this mode of administration. To study toxicology, a pilot toxicology study of RFB4(dsFv)-PE38 was undertaken in cynomolgus monkeys, which like humans but unlike mice have CD22, which binds RFB4. Doses of 100 and 500 microg/kg i.v. QOD x 3 were well tolerated, indicating that a dose that cured tumors in mice was tolerated by primates. Based on these preclinical results, RFB4(dsFv)-PE38 is being developed for the treatment of patients with CD22-positive leukemias and lymphomas.