Preclinical Development Of Bl22

To produce a stable anti-CD22 recombinant immuno-toxin, the variable domains of the anti-CD22 mAb RFB4 were connected by a disulfide bond instead of a peptide linker, and VH was fused to PE38, resulting in BL22 (see Figure 33.1).80 The disulfide bond was created by mutating Arg44 of VH and Gly100 of VL to cys-teines. This technology had been used previously for stabilizing Fvs of a variety of different mAbs, including anti-Tac.8182 Chemical reactions are not needed to make the recombinant immunotoxin, since the disul-fide bond between VL and VH-PE38 forms automatically during in vitro renaturation of the two fragments. Complete regressions in mice of human CD22+ B-cell lymphoma xenografts were observed at plasma levels which could be tolerated in cynomolgus mon-keys.83 CRs of these xenografts were observed whether BL22 was administered by bolus injection q.o.d. X 3 or by continuous infusion. Leukemic cells freshly obtained from patients with CLL and NHL were incubated ex vivo with BL22.72 This study, which showed specific killing of such cells, was important for preclinical development because malignant cells freshly obtained from patients typically display far fewer CD22 sites/cell compared to cell lines.

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