ANTI-TAC(FAB)-PE40, A RECOMBINANT DOUBLE-CHAIN IMMUNOTOXIN WHICH KILLS INTERLEUKIN-2-RECEPTOR-BEARING CELLS AND INDUCES COMPLETE REMISSION IN AN IN-VIVO TUMOR-MODEL

We have produced a single plasmid encoding both the heavy chain Fd dom ain (V-H + CHI) of the anti-interleukin-2 receptor (IL2R) monoclonal a ntibody anti-Tac, and the anti-Tac light chain fused to PE40, a trunca ted derivative of Pseudomonas exotoxin. The active immunotoxin anti-Ta c(Fab)-PE40 could be recovered from E. coil from either periplasm or r enatured inclusion bodies. The double-chain immunotoxin was very cytot oxic toward IL2R-bearing cell lines, human activated T cells and fresh adult-T-cell-leukemia cells. The cytotoxicity was similar to that of anti-Tac(Fv)-PE40, the single-chain recombinant toxin containing only the variable domains of anti-Tac. IL2R-binding affinity was also equiv alent to that of anti-Tac(Fv)PE40, which is one-third that of anti-Tac . The serum half-life in mice was significantly prolonged as compared with anti-Tac(Fv)PE40, with a beta phase of 430 vs. 57 minutes, but th e LD(50)s were equivalent when the immunotoxins were administered in 3 daily doses. Anti-Tac(Fab)-PE40 was very cytotoxic in vitro toward tr ansfected ATAC-4 carcinoma cells which express IL2Rs. In mice bearing ATAC-4 tumors, anti-Tac(Fab)-PE40 showed significant anti-tumor activi ty, inducing complete remissions in 80 and 100% of treated animals at approximately 7 and 14% respectively of the LD(50). Anti-Tac(Fab)-PE40 was much more effective in vitro and in vivo than chemical conjugates between anti-Tac and truncated PE molecules. The recombinant Fab toxi n should be studied further as potential treatment for IL2R-related ma lignancies, particularly if smaller recombinant immunotoxins have insu fficient half-life in humans. (C) 1994 Wiley-Liss, Inc.