Cytotoxic and antitumor activity of a recombinant immunotoxin composed of disulfide-stabilized anti-Tac Fv fragment and truncated Pseudomonas exotoxin.

Disulfide-stabilized Fv (dsFv)-immunotoxins are recombinant immunotoxins in which the inherently unstable Fv moiety, composed of the VH-VL heterodimer, is stabilized by a disulfide bond engineered between structurally conserved framework positions of VH and VL. Anti-Tac(dsFv)-PE38KDEL is composed of such a dsFv, directed to the alpha subunit of the IL2 receptor (IL2R), and containing a truncated form of Pseudomonas exotoxin (PE38KDEL). We have found this new type of immunotoxin to be indistinguishable in its in vitro activity and specificity from its single-chain immunotoxin counterpart, anti-Tac(Fv)-PE38KDEL. We have now examined the therapeutically relevant factors, including stability, pharmacokinetics, and antitumor activity of this new disulfide-stabilized Fv-immunotoxin. We found that anti-Tac(dsFv)-PE38KDEL was specifically cytotoxic to human activated T-lymphocytes in addition to IL2R bearing cell lines. Anti-Tac(dsFv)-PE38KDEL was considerably more stable at 37 degrees C in human serum and in buffered saline than the single-chain immunotoxin, anti-Tac(Fv)-PE38KDEL. The half-life in blood was similar for both immunotoxins (approx. 20 min). The therapeutic potential of the disulfide-stabilized immunotoxin was evaluated using an animal model of immunodeficient mice bearing subcutaneous tumor xenografts of human IL2R-bearing cells. Anti-Tac(dsFv)-PE38KDEL caused complete regression of tumors with no toxic effects in mice. Because dsFv-immunotoxins are more stable and can be produced with significantly improved yields compared to scFv-immunotoxins, dsFv-immunotoxin may be more useful for therapeutic applications.