Ki-4(scFv)-ETA', a new recombinant anti-CD30 immunotoxin with highly specific cytotoxic activity against disseminated Hodgkin tumors in SCID mice.

The human lymphocyte activation marker CD30 is highly overexpressed on Hodgkin/Reed-Sternberg cells and represents an ideal target for selective immunotherapy. We used the murine anti-CD30 hybridoma Ki-4 to construct a new recombinant immunotoxin (rIT) for possible clinical use in patients with CD30(+) lymphoma. Hybridoma V genes were polymerase chain reaction-amplified, assembled, cloned, and expressed as a mini-library for display on filamentous phage. Functional Ki-4 scFv obtained by selection of binding phage on the CD30-expressing Hodgkin lymphoma cell line L540cy was inserted into the bacterial expression vector pBM1.1 and fused to a deletion mutant of Pseudomonas exotoxin A (ETA'). Periplasmically expressed Ki-4(scFv)-ETA' demonstrated specific activity against a variety of CD30(+) lymphoma cells as assessed by different in vitro assays. To evaluate in vivo antitumor activity, severe combined immunodeficient mice challenged with human lymphoma cell lines were treated with the immunotoxin. The blood distribution time t(1/2)alpha of Ki-4(scFv)-ETA' was 19 minutes, and its serum elimination time t(1/2)alpha was 193 minutes. A single intravenous injection of 40 microg rIT 1 day after tumor inoculation rendered 90% of the mice tumor free, extending the mean survival time to more than 200 days compared with 38.1 days in the phosphate-buffered saline control group (P <.001). This new rIT is a promising candidate for further clinical evaluation in patients with Hodgkin lymphoma or other CD30(+) malignancies. (Blood. 2000;95:3909-3914)