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Recombinant Immunotoxins Directed against the c-erb-2/HER2/neu Oncogene Product: In Vitro Cytotoxicity, Pharmacokinetics, and in Vivo Efficacy Studies in Xenograft Models¹

Section of Immunopharmacology and Targeted Therapy, Department of Bioimmunotherapy, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030 [M. G. R., J. W. M., L. C.], and Department of Oncology, Berlex Biosciences, Richmond, California 94804-0099 [J. B.]

TAB-250 and BACH-250 are murine and human chimeric antibodies directed at the extracellular domain of the gp185^c-erb-2 (HER2/neu) growth factor receptor overexpressed in a variety of tumor types, including ovarian and breast carcinoma. The ribosome-inhibiting plant toxin gelonin (rGel) was chemically coupled to both antibodies, and the resulting immunotoxins were purified and tested in vitro against human tumor cells expressing various levels of HER-2/neu and in vivo against human tumor xenograft models. The binding of both BACH-250 and BACH-250/rGel conjugate to target cells was essentially equivalent. Against SKOV-3 cells, the IC₅₀ of BACH-250/rGel was 97 pM (17 ng/ml), whereas BACH-250 and rGel alone showed no cytotoxic effects. There was a clear correlation between expression levels of HER-2/neu and cytoimmunotoxin. Tissue distribution studies showed that the antibody and immunotoxin both concentrate 2–10-fold higher in tumors than in normal tissues, with optimal tumor uptake occurring 48–96 h after administration. Plasma clearance curves for BACH-250 and BACH-250/rGel showed terminal-phase half-lives of 26 and 72 h, respectively. In athymic mice bearing s.c. or i.p. SKOV-3 tumors, immunotoxin treatment slowed tumor growth by 99 and 94% at days 35 and 49 after implantation, respectively, and lengthened the median survival by 40% (from 30 to 50 days) in mice bearing lethal i.p. tumors. We conclude that clinical development of BACH-250/rGel may be warranted in patients with HER2/neu-expressing malignancies.

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