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Synergistic Interaction between Anti-p185^HER-2 Ricin A Chain Immunotoxins and Radionuclide Conjugates for Inhibiting Growth of Ovarian and Breast Cancer Cells That Overexpress HER-2¹

University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030 [F. X., Y. Y., K. W., A. M., R. C. B.]; University of North Carolina, Chapel Hill, North Carolina 27514 [S. A. L.]; and Duke University Medical Center, Durham, North Carolina 27710 [C. M. B., K. O., X. Z., D. S. B., K. D., M. R. Z.]

Radionuclide conjugates or ricin A chain (RTA) immunotoxins that target pl85^HER-2 have partially inhibited the growth of human ovarian cancer xenografts in athymic mice but generally have not cured mice bearing human tumor transplants. The present study was undertaken to explore whether a combination of ionizing radiation and an immunotoxin could exert additive or synergistic cytotoxicity in culture and in vivo against cancer cells that overexpress pl85^HER-2. In cell culture, treatment with 200–2000 cGy external beam irradiation followed by incubation with TA1-anti-pl85^HER-2-RTA immunotoxin (TA1-RTA) produced synergistic inhibition of clonogenic growth of ovarian and breast cancer cells that expressed >10⁶ pl85^HER-2 receptors/cell. The effect on cell survival correlated with an inhibition of DNA repair. A prior study (F. J. Xu et al., Nucl. Med. Biol., 24: 451–460, 1997) compared the biodistribution of radionuclide conjugates prepared with monoclonal antibodies that bind to different epitopes on the extracellular domain of pl85^HER-2 and found optimal tumor uptake with the 520C9 antibody, which did not compete with TA1 for binding to the receptor. In this report, the TA1-RTA immunotoxin and the ¹³¹I-labeled 520C9 radionuclide conjugate could each inhibit the growth of clone-9002-18 xenografts in athymic mice but did not yield long-term survivors using maximally tolerated doses of each agent. When TA1-RTA and ¹³¹I-labeled 520C9 were used in combination, a greater inhibition of tumor growth was obtained than with either single agent. Similarly, survival with the combined treatment was significantly prolonged (P = 0.004) relative to treatment with immunotoxin or radionuclide conjugate alone. After treatment with an optimal combination of immunotoxin and radionuclide conjugate, 50% of mice survived >300 days, whereas controls succumbed with a median survival of 36 days. These results suggest that combinations of immunotoxins and radionuclide conjugates deserve further evaluation for the treatment of cancers that overexpress pl85^HER-2.