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Improved Treatment of Medullary Thyroid Cancer in a Nude Mouse Model by Combined Radioimmunochemotherapy: Doxorubicin Potentiates the Therapeutic Efficacy of Radiolabeled Antibodies in a Radioresistant Tumor Type¹

Departments of Nuclear Medicine [T. M. B., E. W., S. R., S. G., W. B.], and Radiation Oncology [M. R-F., H. S.], Georg-August-University, D-37075 Göttingen, Germany; Garden State Cancer Center, Belleville, New Jersey 07109 [R. D. B., R. M. D.]; and Department of Endocrinology, Ruprecht-Karl-University, Heidelberg, Germany [F. R.]

Whereas in advanced metastatic medullary thyroid cancer (MTC), a variety of chemotherapeutic regimens have achieved only limited success clinically, more recently, radioimmunotherapy (RIT) with ¹³¹I-labeled anti-carcinoembryonic antigen (CEA) monoclonal antibodies (MAbs) has shown promising results. The aims of this study were to compare, in an animal model, the therapeutic efficacy of RIT to clinically used "standard" chemotherapeutic regimens and to evaluate whether combination strategies of both modalities may be feasible and may help to improve therapeutic results in this rather radioresistant tumor type.

Nude mice, bearing s.c. xenografts of the human MTC cell line, TT, were treated either with the ¹³¹I-labeled anti-CEA MAb, F023C5 IgG, or were administered chemotherapeutic regimens that had shown promising results in patients with metastatic MTC (doxorubicin and cisplatinum monotherapy, combinations of both agents, and a 5-fluorouracil/dacarbazine/streptozotocin scheme). Control groups were left untreated or were injected with an irrelevant radiolabeled antibody at equitoxic dose levels. The maximum tolerated dose (MTD) of each agent was determined. Combinations of chemotherapy and RIT were evaluated as well. Toxicity and tumor growth were monitored at weekly intervals.

From the chemotherapeutic agents and schemes tested, doxorubicin monotherapy was the most effective; combination therapies did not result in an increased antitumor efficacy, but they did result in more severe toxicity. At equitoxic doses, no significant difference was found between the therapeutic efficacy of doxorubicin and that of RIT. Myelotoxicity was dose limiting with radiolabeled MAbs (MTD, 600 µCi), as well as with chemotherapeutic regimens containing alkylating agents (cisplatinum, dacarbazine, or streptozotocin). At its MTD (200 µg), doxorubicin caused only mild myelotoxicity, and despite signs of cardiac toxicity, gastrointestinal side effects were dose limiting. Accordingly, bone marrow transplantation (BMT) enabled dose intensification with RIT (MTD with BMT, 1100 µCi), which led to further increased antitumor efficacy, whereas BMT was unable to increase the MTD of doxorubicin. Due to the complementarity of toxic side effects but an anticipated synergism of antitumor efficacy, combinations of RIT with doxorubicin were tested. Administrations of 500 µCi of ¹³¹I-labeled anti-CEA and, 48 h later, 200 µg of doxorubicin (i.e., 83 and 100% of the respective single-agent MTDs), were the highest doses that did not result in an increased lethality; with bone marrow support, 1000 µCi of ¹³¹I-labeled anti-CEA could be combined with 200 µg of doxorubicin (i.e., 90 and 100% of the individual MTDs). Therapeutic results of this combined radioimmunochemotherapy were superior to equitoxic monotherapy with either agent, and indication for synergistic antitumor effects is given. At its respective MTD, radioimmunochemotherapy led to a 36% cure rate if it was given without bone marrow support and to a 85% permanent cure rate if it was given with bone marrow support.

The animal model, as presented in this study, seems to be useful for the preclinical testing of therapeutic agents for the systemic treatment of MTC. At equitoxic doses, RIT with radiolabeled anti-CEA antibodies seems to be equally as effective as chemotherapy with doxorubicin. Combination of RIT and doxorubicin chemotherapy seems to have synergistic therapeutic efficacy, which may be due to a radiosensitizing effect of doxorubicin.

¹ This work was supported in part by a research grant from Henning Berlin (Berlin, Germany) and by Deutsche Forschungsgemeinschaft Grant Be 1689/4-1.

² To whom requests for reprints should be addressed, at Department of Nuclear Medicine, Georg-August-University of Göttingen, Robert-Koch-Strasse 40, D-37075 Göttingen, Germany. Phone: 49-551-39-8510; Fax: 49-551-39-8526.

Received 6/23/97. Accepted 10/ 9/97.

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