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Gene Therapy of a Rat Follicular Thyroid Carcinoma Model with Adenoviral Vectors Transducing Murine Interleukin-12

Thyroid Study Unit, Department of Medicine, The University of Chicago, Chicago, Illinois 60637

Address all correspondence and requests for reprints to: Leslie J. DeGroot, M.D., Thyroid Study Unit, Mail Code 3090, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.

Interleukin-12 (IL12) is a heterodimeric cytokine that plays an important role in the development of cellular immunity. We previously reported the antitumor activity of mouse IL12 (mIL12) transduced by adenovirus in a medullary thyroid carcinoma model. In this study, a rat thyroid follicular cancer cell line (RTC-R2) was employed to develop tumors after sc injection in Wistar rats. In five of five animals, RTC-R2 cells infected with mIL12 transducing adenovirus (AdCMVmIL12) in vitro failed to be tumorigenic in vivo in syngenic rats, whereas four of five animals developed tumors after injection of luciferase transducing adenovirus (AdCMVLuc)-infected cells. After intratumoral treatment with AdCMVmIL12 at 1 x 10⁹ plaque-forming units per rat, 90% (26/29) of animals bearing small (<100 mm³) tumors were apparently cured. Larger tumors treated by injection of AdCMVmIL12 became significantly smaller than AdCMVLuc-treated animals, and growth stabilized. Challenge studies showed that only 3 of 28 animals previously treated and cured with AdCMVmIL12 developed a tumor after sc reinjection of RTC-R2 cells, whereas all animals developed tumors in naïve animals. Thus, AdCMVmIL12-treated animals developed long-term antitumor immunity. We also studied animals with two tumors, injecting virus in one. Tumors regressed at both sites in five of six animals after treatment of one tumor with AdCMVmIL12, and in the sixth animal one site tumor regressed and another tumor continued to grow. In AdCMVLuc-treated animals, both tumors regressed in only one animal, and the reminder continued to grow.

To detect toxicity to liver and other tissues after administration of AdCMVmIL2, the vector was administrated intratumorally or iv at the dose of 2 x 10⁹ plaque-forming units per rat. No change in behavior was observed in any of the treated animals. Rats were killed at different time after virus administration. An overt increase of spleen size was observed 7 d after infection in all animals treated with AdCMVmIL12. All animals given virus IV had some lymphocyte infiltration in the sinusoids and triads of the liver, whereas AdCMVmIL12 injected intratumorally did not cause this effect. Spleens of some virus-treated animals showed decreased white pulp, with apparently increased hematopoiesis. No specific changes were found in lungs and kidneys. Iv administration of AdCMVmIL12 induced a moderate increase of glutamic-oxalacetic transaminase and glutamic-pyruvic transaminase, whereas AdCMVmIL12 injected intratumorally did not.

This study confirms the efficient antitumor activity of an adenovirus expressing mIL12 after in vivo delivery in an animal model and indicates the possibility of application to patients because of the low toxicity.

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