Cyclosporin A (Cy A), a novel immunosuppressive agent with apparently selective inhibitory effects on T lymphocytes and little myelotoxicity, was tested for its effects on a variety of syngeneic animal tumours including sarcomas, carcinomas and a T-cell lymphoma. Cy A, given orally or parenterally in repeated doses, had no effect on the growth rates of any of the tumours tested, but a highly significant effect on metastasis was seen in many cases. All the sarcomas examined in both rats and mice, and also the lymphoma, showed a marked increase in their metastases, in some cases even when administration of Cy A was delayed until after excision of the "primary" tumour implants. In contrast no effect of Cy A on metastasis was observed in animals bearing poorly immunogenic mammary or squamous-cell carcinomas. The metastases developing in Cy A-treated animals, when transplanted into normal syngeneic animals, showed no evidence of enhanced metastatic potential compared with their "parent tumours.
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