Abstract
Six different pure human interferons (IFNs). were tested for anti‐tumour effect against two human carcinomas, breast and bowel, growing in nude mice, in a total of 36 experiments. The IFN‐α mixture, analogue and subtypes showed the greatest activity, particularly against the breast cancer xenograft, whereas IFN‐β and IFN‐γ had little effect. However, circulating IFN could be found in the sera of mice treated with all 3 IFN types. In terms of amount of IFN protein, IFN‐αCon~1~, an IFN‐α analogue, was the most effective, a dose of 0.1 μg/mouse/day being sufficient to induce breast tumour regression, and IFN‐γ the least effective, a dose of 10 μg/mouse/day having no effect on the same tumour. A more detailed comparison of 2 IFN‐α subtypes showed that a daily dose of 1 μg IFN‐αA was more effective than the same dose of IFN‐αD, but as this IFN had approximately 30 times less antiviral activity on human cells than IFN‐αD, these IFNs were probably at least equally effective in terms of human cell units. IFN‐αD stimulated mouse spleen natural killer cell activity but it was not clear whether this stimulation was involved in anti‐tumour activity. We conclude that this model system is useful for investigating direct anti‐tumour activity of a wide range of IFN types and subtypes.