Interferon treatment markedly inhibits the development of tumor metastases in the liver and spleen and increases survival time of mice after intravenous inoculation of friend erythroleukemia cells

To investigate the effect of interferon treatment on the development of tumor metastases, DBA/2 mice were injected i.v. with 2 × 10 6 Friend erythroleukemia cells (FLC) (equivalent to about 5 × 105 LD 50 ). FLC multiplied rapidly in the liver and spleen and all untreated or control treated mice died between 7 and 12 days. Daily treatment of mice with potent preparations of mouse interferon α/β was initiated 3 to 72 he after i.v. inoculation of tumor cells, at times when FLC were already present in the liver and spleen. Interferon treatment resulted in a 100 to 1,000‐fold inhibition of the multiplication of FLC in the liver and spleen and a marked increase in mean survival time. Small numbers of tumor cells persisted in the liver and spleen in some interferon‐treated mice and could be recovered by bioassay several weeks after tumor inoculation‐Most interferon‐treated mice died with tumor in the ensuing months. Three of 34 Interferon‐treated mice were considered cured as they were alive at 386, 325 and 284 days after tumor inoculation. Daily treatment of tumor‐inoculated mice with human recotnbinant interferons αD and αBDDD, which had antiviral activity on mouse cells in culture, also increased the survival time of mice injected i.v. with FLC. The use of the interferon‐resistant 3C 18 line of FLC suggests that the marked inhibition of development of established liver and spleen metastases was not due to a direct effect of Interferon on the tumor cells, but was host‐mediated.