Interleukin-2 (IL-2) can mediate in vivo tumor regression at high doses. To enhance this efficacy, we studied the effect of adding a human hybrid recombinant interferon α A/D (rHuIFN-α-A/D) because of its known in vitro augmentation of immune-mediated tumoricidal activity. C56BL/6 mice bearing established pulmonary metastases induced by the iv injection of the methylcholanthrene-induced fibrosarcoma MCA 106 were treated for 12 days with intraperitoneal injections of (1) Hanks' balanced salt solution, (2) recombinant IL-2, (3) rHuIFN-α-A/D, and (4) a combination of IL-2 and HuIFN-α-A/D. IL-2 and interferon each had some antitumor activity. However, maximal reduction of pulmonary metastases consistently resulted from combining IL-2 with interferon. In two of four experiments, this combination was significantly better compared to either IL-2 or interferon treatment alone. The most potent regiment was 12 days of IL-2 (50,000 units bid) together with rHuIFN-α-A/D (50,000 units ip qd). No consistent pattern of proliferative or cytotoxic activity was found against a panel of stimulator and target cells. These results demonstrate enhanced antitumor efficacy of combining recombinant interferon-α and IL-2 against established pulmonary metastases. Potential clinical applications are suggested by these data.
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