AbstractPreliminary studies involving small numbers of patients have suggested that interleukin-2 (IL-2) administered by continuous infusion in repetitive weekly cycles using doses of 3 X 106U/M2/day is immunologically active and can induce tumor responses in patients with renal cell carcinoma. This study was designed to examine both the immunological and clinical effects of prolonged infusion IL-2 given by repetitive weekly cycles; first at moderate doses for 4 weeks as an inpatient followed by lower doses of IL-2 for up to 5 months. Prolonged IL-2 treatment was investigated because previous studies revealed that patients had a return to their baseline immune status within 4 weeks after completing IL-2 treatment. Twenty-five patients (including 18 with renal cell carcinoma) were treated with one of two regimens utilizing IL-2 as sole therapy. These regimens were designed to induce augmented and prolonged immune activation based upon in vitro and in vivo data. Though patients on both arms of the study demonstrated sustained lymphocytosis, increase in numbers of natural killer cells, and induction of lymphokine-activated killer activity with prolonged IL-2 administration, only 1 out of the 18 patients with renal cell carcinoma demonstrated a sustained partial antitumor response to therapy. Furthermore, several patients demonstrated profound immune activation, without any evidence of tumor regression. The lack of clinical responses in these patients showing marked activation of LAK cytotoxicity suggests that other variables must also influence the likelihood of antitumor effects for patients receiving IL-2 therapy.