In vivo and ex vivo antitumor activity in patients receiving low-dose subcutaneous recombinant interleukin-2

Alterations in cell-mediated cytotoxicity levels were studied in patients receiving recombinant interleukin-2 (rIL-2) via subcutaneous injection. Fourteen outpatients, aged 36-68 years, with progressive metastatic malignancies, were treated with weekly escalated doses of rIL-2, starting at 1.8 IU/m2/day for 6 days a week, up to 14.4 IU/m2/day during the 4th week of therapy. Patients presenting with stable disease thereafter were started on maintenance therapy and received 10.8 IU/m2 once weekly for up to 12 weeks. Patient mononuclear cells were isolated from fresh peripheral blood at various times throughout the treatment. Cells were assayed prior to and after further in vitro stimulation by rIL-2 (600 IU/ml for 7 days). Natural killing (NK) activity was measured by cytolysis of K 562 target cells, and lymphokine-activated killing (LAK) was determined by cytotoxicity against Daudi targets, respectively, in four effector:target ratios (E:T), using a standard 2-hour europium3+ release assay. Spontaneous NK cell function (E:T = 25:1) of freshly isolated peripheral blood mononuclear cells (PBMC) was enhanced significantly after 28 days of therapy (27.8 vs. 9.1% on day 0). LAK activity also markedly increased during therapy (26.2 vs. 5.4% on day 0). Further in vitro culture of these PBMC in the presence of rIL-2 resulted in day 28 non-MHC-restricted cytolytic activity of 63.2% (40.3% on day 0) against K 562 targets, and 64.9% (39.6% on day 0) against Daudi targets. Activation of cytolytic function by rIL-2 appeared to be dose-dependent, as measurable lytic capability decreased throughout maintenance therapy, while neither sex nor tumor entity prior to therapy or clinical response were correlated with cytotoxicity levels. Taken together, our observations demonstrate that stimulation of the non-MHC-restricted pathway of cytolytic activation, as measured by lysis of target cells, arises in patients treated with rIL-2 doses 5- to 30-fold lower than used previously in intravenous protocols, connecting effective clinical response rates with acceptable tolerability.