Effects of Nitric Oxide (NO) Synthesis Inhibition on Antitumor Responses during Interleukin-2 (IL-2) Treatment of Mice

OBJECTIVES: To evaluate if L-arginine: NO pathway is activated in tumor tissues during IL-2 therapy and to evaluate whether IL-2 induced NO synthesis represents an antitumor effector muchanism or an inhibitory factor against therapeutic effects of IL-2. METHODS: Four groups[untreated control, N(G)-monomethyl-L-arginine(MLA) therapy only, IL-2 therapy only, IL-2/MLA therapy groups] of BALB/c mice were injected intraperitoneally with 2×10(6) Meth A tumor cells on day 0. MLA was administered subcutaneously with Alzet continuous infusion pumps on day 2. IL-2 therapy (180,000 IU s.c. every 12h for 5 days) was started on day 3. NO production within ascites tumors was assessed by measuring nitrite concentrations in cultures of ascites cells harvested on day 8. Survival and the rate of body weight increment of the mice were measured to evaluate therapeutic responses. Daily urinary nitrate excretion was monitored to demonstrate the effectiveness of MLA in inhibiting NO synthesis. RESULTS: Nitrite production in supernatants of Meth A ascites cell cultures was 63±14 μM in IL-2 treated mice and 3.2±1.5 μM in untreated controls (p < 0.001). MLA prevented the IL-2 therapy induced increase in nitrite production. IL-2 therapy did not decrease the rate of body weight increment and marginally prolonged mean survival to 18.2 days, compared to 16.6 days in control mice (p=0.255). MLA administration decreased the rate of body weight increment and prolonged mean survival of IL-2 treated mice (21.8 days, p=0.001 versus IL-2 alone). Interestingly, the MLA treatment increased the rate of body weight increment and diminished the survival of control mice to 11.6 days (p=0,003). MLA administration via Alzet continuous infusion pumps achieved appoximately 60% suppression of urinary nitrate excretion by control mice. Subcutaneous IL-2 treatment strongly induced nitric oxide synthesis (up to 3.5 μmoles of urinary nitrate/mouse/day). MLA also effectively suppressed IL-2 induced NO production. CONCLUSION: L-arginine: NO pathway can be activated in malignant ascites, by IL-2 therapy and NO synthesis functions as an inhibitory mechanism against IL-2 induced anti-tumor effects.