Targeted delivery of IFN gamma to tumor vessels uncouples antitumor from counterregulatory mechanisms

Because of its immunomodulatory and anticancer activities, IFN gamma has been used as an anticancer drug in several clinical studies, unfortunately with modest results. Attempts to increase the response by increasing the dose or by repeated continuous injection often resulted in lower efficacy, likely due to counterregulatory effects. We show here that targeted delivery of low doses of IFN gamma to CD13, a marker of angiogenic vessels, can overcome major counterregulatory mechanisms and delay tumor growth in two murine models that respond poorly to IFN gamma. Tumor vascular targeting was achieved by coupling IFN gamma to GCNGRC, a CD13 ligand, by genetic engineering technology. The dose-response curve was bell-shaped. Maximal effects were induced with a dose of 0.005 mu g/kg, about 500-fold lower than the dose used in patients. Nontargeted IFN gamma induced little or no effects over a range of 0.003 to 250 mu g/kg. Studies on the mechanism of action showed that low doses of targeted IFN gamma could activate tumor necrosis factor (TNF)-dependent antitumor mechanisms, whereas high doses of either targeted or nontargeted IFN gamma induced soluble TNF-receptor shedding in circulation, a known counterregulatory mechanism of TNF activity. These findings suggest that antitumor activity and counterregulatory mechanisms could be uncoupled by tumor vascular. targeting with extremely low doses of IFNy.