The study is divided into a sequential dose escalation phase followed by the expansion of the selected dose in a double-blind, placebo-controlled, evaluation phase. Patients with metastatic or inoperable melanoma and renal cell carcinoma will be receiving high dose IL-2 per approved labeling as two 5-day sequences. M40403 will be administered by intravenous infusion over 30 minutes prior to each intravenous administration of high dose IL-2. Sequential panels of patients will receive increasing doses of M40403 along with IL-2 until an active dose is determined and an MTD is reached. Patients will be followed to determine the effects of M40403 on development of markers of IL-2 dose-limiting toxicity including hypotension, tachycardia, index of renal perfusion, cumulative dose of pressor required and cumulative dose of IL-2 administered. Approximately 48 patients will be studied.