CNI-1493, an inhibitor of proinflammatory cytokines, was studied in a Phase I trial in melanoma and renal cancer patients receiving high-dose interleukin 2 (IL-2). Objectives of the study were to define the maximum tolerated dose (MTD) and toxicity of CNI-1493, to assess its pharmacological effects, and to define its pharmacokinetics. Twenty-four patients were treated in sequential cohorts with CNI-1493 doses from 2 through 32 mg/m2 daily. Patients first received only CNI-1493 daily for 5 days. After a 9-day rest, patients received two 5-day courses of IL-2 of 600,000 IU/kg every 8 h for up to 14 doses/course plus daily CNI-1493; courses were separated by a 9-day rest period. CNI-1493 administered alone was well tolerated at doses through 32 mg/m2; MTD was not reached. The only clinical toxicity attributed to CNI-1493 was occasional injection-site phlebitis. Grade 1 creatinine increases occurred in 1 of 7 patients at 4 mg/m2, in 1 of 1 patients at 25 mg/m2, and in 3 of 6 patients at 32 mg/m2 CNI-1493 alone. In combination with high-dose IL-2, CNI-1493 at > or = 25 mg/m2 seemed to exacerbate IL-2-induced nephrotoxicity: grade 3 or 4 creatinine increases developed in 3 of 6 patients at 25 or 32 mg/m2, as compared with 1 of 16 patients at doses < or = 16 mg/m2. The MTD for CNI-1493 given with high-dose IL-2 was 16 mg/m2. The dose-limiting toxicity of IL-2 was hypotension in 63% of patients; overall tolerance to IL-2 was not improved by CNI-1493. However, relative to changes seen in a reference group receiving high-dose IL-2 alone, at doses > or = 4 mg/m2 CNI-1493 did show evidence of pharmacological activity as an inhibitor of tumor necrosis factor production.