3023 Background: Despite the positive impact of targeted therapies on treatment for mRCC, the efficacy of these agents appears to decrease beyond the first-line setting. There is an unmet need for novel therapies after failure of vascular endothelial growth factor (VEGF)-directed agents. rIL-21, a cytokine that enhances CD8+ T-cell and NK cell activity, has single-agent antitumor activity (J Clin Oncol. 2008;26:2034). Based on promising results of a phase I study of rIL-21 plus sorafenib, we initiated a phase II study to explore the safety and efficacy of this combination as second- or third-line treatment for mRCC. Methods: Patients with mRCC received second- or third-line therapy with sorafenib 400 mg PO BID continuously plus rIL-21 30 μg/kg IV on days 1–5 and 15–19 of each 7-week treatment course (TC). Efficacy endpoints included progression-free survival (PFS) and overall response rate (ORR) per RECIST. Response was assessed by the investigator and by independent radiologic review (IRR). Results: 33 patients were enrolled from 14 sites in the U.S. and Canada. Median age was 61 years (range, 46–75); ECOG performance status was 0 (n=15) or 1 (n=18). Patients had received 1 (n=25) or 2 (n=8) prior lines of therapy, including sunitinib (n=19), temsirolimus (n=5), bevacizumab (n=3), everolimus (n=2), IL-2 (n=11), or other (n=4). Grade ≥3 adverse events considered at least possibly related to study drug and occurring in ≥3 patients included hypophosphatemia (33%), hand-foot syndrome (24%), rash (24%), thrombocytopenia (8%), and neutropenia (8%). Twelve patients remain on study; 13 withdrew for progressive disease (PD), 6 for toxicity, and 2 for other reasons. IRR has been performed for the first 23 patients who completed at least 1 full TC, with 6 confirmed PR (26%), 1 unconfirmed PR (4%), 14 SD (61%), and 2 PD (9%). While median PFS cannot yet be determined, 14 of the first 29 patients have completed at least 3 TCs, equivalent to approximately 21 weeks, with SD or better. Conclusions: rIL-21 plus sorafenib is associated with an acceptable safety profile and promising antitumor efficacy in previously treated patients with mRCC. The observed ORR to date compares favorably with the rate previously reported for sorafenib in the first and second-line setting. [Table: see text]