4507^ Background: cG250 (Rencarex) is a chimeric monoclonal antibody that binds to the CAIX cell-surface antigen present on 95% of ccRCC. Its safety and activity in phase II studies provided the rationale to investigate its use as an adjuvant monotherapy in subjects with clinically localized high-risk ccRCC—50% of whom progress to metastatic disease. Methods: We performed an international, prospective, multicenter, phase III trial of the efficacy and safety of adjuvant cG250 vs placebo (randomized 1:1) by assessing disease-free (DFS) and overall survival (OS) in 864 RCC patients after nephrectomy. Inclusion criteria were: 1) T3/T4 N0/M0; 2) any T stage and N+/M0; or 3) T1b/T2 N0/M0 high-grade ccRCC. Treatment consisted of a 50 mg loading dose followed by 23-weekly infusions of 20 mg. DFS and CAIX antigen expression were quantified by an independent radiological review and a central pathologist. CAIX score was derived by multiplying the intensity of staining (1–3) by the fraction of positive of cells (0–1), yielding a range of 0.0–3.0. Results: Baseline demographics were well balanced across groups. Treatment demonstrated an excellent safety profile. There were 360 confirmed recurrences and 181 deaths during follow up. When compared with the placebo group, cG250-treated subjects did not enjoy a statistically significant DFS (HR=0.99, p=0.74) or OS advantage (HR=1.01, p=0.94). Median DFS and OS were over 6 years and never reached irrespective of treatment, respectively. However, on subset analysis, subjects with high CAIX expression (>2.0) who received cG250 experienced a statistically significant treatment effect with improved DFS (HR=0.55; p=0.01). Conclusions: While cG250 appears not to have clinical benefit in the adjuvant treatment for all high-risk patients, cG250-treated patients with a high CAIX score appeared to have a significantly improved DFS. The CAIX score may help in stratifying patients who may benefit from cG250 adjuvant therapy. Notwithstanding this benefit, the surprising median DFS of over 6 years and outstanding OS in high-risk patients represent a significant challenge to adjuvant ccRCC drug development. Clinical trial information: NCT00087022.