Background: Uveal melanoma is the most common primary intraocular cancer in adults. Despite successful treatment of primary tumors, up to 50% of patients later die of distant metastases. Currently, no effective adjuvant treatment is available. Presence of both monosomy 3 and 8q amplification (M3 + 8q amp) or DecisionDx-UM Class 2 in the primary uveal melanoma characterizes a group of patients with high metastatic death rates with reported 2-year survival rates ranging from 50% to 78%. This study aims to decrease or delay the death from metastatic uveal melanoma. Methods: Uveal melanoma patients with high risk for systemic metastasis defined as any of the following were eligible: A) M3 + 8q amp; B) Class 2. Patients must show no evidence of systemic metastasis and they need to be enrolled within 6 months of initial treatment of primary uveal melanoma. Patients were randomized to receive either sunitinib 25 mg daily or valproic acid (VPA) 750 mg daily as adjuvant treatment for 6 consecutive months. Improvement of 2-year overall survival (OS) rate from historical reference (70%) to 85% was the primary endpoint. The secondary endpoints included 1) systemic relapse-free survival (RFS) rate at 18 months, 2) ability to complete adjuvant treatment and, 3) toxicity assessment. The study was not powered to compare the efficacy between each arm. Results: A total of 90 patients were enrolled in this study. Two patients were excluded from the study including one in the sunitinib arm (did not start treatment after randomization) and one in the VPA arm (refused to stop VPA at 6 months). Nine of 45 patients in the sunitinib arm and 4 of 43 patients in the VPA arm could not complete the 6-month treatment due to toxicity (sunitinib n = 6, VPA n = 2) or systemic progression (sunitinib n = 3, VPA n = 2). The rest of patients completed the 6-month course of study treatments and all patients were followed at least for 2 years. With a median follow-up of 40.2 months, the 2-year OS rates of the sunitinib and VPA group were 95.6% (90% CI 86.5-98.6) and 90.7% (90% CI 80.1 - 95.8), respectively. The 18-month RFS rates of the sunitinib and VPA group were 75.6% (90% CI 63.1 - 84.3) and 62.8% (90% CI 49.4 - 73.5), respectively. Conclusions: Although the study is still ongoing, adjuvant sunitinib and VPA were considered to be safe and tolerable treatments for high-risk uveal melanoma. Sunitinib showed a tendency for a better outcome, thus a Cohort 2 was created to investigate the safety and potential improvement of 18-month RFS rate with 12 months of treatment with sunitinib. Clinical trial information: NCT02068586