Background: The overexpression of hypoxia induced factor 1a/2a in ccRCC leads to up-regulation of vascular endothelial growth factor (VEGF) that results in increased angiogenesis, tumor metastasis, and treatment resistance. Using several preclinical xenograft models, it has been demonstrated that therapeutic doses of the selenium-containing molecules, seleno-L-methionine (SLM) and methylselenocysteine (MSC) caused enhanced degradation of HIF1α/2α, down-regulation of oncogenic miRNA-210 and 155, up-regulation tumor suppressor miRNA-664 and LET-7b, and stabilization of tumor vasculature, yielding higher tumor drug uptake and protection from toxic side effects when combined with chemotherapeutic and VEGF-targeted agents. Methods: This is a phase I (3+3 design) dose finding trial of SLM (2500, 3000 or 4000 µg) given orally twice daily for 14 days, followed by once a day in combination with standard dose axitinib to patients with metastatic RCC. Primary endpoint is safety. Secondary endpoint is efficacy including overall response rate (ORR), progression free survival (PFS) and overall survival (OS). Results: To date, 12 evaluable patients (pts) with metastatic RCC who progressed on one or more prior lines of treatment were enrolled. The first 3 pts were treated at 4000 µg, the second and third 3 pts were treated at 2500 and 3000 µg respectively. Additional 3 pts were added to 4000 µg. No dose limiting toxicity (DLT) was seen. Most common AEs included fatigue, diarrhea, hypertension, nausea, anorexia, cough, proteinuria and weight loss. Of the 4000 µg cohort, 2 pts achieved CR with ongoing responses at 31 and 29 months, 1 pt had PR for 24 months and 1 had PD at 3 months, 2 pts are not assessed yet. Of the 2500 µg cohort, 1 pt with ongoing PR for 21 months, 1 pt had stable disease for 6 months, and 1 pt had PD at 2 months. The 3000 µg cohort, one pt has ongoing PR for 12 months; another pt had PR lasting 10 months, the 3^rd pt had SD for 4 months. Conclusions: High dose SLM is safe in combination with axitinib, with promising efficacy. Further data including biomarkers will be presented at the meeting. Clinical trial information: NCT02535533