This study will assess the serum uric acid lowering effects and safety of lesinurad in combination with febuxostat over a long-term timeframe.

This is a Phase 3 extension study to assess the long-term efficacy and safety of lesinurad in combination with febuxostat in subjects who completed the double-blind treatment period in Study RDEA594-304.

Inclusion Criteria:

• Subject is able to understand the study procedures and the risks involved and is willing to provide written informed consent before the first study related activity.
• Subject completed the double-blind treatment period in Study RDEA594-304 and was actively receiving and tolerating study medication (lesinurad or placebo) and febuxostat 80 mg at the Month 12 visit.
• Subject is male or female; female subjects of childbearing potential must agree to use an effective non-hormonal method of birth control during the study and for at least 14 days after the last dose of study medication.

Exclusion Criteria:

• Subject has any medical or psychological condition, which in the opinion of the Investigator and/or the Medical Monitor, might create undue risk to the subject or interfere with the subject's ability to comply with the protocol requirements, or to complete the study.