Adefovir dipivoxil is the diester form of adefovir, an antiviral prodrug that displays immunomodulatory activity. Adefovir is an acyclonucleotide analog that inhibits viral DNA polymerase, acting as a DNA chain terminator. Adefovir is primarily used to treat hepatitis B infection, but also displays activity against retroviruses. Adefovir decreases levels of Treg cells and increases levels of IL-4, IL-2, TNF-α, and IFN-γ. Additionally, this compound may show anti-arthritic activity as well, inhibiting production of NO and expression of RANTES and acting both as a prophylactic or a rescue treatment in animal models of adjuvant-induced arthritis.