Anti-rheumatic effect of quercetin and recent developments in nano formulation

Rheumatoid arthritis (RA) is a common worldwide chronic autoimmune disease, characterised by synovial hyperplasia, inflammatory cell infiltration, pannus formation and destruction of articular cartilage and bone matrix. It is one of the most common forms of osteoarthritis bestowing high rates of both disability and death. Increasing attention has been paid to the use of natural medicines and natural products in the treatment of RA and patients' acceptance has increased year by year because of their high efficacy and safety. Flavonoids are a group of important secondary metabolites occurring in many plants which have rich biological activities such as anti-rheumatic, vasodilator, and anti-tumor effects. Many successful medical treatments of RA appear to be attributable to the application of flavonoids. Quercetin, a representative active member of the flavonoid family, is found abundantly in many plants, e.g. apples, berries, cabbages, onions, and ginkgo. In recent years, progress has been made in the research of its anti-rheumatoid effects which indicate that it is potentially a noteworthy prodrug for the treatment of RA. However, the poor solubility of quercetin affects its bioavailability and clinical efficacy. This review aims to provide an up to date summary of the biological effects and mechanism of action of quercetin for the treatment of RA, and the research progress made towards nano formulations of quercetin to improve its solubility and efficacy.