Despite the introduction of numerous biologic agents for the treatment of 
rheumatoid arthritis (RA) and other forms of inflammatory arthritis, low-dose 
methotrexate therapy remains the gold standard in RA therapy. Methotrexate is 
generally the first-line drug for the treatment of RA, psoriatic arthritis and 
other forms of inflammatory arthritis, and it enhances the effect of most 
biologic agents in RA. Understanding the mechanism of action of methotrexate 
could be instructive in the appropriate use of the drug and in the design of new 
regimens for the treatment of RA. Although methotrexate is one of the first 
examples of intelligent drug design, multiple mechanisms potentially contribute 
to the anti-inflammatory actions of methotrexate, including the inhibition of 
purine and pyrimidine synthesis, transmethylation reactions, translocation of 
nuclear factor-κB (NF-κB) to the nucleus, signalling via the Janus kinase 
(JAK)-signal transducer and activator of transcription (STAT) pathway and nitric 
oxide production, as well as the promotion of adenosine release and expression 
of certain long non-coding RNAs.