AMPD1 and rheumatoid arthritis: Polymorphisms of adenosine monophosphate deaminase 1 (AMPD1), aminoimidazole carboxamide ribonucleotide transformylase (ATIC), and inosine triphosphate pyrophosphatase (ITPA), which are involved in the adenosine pathway, have been associated with good clinical response to MTX in patients with RA in some but not all studies [26,27].