CYP2C9 and rheumatoid arthritis: NAT2 and CYP2C9 enzymes catabolise sulfa-pyridine to excretable hydroxylamine whereas, 5 aminosalicylic acid is responsible for the pharmacotherapeutic effects in RA.4 Mutations in NAT2 and CYP2C9 lead to the accumulation of hydroxylamine which causes toxicity.