CYP2D6 and systemic lupus erythematosus: Pharmacogenetic polymorphisms can affect the rate of HCQ metabolism in vivo and may be an important reason for the differences in blood levels.10 11 HCQ is metabolised by cytochrome P450 (CYP450) enzymes (CYP2D6, CYP2C8, CYP3A4 and CYP3A5) into three products, including desethylhydroxychloroquine (DHCQ), desethylchloroquine (DCQ) and bisdeethylchloroquine, of which DHCQ is the main active metabolite of HCQ.12 13 Specific single nucleotide polymorphisms (SNPs) in CYP450 enzymes may impact HCQ activity, leading to differences in drug concentrations in patients with SLE.