Theoretical metabolism of flupirtine in man (modified from Methling et al. 2009 19). POD, peroxidase; NAT, N-acetyltransferase; UGT, UDP-glycuronosyltransferase; GSH, gluthathione; D13223, major N-actylated metabolite of flupirtine in man; M424 and M466, mercapturic acid derivatives of the quinone diimine