Recent studies of PQ metabolism have revealed interesting new information about the metabolic pathways, suggesting that in humans the two enantiomers of PQ (which is used as a racemic mixture) are differentially metabolized to CPQ . While the R(–) form of PQ is readily converted to CPQ, the S(+) form is much less so, and in fact >95 % of the CPQ circulating is attributable to R(–) PQ. This raises the question as to the primary metabolic route for the S(+) form. Furthermore, other studies revealed several other major metabolites of PQ generated in human hepatocytes  and some of these are formed predominantly or exclusively from one enantiomer of PQ [9, 21].