Plasma Protein Binding of the Enantiomers of Hydroxychloroquine and Metabolites

The in vitro binding of the enantiomers of hydroxychloroquine and its three major metabolites in pooled plasma obtained from four healthy volunteers and the binding of the enantiomers of hydroxychloroquine to purified plasma proteins has been investigated. The plasma protein binding of hydroxychloroquine was found to be stereoselective. The (S)-enantiomer of hydroxychloroquine was 64% bound in plasma, while (R)-hydroxychloroquine was 37% bound. Fifty% of (S)-hydroxychloroquine was bound to a 40 g.l-1 solution of human serum albumin, while only 29% of the (R)-enantiomer was bound. The enantioselectivity of hydroxychloroquine binding was reversed in a 0.7 g.l-1 solution of alpha 1-acid glycoprotein with (R)-hydroxychloroquine being bound to a greater extent than its optical antipode (41% versus 29%). The enantiomers of the metabolites of hydroxychloroquine were bound to a similar extent to plasma and purified plasma proteins. Binding of hydroxychloroquine to plasma and purified proteins was found to be linear over the racemic concentration range of 50 to 1000 ng.ml-1 and hydroxychloroquine metabolite binding to plasma was linear over the range 25 to 500 ng.ml-1.