Hydroxychloroquine (HCQ) is a racemic antiarthritic agent that has a l
ong half-life (t(1/2)) in plasma and accumulates in blood cells. To st
udy the relationships between HCQ concentrations in plasma, serum, and
whole blood and to determine the optimal blood fraction to use for th
erapeutic drug monitoring of the drug, we studied the relative distrib
ution of the HCQ enantiomers in various fractions of human blood under
in vivo and in vitro conditions. Substantially greater concentrations
of both enantiomers were found in serum as compared with plasma becau
se of release via platelet activation. After in vitro incubations of t
he separated blood cells with HCQ, high concentrations of both enantio
mers were found in leukocytes, and low concentrations in erythrocytes
and platelets; the R:S ratio in vitro was near unity in all of the cel
ls examined. Unlike the in vitro cellular uptake, the concentrations o
f HCQ in vivo were significantly lower and stereoselective (R:S ratio
= 2). There was almost no drug in the polymorphonuclear cells (PMN) in
vivo, despite a substantial uptake in vitro after incubation of separ
ated cells. The enantiomeric (R:S) ratio in the urinary excretion of t
he enantiomers was significantly correlated with that in plasma. The p
lasma-protein binding of the enantiomers was stereoselective and compl
imented the cellular uptake findings; the unbound fraction was depende
nt on the plasma concentrations of alpha(1)-acid glycoprotein, but not
albumin. Although concentrations in whole blood correlated well with
those in lymphocytes and monocytes (the proposed site of HCQ action),
stronger correlations were found between concentrations in serum and i
n the mononuclear cells.