12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Malaria

Hydroxychloroquine is a 4-aminoquinoline antimalarial [see Microbiology (12.4)] and antirheumatic agent.

Rheumatoid Arthritis, Systemic Lupus Erythematosus and Chronic Discoid Lupus Erythematosus
The mechanisms underlying the anti-inflammatory and immunomodulatory effects of hydroxychloroquine in the treatment of rheumatoid arthritis, chronic discoid lupus erythematosus and systemic lupus erythematosus are not fully known.

12.2 Pharmacodynamics

The exposure-response relationship and time course of pharmacodynamic response for the safety and effectiveness of hydroxychloroquine have not been fully characterized.

12.3 Pharmacokinetics

Absorption

Following a single 200 mg oral dose of hydroxychloroquine to healthy male volunteers, whole blood hydroxychloroquine Cmax was 129.6 ng/mL (plasma Cmax was 50.3 ng/mL) with Tmax of 3.3 hours (plasma Tmax 3.7 hours). Following a single oral hydroxychloroquine dose of 200 mg, the mean fraction of the dose absorbed was 0.74 (compared to administration of 155 mg of hydroxychloroquine intravenous infusion). Peak blood concentrations of metabolites were observed at the same time as peak levels of hydroxychloroquine.
After administration of single 155 mg and 310 mg intravenous doses, peak blood concentrations ranged from 1,161 ng/mL to 2,436 ng/mL (mean 1,918 ng/mL) following the 155 mg infusion and 6 months following the 310 mg infusion. Pharmacokinetic parameters were not significantly different over the therapeutic dose range of 155 mg and 310 mg indicating linear kinetics.
In patients with rheumatoid arthritis, there was large variability as to the fraction of the dose absorbed (i.e. 30 to 100%), and mean hydroxychloroquine levels were significantly higher in patients with less disease activity.
Distribution

Hydroxychloroquine is extensively distributed to tissues.

Elimination

A half-life of 123.5 days in plasma were observed following a single 200 mg oral hydroxychloroquine dose to healthy male volunteers. Urine hydroxychloroquine levels were still detectable after 3 months with approximately 10% of the dose excreted as the parent drug. Results following a single dose of a 200 mg tablet versus i.v. infusion (155 mg), demonstrated a half-life of about 40 days and a large volume of distribution. Following chronic oral administration of hydroxychloroquine, the absorption half-life was approximately 3 to 4 hours and the terminal half-life ranged from 40 to 50 days.
Metabolism
Significant levels of three metabolites, desethylhydroxychloroquine (DHCQ), desethylchloroquine (DCQ), and bidesethylhydroxychloroquine (BDCQ) were found in plasma and blood, with DHCQ being the major metabolite.

Excretion

Renal clearance in patients with rheumatoid arthritis treated with hydroxychloroquine for at least 6 months was similar to that in single dose studies in healthy volunteers, suggesting that no change in clearance occurred with chronic dosing. Renal clearance of unchanged drug was approximately 16% to 30%.

12.4 Microbiology

Mechanism of Action in Malaria

The precise mechanism by which hydroxychloroquine exhibits activity against Plasmodium is not known. Hydroxychloroquine is a weak base and may exert its effect by concentrating in the acid vesicles of the parasite and inhibiting polymerization of heme. It can also inhibit certain enzymes by its interaction with DNA.

Antimicrobial Activity

Hydroxychloroquine is active against the erythrocytic forms of chloroquine sensitive strains of P. falciparum, P. malariae, P. vivax, and P. ovale. Hydroxychloroquine is not active against the gametocytes and exoerythrocytic forms including the hypnozoite liver stage forms of P. vivax and P. ovale.
Drug Resistance
P. falciparum strains exhibiting reduced susceptibility to chloroquine also show reduced susceptibility to hydroxychloroquine. Resistance of Plasmodium parasites to chloroquine is widespread [see Indications and Usage (1.1)].