Chloroquine phosphate: a long-term follow-up of drug concentrations in skin suction blister fluid and plasma.

Chloroquine is known to bind strongly to melanin and is accumulated in the skin. In dermatology, the drug is mainly used to treat photosensitivity disorders, but it has also been reported to cause sun sensitivity, especially in patients with rheumatoid arthritis. In the present study the concentrations of chloroquine phosphate in plasma and skin suction blister fluid (interstitial fluid in the skin) from 16 patients were studied by HPLC at steady-state (after 2 months' ingestion of 250 mg of the drug daily) and 2, 4 and 6-7 months after cessation of therapy. At steady-state the concentrations were similar in the two compartments, whereas after discontinuation the drug remained much longer in the skin than in the plasma. In tests using cow's eye melanin in vitro, UV irradiation failed to interact with the binding of chloroquine to melanin. It is speculated that the prolonged storage of the drug in the skin could be of importance for its therapeutic as well as adverse effects.