Objective: A previous study showed the inhibitory effects of loratadine on histamine-induced wheal, flare and itch in human skin to be very variable between individuals. It was hypothesised that this variability may have been due to differences in the rates of metabolism of loratadine to its active form, desloratadine. This double blind, crossover study examined the effects of desloratadine in 12 healthy volunteers. Levocetirizine was used as a comparator.
Methods: Desloratadine (5 mg), levocetirizine (5 mg) or placebo was taken orally 4 h before an intradermal injection of histamine (20 microL, 100 microM) or vehicle control into the forearm skin. Flare areas were assessed by scanning laser Doppler imaging before and at 30 s intervals for a period of 9 min. Wheal areas were measured by planimetry at 10 min. Itch was scored every 30 s for 5 min using a visual analogue scale.
Results: Following placebo administration, the mean (+/- SEM) wheal area at 10 min was 79.3 +/- 6.9 mm(2), mean flare area for the first 5 min following challenge 26.6 +/- 2.7 cm(2), and itch score for the same period 48.5 +/- 7.6%. The effects of desloratadine were variable between individuals, mean reductions in the wheal and flare areas being 17% (P = 0.033) and 12% (P = 0.036). Desloratadine did not reduce itch significantly. Levocetirizine was more consistent in its effects, mean reductions in wheal, flare and itch being 51%, 67% 78% respectively (all P < 0.001).
Conclusions: A single dose of 5 mg levocetirizine produced more consistent and greater inhibitory effects on histamine-induced wheal, flare and itch than did 5 mg desloratadine. The difference is suggested to reflect the basic pharmacokinetics of the two drugs.