The irritancy of anthralin is inhibited by repeat applications of a subirritant concentration

Anthralin is a safe, effective treatment for psoriasis, but its efficacy is hampered by the side-effects of irritation and staining of the uninvolved skin. To avoid burning, it is customary to start at low concentrations and increase every 48-72 h until the therapeutically effective concentration is reached, which takes time and appears to prolong treatment. We felt that if the minimal erythema concentration (MEC) of anthralin could be determined initially in an individual, this ought to be near or at the final achievable therapeutic concentration. Hence, by analogy with ultraviolet therapy, treatment could start just below this concentration and thus avoid delay. A series of concentrations of anthralin in Lassar's paste was applied to the back for 3 h, and erythemal responses assessed at 24 and 48 h. MECs (0.015-0.03%) were far below those usually reached during normal therapy. To test the possibility that the skin was adapting to anthralin, we pretreated areas of skin with a subirritant concentration of anthralin (0.007%) for 3 h on 2 consecutive days prior to application of the full dose series. On the pretreated areas, the MEC increased fourfold from 0. 015% to 0.06% (P < 0.01); the concentration of anthralin required to produce the mid-point on the dose-response curve increased from 0. 06% to 0.25% (P = 0.01), demonstrating a clear adaptive response. One pretreatment produced a 52% reduction in erythema compared with control challenge, and maximal 61% inhibition was seen after three applications. Pretreatment with a subirritant concentration of a control irritant, croton oil, had no effect on the response to anthralin and vice versa. Pretreatment of skin with danthron, the non-irritant oxidation product of anthralin, had no effect, suggesting that the attenuation effect is specific to native anthralin. To see whether the attenuation might be due to modulation of xenobiotic metabolizing enzymes, skin was pretreated with inducers and inhibitors of the cytochrome P450 and NADPH-dependent quinone reductase (NDQR) enzyme systems. However, no effect was seen. In conclusion, we have shown that the irritant response to anthralin is attenuated by repeated applications of a subirritant concentration of anthralin; this is not a non-specific response to all irritants, but a specific property of native anthralin, and the enzymes P450 and NDQR are apparently not responsible for this effect.