Ultraviolet B irradiation inhibits the induction of photoallergy to systemically administered quinidine in the mouse.

In albino mice photosensitized to quinidine, 100 mg/kg by intraperitoneal injection, pretreatment of the induction area with ultraviolet B (UVB) on 3 consecutive days was shown to significantly reduce the inflammatory response when the mice were challenged at a distant site 1 week later. Mice controlled for phototoxicity did not react. The inhibition was dose-dependent within the UVB dose range tested (0.05-1.0 J/cm2 x 3), being almost complete with the highest doses. Inhibition, although somewhat less pronounced, was also seen when an area on the back, distant from the induction site, was preirradiated following a similar protocol. The reduced response at elicitation persisted when the time to challenge was increased up to 4 weeks. At 5 weeks, a second attempt to photosensitize the previously inhibited animals failed, suggesting that a state of tolerance had been acquired. The timing of the UV exposure in relation to the photoactive chemical is critically important in determining whether an exposure promotes or inhibits photoallergic sensitization.