The potential of
Kathon biocide, an aqueous
solution containing, as active ingredients (a.i.), a mixture of
5-chloro-2-methyl-4-isothiazolin-3-one and
2-methyl-4-isothiazolin-3-one (14.4% a.i.), to produce delayed
contact dermatitis, a sensitization response, was evaluated in outbred Hartley guinea pigs by a modified Buehler's occluded epicutaneous patch technique. The relationship of the response as a function of induction/elicitation concentrations was investigated. Groups of guinea pigs received 9 induction doses of the
biocide, 3 times a week, at concentrations ranging from 25-2000 ppm a.i. These guinea pigs were challenged with the
biocide at concentrations ranging from 20-2000 ppm a.i., and the application sites were scored for
erythema 24 and 48 h after the challenge. The incidence of delayed
contact dermatitis in induced guinea pigs was dependent on both the induction and challenge concentrations. The EC50 (concentration at which delayed
contact dermatitis was seen in 50% of the population) for induction at a challenge concentration of 2000 ppm a.i., a nonirritating concentration, was estimated to be 88 ppm a.i. with a slope of 3.47 probits/unit log concentration. The EC50 for elicitation at an induction concentration of 1000 ppm a.i. was estimated to be 429 ppm a.i. with a slope of 2.74 probits/unit log concentration. These data demonstrate that for
Kathon biocide, there is an induction/elicitation concentration dependency for delayed
contact dermatitis response, and there is a "no response concentration" zone where the
biocide can be used without concern for clinically significant delayed
contact dermatitis. In comparison with a previous study, these data also suggest that the number of induction doses may be an important factor in demonstrating the sensitization potential of a chemical.