D&C nos. 10 and 11: chemical composition analysis and delayed contact hypersensitivity testing in the guinea pig

D&C Yellow no. 11 was found to consist predominantly of quinophthalone together with other minor components, though not 6'-methylquinophthalone. Quinophthalone was found to be a strong sensitizer in guinea pigs by means of a modified testing technique, and its threshold concentration for induction and challenge was 10 ppm. Purified D&C Yellow no. 10, which was found to consist of 6 derivatives of sodium sulfonated quinophthalone, did not sensitize or elicit a cross-reaction in quinophthalone-sensitized guinea pigs even at a challenge concentration of 5%. Commercial D&C Yellow no. 10 contained 0.95-170 ppm of quinophthalone, and it is suggested that skin sensitization by D&C Yellow no. 10 is caused by contaminating quinophthalone.