Diphenylcyclopropenone (DPCP) has recently been reported to be a potent contact sensitizer for the treatment of alopecia areata. DPCP is nonmutagenic in the Ames assay. Studies on chemical purity, stability, and mechanisms of action are lacking. DPCP is synthesized by cyclization of α,α′-dibromodibenzylketone. The latter elicits a reaction in guinea pigs sensitized to DPCP. We examined a commercial sample of DPCP by gas chromatography-mass spectrometry for the presence of contaminants. The stability of dilute solutions of DPCP in ethanol and cyclohexane to fluorescent light, sunlight, and heat was followed by ultraviolet (UV) spectrometry. The isosbestic point and half-life for this reaction were determined. We found no original or intermediate reagents in our sample. DPCP was found to decompose, forming diphenylacetylene on exposure to both sunlight and fluorescent light in less than 2 weeks. Samples shielded from the light at -70° C and at room temperature were stable over a 4-week period. The extent of decomposition was the same in both ethanol and cyclohexane, It is possible that DPCP may act as a prosensitizer with the actual sensitizer liberated by a photoactivated intermediate (stable, metastable, or unstable) or a combination of these.
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