Different physical forms of maleopimaric acid give different allergic responses

Endo‐maleopimaric acid (MPA) is a contact allergen formed when colophonium is “modified” with maleic anhydride or fumaric acid. Previous patch testing showed a higher allergic response to petrolatum (pet.) preparations of MPA in amorphous form compared to MPA in crystalline form. In the present study, the impact of the physical form of MPA on the allergic response was investigated. Since the amorphous form is difficult to standardize, crystalline MPA mechanically incorporated or dissolved in pet. was used. A lower eliciting capacity was obtained from crystalline MPA, compared to that obtained from dissolved MPA, in guinea pigs intradermally induced with MPA. Using 3 H‐MPA a 3× difference in the dissolution into synthetic sweat from MPA dissolved in pet., compared to MPA mechanically incorporated, was demonstrated. A difference in bio‐availability between dissolved and crystalline MPA could therefore be assumed. Crystalline MPA had a low sensitizing capacity compared to that seen for amorphous MPA in previous studies. The amorphous form of MPA is likely to have a larger surface area than crystalline MPA, with less ordered molecules, resulting in a higher dissolution rate and a greater bio‐availability. Modified colophonium exists as amorphous solids and as viscous liquids. Thus, exposure will probably be to non‐crystalline MPA and cases of contact allergy could be overlooked when screening with crystalline MPA.