Acetophenone Azine

Abstract

Acetophenone azine (CAS 729-43-1) was recently discovered as a potent allergen in shin pads and footwear containing the foam elastomer ethyl vinyl acetate. The compound is probably not intentionally added to ethyl vinyl acetate but is likely the result of reactions between other additives that take place during the manufacturing process. A patch testing concentration of 0.1% in acetone or petrolatum is recommended. Acetophenone azine should be part of patch testing shoe series, as well as plastics and glues series. The compound is not currently available from patch testing materials suppliers, a situation that hopefully will be corrected.

Beginning in 2016, a small number of publications have described the occurrence of approximately a dozen cases of severe allergic contact dermatitis secondary to shin pads or footwear.^1–6 A common characteristic of the responsible objects was the presence of cushioning foam made of ethyl vinyl acetate (EVA), a largely used thermoplastic, elastomeric copolymer of ethylene and vinyl acetate.

CASE REPORTS

The first reported case was a 13-year-old boy who wore shin pads when playing soccer. He developed a contact dermatitis that was initially located on both shins (Fig. 1) but eventually became progressively more severe and widespread (Fig. 2). Comprehensive patch testing was carried out in multiple sessions with the European baseline series, rubber series, plastics/glues series, and shavings from the EVA foam lining the inner side of the shin guards. The only positive reaction was to the EVA foam. Chemical analysis of this material by high-performance liquid chromatography revealed the presence of acetophenone azine. The chemical was bought from Sigma-Aldrich (St Louis, MO) and was diluted in water and acetone to prepare stock solutions at 1.0% wt/vol. The patient was then tested with serial dilutions of these stock solutions, which gave positive reactions down to a concentration of 0.1% in water and 0.001% in acetone (Fig. 3).¹

Figure 1: Case 1. Contact dermatitis localized to the area covered by shin pads. Reproduced with permission from John Wiley & Sons A/S.

Figure 2: Case 1. Widespread extension of the initial dermatitis.

Figure 3: Case 1. Results of patch tests with dilutions of acetophenone azine in water and acetone. Reproduced with permission from John Wiley & Sons A/S.

Additional reports have described similar cases, mostly in young patients wearing not only shin pads but also sports shoes or flip-flops, either de novo or after sensitization from shin pads.^2–6 When secondary to footwear, the dermatitis presented either as dyshidrosiform, vesiculobullous eczema, sometimes accompanied by palmar lesions, or as plantar hyperkeratotic dermatitis. Widespread dissemination was also often seen in these cases. Some patients healed with scarring and marked postinflammatory hypopigmentation.^2,4

These patients were all children or teenagers, except for a 29-year-old male hockey player who developed contact dermatitis to his shin pads and subsequently to his sneakers.³ All patients were Europeans, with the exception of a Canadian soccer-playing child who became sensitized to his shin pads and thereafter reacted to 2 brands of sports shoes.⁶ Chemical analysis revealed a higher amount of acetophenone azine in shin pads than in footwear.³ This, coupled with friction and sweating, a wider area of exposure over thinner skin than the sole of the foot, probably explains why most subjects became sensitized from their shin pads. We believe that primary sensitization from other EVA-containing items will ultimately be reported.

WHAT IS ACETOPHENONE AZINE?

The chemical formula of acetophenone azine is C₁₆H₁₆N₂, and its CAS is 729-43-1 (Fig. 4). It is also known as 1-phenylethan-1-one (1-phenylethylidene), hydrazone, or methylphenyl ketazine. Whereas ketazines in general may be used as industrial biocides,⁷ acetophenone azine most of the time is used as an intermediate in organic synthesis. As found on the Internet, section 11 of its Material Safety Data Sheet clearly mentions that “prolonged or repeated exposure may cause allergic reactions in certain sensitive individuals. The preceding data, or interpretation of data, was determined using Quantitative Structure Activity Relationship (QSAR) modeling.”⁸

Figure 4: Molecular structure of acetophenone azine.

The presence of this compound in sports equipment and footwear is poorly explained. It is likely not intentionally added to EVA but is believed to be generated during the manufacturing process, probably from reactions between other additives. It is hypothesized that acetophenone arises from the degradation of the initiator dicumylperoxide and hydrazine from the foaming agent azodicarbonamide.²

PATCH TESTING

Acetophenone azine is not currently available from suppliers of patch testing materials and needs to be obtained from distributors of chemical products. Based on the available publications, a concentration of 0.1% in acetone or petrolatum is recommended, the latter vehicle potentially conferring greater stability to the preparation. From the clinical presentation of the reported cases, acetophenone azine seems to be a potent allergen, likely present in numerous items such as sneakers, flip-flops, ski boots, insoles, shin pads and other protective sports equipment, swimming goggles, bicycle saddles, and so on (Table 1). With the COVID-19 pandemic, EVA is liable to be found in some personal protective equipment, such as the headband of face shields.⁹ Given the recent discovery of this allergen, it is presumed that cases of allergic contact dermatitis would have been missed and labeled irritant contact dermatitis or dyshidrosis. The inclusion of acetophenone azine in shoe series, or in any plastics and glues series, seems highly desirable.

TABLE 1 - Summary of Published Cases

Reference	Country of Origin	Age	Sex	Source	Test Materials	Concentration, %	Vehicle	Patch Tests Results
								D2	D3/D4
Raison-Peyron et al1	France	13	M	Shin pads	Shin pads	100	Aqua	++	+++
					Shin pads	100	Acetone	++	+++
					Shin pads	100	Ethanol	++	+++
					AA	1	Aqua	++	++
					AA	0.1	Aqua	+	+
					AA	0.01	Aqua	−	−
					AA	0.001	Aqua	−	−
					AA	0.0001	Aqua	−	−
					AA	1	Acetone	++	++
					AA	0.1	Acetone	++	++
					AA	0.01	Acetone	+?	+
					AA	0.001	Acetone	−	+?
					AA	0.0001	Acetone	−	−
Raison-Peyron et al2	France	11	M	Shin pads	Shin pads	100	Aqua	++	++
					Shin pads	100	Acetone	++	++
					Shin pads	100	Ethanol	++	++
					Flip-flops	100	Aqua	++	++
				Flip-flops	Flip-flops	100	Acetone	++	++
					Flip-flops	100	Ethanol	++	++
					AA	0.1	Acetone	++	++
					AA	0.01	Acetone	++	++
		12	M	Sneakers	Sneakers	100	Aqua	++	++
					Sneakers	100	Acetone	+	+
					Sneakers	100	Ethanol	−	−
					AA	0.1	Acetone	++	++
					AA	0.01	Acetone	++	++
De Fré et al3	Belgium	29	M	Shin pads	Shin pads	100	Acetone	+	++
				Sports shoes	Sports shoes	100	Acetone	+	++
					AA	0.1	Acetone	++	++
					AA	0.01	Acetone	+	+
Koumaki et al4	United Kingdom	17	M	Shin pads	Shin pads	100	Aqua	++	+
					AA	0.1	Acetone	++	++
					AA	0.01	Acetone	+	−
					AA	0.001	Acetone	+	−
					AA	0.0001	Acetone	−	−
					AA	0.00001	Acetone	−	−
Darrigade et al5	France and Belgium	7	M	Shin pads	Shin pads	100	As is	−	+
					AA	0.1	Petrolatum	−	+
		12	M	Shin pads	Shin pads	100	As is	++	++
				Flip-flops	Flip-flops	100	As is	++	++
					AA	0.1	Petrolatum	++	++
		12	M	Shin pads	Shin pads	100	As is	++	++
				Sneakers	Sneakers	100	As is	++	++
					AA	0.1	Petrolatum	++	++
		14	M	Shin pads	Shin pads	100	As is	?	++
				Sneakers	Sneakers	100	As is	?	++
				Flip-flops	Flip-flops	100	As is	?	++
					AA	0.1	Petrolatum	−	++
		13	M	Shin pads	Shin pads	100	As is	+	+
					AA	0.1	Petrolatum	−	+
		13	M	Shin pads Nike	Shin pads Puma	100	Aqua	++	++
				Shin pads Puma	Shin pads Nike	100	Aqua	++	++
				Sneakers	Sneakers	100	Aqua	?	+
				Sneakers	Sneakers	100	Aqua	+	++
					AA	0.1	Petrolatum	+++	+++
Besner Morin et al6	Canada	6	M	Shin pads	Shin pads	100	Aqua	+	+
				Sneakers	Adidas cleats	100	Aqua	−	+
				Soccer cleats	AA	1	Petrolatum	+	+
					AA	0.1	Petrolatum	+	+

Before application for patch testing, some pieces of shin pads, sneakers, flip-flops, or cleats were moistened either with water (aqua), acetone, or ethanol.

AA, acetophenone azine; D2, day 2; D3/D4, day 3/day 4; M, male.

A study published in 2018 by ANSES (a French agency for food, environmental, and occupational health and safety), entitled “Assessment of the Skin Sensitizing/Irritant Effects of Chemicals Found in Footwear and Textile Clothing,” revealed that 14% of sampled footwear contained acetophenone azine.¹⁰ Recently, the French authorities have filed a request to the European Chemicals Agency to register acetophenone azine as a cutaneous sensitizer in the Registration, Evaluation, Authorization and Restriction of Chemicals regulation.

CONCLUSIONS

The discovery of acetophenone azine as a potent, previously unrecognized allergen is another example of the beneficial results of fruitful collaboration between dermatologists and chemists. Given the wide use of EVA foam, especially in sports equipment, the suppliers of patch testing materials should make acetophenone azine 0.1% petrolatum rapidly available to the dermatology community.