Damage control: Sunburn cell assay used to demonstrate antioxidant abilities of agents

August 1, 2009
Zoe Diana Draelos, M.D.

Zoe Diana Draelos, M.D., is a consulting professor of dermatology, Duke University School of Medicine, Durham, N.C. She is investigator, Dermatology Consulting Services, High Point, N.C., and a Dermatology Times Editorial Advisor and co-medical editor.

Antioxidants function to donate an electron to an unstable, highly energetic oxygen species. Thus, they prevent damage. In order to determine the value of a certain antioxidant ingredient, it is necessary to have an evidenced-based, validated method for comparison.

Key Points

Q How can the antioxidant ability of an oral or topical agent be physiologically demonstrated?

The current methodology used to demonstrate oral and topical antioxidant abilities is known as the sunburn cell assay. This is performed by selecting subjects with Fitzpatrick type I skin and determining the minimal erythemal dose (MED) required to induce reddening (sunburn) of the skin. Once this dose has been determined, it's important for all subjects to receive the proper amount of radiation so the degree of skin-reddening can be standardized. If subjects with Fitzpatrick type I skin are chosen, an adequate radiation dose is 2MED.

Twenty-four hours after radiation, the subjects are invited to return, and a skin biopsy is taken from the reddened area of skin. It is preserved in formalin and stained with H&E. The sunburn cells, which appear as deeply pink globes within the dermis, are then counted.

Also known as apoptotic cells, the sunburn cells have been sublethally injured by the UVB radiation. They have prematurely entered the cell death cycle, a mechanism used by the body to remove damaged, dysfunctional cells.

This UVB-induced damage can be prevented by antioxidants, which can be administered topically or orally. If topical antioxidants are tested, they can be applied to the area where the UVB radiation will be administered. If administered orally, a systemic would be expected, and the biopsy can come from any desired location.

It is key in sunburn cell studies to have a control, which can be two biopsies from the same individual; one from a UVB-radiated treatment site and one from a UVB-radiated nontreatment site; or two randomized groups can be enrolled in the study, one using a placebo cream and the other using the antioxidant-containing cream.

The sunburn cell counts are compared between the two sites. If an antioxidant is effective, there will be a statistically significant reduction in the sunburn cell count with treatment.

The sunburn cell assay is an invasive, challenging test to perform. It requires special equipment and subjects willing to undergo a skin biopsy. But it is the best test, since it assesses a physiologic reaction to a relevant skin oxidative insult.

Why is it so hard to demonstrate the antioxidant ability of a substance in clinical trials? Because antioxidants prevent damage - they do not repair damage. Since oxidative damage is cumulative over a lifetime, it takes many years to visibly see the effect of oxidation prevention.

A good study would probably give oral or topical antioxidants over a 10-year period to determine the benefits of the treatment. This is impractical, especially since there appears to be tremendous interindividual variability in resistance to oxidative stress. The study would probably need to enroll more than 500 subjects as well.

Zoe Diana Draelos, M.D., is a Dermatology Times editorial adviser and investigator, Dermatology Consulting Services, High Point, N.C. Questions may be submitted via e-mail to zdraelos@northstate.net

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