Topical distribution of acyclovir in normal equine skin and equine sarcoids: An in vitro study

- Sarcoid skin is less permissive for acyclovir permeation compared to normal skin.
- Even in sarcoid skin, considerable acyclovir concentrations are found in the dermis.
- Research should now focus on the susceptibility of BPV DNA polymerase for acyclovir.

Topical acyclovir application is an owner-friendly treatment for occult equine sarcoids, without the caustic side-effects other topical treatments have. Variable clinical success rates have been described, but it is not known to what rate and extent acyclovir penetrates in and through equine skin from a topical formulation.In the current study, an in vitro Franz diffusion model was used to determine the permeation parameters for a generic 5% acyclovir cetomacrogol cream for both healthy and sarcoid equine skin. The distribution of acyclovir between different layers of both skin types was also evaluated.While acyclovir penetrated through both skin types, significantly less acyclovir permeated to the deep dermis of sarcoid skin (197.62 ng/mm3) compared to normal skin (459.41 ng/mm3). Within sarcoid skin samples, significantly higher acyclovir concentrations were found in the epidermis (983.59 ng/mm3) compared to the superficial dermis (450.02 ng/mm3) and the deep dermis. At each sample point, significantly more acyclovir permeated to the receptor fluid through normal skin compared to sarcoid skin, which is reflected in the significantly higher permeation parameters of normal skin.Normal skin was found to be more permissive for acyclovir, but even in sarcoid skin, enough acyclovir reached the deep dermis to treat a Herpes simplex virus infection. In the case of equine sarcoids, the treatment is aimed at the Bovine papillomavirus and no information is available on the susceptibility of the DNA polymerase of this virus for acyclovir. Therefore, further research is needed to determine the efficacy of acyclovir to treat equine sarcoids.