Recent research shows that the rate at which medications are absorbed through the skin may differ depending on which part of the body is being treated.
Medications are often applied topically to treat localised problems. The aim is to provide high levels of the active ingredient in the affected area. To achieve similar levels by giving the drug systemically (by injection or by mouth) would require a larger dose.
As the drug is applied directly, there is no need for high levels in the blood. This is particularly useful if the drug being used has adverse effects when given systemically. The lower blood levels also means it takes less time for the drug to be removed from the blood stream. This is an important consideration in competition horses that are likely to be subjected to routine “dope” tests.
Hydrocortisone (cortisol) is a potent anti-inflammatory drug that is widely used in topical preparations. But little work has been carried out to assess how well it is absorbed through the skin.
Dr Paul Mills and Dr Sheree Cross of the School of Veterinary Science and the Therapeutics Research Unit at the University of Queensland, have been investigating how quickly hydrocortisone penetrates the skin in different parts of the horse’s body.
The laboratory study compared the rate at which hydrocortisone penetrated skin samples taken from the chest, groin and lower leg (in front of the cannon).
They used a saturated solution of hydrocortisone dissolved in 50% ethanol. This was because previous work in the same laboratory had shown (in dogs) that hydrocortisone was absorbed more quickly when dissolved in 50% ethanol than in either 50% propylene glycol or phosphate buffered saline.
They measured the amount of hydrocortisone that remained in the skin and passed through after 24 hours. There was a significant difference between skin from different areas of the horse. Hydrocortisone was absorbed almost twice as quickly through the skin of the lower leg than in the other areas tested.
The scientists conclude that hydrocortisone in ethanol was absorbed more rapidly through the skin of the lower leg than through either of the other two places they tested. Depending on the blood flow in the area, this could result in higher levels in the blood, or higher concentrations in the tissue.
So it is likely that hydrocortisone preparations will penetrate the skin better in the lower leg than in the chest or groin. This may result in higher concentrations in the tissues, and greater clinical effect, when applied to the lower leg. Conversely the lower concentrations that are achieved in other parts of the body may result in a less favourable response to treatment.
Dr Mills adds, "I think that a number of owners and competitors within the equine industry need to be more aware of the transdermal penetration of drugs, even
formulations that were only intended for topical (external) effects - this is particularly if they are at higher levels of competition where testing for drugs is common."
For more details see:
Regional differences in the in vitro penetration of hydrocortisone through equine skin.
PC Mills, SE Cross.
J Vet Pharmacol Therap (2006) 29, 25 - 30.