Veterinary drug delivery: potential for skin penetration enhancement

A range of topical products are used in veterinary medicine. The efficacy o f many of these products has been enhanced by the addition of penetration e nhancers. Evolution has led to not only a highly specialized skin in animal s and humans, but also one whose anatomical structure and skin permeability differ between the various species. The skin provides an excellent barrier against the ingress of environmental contaminants, toxins, and microorgani sms while performing a homeostatic role to permit terrestrial life. Over th e past few years, major advances have been made in the field of transdermal drug delivery. An increasing number of drugs are being added to the list o f therapeutic agents that can be delivered via the skin to the systemic cir culation where clinically effective concentrations are reached. The therape utic benefits of topically applied veterinary products is achieved in spite of the inherent protective functions of the stratum corneum (SQ, one of wh ich is to exclude foreign substances from entering the body. Much of the re cent success in this field is attributable to the rapidly expanding knowled ge of the SC barrier structure and function. The bilayer domains of the int ercellular lipid matrices within the SC form an excellent penetration barri er, which must be breached if poorly penetrating drugs are to be administer ed at an appropriate rate. One generalized approach to overcoming the barri er properties of the skin for drugs and biomolecules is the incorporation o f suitable vehicles or other chemical compounds into a transdermal delivery system. Indeed, the incorporation of such compounds has become more preval ent and is a growing trend in transdermal drug delivery. Substances that he lp promote drug diffusion through the SC and epidermis are referred to as p enetration enhancers, accelerants, adjuvants, or sorption promoters. It is interesting to note that many pour-on and spot-on formulations used in vete rinary medicine contain inert ingredients (e.g., alcohols, amides, ethers, glycols, and hydrocarbon oils) that will act as penetration enhancers. Thes e substances have the potential to reduce the capacity for drug binding and interact with some components of the skin, thereby improving drug transpor t. However, their inclusion in veterinary products with a high-absorbed dos e may result in adverse dermatological reactions (e.g., toxicological irrit ations) and concerns about tissue residues. These a-re important considerat ions when formulating a veterinary transdermal product when such compounds ate added, either intentionally or otherwise, for their penetration enhance ment ability. (C) 2001 Elsevier Science B.V. All rights reserved.