Cutaneous blood flow systemic absorption

Drugs are applied topically primarily for local effects however, this route can be used to administer drugs for systemic action. Few drugs readily penetrate intact skin. The absorption of drugs that do penetrate the skin is proportional to the surface area over which they are applied and to their lipid solubility. Increased cutaneous blood flow also enhances absorption. Systemic toxicity can become evident when highly lipid-soluble substances (e.g. lipid-soluble insecticides) are absorbed through the skin. Controlled-release patches are now commonly used in human medicine for transcutaneous drug administration. 

A 2% minoxidil (Rogaine) solution has been approved by the FDA for treating alopecia. Minoxidil causes vasodilation. This increases cutaneous blood flow and stimulates hair-follicle growth. However, alopecia returns within 3 to 4 months after the patient stops using minoxidil. Systemic absorption of minoxidil is minimal and adverse reactions seldom occur. 

Determination of the effects of changes in blood flow through the various regions of the cutaneous microvasculature is obviously not possible using traditional Franz-type isolated membrane diffusion cell studies. The ultimate goal of experimental systems is usually to allow quantitative prediction of the absorption and distribution of topically applied solutes that wfll be applicable to the in vivo situation. Therefore, we can deduce that studies examining the effects of changes in cutaneous blood flow are limited to experimental models in which the microvasculature has been preserved and can be effectively perfused and manipulated. Models reported in the htraature to date include isolated perfused tissue models, anesthetized animal studies, and more recently human and animal cutaneous microdialysis studies. 

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