Bioavailability in Delivery to the Skin

Abstract

After oral administration of drugs, they undergo various phases of pharmacokinetics such as absorption, distribution, metabolism, and excretion (ADME). Absorption may also involve liberation of active ingredients from their respective formulation or delivery system. In practical terms, the bioavailability of the drug is the fraction of the intact drug, in its active form, that reaches the site of action. Typically, in oral drug administration, it must travel through the acidic pH of the stomach, the intestine wall, to blood circulation and finally to the target site. During this journey, drugs could be chemically metabolized in the intestine and liver. Also, the intestinal wall poses a permeability barrier to the drugs which depends upon the physiochemical characteristics of the molecules. Similarly, transdermal administration also has a barrier, the epidermal/dermal drug barrier. As discussed in previous chapters, the stratum corneum (SC), the top layer of the skin, provides the main barrier to the skin. Both skin and intestine wall contain lipid bilayers through which drugs must pass. The permeability of the intestine wall is much higher compared to the SC in the skin. There have always been challenges to delivering the therapeutic amounts of drugs to targets. Bioavailability in transdermal drugs delivery is mostly limited by instability of the compound and by the skin barrier.