4A, there are a range of epidermal, dermal, and appendageal target sites that include the keratinocytes, Langerhans cells, melanocytes, Merkel cells, and blood vessels, as well as deeper tissues such as muscle. A counterbalancing consideration is the potential local and systemic adverse effects for the potential drug.
Transcellular route transports the drug molecule into the bilayered lipid cells found in stratum corneum. Inside of the bilayered lipids in the stratum corneum is a water-soluble environment, and the drug molecules will diffuse through these bilayered lipids into deeper area of the skin.
In general, the rate of absorption of chemicals through skin follows the following scheme from fastest to slowest: Scrotal > Forehead > Armpit ≥ Scalp > Back = Abdomen > Palm = under surface of the foot.
Topical administration of the drug is employed to treat anterior segment diseases, most often in the form of eye drops. For this route, the site of action is generally the different layers of the conjunctiva, cornea, sclera, and other anterior segment tissues like iris and ciliary body (anterior uvea).
In pharmacotherapy, drugs are mostly taken orally to be absorbed systemically from the small intestine, and some drugs are known to have preferential absorption sites in the small intestine.
However, about 95 percent of nutrient molecules are absorbed in the small intestine. The absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. However, there are a few exceptions.
This process begins with the administration of the drug, its absorption, distribution, metabolization, and excretion through the body. This phase of drug action is called pharmacokinetics.
Topical and Transdermal Medications. Topical medications are medications that are administered via the skin or mucous membranes for direct local action, as well as for systemic effects. An innunction is a medication that is massaged or rubbed into the skin and includes topical creams such as nystatin antifungal cream.
Sublingual administration involves placing a drug under the tongue and buccal administration involves placing a drug between the gums and cheek. The sublingual and buccal routes are considered promising alternatives to the traditional oral route for drug delivery.
For systemic activity, the transdermal patches or cream formulations deliver the drug into the systemic circulation through uptake in blood vessels in the dermis (Benson, 2005). (See Figure 5-2 for an illustrative example of the systemic absorption of a topical active ingredient.)
Because most absorption occurs in the small intestine, gastric emptying is often the rate-limiting step. Food, especially fatty food, slows gastric emptying (and the rate of drug absorption), explaining why taking some drugs on an empty stomach speeds absorption.
The drug initially penetrates through the stratum corneum and then passes through the deeper epidermis and dermis without drug accumulation in the dermal layer. When drug reaches the dermal layer, it becomes available for systemic absorption via the dermal microcirculation [8,9].
The ideal application site for transdermal medications is one that has minimal hair and that cannot be easily licked or rubbed.
Pharmacokinetics related to topical applications of drugs describes the time-dependent drug concentration following the application of the drug to the skin surface, its subsequent passage through the skin barrier into the underlying skin layers, and its distribution into the systemic circulation.
One way we achieve this is through novel delivery systems. These systems involve incorporating active ingredients into carriers that facilitate their penetration into the skin layers. For instance, liposomes are microscopic vesicles that can easily transport drugs across cell membranes, enhancing their absorption.
One reason for selecting the sublingual route is to avoid drug destruction. Because gastric acid and intestinal and hepatic enzymes are bypassed, sublingual absorption can be more efficient overall for certain drugs than intestinal uptake. The onset of drug effect may also be quicker than with oral ingestion.
Buccal – The cheek-side of the tooth. This surface is also referred to as the facial surface when referring to the front teeth. Lingual – The part of the tooth that is closest to the tongue.
Ointments are used topically on a variety of body surfaces. These include the skin and the mucous membranes of the eye (an eye ointment), chest, vulva, anus, and nose.
Applying medication to the skin or mucous membranes allows it to enter the body from there. Medication applied in this way is known as topical medication. It can also be used to treat pain or other problems in specific parts of the body. Topical products aren't only used to treat medical problems, though.
For example, less than 2% of a topically applied corticosteroid such as hydrocortisone is absorbed after a single application left on the skin for more than 1 day. Furthermore, peak rates of absorption are reached up to 12–24 hours after application.
The small intestine, however, has the largest surface area for drug absorption in the GI tract, and its membranes are more permeable. For these reasons, most drugs are absorbed primarily in the small intestine.
Inhibition of drug metabolism is a frequent cause of drug interactions. Most metabolic interactions are due to competition for the cytochrome P450 enzyme (CYP), which is expressed in the liver and catalyzes the phase I oxidation of more than half of all medical drugs (16).
For inhaled formulations of a drug substance, the balance between desired local activity and undesired systemic activity can be expressed with an L:T ratio, where L stands for the local bioavailability and T stands for the total systemic bioavailability.