Definition, Classification, Mechanism and Factors Influencing the Dermal Penetration of Drugs

A percutaneous penetration (penetration of drugs) test measures the amount of substance being absorbed into the skin by examining its penetration into the skin through the skin barrier. Due to its size and thickness, the skin plays a key role in how drugs permeate and penetrate the body. Furthermore, transdermal drug delivery works in conjunction with dermal infection management and the achievement of sustained plasma drug levels. TDDS must be tested for percutaneous penetration of medicines through the skin to be suitable for human use. Several compounds, such as sunscreen, insect repellent, and antiseptics, are meant to remain on the skin surface (topical) while others penetrate the layers of the skin (transdermal) to communicate with target tissues within or just below the skin.

Transdermal delivery refers to the delivery of a substance through the skin to the bloodstream, which produces a therapeutic effect, such as nicotine withdrawal effects. In addition to treating psoriasis, fungal infections, lesions, and ulcers, it demonstrates the benefit of treating other types of skin infections. Initial penetration occurs in the stratum corneum, followed by penetration through deeper epidermis and dermis layers without accumulation in the dermis. Dermal microcirculation allows a drug to be absorbed by the system once it reaches the dermal layer.

The biphasic vesicles-a lipid-based, topical delivery system-have recently been found to be capable of delivering large molecules and macromolecules to the skin. Biphasic vesicles may contribute to the revival of drugs that have been held back by problems with delivery. In addition, biphasic vesicles enable the delivery of hard-to-deliver molecules combined with new drugs, enabling safe and non-invasive delivery through the skin. Drugs are delivered into the skin to treat dermatological conditions, to administer vaccines, or to achieve cosmetic results

Permeation enhancers

Making it more difficult for drugs to penetrate the skin, Besides reversibly modifying the structure of the stratum corneum by reversibly disarranging the lamellar packing, penetration enhancers increase the thermal activity of a drug and hence its permeation through the skin. An alternative class of penetration enhancers increases the amount of solubilized drug molecules at the skin's surface, which results in the enhanced permeability of lipophilic drug molecules.

Transdermal drug delivery systems (TDDS) provide a means of sustaining the release of drugs along with reducing the intensity of their action and thus reducing the risk of side effects associated with oral administration. Drugs delivered by transdermal means are discrete, self-contained dosage forms. A controlled rate of drug delivery is provided through intact skin into the circulation system. For most drugs, the rate at which they are delivered is controlled by the skin or membrane. An adhesive transdermal patch is a medicine patch applied to the skin for delivering specific doses of medication by way of the skin into the bloodstream. In many cases, this promotes the healing of an injury.

Delivery of drugs via transdermal patches

Pros of transdermal drug delivery - Prevents first-pass metabolism - Gastric incompatibility is avoided - Duration of activity that is predictable and prolonged - Reducing side effects to a minimum - It enables the use of drugs that have short biological half-lives and a narrow therapeutic window - Improvements in physiology and pharmacology - Preventing drug levels from fluctuating - Maintaining plasma concentrations of powerful drugs - It is easy to discontinue therapy - There is greater patient compliance as there are no multiple doses required - Delivery of the drug to a specific site more efficiently - The system should be self-administered - Improve treatment outcomes.

Several of the formulation components can cause skin irritation by transdermal drug delivery systems. If the drug is bound to the skin, the dose may be dumped. The use of the drug is limited to chronic conditions involving the use of drugs for an extended period. Drug lag times can be variable and can range from several hours up to days depending on the candidate. A change in metabolic activity of the skin can affect therapeutic efficacy. There are only a few medications that can be applied transdermally. However, ionic drugs cannot be treated via transdermal means.

Mechanisms and factors influencing dermal penetration of drugs

The skin's ability to absorb substances depends on several factors:

• Concentration
• Molecular Weight of the molecule
• Duration of contract
• Solubility of medication
• The physical condition of the skin
• Part of the body is exposed including the amount of hair on the skin

It is primarily the epidermis that is responsible for absorbing small amounts of chemicals into the body quickly through glands or hair follicles. This is the layer that forms the outermost layer of the epidermis and is the barrier that limits a substance's ability to penetrate it. A substance has no significant effect on the rest of the epidermal layer and corium once it has gone through the stratum corneum. There are primarily lipophilic cholesterols, cholesterol esters, and ceramides (fatty acids) in the stratum corneum. Hence, lipid-soluble chemicals pass through the membrane and enter the circulatory system faster; however, nearly every molecule penetrates it to some extent. Moreover, penetration depends on the movement of interstitial fluid and effective blood flow.

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