Understanding Transdermal Drug Delivery: Penetration into Epidermis and Dermis and the Carry Over Effect

The drug builds up in the epidermis and dermis from the first patch, which continues to release the drug into the systemic circulation until the next patch begins releasing its contents – “carry over”

When discussing the concept of a drug building up in the epidermis and dermis, it is important to understand how transdermal drug delivery systems work

When discussing the concept of a drug building up in the epidermis and dermis, it is important to understand how transdermal drug delivery systems work. Transdermal patches are designed to deliver medications directly through the skin and into the bloodstream.

The outermost layer of the skin is known as the epidermis, while the layer underneath it is called the dermis. These layers are composed of various skin cells and structures that create a barrier between the external environment and the underlying tissues. One of the functions of the skin is to prevent the absorption of foreign substances, including drugs.

Transdermal patches are constructed with a specific formulation that allows for controlled drug release. The active drug is typically embedded within a matrix or reservoir within the patch. The patch adheres to the skin and the drug is gradually released through the skin layers.

When a transdermal patch is applied, the drug initially starts to penetrate the epidermis. The drug molecules can pass through the intercellular spaces of the epidermis, which is composed of tightly packed cells, as well as through the sweat glands and hair follicles, which provide additional pathways for drug absorption.

As the drug molecules traverse the epidermis, they then reach the dermis, which is a deeper layer of skin. The dermis is more vascularized compared to the epidermis, meaning it has a greater network of blood vessels. This vascularity allows for enhanced drug absorption into the systemic circulation. The drug molecules diffuse through blood vessels in the dermis and enter the bloodstream.

Now, with regard to the term “carry over,” it refers to the phenomenon where a drug continues to be released from the previous patch and enters the systemic circulation even when a new patch is applied. This is due to the residual drug remaining in the skin layers after patch removal. The drug may have permeated deeply into the epidermis and dermis during its wear time, and some of it may remain within the skin after patch removal.

The carry over effect can be significant depending on the drug’s formulation and its characteristics, such as its molecular properties and the patch design. The residual drug in the skin layers can continue to release into the systemic circulation until it is fully depleted. This means that the drug concentration in the blood may remain elevated for some time after removing the patch, contributing to a sustained therapeutic effect.

It is worth noting that the carry over effect is carefully considered during the development of transdermal drug products to ensure consistency and predictability of drug release. Manufacturers aim to minimize or control the extent of carry over to maintain appropriate drug levels in the body and prevent excessive drug accumulation.

In conclusion, transdermal drug delivery systems allow for the drug to build up in the epidermis and dermis upon patch application. The drug molecules penetrate through the layers of the skin, reaching the dermis where they enter the systemic circulation via blood vessels. The carry over effect occurs when residual drug present in the skin continues to release into the bloodstream after patch removal, contributing to a sustained drug effect.

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