Smoking: pharmaceutics

People like Tracy often use nicotine patches to help give up smoking. In this video, you’ll learn a bit about skin to help you understand how those patches work. While our skin helps maintain our body temperature, regulates water loss, and synthesises vitamin D, perhaps its most important function is to protect our body. Thus, it is a natural barrier to the movement of drugs into our body. In this schematic of the cross section of skin, we can see a hair follicle, the blood supply, and sweat glands. Of most interest to us, are the three sections that make up skin. The epidermis, the dermis, and the subcutaneous section.

A microscopic cross-section of skin clearly shows the outermost layer of the epidermis, called the stratum corneum. Which is Latin for horny layer. The stratum corneum provides much of the skin’s barrier properties. And is the major barrier to drug penetration. The stratum corneum consists of dead cells, corneocytes, embedded within lipids. This is often referred to as a bricks and mortar structure, where the corneocytes are the bricks and the lipids are the mortar. This bricks and mortar structure is an effective barrier. Here we can see the corneocytes as the bricks, and the lipid matrix as the mortar. Molecules are thought to need to move through the lipid, or mortar phase. Molecules must be small and soluble in lipids.

But also soluble in water so they can enter the blood system for transport to receptors. Nicotine has these properties. Drawn from a patch, moves through the skin by diffusion. Diffusion is simply the movement of molecules from areas of high concentration to areas of a low concentration.

We can see that the drug follows a complex path through the lipids. As Kirstie told us, Tracy is using a nicotine patch. Let’s learn how these patches work. While there are many different designs, patches for the delivery of drug through the skin basically come in two types. Matrix systems. And ones including a rate controlling membrane. The matrix system consists of an impermeable backing. That is the top layer that you can see when you look at a patch. And the drug dissolved in an adhesive layer underneath. Systems with the rate controlling membrane include, in addition to the adhesive layer and the impermeable backing, a rate controlling membrane and a drug reservoir. Nicotine patches are usually made this way.

The rate controlling membrane does just that. It controls the release of drug to the skin to try to maintain a constant level of Nicotine. The separate drug reservoir is needed to contain to drug load needed to keep releasing the drug for the whole time the patch is worn. Different strength nicotine patches usually release a constant amount of Nicotine put area, that is per square centimetre. This means that different dose or strength patches are different in size. As bigger patches are needed to release more Nicotine over time. In this video, you’ve learned that while our skin is a barrier, drugs can move through it if they have the necessary chemical properties.