Semisolid Preparations 4: Drug permeability

Summary

This lecture covers the essential concepts behind semi-solid preparations, focusing on skin structure, drug release, and penetration mechanisms. The skin's layers—epidermis, dermis, and subcutaneous fat—are explored in relation to how drugs are absorbed through them. Key factors influencing drug penetration, such as the chemical properties of drugs, pH, viscosity, and skin conditions, are discussed. The role of vehicles, excipients, and surfactants in enhancing drug release and absorption is emphasized. The lecture provides insights into optimizing formulations for better drug penetration, offering a detailed understanding of semi-solid product formulation and drug delivery systems.

Takeaways

• 😀 The skin consists of three main layers: epidermis, dermis, and subcutaneous fat. The stratum corneum is the outermost and most important barrier for drug penetration.
• 😀 Drug penetration into the skin occurs through different routes, including transcellular and intercellular routes. Both routes involve navigating through the stratum corneum.
• 😀 The stratum corneum, made up of keratinized cells, acts as a barrier for drugs. To penetrate, drugs must be balanced in hydrophobic and hydrophilic properties.
• 😀 Drug release from a semi-solid base depends on the chemical properties of the drug and its interactions with other ingredients in the formulation.
• 😀 Factors such as the drug's chemical activity, its ability to bind with excipients, and its solubility can influence the rate of drug release from the base.
• 😀 pH plays a significant role in drug absorption. Maintaining a skin-friendly pH (around 5.5) is ideal to ensure drug activity and absorption.
• 😀 The viscosity of the base formulation affects drug release. Lower viscosity allows for better spreadability and faster release of the active ingredients.
• 😀 Skin hydration is crucial for enhancing drug penetration. Using bases that promote hydration, such as water-in-oil or hydrocarbon bases, can increase drug absorption.
• 😀 Lipophilic drugs, which have higher lipid solubility, tend to penetrate the skin more easily, especially when formulated to move through hair follicles or sebaceous glands.
• 😀 Emulsifiers, surfactants, and humectants can improve drug penetration by altering the skin's permeability, ensuring the drug has better contact with the skin surface.
• 😀 Skin conditions (e.g., dryness, injury, inflammation) significantly affect drug penetration. Damaged skin may increase absorption rates, while thicker skin may slow it down.

Q & A

What are the three major components of human skin?

The three major components of human skin are the epidermis, dermis, and fat layer.

Which layer of the skin is primarily responsible for preventing substances, including active ingredients, from entering?

The stratum corneum, the outermost layer of the epidermis, is primarily responsible for preventing substances, including active ingredients, from entering the skin.

What is the difference between transcellular and intercellular routes of drug penetration?

The transcellular route involves the drug passing through the keratinocytes in the stratum corneum, while the intercellular route involves the drug traveling through the lipid layer between cells.

How does the pH of the vehicle affect drug release and penetration?

The pH of the vehicle plays a role in the ionization of drugs. If a drug is ionized, it may have reduced activity and slower penetration. Ideally, the formulation's pH should remain around 5.5, which is the normal pH of the skin.

How does skin hydration influence drug penetration?

Skin hydration enhances drug penetration. Hydrated skin allows the drug to diffuse more easily and improves its absorption into the skin.

What role does the viscosity of a semi-solid base play in drug release?

The viscosity of the semi-solid base affects drug release. Lower viscosity typically results in faster drug release. It can be adjusted by adding liquid ingredients like macrogol or isopropyl myristate to reduce viscosity.

What is the effect of molecular size on drug penetration?

Larger molecular size tends to slow down drug penetration. Reducing the size, especially in suspensions, can help improve penetration.

Why are lipophilic drugs more likely to penetrate the skin through hair follicles?

Lipophilic drugs, due to their higher lipid solubility, can easily penetrate the skin through hair follicles, which contain lipids that facilitate this process.

How can the addition of surfactants improve drug release from a semi-solid base?

Surfactants in the semi-solid base can enhance the drug's release by improving contact between the drug and the skin's hydrophobic surface, increasing solubility and absorption.

What is the function of penetration enhancers in drug formulations?

Penetration enhancers, such as propylene glycol or ethanol, alter the skin's permeability by modifying its structure, allowing for faster and more efficient drug absorption.