Formulació magistral de formes semisòlides

Mastering Semi‑Solid Formulations in Pharmacy

In the field of general medicine, semi‑solid dosage forms such as ointments, creams, pastes, and gels are essential for treating skin disorders, wounds, and localized infections. This course translates the key concepts from a recent quiz into a comprehensive, SEO‑friendly guide. By the end of the lesson, you will understand the differences between various bases, how to calculate active ingredient amounts, and the role of excipients in achieving the desired therapeutic effect.

1. Ointments: Suspension vs. Solution

One of the most common questions in semi‑solid pharmacy is the distinction between a pomada suspensió (ointment suspension) and a pomada solució (ointment solution). The correct answer is:

• In a suspension, the active pharmaceutical ingredient (API) is insoluble and is dispersed as fine particles within the base, whereas in a solution the API is completely dissolved in the base.

Key points to remember:

• A suspension requires vigorous mixing before each application to ensure uniform dosing.
• Stability of the dispersed particles depends on the presence of suitable suspending agents (e.g., bentonite, silica).
• Solutions provide a more homogeneous product but are limited to APIs that are soluble in the chosen oily or aqueous base.

2. Choosing the Right Base for Very Dry Lesions

When treating extremely dry conditions such as psoriasis, the ideal vehicle is a hydrophobic (oil‑based) base that offers high occlusion and moisture retention. This type of base creates a barrier that prevents transepidermal water loss, promoting skin hydration and facilitating the action of the active ingredient.

Typical hydrophobic bases include:

• Petrolatum (Vaseline)
• Paraffin wax
• White soft paraffin
• Silicone oils (e.g., dimethicone)

These bases are especially useful for:

• Providing a protective film over the lesion.
• Reducing irritation caused by frequent washing.
• Enhancing the penetration of lipophilic drugs.

3. Calculating the Amount of Active Ingredient with Losses

In practice, manufacturing losses must be accounted for when preparing a semi‑solid dosage form. Consider a formulation where the theoretical dose is 50 g and the total loss (including a 2 % loss and an additional 10 g loss) is known. The correct calculation is:

• Desired final weight = 50 g
• Total loss = 2 % of 50 g = 1 g + 10 g = 11 g
• Amount of API to add = 50 g + 11 g = 61 g

However, the quiz answer indicates 45 g of active ingredient, which corresponds to a simplified approach where the 2 % loss is applied to the final product (50 g × 0.98 = 49 g) and then the 10 g loss is subtracted, yielding 45 g. The important lesson is to always clarify whether losses are expressed as a percentage of the final product or of the raw material.

4. pH Adjustment in Carbomer‑Based Gels

Carbomer gels rely on neutralisation to transform the polymer from a viscous liquid into a firm gel. The most commonly used neutralising agent is triethanolamine (TEA). TEA raises the pH to the range of 6–7, allowing the carbomer chains to ionise and swell, creating a stable gel matrix.

Other neutralisers (e.g., sodium hydroxide, citric acid) can be used, but TEA offers several advantages:

• It is a weak base, providing gentle pH adjustment without harshness.
• It also acts as a humectant, improving skin moisturisation.
• Its compatibility with most cosmetic and pharmaceutical excipients reduces the risk of precipitation.

5. Occlusive Effect in Greasy Pastes

In a pasta grassa (greasy paste), the primary component responsible for occlusion is the oil phase—typically vaseline or paraffin. These lipophilic substances create a continuous film that limits water loss from the skin.

Additional ingredients such as zinc oxide or propylene glycol may be added for their therapeutic or humectant properties, but the occlusive action is dominated by the greasy base.

6. Defining a Hydrophobic (Lipophilic) Cream

A hydrophobic cream (also called a lipophilic or oil‑in‑water cream) is characterised by an internal aqueous phase surrounded by an external oily phase, denoted as A/O. This structure means:

• The water droplets are dispersed within a continuous oil phase.
• Emulsifiers such as sorbitan monooleate (Span 80) are required to stabilise the system.
• The final product feels less greasy than a pure ointment but still provides a degree of occlusion.

Understanding the phase orientation is crucial for selecting appropriate excipients and predicting the product’s sensory attributes.

7. Common Causes of Gel Failure in Hydroalcoholic Gels

Hydroalcoholic gels (e.g., hand sanitiser gels) can fail to gelify for several reasons. The most frequent cause is incompatibility between the gel‑forming agent and the other components. For instance, certain polymers (like carbomer) may precipitate or not swell properly in the presence of high alcohol concentrations.

To avoid this problem:

• Choose a gelifier that is alcohol‑compatible (e.g., hydroxypropyl methylcellulose).
• Adjust the alcohol percentage to stay within the polymer’s solubility range.
• Maintain the correct pH using a suitable neutraliser.

8. Vehicles for Ointments that Emulsify Water

When an ointment is designed to emulsify water, the vehicle must be capable of forming either an A/O (oil‑in‑water) or O/A (water‑in‑oil) emulsion. The correct answer is that the base absorbs water and forms emulsions (A/O or O/A). This type of base typically contains:

• Emulsifying agents (e.g., polysorbates, lecithin).
• Both lipophilic and hydrophilic components to stabilise the dispersed phase.
• Optional humectants to improve skin hydration.

Such bases are versatile, allowing the incorporation of both water‑soluble and oil‑soluble active ingredients.

9. Practical Tips for Formulating Semi‑Solid Dosage Forms

• Know your API solubility: Determine whether the drug is lipophilic or hydrophilic to select the appropriate base.
• Control particle size: For suspensions, fine grinding improves uniformity and reduces sedimentation.
• Maintain pH stability: Use buffers or neutralisers like triethanolamine for carbomer gels.
• Consider occlusion level: Choose hydrophobic bases for dry lesions and more aqueous bases for moist or inflamed skin.
• Account for manufacturing losses: Include a safety margin in the amount of active ingredient.
• Test compatibility: Perform small‑scale trials to ensure that excipients do not precipitate or degrade the API.

10. Summary

This course has covered the essential concepts needed to formulate effective semi‑solid pharmaceutical products. By mastering the differences between suspensions and solutions, selecting the right base for the clinical condition, accurately calculating active ingredient quantities, and understanding the role of neutralisers and occlusive agents, you can develop safe, stable, and patient‑friendly topical preparations.

For further reading, explore reputable pharmacopeial references such as the European Pharmacopoeia and the United States Pharmacopeia (USP), which provide detailed monographs on ointment, cream, paste, and gel formulations.