Topical formulations are becoming increasingly popular in the pharmaceutical industry. Not only have they allowed the delivery of the drug substance to a specific site, but they also avoid fluctuations in drug levels, inter and intra-patient variations, and improve the patient’s compliance with the treatment. In a topical formulation, the skin acts as a perfect medium for the delivery of the drugs having both local and systemic effects, but also acts a mechanical barrier for the penetration of some.
This is so because the ingredients in the topical applications not only react amongst themselves, but also with the skin of the patient. For example, the drug substance used in a psoriasis treatment may be effective simply due to the hydrating or soothing effects of the formulation. Whether the molecule maintains its purity, potency, and delivery to the right target site may be masked by the ingredients that surround it. Both the mechanical barrier properties of the top layer of skin and the physicochemical properties of the drug affect the transportation of the drug substance from the formulation to the site of action.
Thus, to have the desired benefit from topical applications, their right formulation is absolutely essential. Even a small change in the formulation of the topical drugs can make a huge difference in their efficacy.
Therefore, with topical applications, the right formulation is as important as the molecule itself to ensure that the drug substance is delivered to the right target site and maintains the dosage integrity, drug transport, and active duration. Moreover, in addition to the ingredients in the formulation, factors like pH, viscosity, and the relative amounts of oil, water, surfactants, stabilizers, droplet size, ionic nature, and the method of preparation can also influence skin absorption and efficacy.
This makes preformulation studies and careful selection of excipients highly important in the formulation of topical applications.
The right formulation and selection of excipients for topical application is extremely important as it may alter the following:
- Its ability to permeate through the skin
- Its stability through shelf life
- Its ability to not metabolize in skin
- Its ability to stay dissolved at right concentrations
- Its capability to achieve the desired release rate
Besides the excipients, attention must be given to the choice of permeation enhancers in the early stages of topical drug development. The bioavailability of the drug in the desired location, i.e. the epidermis or dermis must be considered when choosing the right excipient and permeation enhancer. An excipient that forms a depot of the drug concentration in the dermis or epidermis may not be ideal for the formulation of a topical drug.
All the more, the pH of the formulations, particularly how they react to the pH factors in different layers of the skin, is very important.
To conclude, the right formulation for topical drug development is determined by the properties of the drug substance, excipients, and the intended target area. Formulations can be improved by considering both the properties of the skin and the drug substance/excipients. And all this can be analysed during skin-permeation studies.
The right formulation and delivery method in topical drug development can pass or fail the drug substance. Getting the preparation right from the formulation development services experts saves money and time.
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