Review of Changes in Topical Drug Product Classification - Pharmaceutical Technology

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Review of Changes in Topical Drug Product Classification
This article summarizes the classification systems for topical liquid and semisolid dosage forms used for dermatological application and notes some differences between FDA and USP classification.


Pharmaceutical Technology
Volume 32, Issue 10, pp. 66-74


Dosage forms to deliver pharmaceutical actives to the skin (definitions provided in Reference 5. Bolded entries are terms being phased out of USP titles)
If the dosage form is a liquid or semisolid, and not listed in Tables, then the next question in the decision tree is, Is it a liquid or a semisolid? (4). To help answer this question, Buhse et al. and CDER Data Sandards Manual define liquid as being pourable; it flows and conforms to its container at room temperature, and displays Newtonian or pseudoplastic flow behavior. A semisolid is defined as not being pourable; it does not flow or conform to its container at room temperature. It does not flow at low-shear stress and generally exhibits plastic flow behavior (4, 6). Thus to distinguish between a liquid and a semisolid, a rudimentary understanding of plastic and pseudoplastic flow behavior and Newtonian liquids is required. Because these references do not define these terms, it appears in keeping with FDA's interpretation to use descriptive definitions.

Newtonian fluids such as water and solutions containing only low molecular-weight material are characterized by viscosity that is independent of the rate of stirring. Thus, a plot of shear rate versus shear stress is linear and passes through the origin. Pseudoplastic flow behavior is characterized by viscosity decreasing with the rate of shear (i.e., shear thinning). Plastic flow behavior is characterized by a material behaving as a solid until a threshold stress is reached beyond which the material flows. The slope of the rheogram can be extrapolated to determine the yield value. Because a threshold stress is required before flow, the plot of shear rate versus shear stress can not pass through the origin for a material having plastic flow. Based on the data presented by Buhse et al., the plot of shear rate versus shear stress for both pseudoplastic liquids and plastic semisolids are characterized by a yield value (D/cm2 ) and a curved response to changes in shear rate (4). In a presentation, Buhse suggested that semisolids have yield values ≥ 200 D/cm2 , and liquids capable of conforming to the container have yield values <200 D/cm2 (12).

Solutions, suspensions, and lotions

Once it is decided that a liquid is being applied to the skin surface, the next question in the decision tree is, Is the liquid clear and homogeneous, a solid dispersed in a liquid, or an emulsion? (4). If the liquid is clear and homogeneous, then the dosage form is classified as a solution. A solution is defined as "a clear, homogeneous liquid dosage form that contains one or more chemical substances dissolved in a solvent or mixture of mutually miscible solvents." If the product is a solid dispersed in a liquid, then the dosage form is classified as a suspension. Suspension is defined as "a liquid dosage form that contains solid particles dispersed in a liquid vehicle." If the liquid is an emulsion, then the dosage form is classified as a lotion.


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