An emulsion is a biphasic system consisting of at least two immiscible liquids (or two liquids that are saturated with each other), one of which (the dispersed phase) is finely and uniformly dispersed as globules within the other liquid phase (the continuous phase), generally stabilized by an emulsifying agent. The dispersed phase is the discontinuous or internal phase, and the dispersion medium is the continuous or external phase.
Emulsion dosage forms are designed to meet the following quality attributes:
- 1 1. Uniformity of content (dose-to-dose within the same bottle and bottle-to-bottle)
- 2 2. Separation volume or creaming
- 3 3. Dispersed phase size distribution
- 4 4. Drug concentration
- 5 5. Palatability
- 6 6. Redispersability
- 7 7. Absence of phase separation
- 8 8. Deliverability
- 9 9. Flow
- 10 10. Lack of microbial growth
- 11 11. Physical integrity
- 12 12. Adhesion to the package
- 13 13. Leachables and extractables
- 14 14. Chemical stability
- 15 15. Drug release
- 16 Source
1. Uniformity of content (dose-to-dose within the same bottle and bottle-to-bottle)
All the doses dispensed from a given multidose container should have acceptable uniformity of drug content. In addition, the drug content must be uniform between different bottles of a given batch of emulsion.
2. Separation volume or creaming
Once an emulsion has been left undisturbed for some time, it may show some degree of separation of the dispersed phase from the dispersion medium. For example, in the case of an o/w emulsion, creaming of an emulsion is sometimes observed, which indicates a higher concentration of the dispersed oil phase in the top layer of the emulsion. This top phase is visually distinguishable from the bottom layer due to greater light obscuration and diffraction by a higher concentration of the dispersed phase globules. The more the concentration of the dispersed phase, the more the light obscuration in a smaller thickness of the creamed layer and greater the instability. Thus, the proportion of the volume occupied by the separated phase is an indicator of physical instability of the emulsion. The higher this volume, the more stable is the emulsion.
3. Dispersed phase size distribution
Size distribution of dispersed phase should remain fairly constant during the shelf life of the emulsion. Brownian motion, agitation during handling, and gravitational motion of the dispersed phase lead to collisions of globules with each other, which can cause coalescence or agglomeration resulting in an increase in the size of the dispersed phase.
A change in the dispersed phase globule size on storage is indicative of inherently low physical stability of the emulsion.
4. Drug concentration
In cases where drug concentration in the emulsion is close to the drug solubility, crystallization can sometimes occur due to temperature fluctuations during storage, preferential evaporative loss of one phase, incompatibility with packaging components, or
unintended nucleation. Crystal growth can be inhibited by the use of appropriate solubilizers and surfactants, and by formulating an emulsion at a lower concentration than the drug’s thermodynamic solubility. Changes such as drug crystallization or evaporative loss of the continuous phase can reflect changes in the drug concentration during storage stability or shelf life.
Use of an emulsion dosage form can improve the palatability of particularly bitter drugs by dissolving them in the dispersed phase. However, incorporation of the drug in the dispersed phase may not be adequate because some drug would inadvertently partition into the continuous phase depending on the partition coefficient (logP) of the compound.
Palatability of the emulsion can be increased by the use of sweeteners, flavors, and colorants. In certain cases, specialized taste-masking approaches, such as complexation, may be needed. These considerations, of course, are not pertinent for parenteral emulsions. In the case of parenteral emulsions, tissue irritability and osmotic pressure are important considerations.
A separated or creamed emulsion should readily redisperse upon gentle shaking of the container.
7. Absence of phase separation
Coalescence leading to phase separation is irreversible. Although creaming of an emulsion is, to some extent, unavoidable, the dispersed phase should not coalesce and separate from the dispersion medium. This needs to be designed into the formulation by the use of right surfactants in an appropriate concentration.
The labeled number of doses and the labeled amount of emulsion should be deliverable from a bottle under the normal dispensing conditions by a patient. This is usually ensured by pouring out the labeled number of doses from the container and ensuring that the
remaining dose can be poured completely within a reasonable period of time.
The emulsion must not be too viscous to pour freely from a bottle or to flow through a needle syringe or an IV infusion set (for parenteral emulsions).
10. Lack of microbial growth
Use of antimicrobial preservatives could be sufficient for oral and topical emulsions, whereas parenteral, nasal, and ophthalmic suspensions must be sterile.
11. Physical integrity
The dosage form should not show any unexpected change in color, or any other change in physical appearance or perception of the dosage form, such as odor, that may alarm the patient and/or the health-care provider with respect to the physical integrity of the emulsion.
12. Adhesion to the package
Preferential adsorption or adhesion of one phase or component of the emulsion, such as the drug, the chelating agent, or the emulsifier, can adversely affect the uniformity and stability of an emulsion.
13. Leachables and extractables
Primary packaging components of the emulsion can leach out small amounts of chemical components used in the manufacturing of those components. This behavior can be exacerbated at certain pH values of the formulation.
The packaging components must be selected appropriately, and their compatibility with the emulsion determined to make sure no chemical compounds leach into or are extracted by the emulsion from the container on storage.
14. Chemical stability
There should not be any unacceptable chemical degradation of the drug during the shelf life of the product under recommended packaging and storage conditions. The drug product must meet the predetermined requirements of maximum levels of known and unknown impurities.
15. Drug release
Since an emulsion contains the drug in the dispersed phase, the release of drug from the dispersed phase into an aqueous solution in an appropriate dissolution vessel is quantified and controlled as an indicator of its bioavailability. This could be particularly important for semisolid emulsions.
In addition, topical emulsions should be fluid enough to spread easily but not so fluid that the emulsion runs off the surface too quickly. The emulsion must dry quickly and provide an elastic film that should not be too oily. In addition, the dosage form must have pleasant color and odor, although a sweetener is not needed.
- Mahato, R. and Narang, A. (2018). Pharmaceutical Dosage Forms and Drug Delivery (3rd ).New York: Taylor & Francis Group, LLC.
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