An emulsion consists of at least two immiscible liquid phases, one of which is dispersed as globules (dispersed phase) and into the other liquid phase (continuous phase).
Emulsions
Introduction
An
emulsion consists of at least two immiscible liquid phases, one of which is
dispersed as globules (dispersed phase) and into the other liquid phase
(continuous phase). Emulsions are thermodynamically unstable and are usually
stabilized by the presence of an emulsifying agent. The process of forma-tion
of an emulsion is termed emulsification. The diameter of the dispersed phase
globules is generally in the range of about 0.1 to 10 μm, although it can be as small as
0.01 μm or as large as 100
μm. Emulsions in
which the size of the dispersed phase is so small, in the nanometer range, that
all of the dispersed phase is subvisible appear as a homogeneous, isotropic
liquid. Such systems are called microemulsions or nanoemulsion. Emulsified
sys-tems range from lotions of relatively low viscosity to ointments and
creams, which are semisolid in nature. Pharmaceutical emulsions are used for
the administration of nutrients, drugs, and diagnostic agents. Topical creams
and lotions are popular forms of emulsions for external use.
The
main advantages of emulsions as drug delivery systems include the following:
1. Increased drug
bioavailability: Many drugs are highly hydrophobic, with high log P values
(partition coefficient between oil and water). These drugs are usually poorly
soluble in water but readily soluble in oils. Formulation of drug-dosage form
as an emulsion allows the administration of a hydrophobic drug in a
soluble/dissolved state. This can improve the oral bioavailability of a
biopharmaceutics clas-sification system (BCS) class II (low solubility, high
permeability) and class IV (low solubility, low permeability) drugs because
absorption from an emulsion does not require the dissolution step. In addition,
oral administration of a drug as an emulsion involves coadministra-tion of
lipid, which increases bile secretion. Higher concentration of bile in the
gastrointestinal (GI) fluids further enhance oral absorption of hydrophobic
drugs.
2. Increased drug
stability:
Drugs that are more stable in an oily phase
compared to an aqueous medium can show improved stability in an emulsion
dosage form.
3. Prolonged drug
action:
The oily phase can serve as a reservoir of the drug, which slowly partitions into the aqueous phase for
absorption. This phenomenon, especially with semisolid emulsions, can help
pro-long drug action. For example, intramuscular injection of an emul-sion can
prolong drug absorption time.
A
key disadvantage of using emulsions as a delivery system is their physical
instability and complex, energy-intensive manufacturing process. In addi-tion,
solid dosage forms are generally preferred over liquid dosage forms for oral
administration.
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