Suspensions can be classified based on the characteristics of the dis-persed phase or the dispersion medium, and also based on their route of administration.
Types of
suspensions
Suspensions
can be classified based on the characteristics of the dis-persed phase or the
dispersion medium, and also based on their route of administration.
Based
on the particle size (diameter) of the dispersed phase, suspensions can be
classified as (1) coarse suspension (> 1 μm), (2) colloidal dispersion (< 1
μm), or (3)
nanosuspension (10–100 nm). Based on the concentration of the dispersed phase,
highly concentrated suspensions are termed as slur-ries (> 50 % w/w), and
certain suspensions are considered dilute suspensions (2%–10% w/w). Based on
the type of the dispersion medium, suspensions can be aqueous or nonaqueous.
Identifying the physical state of the dispersion medium allows the suspensions
to be classified as solid-in-liquid or solid-in-gas (aerosols) suspensions.
Based
on the route of administration, suspensions can be classified as oral, topical,
ophthalmic, otic, or nasal suspensions. Each of these present unique challenges
and requirements in terms of desired quality attributes. These are briefly
described as follows:
1. Oral suspensions: Suspensions meant
for peroral route of admin-istration are usually liquid preparations in which
solid particles of the active drug are dispersed in a sweetened, flavored,
sometimes colored, and usually viscous vehicle. For example, amoxicillin oral
suspension contains 125–500 mg dispersed active pharmaceutical ingredient (API)
per 5 mL of suspension. When formulated for use as pediatric drops,
concentration of suspended API is increased to allow lower volume of
administration for pediatric doses. Antacids and radioopaque suspensions
generally contain high concentrations of dispersed solids.
2. Topical
suspensions:
Lotions are externally applied suspensions.
These are designed for dermatologic, cosmetic, and protective pur-poses.
Topical suspension formulations need to pay particular atten-tion to the lack
of grittiness and smooth feel on the skin. These suspensions are typically
colored and may have some perfume, but do not need sweeteners and flavors
typically used for oral administration.
3. Injectable
suspensions:
Parenteral suspensions may contain from 0.5%
to 30% w/w of solid particles. Viscosity and particle size are significant
factors because they affect the ease of injection and the availability of the
drug in depot therapy. Most parenteral suspen-sions are designed for
intramuscular or subcutaneous administration. For example, procaine penicillin
G suspension is intended for intra-muscular administration. Sterility is an
important consideration for parenteral suspensions. Being a suspension dosage
form, they cannot be sterilized by terminal filtration. Thus, the use of
sterile API and aseptic processing is required for their manufacturing. In
addition, antimicrobial preservatives are not recommended for intravenous (IV)
suspensions.
4. Otic suspensions: These are intended
for administration into the ear. Most
otic suspensions are antibiotics, corticosteroids, or anal-gesics for the
treatment of ear infection, inflammation, and pain. For example, cortisporin
otic suspension contains polymixin, neomycin, and hydrocortisone for antibiotic
and anti-inflammatory effect. Otic suspensions are generally formulated as
sterile suspensions since they come in contact with the mucosal surface.
5. Rectal
suspensions:
Local administration through the rectal cavity is used for the treatment or management of local disorders of the
colon. For example, 5-acetyl salicylic acid (5-ASA) rectal suspension enema is
used as an anti-inflammatory treatment for ulcerative coli-tis. Formulation and
quality considerations for rectal suspensions are similar to the oral
suspensions.
6. Aerosols: Aerosols are
suspensions of drug particles or drug solution in the air and are used for inhalation of drug delivery to the
lung. Volatile propellants are frequently used as vehicles for pharmaceutical
aerosols.
7. Liposomes and
micro-/nanoparticles: Suspensions of liposomes, microspheres, microcapsules, nanospheres, or nanocapsules are used
for targeted and controlled delivery of drugs. These are usually intended for
parenteral administration.
8. Vaccines: Vaccines are used
for the induction of immunity and are often
formulated as suspensions. For example, cholera vaccine and tetanus vaccine are
suspensions.
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