Types of suspensions

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.