Semisolid Dosage Forms: Ointments, Creams, Gels, and Suppositories

Semi-Solid Dosage Forms

Introduction to Semi-Solids

Semi-solid dosage forms are pharmaceutical preparations that exist in a state between solids and liquids. They are widely used for topical and mucosal drug delivery, offering both local and systemic effects depending on the formulation and application site.

• 4th state of matter: Semi-solids hold their shape when undisturbed (e.g., from whipped cream to chapstick).

• Can be pharmaceutical or cosmetic products.

Functions of Dermatologicals

• Protection: Shield injured areas from the environment.

• Skin hydration (emollient): Maintain or restore skin moisture.

• Vehicle for medication transport: Deliver drugs locally or systemically.

• Prophylactic: Prevent infection (e.g., Neosporin).

Mechanism of Skin Penetration

• Drugs can penetrate the skin via transcellular (through cells) or intercellular (between cells) diffusion, or through appendages (hair follicles, glands).

• Penetration is influenced by the physicochemical properties of the drug, the vehicle, and the condition of the skin.

Types of Semi-Solids

• Creams

• Gels

• Ointments

• Pastes

These may contain zero, one, or more active ingredients and typically include excipients such as emulsifiers, viscosity-increasing agents, antimicrobials, and antioxidants.

Special Attributes of Semi-Solids

• Bioavailability depends on area of application, thickness of residue, and formulation.

• Should be free of pathogens and contamination.

• Require chemical and physical stability.

• Can exert local or systemic effects.

Signs of Physical Instability

• Change in consistency

• Discoloration

• Emulsion breakdown

• Crystal growth

• Shrinking due to water evaporation

• Microbial growth

• Change in odor

Factors to Consider in Formulation

• Purpose of treatment (e.g., skin disease)

• Need for both aqueous and lipid solubility for absorption

• Animal fats can enhance penetration when combined with water

• Greasy bases may interfere with normal skin functions

Pharmaceutical Factors Affecting Drug Penetration

• Stability of the base (e.g., rancidity, antioxidants)

• Rate of drug release

• Solvent properties (most drugs are insoluble in ointment bases)

• Emulsifying properties

• Consistency and ability to withstand temperature changes

Emulsions

Emulsion: A thermodynamically unstable system of at least two immiscible liquid phases, with one dispersed as globules in the other.

• Oil-in-water (o/w): Water is the bulk phase (e.g., milk).

• Water-in-oil (w/o): Oil is the bulk phase (e.g., butter).

• Multiple emulsions: Delayed action delivery systems.

• Microemulsions: Formed spontaneously when components are mixed in appropriate ratios.

Rheology and Types of Flow

Rheology is the study of deformation and flow properties of matter, including solids, liquids, and gases.

• Viscosity: Resistance to flow or thickness.

• Newtonian fluids: Constant viscosity regardless of applied stress.

• Non-Newtonian flows:

  • Plastic flow: No flow until a certain stress threshold is reached (e.g., ointments).

  • Pseudoplastic flow: Shear-thinning; viscosity decreases with increased shear (e.g., creams).

  • Dilatant flow: Shear-thickening; viscosity increases with increased shear (e.g., pastes, whipped cream).

  • Thixotropy: Time-dependent shear thinning; viscosity decreases over time with agitation (e.g., gels, lotions).

Ointments

USP Definition: Semisolid preparations intended for external application to the skin or mucous membranes.

Types of Ointment Bases

• Oleaginous bases

• Absorption bases

• W/O emulsion bases

• O/W emulsion bases

• Water-soluble bases

Oleaginous Bases

• Hydrophobic, anhydrous, not water washable

• Oily, occlusive, greasy, thick, stiff

• Poor drug release

• Examples: Hydrocarbons (petrolatum), animal/vegetable oils, synthetic esters

Absorption Bases

• Intermediate properties between oleaginous and w/o bases

• Can absorb water (e.g., Aquaphor contains 41% petrolatum)

Emulsion Bases

• W/O bases:

  • O/W bases: >45% water, water washable, non-greasy, easy to spread

Water-Soluble Bases

• Water washable and soluble

• Lipid-free, non-greasy, moderate to easy spreadability

• Examples: Polyethylene glycol (PEG), polyoxyl 40 stearate

Selection Criteria for Ointment Bases

• Desired drug release rate

• Type of treatment (localized or systemic)

• Stability of API in the base

• Compatibility with other ingredients

• Ease of removal and application

• Need for occlusion or spreadability

Generalizations for Penetration or Systemic Absorption

• Larger surface area increases absorption

• Increased skin hydration enhances absorption

• Rubbing in the formulation enhances absorption

• Longer contact time increases absorption

Compounding Ointments

• Incorporation: Levigation (reducing particle size), followed by geometric dilution

• Fusion: Separate heating of aqueous and oil phases, then mixing and cooling

Tools for Compounding

• Ointment slab and spatulas

• Ointment pad

• Ointment mill

• Unguator

Ideal Properties of Semi-Solid Dosage Forms

• Smooth texture, elegant appearance

• Non-dehydrating, non-gritty, non-greasy, does not stain

• Non-irritating, does not alter skin function

• Miscible with skin secretions, low sensitization index

• Easily applicable, efficient drug release, high washability

Advantages and Disadvantages

Advantages	Disadvantages
Avoids first-pass metabolism Site-specific action Convenient for topical conditions More stable than liquid forms	May cause staining Bulky to handle Possible contamination with fingers Less stable than solid forms Can be greasy, may cause irritation or allergies

Summary Table: Types of Ointment Bases

Base Type	Water Content	Washability	Greasy	Drug Release	Example
Oleaginous	None	No	Yes	Poor	Vaseline
Absorption	Can absorb	No	Yes	Fair	Aquaphor
W/O Emulsion	<45%	No	Yes	Fair	Hydrophilic Petrolatum
O/W Emulsion	>45%	Yes	No	Good	Hydrophilic Ointment
Water-soluble	May/may not	Yes	No	Good	PEG Ointment

Additional info: This summary covers the foundational principles of semi-solid dosage forms, their pharmaceutical properties, and practical considerations for formulation and compounding, as relevant to Anatomy & Physiology and pharmaceutical sciences.