Semisolid Preparations & Ointments Comprehensive Notes

Overview of Pharmaceutical Semisolid Preparations

• Semisolids = ointments, pastes, cream emulsions, gels, rigid foams.
• Key shared property: adherence to application site until washed/worn off.
• Result of plastic (Bingham) rheologic behavior → material holds shape as a thin film until external force ➔ flow.

Skin Anatomy, Function & Barrier Properties

• Largest multilayered organ; adult (w/o fat) ≈ 8\,\text{lb}; surface area > 20{,}000\,\text{cm}^2.
• Physiologic roles: barrier to chemicals/physics, UV shield, thermoregulation, microbial defense, BP regulation.
• Three principal tissue layers
• Epidermis (outer)
• Dermis (corium)
• Subcutaneous fat (hypodermis)
• Outermost epidermal sub-layer = Stratum Corneum (SC, “horny layer”)
• Dead, flattened keratinized cells; density =1.55.
• Diffusion coefficients \sim 10^3 × lower than other skin tissues → rate-limiting barrier.
• Dry composition: 75\text{–}85\% protein, 15\text{–}20\% lipid, \approx15\% water.
• Barrier restored when extracted lipids are replaced → lipid type/distribution governs membrane permeability.
• Below SC: metabolically active epidermis; basal (germinative) layer adjacent to dermis; cells migrate outward & die.
• Dermis
• ≈ 1/8-inch thick; ≈ 80\% protein in mucopolysaccharide ground substance.
• Contains vasculature, lymphatics, nerves, hair follicles, eccrine & apocrine sweat glands, sebaceous glands.
• Hair follicles occupy ≈ 1/1000 total skin surface; absent on soles, palms, lip vermilion, select genital regions.
• Eccrine glands (heat control) concentrated in palms/soles; apocrine larger (×10) in axillae, anogenital, areola.

Percutaneous Absorption & Pathways

• Therapeutic targets generally viable epidermis/upper dermis.
• Overall sequential events
1. Drug dissolves in vehicle.
2. Diffuses through vehicle → skin interface.
3. Two entry routes
• Transepidermal (intercellular + transcellular across SC)
• Transfollicular (via hair follicles/sebaceous glands)
4. Partition into SC → viable epidermis → dermis → systemic circulation.
• Penetration-Enhancement Strategies
• Drug modification: Prodrugs (↑ lipophilicity, ↓ size, ↓ ionization); Ion-pair complexes.
• Vehicle modification: Liposomes, nanoparticles, coacervates, nanoemulsions.
• SC modification: Hydration, chemical accelerants.
• Barrier bypass: Microneedles, ultrasound, iontophoresis, electroporation, laser/thermal ablation, magnetophoresis.
• Definitions & Illustrative Examples
• Prodrug – inactive molecule converted in vivo to active form; e.g., esterified steroids → higher SC permeation.
• Ion Pair – oppositely charged ion + drug ⇢ neutral complex → ↑ lipid solubility.
• Coacervates – phase-separated droplets encapsulate drug, shield & ferry across SC.
• Eutectic mixture – combo with melting point lower than each component (e.g., Lidocaine 68^{\circ}C + Prilocaine 58^{\circ}C → EMLA cream) → more fluid, better diffusion.
• Ablation – laser/thermal removal of SC loci to form micro-channels.
• Iontophoresis – low-voltage current propels charged drugs; enhances ionic mobility.
• Electroporation – high-voltage pulses create transient lipid pores → passage of large/neutral molecules.
• Lipid-Level Mechanisms (Fig. concepts)
• Lipid extraction (e.g., alcohols, surfactants) – removes lipids → porous SC.
• Phase separation (oleic acid) – disrupts bilayer packing.
• Water-pool formation (DMSO, PEG, PG) – ↑ SC hydration/swelling.
• Fluidization (menthol, terpenes) – ↑ membrane fluidity.
• Polarity alteration (SLS) – changes dielectric environment.
• Lipid vesicle fusion (liposomes) – merge with SC lipids releasing drug.

Factors Influencing Skin Penetration

• Drug-related (primary)
• Hydration of SC – most critical; occlusion can raise water content from 5\text{–}15\% → \le 50\%.
• Solubility in vehicle determines concentration at interface.
• Octanol/water partition coefficient ((K_{o/w})) governs diffusion rate.
• Molecular weight – inverse relation; small molecules diffuse faster. Very large (proteins/polysaccharides) ≈ impermeable.
• Vehicle-related (secondary)
• Water activity & SC partitioning; greases/oils → maximal occlusion & hydration.
• Chemical accelerants: DMSO > DMF > DMA > urea > propylene glycol > surfactants.
◦ DMSO/DMF/DMA – hygroscopic; swell SC, extract structural lipids.
◦ Surfactants – alter water state; anionics (laurate) > cationics > non-ionics; effectiveness ↓ as pH ↑ (~11).
• Physiologic variables: skin integrity, age, anatomic site area, SC thickness, species differences, temperature, moisture.

Ointments: Definition, Uses & USP Base Classification

• Ointment = semisolid for external skin/mucous application; medicated or unmedicated (protectant, emollient, lubricant).
• USP base groups
1. Oleaginous (hydrocarbon) bases
2. Absorption bases
3. Water-removable bases
4. Water-soluble bases

1. Oleaginous Bases

• Hydrocarbon, occlusive, emollient, non-washable; incorporate only small water qty.
• Examples: Petrolatum, White Petrolatum, Yellow Ointment, White Ointment.
• Petrolatum USP – melts 38\text{–}60^{\circ}C; aka petroleum jelly, Vaseline®.
• White Petrolatum – decolorized; aesthetically preferred.
• Yellow Ointment (Simple Ointment) formula for 1000\,g:
• Yellow wax 50\,g + Petrolatum 950\,g.
• Levigating agent for powders: mineral oil.

2. Absorption Bases

• Type A: permit aqueous incorporation → form \text{W/O} emulsion (e.g., Hydrophilic Petrolatum).
• Type B: are pre-formed \text{W/O} emulsions, can absorb more water (e.g., Lanolin).
• Provide moderate occlusion; difficult to rinse with plain water.

3. Water-Removable Bases (Creams)

• Oil-in-water ((\text{O/W})) emulsions; water-washable, can be diluted; absorb exudates.
• Prototype: Hydrophilic Ointment USP.

4. Water-Soluble Bases

• No oleaginous components; “greaseless”; fully water-washable; limited capacity for added water.
• Best for incorporating solids; prototype: Polyethylene Glycol (PEG) Ointment NF.

Preparation Methods of Ointments

A. Incorporation (“cold blending”)

• Components mixed until uniform via mortar & pestle, spatula on slab, Unguator®, electronic mortar, or ointment mill.
• Solids
• Geometric dilution of powders with base.
• Particle-size reduction by levigation with compatible agent (mineral oil for oleaginous, glycerin for aqueous ext. phase). Volume of levigating agent ≈ volume of solid.
• If soluble in innocuous solvent, dissolve → add solution to base (mortar preferred for large liquid volumes).
• Pulverization by intervention for gummy substances (e.g., camphor): dissolve → spread → evaporate → triturate.
• Liquids
• Consider base capacity: oleaginous accept minimal water; hydrophilic bases accept more.
• Strategy: incorporate aqueous solution into small hydrophilic base portion → blend into hydrophobic base.
• Ointment mills/roller mills – ensure smooth texture & uniformity.

B. Fusion (“hot blending”)

• Melt all/some components; cool with constant stirring; add heat-labile/volatile substances at lowest safe T.
• High-melting excipients (beeswax, paraffin, stearyl alcohol, high-MW PEGs) favor fusion.
• Process options
1. Heat highest-melting ingredients to lowest necessary T → add others during cooling.
2. Melt lowest-melting first → sequentially add higher-melting.
3. Simultaneous melt with gradual T increase.
• Large-scale: steam-jacketed kettles; finish via ointment mill.

C. Chemical Reaction Method

• Product formed by in-situ reaction, e.g., Iodine Ointment.
• Iodine low solubility in fats, but dissolves readily in concentrated KI solution forming soluble polyiodides KI\cdot I2,\ KI\cdot 2I2,\ KI\cdot 3I_2 which are incorporated into absorption base.

D. Emulsification Method

• Melt fats/oils/waxes at 70^{\circ}C; heat aqueous phase (heat-stable solutes) to same T.
• Slowly add aqueous to oil phase with continuous stirring; cool until semisolid.

Compendial & Quality Requirements

• USP Tests
• Microbial content (non-sterile except ophthalmic); antimicrobial preservatives (methyl-/propyl-paraben, phenols, benzoic acid, sorbic acid, quaternary ammonium salts) for water-containing products.
• Specific microbial limits: Betamethasone Valerate Ointment must be free from S.\,aureus & P.\,aeruginosa.
• Minimum fill, packaging, storage, labeling.
• Ophthalmic: sterility + metal particle limits.
• Packaging & Storage
• Large-mouth jars or collapsible tubes (metal/plastic).
• Well-closed, cool environment; light-resistant/opaque containers for photosensitive drugs.
• Labeling – USP-mandated info on storage, dosing, administration where applicable.
• Additional Manufacturer Controls
• Viscosity testing (rheology) for consistency.
• In-vitro drug-release assays (diffusion cell, Franz cell) to assure batch uniformity.

Practical/Clinical Takeaways & Connections

• Choosing base impacts hydration, occlusion, washability, drug stability, patient acceptability.
• Occlusive dressings (plastic films) markedly ↑ topical steroid absorption via SC hydration.
• Skin penetration science parallels oral absorption (diffusion, partitioning) yet uniquely dominated by SC hydration & lipid architecture.
• Emerging enhancement technologies (microneedles, electroporation, nanoparticles) bridge topical & transdermal systemic delivery realms.
• Ethical Consideration: Some powerful accelerants (e.g., DMSO) remain experimental due to potential toxicity; prudent formulation & patient counseling required.