US Pharm. 2013;38(12):36-37.
Method of Preparation: Calculate the quantity of each ingredient for the amount to be prepared. Accurately weigh or measure each ingredient. Using a magnetic stirrer, add about 80 mL of propylene glycol to an appropriate-sized beaker. Slowly sprinkle the hydroxypropyl cellulose 1,500 cps onto the stirred propylene glycol and mix well. Combine the benzocaine, lidocaine, and tetracaine powders to form a eutectic. Add to the gel mixture and mix thoroughly. Add sufficient propylene glycol to final volume and mix well. Allow mixture to set to remove any air bubbles. Package and label.
Use: This preparation has been used as a topical anesthetic prior to various procedures.
Packaging: Package in tight, light-resistant containers.
Labeling: Keep out of reach of children. Discard after ______ [time period].
Stability: A beyond-use date of 30 days may be used for this preparation.1
Quality Control: Quality-control assessment can include theoretical weight compared with actual weight, pH, specific gravity, active drug assay, color, clarity, texture-surface, texture-spatula spread, appearance, feel, rheologic properties, and physical observations.2
Discussion: A number of different topical anesthetic agents are available or can be compounded. Characteristics to be aware of include onset of action and duration of action. Advantages of this preparation are that the vehicle does not evaporate, as a water-containing vehicle does, and that propylene glycol is a penetration enhancer.
Benzocaine (ethyl aminobenzoate, C9H11NO2, MW 165.19) occurs as small, white crystals or as a white, crystalline, odorless powder. It is stable in air and exhibits local anesthetic properties when placed on the tongue. It is very slightly soluble in water (1 in 2,500) and freely soluble in alcohol (1 in 5-8) and ether (1 in 4). Benzocaine is sparingly soluble in almond and olive oil (1 in 30-50), and it will dissolve in dilute acids. It should be protected from light and stored in tight containers. Benzocaine is a topical anesthetic of the ester type with low systemic toxicity.1
Lidocaine (lignocaine, C14H22N2O, MW 234.34) occurs as a white to slightly yellow, crystalline powder with a characteristic odor. It has a melting range of 66°C to 69°C. Lidocaine is practically insoluble in water and very soluble in alcohol; it dissolves in oils. It can be mixed with prilocaine to form a eutectic with a lower melting point than that of either of the ingredients, and the eutectic can be incorporated into topical dosage forms. The base form is absorbed faster when applied topically.1
Tetracaine (C15H24N2O2, MW 264.46) occurs as a white or light yellow, waxy solid that is soluble in alcohol and ether; it is very slightly soluble in water. It should be preserved in tight, light-resistant containers. Tetracaine melts between 41°C and 46°C and is soluble 1 g in 1,000 mL water, 5 mL alcohol, and 2 mL ether. It is used as a local topical anesthetic.1,3
Hydroxypropyl cellulose 1,500 cps is a white to slightly cream-colored, practically odorless and tasteless powder or granular solid. It is widely used in oral and topical pharmaceutical formulations. The pH of a 1% w/v aqueous solution is in the range of 5.0 to 8.5. Hydroxypropyl cellulose is soluble 1 in 2 parts water, 1 in 2.5 parts ethanol, and 1 in 5 parts propylene glycol, but it is practically insoluble in glycerin and oils. Hydroxypropyl cellulose is hygroscopic after drying.1,4
Propylene glycol (C3H8O2) occurs as a clear, colorless, viscous, practically odorless liquid with a sweet taste, somewhat resembling glycerin. It has a specific gravity of 1.038 g/mL and is miscible with acetone, chloroform, 95% ethanol, glycerin, and water. Propylene glycol is not miscible with fixed oils or light mineral oil; it will, however, dissolve some essential oils. A 2% v/v aqueous solution of propylene glycol will be iso-osmotic with serum. Propylene glycol is used as a humectant in topicals (~15% concentration); as a preservative in solutions and semisolids (15%-30% concentration); and as a solvent or cosolvent in aerosols (10%-30% concentration), oral solutions (10%-25% concentration), parenterals (10%-60% concentration), and topicals (5%-80% concentration). Propylene glycol is actually a better solvent than glycerin. It is similar to ethanol as an antiseptic and also is used in cosmetics and in the food industry as a vehicle for flavors and emulsifiers. Propylene glycol is stable and may be mixed with numerous other solvents. Aqueous solutions of propylene glycol can be sterilized by autoclaving. Since propylene glycol is hygroscopic, it should be stored in an airtight container and protected from light. Incompatibilities include potassium permanganate.1,5
1. U.S. Pharmacopeia 36/National Formulary 31. Rockville, MD: U.S. Pharmacopeial Convention, Inc; 2013:335-398,1245,1264,1265,1268,1284,1292.
2. Allen LV Jr. Standard operating procedure for performing physical quality assessment of ointments/creams/gels. IJPC. 1998;2:308-309.
3. Tetracaine. In: Allen LV Jr, ed. Remington: The Science and Practice of Pharmacy. 23rd ed. London, England: Pharmaceutical Press; 2013:1777-1778.
4. Kabir MS, Reo JP. Hydroxypropyl cellulose. In: Rowe RC, Sheskey PJ, Quinn ME, eds. Handbook of Pharmaceutical Excipients. 6th ed. Washington, DC: American Pharmaceutical Association; 2009:317-322.
5. Weller PJ. Propylene glycol. In: Rowe RC, Sheskey PJ, Quinn ME, eds. Handbook of Pharmaceutical Excipients. 6th ed. Washington, DC: American Pharmaceutical Association; 2009:592-594.
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