By Melissa Merrell Rhoads, PharmD, PCCA Director of Formulation Development

Our Clinical Services team regularly receives questions from PCCA member pharmacies about compounding with lidocaine (the “free base”) and lidocaine HCl (the salt form). My colleague Andrew Glasnapp recently wrote an article for The PCCA Blog outlining the clinical considerations, common dosages and other important factors for compounding with both lidocaine and lidocaine HCl (there’s a link to it at the end of this article). To build on the information he covered, I would like to provide two technical tips for compounding with lidocaine and lidocaine HCl: how to create a eutectic mixture and how to choose which form to use in topical compounds.

Creating a Eutectic Mixture

Some drug powders, when mixed together in a dry state, lower each other’s melting points to below room temperature, causing them to melt (liquify). This is called a eutectic mixture, and it occurs with chemicals like camphor, menthol, phenol, chloral hydrate, hydroquinone and aspirin as well as benzocaine, lidocaine and tetracaine. Compounders can use this to their advantage in certain situations.

Combining anesthetics such as lidocaine, benzocaine and tetracaine or lidocaine and prilocaine is common practice in the compounding pharmacy, mainly so that practitioners can anesthetize the skin prior to needle sticks or for laser hair reduction and other dermatological procedures. When higher concentrations of these active pharmaceutical ingredients (APIs) are required, forming a eutectic mixture initially may help to keep the preparation from crystallizing over time in some cases.

For topical creams, forming a eutectic mixture may be desirable because it eliminates the need to levigate (wet) the powders since there is no longer any surface air to displace. A drug in a liquid state should show superior absorption compared to a solid drug powder as well.

You can create a eutectic mixture by triturating these APIs together in a mortar and pestle. This can only be achieved using the “free base” forms, not the hydrochloride salt forms. This process takes a few minutes to achieve and requires some physical force. For example, a typical 100 Gm BLT (benzocaine, lidocaine and tetracaine) formula may require vigorously triturating for 5-7 minutes, depending on the concentrations. The mixture will first become sticky and then will become a viscous liquid. Many of our formulas use a procedure to create a eutectic mixture. PCCA members with Clinical Services support can see some lidocaine formulas that use eutectic mixtures in our formula database.

Choosing Which Form of Lidocaine to Use in Topical Compounds

When working with Lidocaine Hydrochloride USP Monohydrate (PCCA #30-1213) versus Lidocaine USP (PCCA #30-1031), you will see little difference in the physical characteristics after compounding most topical cream or gel preparations. The intended use of the compound can help you determine which form to use (see Andrew Glasnapp’s article linked below for more information). Additionally, if using one form of lidocaine causes instability of a compound, such as a cream liquifying, you can often substitute the other form to create a more stable final preparation. Stability can depend on the combination of other APIs or the topical base being used in the formulation.

Certain bases should not be used with the salt forms of the “‘caines,” such as lidocaine, tetracaine and benzocaine. For example, VersaBase^® Gel (PCCA #30-3656) can only hold approximately 2% maximum salt (HCl) load, and Plasticized^™ Base (PCCA #30-3211) has a low salt-load capacity as well. PCCA formulas provide guidance on which PCCA bases will hold certain combinations and concentrations of anesthetics. Regardless of the base or formula, you should always know the capabilities of the base you’re using to ensure the stability of your compounded preparation.

Also on The PCCA Blog: Lidocaine vs. Lidocaine HCl in Pharmacy Compounding

Melissa Merrell Rhoads, PharmD, PCCA Director of Formulation Development, received her pharmacy degree from Mercer University in Atlanta, Georgia, in 1995. She currently is involved with and oversees the development and implementation of new formulas at PCCA. She had more than six years of compounding experience with pharmacies in Georgia and Florida prior to joining the PCCA staff in 2004. Her areas of interest include women’s health, veterinary and pain management compounding.

A version of this article originally appeared in PCCA’s members-only magazine, the Apothagram.

These statements are provided for educational purposes only. They have not been evaluated by the Food and Drug Administration, and are not to be interpreted as a promise, guarantee or claim of therapeutic efficacy or safety. The information contained herein is not intended to replace or substitute for conventional medical care, or encourage its abandonment.