Fixed-Oil Excipient Monographs: Development of USP Fixed-Oil Reference Standards

This article summarizes the development and modernization of the United States Pharmacopeia-National Formulary (USP-NF) fixed-oil excipient monographs. This article contains bonus online-exclusive material.
Apr 02, 2013
Volume 37, Issue 4

This article summarizes the development and modernization of the United States Pharmacopeia–National Formulary (USP−NF) fixed-oil excipient monographs. Fats and fixed oils are processed from natural sources and have complex chemical compositions. As part of the public standards-setting processes, USP staff and the Excipients Expert Committee have formulated a strategic analytical testing plan for fixed-oil excipients. The plan balances modernization with ease of adoption and method simplicity. Modernization introduces more specific compositional methods that can identify as well as quantify the analyte(s) of interest. A combination of simple orthogonal methods uses existing instrumentation that can be applied to a multitude of fixed-oil monographs and encourages ease of adoption. This plan also has helped make possible the development of USP Reference Standards for such complex excipients because the proposed analytical methods provide a comprehensive understanding and characterization of fixed oils. This is an extended version of “Fixed-Oil Excipient Monographs” published in Pharmaceutical Technology’s April 2013 issue.


The US Pharmacopeia (USP) Monographs—Excipients Expert Committee (EXC EC) for the current 2010–2015 revision cycle is responsible for the 31 United States Pharmacopeia–National Formulary (USP−NF) monographs with “oil” in the monograph title (see Table I) in USP 35−NF 30 through the Second Supplement (1). The 31 oil excipients include vegetable oils (edible), petrochemical oils, and essential oils. All vegetable oils are termed “fixed oils” in USP−NF. The term fixed oils distinguishes them from the relatively volatile petrochemical oils and essential oils. Fixed oils are obtained by expression or extraction. Their consistency varies with temperature. Some are liquid (oils), others are semisolid (fats), and still others are solid (tallows) at ambient temperature. Most fixed-oil excipients included in USP−NF are refined oils.

Table I: Excipient monographs that have “oil” in the titles and that fall under the responsibility of the excipients expert committee.
Twenty-four of the 31 USP–NF oil excipients are fixed oils or fixed-oil derivatives. Most fixed oils consist mainly of triglycerides (or triacylglycerols). The four USP−NF mineral oils in Table I are classified as petrochemical oils: each is defined as a purified mixture of liquid hydrocarbons obtained from petroleum. The three volatile essential oils (see Table I) are composed primarily of acyclic monoterpenoids, terpenes, and their derivatives or aromatic compounds and their derivatives and contain lesser quantities of alcohols, aldehydes, esters, and/or phenols.

Three of the 22 USP−NFfixed oils—castor oil, soybean oil, and safflower oil—are designated as USP, and the remaining fixed oils are designated as NF. The USP−NFfixed oils can have different functions. The majority are used as oleaginous vehicles or solvents in drug formulations. Some, however, are used as emulsifying agents, flavors and perfumes, ointment bases, plasticizers, stiffening agents, tablet or capsule lubricants, wetting or solubilizing agents, or coating agents.

Because of the complex and multicomponent nature of fixed oils as well as the lack of a thorough understanding of these materials, the monograph definition usually identifies the source as a means of defining the excipient. In contrast, the monograph definition for a single-component, well-characterized, and highly purified excipient indicates the acceptance criteria for the assay that is reflective of its content and purity. The use of any permitted additives also is included in the monograph’s definition (2) and labeling sections.

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