The FDA issued a final guidance to explain the latest changes to the 503A bulk drug substances list to small businesses.

The final rule, published in February and effective March 21, defines the criteria the agency uses to determine which bulk drug substances will be included on the 503A bulks list—allowing them for use in drug compounding operations. The final rule names six bulk drug substances that have been placed on the list and four that were deemed ineligible.

The agency added cantharidin, diphenylcyclopropenone, N-acetyl-D-glucosamine (NAG), squaric acid dibutyl ester and thymol iodide to the bulks list for topical use only; Brilliant Blue G (Coomassie Brilliant Blue G-250) was also selected for inclusion. But it rejected four other bulk drug substances—oxitriptan, piracetam, silver protein mild and tranilast—finding them inappropriate for use in compounding drug products.

The guidance explains that FDA appraises nominated bulk drug substances using four criteria: their physical and chemical characteristics, their safety in compounded drug products, their evidence of effectiveness, and their historical use in compounded drug products.