The role of glycerol in allergen extracts Jay E. Slater, MD Laboratory of Immunobiochemistry FDA/CBER/OVRR/DBPAP

n Glycerol n Glycerin n Glycerine n 1,2,3-propanetriol n Trihydroxypropane

n Solvent n Humectant n Plasticizer n Emollient n Sweetener n Manufacture of NitroglycerolNitroglycerol CosmeticsCosmetics Liquid soapsLiquid soaps InksInks LubricantsLubricants GluesGlues CementsCements

Note: manufacturers may produce more than one vehicle for a product

Why is glycerol added to allergen extracts? n Protein stabilizer n Protease inhibitor n Bacteriostatic

How does glycerol stabilize proteins? n Freezing point depression n Conformational alterations Preferential hydrationPreferential hydration Domain interface changesDomain interface changes Reduction of backbone mobilityReduction of backbone mobility n Changes in association constants

Expiry dating n Standardized Based on real-time stabilityBased on real-time stability n Unstandardized CFR CFR Glycerol: 50% vs <50%Glycerol: 50% vs <50%

Date of manufacture 18 months 36 months < 50% glycerol

Date of manufacture 36 months 72 months 50% glycerol or greater

Glycerol inhibits particulate formation in allergen extracts n In one series of obsevations, particulates appeared in 95% of aqueous extracts but only 50% of glycerinated extracts n Particulates in glycerinated extracts are significantly smaller

Glycerol is a preservative n “Products in multiple-dose containers shall contain a preservative, except that a preservative need not be added to…an Allergenic Product in 50 percent or more volume in volume (v/v) glycerin.” 21 CFR

Negatives n Pain on injection Glycerol dose relatedGlycerol dose related Not volume relatedNot volume related Van Metre et al JACI 1996;97:1033-9

More negatives n Induration (“glycerin button”) n Bruising

Summary n Glycerol Stabilizes allergens, especially at suboptimal temperaturesStabilizes allergens, especially at suboptimal temperatures May inhibit proteasesMay inhibit proteases Inhibits bacterial growthInhibits bacterial growth Inhibits precipitate formationInhibits precipitate formation n Only real drawback is pain on injection