GSK Study Questions Bioequivalency of Generic Cold Sore Creams

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ePT--the Electronic Newsletter of Pharmaceutical Technology

GSK Study Questions Bioequivalency of Generic Cold Sore Creams

A recent study published in the International Journal of Pharmaceutics (1) finds wide variations in the bioequivalency of several generic topical acyclovir creams used to treat recurrent Herpes labialis.

The research team members, most of whom are from GlaxoSmithKline (GSK), studied 139 generic versions of GSK’s “Zovirax” cream. Though Zovirax contains 40% propylene glycol, 111 of these creams contain less that 20% of antimicrobial preservative and solvent, according to the study data. 

To test bioavailability, the group compared the innovator cream with two generic versions that each contained 15% propylene glycol. In addition, 10 generic creams that contained <15% of this ingredient were tested in an independent laboratory. All studies were conducted on human skin and the acyclovir creams were analyzed by liquid chromatography and mass spectroscopy.

According to the research team, the innovator drug delivered 7.5-fold more acyclovir than the generic “equivalents,” which the study attributed to variations in the amount of propylene glycol.

The group’s conclusion that the marketed, generic versions of the acyclovir creams are not bioequivalent to Zovirax bolsters the notion that a drug’s formulation can influence its bioavailability, and thus its therapeutic bioequivalency. FDA requires that the innovator drug and the follow-on biologic must be therapeutically equivalent, which includes both bioequivalency and pharmaceutical equivalency (2).



1. L. Trottet, “Are All Aciclovir Cream Formulations Bioequivalent?” Int. J. Pharm.304 (1–2), 63–71 (2005).

2. A.K. Bansal and V. Koradia, “The Role of Reverse Engineeing in the Development of Generic Formulations,”

Pharm. Technol.



(8), 50–55 (2005).