A study published last week found wide variations in the level of propylene glycol (PG) in 139 generic topical aciclovir creams, which lead to significant differences in skin penetration of aciclovir, the API used to treat cold sores.
Three comparative studies also found that the branded drug, Zovirax (40 per cent PG), delivered up to five times more aciclovir directly into the skin and therefore had the greatest bioavailability, compared with generic formulations with less than 20 per cent PG.
This study supports the theory that formulation can play an important part in bioequivalence and hence therapeutic equivalence of a drug product, leading to speculation about the efficacy of generic drugs.
By definition, a generic drug is an exact chemical copy of a brand-name drug, identical in dosage, safety, strength, administration, quality, performance, and intended use, although certain excipients (non-active ingredients) may be different.
The results of this study are set to spark concern amongst health professionals who, with greater understanding of the formulations and efficacy of brand name drugs, may err on the side of caution and steer away from recommending generics drugs.
This comes at a time when generic drug use is being heavily promoted as a cost-saving measure, as the savings opportunity has never been greater, with over $50 billion worth of branded drugs due to lose patent exclusivity over the next five years.
The generics drugs industry has been growing rapidly, as more and more branded drugs are coming off patent, rival manufacturers, particularly in Asia, are scrambling to produce generic versions at a fraction of the branded drug price.
However, the results of studies such as this, which was published in the International Journal of Pharmaceutics last week, may shake consumer confidence that a generic drug is as effective as its branded counterpart.
Zovirax (5% aciclovir) topical cream was the world's first clinically proven antiviral treatment for cold sores, introduced in 1981, and now widely available as an OTC brand.
It is known that PG plays an important role in skin penetration, optimising and maintaining levels of aciclovir in the skin. Based on the mode of action of aciclovir, the greater the concentration and time the compound is present in the skin during viral reactivation of a cold sore, the greater the potential for clinical benefit.