Adapalene

What is adapalene?

Adapalene is a third-generation synthetic retinoid that binds selectively to retinoic acid receptors β and γ (R). It is available in topical formulations, typically as a gel, cream, or lotion.

Is adapalene stable?

Adapalene is remarkably stable compared to tretinoin, which is very sensitive to light and oxidation (R). It remains stable even in the presence of light or the oxidising agent benzoyl peroxide (R, R). This is due to the unstable double bonds in tretinoin being replaced by aromatic rings (R).

Does adapalene get absorbed?

Adapalene is lipophilic (R), which helps it better penetrate the similarly lipophilic stratum corneum. However, it is hardly absorbed once it has penetrated the stratum corneum. 1 study showed that only 0.01% of the applied dose penetrated through the skin (R). Instead, significant quantities of adapalene were trapped in the epidermis, dermis and hair follicles, which is ideal as it allows adapalene to exert maximal effect locally (R).

The hair follicles of the skin are particularly attractive sites for adapalene because they can act as long-term reservoirs. This is because drugs in the hair follicles are depleted primarily through the slow process of sebum production and hair growth, in contrast to the stratum corneum, where they are continuously depleted due to desquamation (R). Formulations of adapalene in microparticles (R, R), microemulsions (R), liposomes (R) and soybean oil-cyclodextrin complexes (R) have therefore been developed to enhance its delivery into the hair follicles.