Topical delivery of active ingredients occurs via partition
and diffusion through the skin. Skin penetration depends on
the logarithm of partition coefficient (log P) and the molecular
weight of the compound. Compounds with molecular weights
less than 500 Da or with a log P of approximately 1 to 4 are
most likely to be absorbed through the skin.1–3) Thus, most
of the drugs used in transdermal therapeutic systems are designed
in accordance with these physicochemical parameters.4)
Many trials have investigated methods that might enhance the
skin penetration of poorly absorbed compounds. For example,
microemulsion applications have been used to enhance the
skin delivery of polyphenols.5) The effects of surfactants and
lipids of microemulsions on skin permeation of octylmethoxycinnamate
have also been investigated.6)
Stearyl glycyrrhetinate (SG) (Fig. 1) is a derivative of glycyrrhetic
acid, which is a well-known component of licorice
and has anti-inflammatory and anti-viral properties.7,8) It is
a lipophilic compound that is frequently used in cosmetics
such as emulsions and creams. The molecular weight of SG is
723 Da and its log P is 15.6.9) The properties of SG are outside
the range of appropriate molecular weights and log P. However,
the target site of cosmetics is mainly the epidermis, and it
is important to deliver the active ingredients to the epidermis.
To date, there have been no reports on the skin penetration
of SG. Thus, we examined whether emulsifiers or oils could
improve SG skin penetration when applied as oil-in-water
(O/W)-type emulsions under finite dose conditions.