Oxide terpenes as human skin penetration enhancers of haloperidol from ethanol and propylene glycol and their modes of action on stratum corneum

In this study, two terpenes with the same functional group; limonene oxide and pinene oxide were used at 5% w/v concentration in 50% v/v ethanol and 100% v/v propylene glycol (PG) to enhance the in vitro permeation of haloperidol (HP) through the human epidermis (or stratum corneum, SC). The enhancement mechanism of terpenes from both solvents was elucidated with HP-SC binding studies, Fourier transform infrared spectroscopy and differential scanning calorimetry. The enhancement activity of these terpenes was higher in 50% v/v ethanol than in 100% v/v PG. These terpenes in 50% v/v ethanol were predicted to provide the required therapeutic plasma concentration and daily-permeated amounts of the drug. Limonene oxide showed higher enhancement in both solvents, which was attributed to its less bulky structure. The terpenes in both solvents did not increase the partition of HP. Instrumental studies showed that these terpenes in 50% v/v ethanol extracted the SC lipids, disrupted the bilayer packing and partially fluidised the lipids. Limonene oxide in 100% v/v PG possibly disrupted the lipid bilayer, whilst leaving the overall bilayer structure intact and pinene oxide in the same vehicle fluidised the lipids within the ordered environment. This study showed that the mode of interactions of terpenes with SC were different in two solvent systems.