The Transdermal Permeability of Terpenoids as an Insect Repellent

Abstract

Malaria is a communicable disease caused by the multiphase Plasmodium parasite. The disease is characterized as a significant global health concern that presents as a cyclical fever. The rapid development of resistance to antimalarial drugs and insecticides underscores the need to enhance the efficacy of repellents as a tool in vector control. Terpenoids, a large and diverse class of secondary plant metabolites have shown significant potential as an insect repellent, thus highlighting the need to further investigate the compounds. A method that is often used to accurately measure the terpenoids and assesses their effect on the skin barrier as well as their permeation through the skin layers is High Performance Liquid Chromatography (HPLC) methods. HPLC methods were developed to optimise detection of the terpenoids. The HPLC validation was conducted according to The International Council for Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) guidelines. The parameters include linearity, lowest detectable (LOD), and quantifiable (LOQ) amounts, precision, accuracy, stability, and robustness. Acceptance criteria were expressed as relative standard deviation (%RSD). Linearity was established from the coefficient of determination (𝑅2) of the line of regression from a calibration curve. Ex vivo diffusion and skin retention studies were performed using porcine skin samples mounted into the flow cells of a 7-in-line flow-through diffusion system. Skin integrity after the application of the terpenoids was assessed using the Transepithelial electrical resistance (TEER) measurements. The repellency was evaluated employing a WHO repellency bioassay at 1 mg/mL, 5 mg/mL, and 10 mg/mL. Reproducible and rapid HPLC methods were developed to detect the terpenoids accurately. The calibration curve for all the terpenoids were linear (𝑅2>0.9991). Precision, accuracy, and robustness exhibited a %RSD <2.0% across all compounds. The positive control (N, N-diethyl- meta-toluamide) DEET diffused through the skin and the average steady-state flux (Jss) was calculated to be 632 ± 42μg/𝑐𝑚2/min. The terpenoids did not diffuse across the skin into the acceptor chambers, they however accumulated within the skin layers. The compounds demonstrated significant repellency with (-)-Beta pinene, (-)-alpha pinene and linalyl acetate producing a 100% repellency percentage. The described methods can be successfully applied for routine analysis of the terpenoids due to them being simple, specific, precise, accurate, and robust. The skin integrity after the application of DEET showed a 51% decrease whereas the terpenoids produced a negligible effect on skin integrity. At the termination of the ex vivo permeation studies the terpenoids were not detected, indicating very low possibility of a iv systemic effect. The terpenoids have repellency potential with low risk of systemic adverse effects and warrant further investigation