Investigating the absorptive and secretory transport of selected anti-malarial drugs

Abstract

Malaria is one of the predominant infectious diseases affecting more than 200 million people a year worldwide. Artemisinin combination therapy (ACT) is currently the frontline treatment against the Plasmodium falciparum parasite, but reports of resistance developed against the ACT has started to surface in Cambodia and Thailand. New derivatives of artemisinin, such as such as artemisone and artemiside, are therefore being established in an effort to overcome this resistance, whereas artemether is already being used clinically. Knowledge regarding the membrane permeability properties of the newly developed anti-malarial drugs is limited, and treatment failure caused by sub-optimal drug concentrations at the site of action can give rise to drug resistance. The aim of this study was to use the Caco-2 cell-based in vitro model to evaluate intestinal membrane permeability and P-glycoprotein (P-gp) related efflux of these orally administered drugs, which can provide an estimation of the bioavailability of the compounds. The Caco-2 model was validated to confirm monolayer integrity using Lucifer yellow, and subsequently the expression of P-gp was confirmed with the P-gp substrate, vinblastine. Furthermore, high-performance liquid chromatography (HPLC) analytical methods to quantify the various compounds studied were validated. Bi-directional transport of the selected anti-malarial drugs across Caco-2 cell monolayer was measured in the presence and absence of verapamil, a known P-gp inhibitor, and samples obtained during the transport studies were analysed. The relative percentage transport of each compound in both directions were used to calculate the apparent permeability coefficient (Papp) values, as well as the efflux ratios (ER) for each anti-malarial. The relative percentage transport of artemisone and artemether appeared to be moderate in comparison to the reference compounds, caffeine and atenolol, while very low permeation levels of artemiside could be detected. The bi-directional transport results revealed that artemether was probably a very poor P-gp substrate, while artemisone and artemiside were both susceptible to P-gp mediated efflux to some extent. In an effort to potentially increase the bioavailability of these P-gp substrates, the potentially beneficial herb-drug interaction with piperine extracted from black pepper was investigated. Piperine has been shown to inhibit P-gp, which could then potentially reduce the efflux of artemisone and artemiside. It reduced the efflux of artemisone slightly more than verapamil, but solicited an increase in artemiside efflux, thus revealing the possible involvement of other efflux transporters. The transport studies of the artemisinin derivatives clearly suggests these compounds to have moderate to low bioavailability, and low affinity for the P-gp efflux protein.