In-vitro prediction of the membranotropic action of emerging organic contaminants using a liposome-based multidisciplinary approach

Abstract

According to ISO 17402:2008 more knowledge is needed on processes controlling bioavailability of organic species so as to close the still existing gap between chemical measurements and biological effects. The bioavailability concept encompasses the investigation of the degree of penetration of target species across biological membranes. In addition, REACH (Registration, Evaluation, Authorisation and restriction of Chemicals) guidelines promote the use of in-vitro methods against conventional ecotoxicological tests because of the ethical controversy of in-vivo tests. This work is aimed at filling the gap by proposing a multidisciplinary approach based on high-resolution and low-resolution empirical techniques, and theoretical quantum mechanics for the in-vitro investigation of the bioavailability and membranotropic effects of organic emerging contaminants, including bioaccumulation, via passive diffusion across lipid bilayers. Phosphatidylcholine (PC) liposomes are selected as biomembrane surrogates, and contaminant effects are explored by (i) fluorescence anisotropy and generalized polarization assays using membrane fluorescence probes (laurdan and prodan) and UV-Vis spectroscopy, (ii) ¹H NMR measurements to ascertain supramolecular interactions with PC and (iii) molecular dynamics simulations. In particular, un-regulated model compounds with distinct physico-chemical properties that are representative of three different classes of emerging contaminants in environmental compartments are chosen for validation of the holistic approach: (i) diclofenac as a model of anti-inflammatory drug; (ii) triclosan as an anti-microbial agent; and (iii) bisphenol A as a plastic-borne compound, and compared with chlorpyrifos as a legacy insecticide. Laurdan anisotropic measurements are in good agreement with ¹H NMR data and both approaches pinpoint that triclosan and chlorpyrifos are highly bioaccumulative in membranes. Molecular dynamic studies indicate that the lateral diffusion of the lipid bilayer is much lower with the incorporation of either triclosan or chlorpyrifos into the bilayer. The theoretical simulations also allowed estimating absolute bioavailability data under passive diffusion (<0.1%, 63%, 73% and 89% for diclofenac, bisphenol A, triclosan and chlorpyrifos, respectively) given as the percentage of time that a given species is located in the region of the fatty acyl chains. Our findings indicate that PC-based liposome assays serve as a fast and cost-effective in-vitro approach, notwithstanding its low resolution features, for environmental bioavailability studies of emerging contaminants for which insufficient or inconsistent ecotoxicological data are identified in the literature.

Keywords: Bioavailability; Emerging contaminants; Lipid bilayer; Liposome; Membrane; Permeability.