Environmental fate of chiral pharmaceuticals: determination, degradation and toxicity

Pollution of the aquatic environment by pharmaceuticals is of major concern. Indeed pharmaceutical pollutants have several undesirable effects for many organisms, such as endocrine disruption and bacterium resistance. They are resistant to several degradation processes, making their removal difficult and slow. Pharmaceuticals reach the environment due to their inefficient removal by waste water treatment plants (WWTP), and by improper disposal of unused medicines. In aquatic environments pharmaceuticals reach concentrations at trace levels of ngL-1-μgL-1 range. Many pharmaceutical pollutants are chiral. They occur in nature as a single enantiomer or as mixtures of the two enantiomers, which have different spatial configuration and can thus be metabolized selectively. In spite of similar physical and chemical properties, enantiomers have different interactions with enzymes, receptors or other chiral molecules, leading to different biological response. Therefore they can affect living organisms in a different manner. The fate and effects of enantiomers of chiral pharmaceuticals in the environment are still largely unknown. Biodegradation and toxicity can be enantioselective, in contrast to abiotic degradation. Thus accurate methods to measure enantiomeric fractions in the environment are crucial to better understand the biodegradation process and to estimate toxicity of chiral pharmaceuticals. We review (1) general properties of chiral compounds, (2) current knowledge on chiral pharmaceuticals in the environment, (3) chiral analytical methods to determine the enantiomers composition in environmental matrices, (4) degradation and removal processes of chiral pharmaceuticals in the environment and (5) their toxicity to aquatic organisms. The major analytical methods discussed are gas chromatography (GC), high performance liquid chromatography (HPLC), electrochemical sensors and biosensors. These chiral methods are crucial for the correct quantification of the enantiomers regarding that if an enantiomer with more or less toxic effects is preferentially degraded, the assessed exposure based on measurements of achiral methodologies would overestimate or underestimate ecotoxicity. The degradation and biodegradation is discussed using few examples of important therapeutic classes usually detected in the aquatic environment. Few examples of ecotoxicity studies are also given on the occurrence of enantiomers and their fate in the environment which differs with regard to undesirable effects and to biochemical processes.