Enantioselective occurrence and fate of chiral drugs in the aqueous environment

Drugs are potentially hazardous biologically active emerging contaminants as many of them are ubiquitous and persistent with suspected or identified toxicity towards aquatic organisms. Additionally, due to their continuous introduction into the environment and synergistic effects through combined parallel action, even drugs of a low persistence might cause unwanted effects in the environment. Hundreds of tonnes of these compounds are dispensed in communities every year and subsequently enter the environment through wastewater. Several groups of drugs such as beta-blockers, antibiotics or analgesics have been studied before in the environment but surprisingly their chiral character has been overlooked by environmental researchers. More than half of the drugs currently in use are chiral compounds and many of those are distributed as racemates consisting of an equimolar mixture of two enantiomers. A chiral molecule has at least one chiral centre (usually asymmetric carbon) as a result of which it shows optical activity. It exists in the form of two enantiomers, being the non-superimposable mirror images of each other. Enantiomers have the same chemical formula and physicochemical properties but they differ in their optical activity and spatial arrangement. The enantiomers of chiral compounds differ in interactions with chiral environments such as enzymes in the body. Therefore in biological systems they can be recognised as two different substances that elicit different responses: one enantiomer of the same drug may produce the desired therapeutic activity, while the other may be inactive or even toxic. The ratio of active/inactive enantiomer of the chiral drug can change significantly after its administration, metabolism in and excretion from the body. It can be subsequently altered during biological wastewater treatment and when the drug is already present in the environment. This is because degradation of enantiomers can be stereo-specific and can in some cases lead to an increase in the drug's toxicity. Research in this area is important as with the ageing population in western countries and an increase in consumption levels in the developing world increasing quantities of drugs are entering the environment and, to date, studies of the prevalence and characteristics of these contaminants in the environment have been limited in scope. Furthermore, existing reports, due to their non-enantiospecific analytical methodology, do not tackle the problem of chirality of drugs, so these studies cannot unequivocally differentiate between biological (enantioselective) and abiotic (non-enantioselective) processes. Therefore this project aims to identify chiral drugs in the aqueous environment and to test the hypothesis that their distribution in the aqueous environment is stereoselective and that stereoselective mechanisms governing their fate are biological in nature. The hypothesis will be tested through a series of analyses of surface water samples in West Yorkshire (the River Calder), concentrating on densely populated areas where the environment has the greatest potential to be contaminated with drugs. Analysis of contamination will be undertaken taking account of local wastewater treatment activity, thus achieving a more accurate understanding of the risk associated with the presence of chiral drugs in the environment. To achieve this a multi-residue analytical methodology that will allow for simultaneous identification and quantification of trace concentrations (low ppt levels) of chiral drugs of abuse in environmental matrices will be established. Verification of the stereoselectivity of the degradation of chiral drugs in the aqueous environment after taking into consideration non-stereoselective abiotic (photochemical processes, hydrolysis, sorption) and stereoselective biological (microbial) variables will also be undertaken at the laboratory scale with the usage of microcosm protocol and model compounds.