Enantioselective occurrence and fate of chiral drugs in the environment

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Funder: Natural Environment Research Council

Project No: NE/I000534/1 (project details)

The importance of chirality

Chirality plays an important role in the life of plants and animals but it is also vital in the agricultural, pharmaceutical and chemical industries. The phenomenon of chirality is also of growing importance in the field of environmental pollution and its effects on human health. More than half of the pharmacologically active compounds (PACs) currently in use are chiral compounds and many of those are marketed as racemates consisting of an equimolar mixture of two enantiomers (1).

A chiral molecule usually has at least one chiral centre (e.g. asymmetric carbon) as a result of which it shows optical activity. It exists in the form of two enantiomers (if only one chiral centre is present), being the non-superimposable mirror images of each other. Enantiomers of the same chiral molecule have similar physico-chemical properties but may differ in their biological properties. Distribution, metabolism and excretion usually favour one enantiomer over the other. This results from the fact that enantiomers stereoselectively react in biological systems, e.g. with enzymes. Additionally, due to different activity, chiral molecules can differ in toxicity. Thalidomide is an excellent example. A therapeutic (+)-thalidomide is harmless but in the human body it undergoes in vivo inter-conversion leading to toxic (-)-enantiomer, which leads to malformations of embryos if administered to pregnant woman (1).

Therefore, the enantiomeric composition of a chiral molecule can change throughout its environmental life-cycle. It can be altered after its administration as a result of its metabolism in the body. Enantiomeric composition of the chiral molecule can be subsequently changed during wastewater treatment and in the environment. Therefore the very same chiral molecule might have different activity/toxicity at different stages of its environmental life cycle, which will depend on its origin and exposure to environmental factors.

Chiral PACs in the environment and their undiscovered enantiomer dependent fate

PACs are emerging environmental contaminants. Thousands of PACs are approved for human/veterinary use, although only a very small percentage of these compounds have been studied in the environment. Some of the most commonly used PACs are sold in hundreds of tonnes/year in the UK alone. Usage of PACs is likely to increase in the future due to an ageing population in western countries and an increase in consumption levels in the developing world.

PACs enter the environment mainly through insufficiently treated sewage, waste effluents from manufacturing processes, runoff and sludge. They are ubiquitous and persistent with synergistic properties. PACs have also been detected in drinking water, which poses a direct risk to humans.

Surprisingly, the environmental fate and toxicity of PACs are assessed without taking into consideration their enantiomeric forms. This might lead to a significant under or overestimation of toxicity of chiral PACs and to incorrect environmental risk assessment as chiral PACs are likely to be present in the environment in their non-racemic forms.

Fluoxetine is a great example. It is one of the most toxic PACs reported so far. Its toxicity is assessed for the racemate; however, recent research indicates that toxicity of fluoxetine is enantiomer dependent: S-fluoxetine is 9.4 times more toxic than R-fluoxetine in Pimephales promelas (2). This enantiomer dependant toxicity of fluoxetine is of vital importance if it is not released to the environment in a racemic form. According to preliminary studies (3), raw sewage was enriched with R(-)-fluoxetine, but WWT led to an enrichment of fluoxetine with S(-)-enantiomer. Additionally, as fluoxetine has been found in tap water (1), human exposure to this drug might be higher than expected. It is hypothesised that other chiral PACs will also show enantiomer- specific fate.

Project objectives:

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 project will be undertaken taking into account the following objectives:

Objective 1: To establish and validate multi-residue analytical methods for the quantification of chiral drugs using SPE-chiral-LCMS/MS instrumentation.

Objective 2: To analyse enantiomers of chiral drugs and their metabolites in the aqueous environment and to test the hypothesis that distribution of chiral drugs in the aqueous environment is stereoselective.

Objective 3: To verify stereoselectivity in degradation pathways of chiral drugs in surface water in microcosm experiments in order to test the hypothesis that stereoselective mechanisms governing their fate are biological in nature.

Project outcomes:

  1. Development of novel methodology for enantiomeric profiling of chiral drugs in aqueous environmental matrices.

A novel multi-residue methodology for enantiomeric profiling of chiral drugs in different environmental matrices utilising for the first time high resolution QTOF MS was developed. This method allows for both target analysis and screening of unknowns and is of key importance if mechanisms of degradation are studied.

  1. Verification of enantiomer-specific fate of chiral drugs in the UK aqueous environment.

Enantiomeric profiling of chiral drugs of abuse in the environment has never been a subject of investigation before. It revealed the enantiomer-specific fate of all studied drugs. The extent of stereoselectivity depended on several parameters including: type of a chiral drug, wastewater treatment technology used and season.

  1. Discovery of enantiomer-specific biotransformation of chiral drugs in river microcosms.

Laboratory microcosm experiments undertaken for drugs of abuse proved, in the first ever study of this kind, the hypothesis that stereoselective mechanisms governing fate of chiral drugs of abuse are biological in nature.

This ground-breaking project proved for the first time that chiral drugs of abuse are subject to enantiomer-specific processes occurring in the environment and that the enantiomeric composition of a chiral drug can change throughout its environmental cycle. Knowing that two enantiomers of the same chiral drug usually differ in potency and toxicity (e.g. S(+)-amphetamine has twice as high stimulant activity than R(−)-amphetamine), the very same chiral compound might have different activity/toxicity at different stages of its environmental life cycle, which will depend on its origin and exposure to environmental factors. The above is of critical significance in the environmental risk assessment of pharmacologically active compounds, which currently does not take into account enantiomerism of pollutants and potentially leads to a significant under or overestimation of toxicity of chiral drugs.

References:

(1) B. Kasprzyk-Hordern, Chemical Society Reviews 39 (2010) 4466.

(2) Stanley, J.K., Ramirez, A.J., Chambliss, C.K., Brooks, B.W., Chemosphere 69 (2007) 9-16.

(3) S.L. MacLeod, P. Sudhir, C.S. Wong. Journal of Chromatography 1170 (2007) 23.

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