Australasian Science: Australia's authority on science since 1938

Chemicals Undergo Toxic Inversion

By Stephen Luntz

Pharmaceuticals in wastewater can become converted to toxic forms.

Even non-toxic pharmaceuticals escaping into the water supply may be unsafe, Dr Stuart Khan of the University of NSW Water Research Centre has revealed in the journal Water Research.

Many chemicals exist in two forms known as enantiomers, mirror images that cannot be superimposed on each other. The transformation from one enantiomer to another is known as a chiral inversion and can be triggered by bacteria. While some enantiomers have identical effects on the human body, some are harmless or beneficial while their reflection is toxic.

Consequently some drugs, such as the anti-inflammatory naproxen, have to be carefully manufactured to only contain a particular enantiomer, in naproxen’s case the S-form. However, when Khan examined the output of a water treatment plant he found quantities of the liver toxin R-naproxen.

Khan says he has long been interested in what happens to pharmaceuticals in the wastewater system. “I saw a paper that monitored the change of ibuprofen through a plant, using quantities in and out as a measure of effectiveness. They found higher levels of one form afterwards. However, because ibuprofen is dispensed in both forms it could have been that bacteria selectively take up more of one than the other,” Khan says.

Khan’s measurements in the sewerage system revealed only S-naproxen, making it clear that the transformation is taking place within the treatment plant. Closely related chemicals have been observed experiencing chiral inversions in the gut, but naproxen could not be used if this occurred to any significant extent.

“Once we observed this we looked around and found bacteria that do this conversion, but we don’t know if they are the ones doing it in the plants,” Khan says.

Two other drugs Khan tested showed a change in ratios, suggesting that an inversion was occurring. Since both are believed to be safe in either form the finding was not of direct concern, although it does suggest that naproxen may not be exceptional.

“Generally plants are not designed to remove low traces of pharmaceuticals,” Khan says. “However, we’ve seen from the way plants perform around the world that it is possible to tweak them to improve the removal, or you can use reverse osmosis which is much more effective at keeping chemicals out.”

In Europe, environmental risk assessments are conducted on pharmaceuticals. Khan says that the possibility of dangerous conversions should be added to this process. However, no such assessments are currently done in Australia.

On the other hand, Khan says there is a more general question. “How are we going to deal with the transformation and degradation products of pharmaceuticals? Chiral inversion is just one process by which these can be formed. Risk assessment is not sophisticated enough to deal with this at the moment.”