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Design for biodegradability
Design for biodegradability
Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
Pharmaceuticals are excreted from the body and enter the environment via the waste water system. This presents a risk to species. This is elaborated in a dedicated section on pharmaceuticals in the environment. One solution being put forward to reduce the burden of pharmaceuticals in the environment is the development of drug molecules with properties optimised for rapid and complete biodegradation after excretion. This design concept is called Benign by Design.
Biodegradation: When organic chemicals (e.g. drug molecules) are mineralised by microorganisms. This process makes water and carbon dioxide, but sulphates or nitrates may also be formed depending on what atoms were present in the original compound.
Transformation products: Commonly, the process of mineralisation by biodegradation is incomplete, and as a consequence the environment is exposed to products from the partial breakdown of a chemical (so called 'transformation products'). To understand the effect of pharmaceuticals in the environment, we must consider the fate of these transformation products as well as the parent molecule.
Persistent: The opposite of biodegradable. Persistent chemicals are not biodegradable in an acceptable timescale.
In the context of pharmaceuticals, biodegradation in the environment by microorganisms is not be confused with metabolism in humans or animals (resulting in potentially different breakdown products called metabolites).
Any chemical that inevitably enters the environment must achieve fast and complete mineralisation to avoid incomplete degradation and persistent transformation products which may have a negative impact on the environment. The complexity of medicinal products and their metabolites and transformation products contributes to a cocktail of environmental pollutants which makes the assessment of environmental risks more challenging.
Benign by Design takes advantage of the fact that (bio)degradability is strongly dependent on the ambient conditions in addition to the inherent properties of the molecule. Since the ambient conditions are changing along the lifecycle of an drug molecule (from production to transport and storage to application in the patient then excretion and finally exposure to the environment), the (bio)degradation potential varies as well.
Another important principle of Benign by Design is that small changes to chemical structure have an impact on the biodegradation rate. As a consequence, it is possible to keep the functional parts of molecules while increasing the rate of biodegradation through slight molecular changes.
Environmental design rules
Environmental design rules
Rules of thumb can create Benign by Design principles to increase the chances that an active pharmaceutical ingredient (API), its eventual metabolites and its transformation products will not be persistent. Several examples have been proposed, including:
Introduce easily biodegradable functional groups into molecules.
Avoid very stable functional groups that resist biodegradation.
Design molecules to have high affinity to wastewater sludge, therefore removed in water treatment plants.
Make chemicals that are sensitive to light. Photodegradation is an alternative to biodegradation.
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