Thalidomide: The dangers of stereoisomerism
What was once believed to be an effective antiemetic drug for morning sickness, in reality, led to tragedy. Widely popular in the 1950s, Thalidomide was once an over-the-counter medication often used to treat pregnant women due to its ability to function as a sedative and tranquilliser. It quickly spread and was marketed under different names in 46 different countries. At the time, scientists remained unaware that the effects of drugs and other harmful substances could pass from the mother to her foetus via the placenta and subsequently ceased to recognise the dangers Thalidomide posed on developing foetuses. The widespread use of Thalidomide resulted in the deaths of over 10,000 newborn children as it was later discovered that the drug was teratogenic, thus causing severe birth defects. Such birth defects include the shortening and/or absence of limbs, sensory impairment, facial disfigurement, and damage to the brain, internal organs as well as the ears and eyes (American Chemical Society, n.d.).
Thalidomide exists in 2 forms as it has 2 stereoisomers, one of which is responsible for the adverse effects on foetal development. These 2 forms are called R-Thalidomide and S-Thalidomide, and are enantiomers of one another (see fig. 1). To understand what exactly this means and how these 2 compounds differ, it is important to understand stereoisomerism and chirality. Isomers refer to compounds which have the same molecular formula as one another ie. the same number of specific elements of which it is made up. There are 2 types of isomers: 1) structural isomers and 2) stereoisomers. This article is only concerned with the latter. Compounds which are stereoisomers of each other are those which have different spatial arrangements ie. the way in which atoms are arranged in the space surrounding the organic compound/its carbon chain but have the same connectivity (the way the atoms are connected together). Stereoisomers can be broken down into two groups: 1) conformational isomers and 2) configurational isomers. Configurational isomers are those which can only be interconverted through the breaking of bonds whereas conformational isomers can do so by simply rotating around a 𝛔 bond.
Fig. 1 - The 2 enantiomers of Thalidomide
The (+) and (-) refer to the direction in which each enantiomer rotates plane-polarised light. A clockwise direction means a (+) enantiomer while an anti-clockwise direction means a (-) enantiomer.
One type of configurational isomerism is optical isomerism. Optical isomers exist in 2 types of pairs: 1) enantiomers (which applies to Thalidomide) and 2) diastereomers. They can rotate plane-polarised light, which is relevant when determining whether the compound is a (+) or (-) enantiomer. Enantiomers are compounds which are mirror images of each other and are non-superimposable; they cannot align with each other to become replicas either through translation or rotation. Most of their chemical and physical properties are identical except for their reactions with other optical isomers whilst Diastereomers, on the other hand, are compounds which are not related by a mirror image and have different physical and chemical properties (Bylikin et al., 2014).
Chirality is when compounds cannot be superimposed on their mirror image. Compounds are chiral because of the presence of an asymmetric carbon or also known as a stereogenic centre where a central carbon has 4 different atoms/molecules bonded to it (see fig. 2). This is important because Thalidomide’s chiral carbon centre enables it to form R- and S- enantiomers (Chemistry Steps, 2017). Absolute configuration or the R and S system for naming compounds is interested in the placement of each particular element in space. For a compound to either be an R or S enantiomer depends on the direction in which each of the elements in the enantiomer is counted based on its atomic number (Chemistry Steps, 2019). A clockwise direction means that the compound is an R-enantiomer and conversely, an anti-clockwise direction indicates an S-enantiomer (see fig. 3).
The human body consists of various enzymes which are chiral. Enzymes are made up of proteins which have different structures and consequently different functions as they can fold differently due to specific bonds that can form between their R-groups or variable side chains. This is important because it means that enzymes are highly specific and complementary to their substrates as well as take into account their stereochemistry. Enzymes can distinguish between and recognise enantiomers despite them being mirror images of one another and in most cases, will only be able to bind to one (Chemistry LibreTexts, 2014). An easy way to illustrate this is through our hands. If you place both your palms together, they are mirror images of one another (much like enantiomers) but your left hand would cease to fit into a softball glove specifically designed to be right-handed. Continuing with this analogy, the softball glove represents an enzyme’s active site which is specific to certain substrates (or hands), and so certain enzymes may only catalyse specific reactions. Structure is essential in terms of living organisms because structure always relates to function; different structures have been adapted to carry out specific functions within the body effectively.
It is intuitive to assume that Thalidomide’s teratogenic properties and its detrimental effects could have been mitigated had pure R-Thalidomide been administered, however, once introduced into the body, R-Thalidomide rapidly converts into its harmful counterpart. This is because of the acidity of the proton at Thalidomide’s stereocenter which makes it more susceptible to inversion. When the proton at the stereocenter is removed, a negatively charged intermediate is formed. This intermediate is potentially vulnerable to nucleophilic attack by hydroxide ions as it can further lower the enantiomerization energy barrier resulting in the hydrolysis of the amide bond (Tian et al., 2012). The hydrolysis in turn ultimately causes the inversion of the stereocenter and thus the formation of the opposite enantiomer. In-vivo, Thalidomide, therefore, exists as a racemic mixture so despite not ingesting the S-Thalidomide enantiomers directly, its adverse effects can still be caused.
I have yet to address how exactly Thalidomide causes birth defects and will do so in this section. While Thalidomide’s mechanism and action in the body still largely remain unknown, there is strong evidence to suggest that Thalidomide promotes the degradation of transcription factors (which determine whether genes are switched on or off) such as SALL4. SALL4 is involved in the process of forming tissues and organs during embryonic development so the degradation and loss of this transcription factor results in abnormal changes in limb development and foetal growth. Individuals who are not affected by Thalidomide but carry a mutated gene for SALL4 are generally born with deformities including the loss of thumbs, eye and ear defects, underdeveloped limbs and congenital heart disease which greatly resemble the defects present in children whose mothers took Thalidomide whilst pregnant (Fischer, 2018).
Today, Thalidomide has served as an important lesson in the development of pharmaceuticals and shaped modern clinical trials. Although it is no longer being used to treat morning sickness, Thalidomide is still being used presently to treat multiple myeloma or cancer of the plasma cells as well as leprosy, or as it is now more commonly known - Hansen’s disease (Mayo Clinic, 2019). When Thalidomide binds to a protein called cereblon, it prevents other substrates from binding and being degraded as well as promotes transcription factors such as IKZF1 and IKZF3 to the enzyme’s active site which inhibits the growth of tumours and thereby is an effective treatment for multiple myeloma (Stewart, 2014b). Concerning Hansen’s disease, Thalidomide inhibits the gene expression of the pro-inflammatory cytokine, tumour necrosis factor-α (TNF-α), which is involved in nerve damage.
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