50 years ago, pregnant women who were prescribed thalidomide for morning sickness gave birth to children with birth defects. Thalidomide is a mixture of two enantiomers; one reduces morning sickness, but the other causes severe birth defects. Today, the FDA has approved this drug for non-pregnant individuals with Hansen’s disease (leprosy) or newly diagnosed multiple myeloma, a blood and bone marrow cancer. The beneficial enantiomer can be synthesized and given to patients, but over time, both the beneficial and the harmful enantiomer can be detected in the body. Propose a possible explanation for the presence of the harmful enantiomer.

I would suggest this as the best reason behind the detection of the harmful enantiomer in the body after the prescription of the beneficial one is that the human liver contains an enzyme that can convert the "good" R-enantiomer into the "bad" S-enantiomer.

Explanation:

Thalidomide (C13H10N2O4) is one of the most controversial drugs that exist nowadays due to the unfortunate error of prescribing this drug to pregnant women because its harmful effects were unknown at that time.

This drug possesses two enantiomers (optical isomers with mirror-image structures). Thalomide has a racemic mixture of both enantiomers, that is, an equal ratio of both 1:1. The R-enantiomer is a highly effective sedative with sleeping effects; whereas the S-enantiomer is teratogenic, i.e. it causes birth deffects by inhibiting the growth of new blood vessels.

After this disaster, researchers decided to prescribe this drug to patients with leprosy, cancer, and other diseases but, of course, not to pregnant women because as it turns out, the "good" enantiomer is also a great drug to treat them.

While they suggested the best way to prevent the presence of the "bad" enantiomer was to separate them into different drugs, they discovered that the human liver contains an enzyme that can convert the "good" R-enantiomer into the "bad" S-enantiomer.

Although there are still many research studies done because its mechanism is still not understood completely, I would suggest this as the best reason behind the detection of the harmful enantiomer in the body after the prescription of the beneficial one.