• Most of the drugs that are used to treat diseases contain?one or more?chiral centres
• These drugs can therefore exist as?enantiomers?which differ from each other in their ability to rotate plane polarised light
• Another crucial difference between the enantiomers is in their potential?biological?activity?and therefore their?effectiveness?as medicines
• Drug compounds should be prepared in such a way that only one of the optical isomer is produced, in order to increase the drugs’ effectiveness
  • Some drug enantiomers can have very harmful side effects

Potential biological activity of enantiomers

• If?conventional organic reactions?are used to make the desired drug, a?racemic mixture?will be obtained
  • In a racemic mixture, there are equal amounts of the two enantiomers
• The?physical?and?chemical?properties of the enantiomers are the same, however, they may have opposite biological activities
• For example, the drug?naproxen?is used to treat pain in patients that suffer from arthritis
  • One of the enantiomers of naproxen eases the pain, whereas another enantiomer causes liver damage
• One enantiomer of a drug used to treat tuberculosis is effective whereas another enantiomer of this drug can cause blindness
• Thalidomide is another example of a drug that used to be used to treat morning sickness, where one of the enantiomers caused very harmful side effects for the unborn baby

Separating racemic mixtures

• Due to the different biological activities of enantiomers, it is very important to?separate?a racemic mixture into?pure single enantiomers?which are put in the drug product
• This results in?reduced side-effects?in patients
  • As a result, it protects pharmaceutical companies from legal actions if the side effects are too serious
• It also?decreases?the patient’s?dosage?by half as the pure enantiomer is more?potent?and therefore reduces production costs
  • A more potent drug has a better?therapeutic activity

Chiral catalysts

• In order to produce single, pure optical isomers,?chiral catalysts?can be used
• The benefits of using chiral catalysts are that only?small amounts?of them are needed and they can be?reused
  • For example, an organometallic ruthenium catalyst is used in the production of?naproxen?which is used in the treatment of arthritis

The organometallic ruthenium catalyst is a chiral catalyst which ensures that only one of the enantiomers is formed which can be used in treating arthritis

• Enzymes?are excellent biological chiral catalysts that promote?stereoselectivity?and produce single-enantiomer products only
  • Stereoselectivity?refers to the preference of a reaction to form one enantiomer over the other
• Due to the?specific?binding site of enzymes, only one enantiomer is formed in the reaction
• The enzymes are fixed in place on?inert supports?so that the reactants can pass over them without having to later separate the product from the enzymes
• The?disadvantage?of using enzymes is that it can be?expensive?to isolate them from living organism
  • Therefore, more research has recently been carried out into designing?synthetic enzymes
• Although using enzymes to produce pure enantiomers in drug synthesis takes longer than conventional synthetic routes, there are many advantages to it in the long run
  • For example, using enzymes to synthesise drugs is a?greener?process as fewer steps are involved compared to conventional synthetic routes