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One enantiomer of a drug may have a desired beneficial effect while the other may cause serious and undesired side effects, or sometimes even beneficial but entirely different effects. [1] The desired enantiomer is known as an eutomer while the undesired enantiomer is known as the distomer. [ 2 ]
The eutomer is the enantiomer having the desired pharmacological activity, [4] e.g., as an active ingredient in a drug. The distomer , on the other hand, is the enantiomer of the eutomer which may have undesired bioactivity or may be bio-inert.
In case of penicillamine, the (S-isomer is used in the treatment of primary chronic arthritis, whereas the (R)-isomer has no therapeutic effect, as well as being highly toxic. [30] In some cases, the less therapeutically active enantiomer can cause side effects. For example, (S-naproxen is an analgesic but the (R-isomer causes renal problems. [31]
Selenomethionine (SeMet) is a naturally occurring amino acid.The L-selenomethionine enantiomer is the main form of selenium found in Brazil nuts, cereal grains, soybeans, and grassland legumes, while Se-methylselenocysteine, or its γ-glutamyl derivative, is the major form of selenium found in Astragalus, Allium, and Brassica species. [1]
The (R)-enantiomer has the desired sedative effect while the (S)-enantiomer harbors embryo-toxic and teratogenic effects. Attempting to extract solely R-thalidomide does not remove the risk of birth defects, as it was demonstrated that the "safe" R-thalidomide undergoes an in vivo chiral inversion to the "teratogenic" S-thalidomide.
However, studies have shown that only the (S)-(+) enantiomer (escitalopram) is responsible for the drug's beneficial effects. [10] [11] Drug safety: D‑penicillamine is used in chelation therapy and for the treatment of rheumatoid arthritis whereas L‑penicillamine is toxic as it inhibits the action of pyridoxine, an essential B vitamin. [12]
The drug was withdrawn from world market when it became evident that the use in pregnancy causes phocomelia (clinical conditions where babies are born with deformed hand and limbs). Later in late 1970s studies indicated that the (R)- enantiomer is an effective sedative, the (S)-enantiomer harbors teratogenic effect and causes fetal abnormalities.
Levofloxacin as an example of a chiral switch. A chiral switch is a chiral drug that has already approved as racemate but has been re-developed as a single enantiomer. [1] [2] The term chiral switching was introduced by Agranat and Caner in 1999 [3] to describe the development of single enantiomers from racemate drugs.