Chiral drugs

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Chemical compounds that come as mirror-image pairs are referred to by chemists as chiral or handed molecules. [1] Each twin is called an enantiomer. Drugs that exhibit handedness are referred to as chiral drugs. Chiral drugs that are equimolar (1:1) mixture of enantiomers are called racemic drugs and these are obviously devoid of optical rotation. The most commonly encountered stereogenic unit, [2] that confers chirality to drug molecules are stereogenic center. Stereogenic center can be due to the presence of tetrahedral tetra coordinate atoms (C,N,P) and pyramidal tricoordinate atoms (N,S). The word chiral describes the three-dimensional architecture of the molecule and does not reveal the stereochemical composition. Hence "chiral drug" does not say whether the drug is racemic (racemic drug), single enantiomer (chiral specific drug) or some other combination of stereoisomers. To resolve this issue Joseph Gal introduced a new term called unichiral. [3] [4] Unichiral indicates that the stereochemical composition of a chiral drug is homogenous consisting of a single enantiomer.

Contents

Many medicinal agents important to life are combinations of mirror-image twins. Despite the close resemblance of such twins, the differences in their biological properties can be profound. In other words, the component enantiomers of a racemic chiral drug may differ wildly in their pharmacokinetic, pharmacodynamic profile. [5] [6] [7] [8] The tragedy of thalidomide illustrates the potential for extreme consequences resulting from the administration of a racemate drug that exhibits multiple effects attributable to individual enantiomers. [9] With the advancements in chiral technology and the increased awareness about three-dimensional consequences of drug action and disposition emerged specialized field "chiral pharmacology". Simultaneously the chirality nomenclature system also evolved. A brief overview of chirality history and terminology/descriptors is given below. A detailed chirality timeline is not the focus of this article.

Chirality: history overview

Louis Pasteur - pioneering stereochemist Louis Pasteur, foto av Paul Nadar, Crisco edit.jpg
Louis Pasteur - pioneering stereochemist

Chirality can be traced back to 1812, when physicist Jean-Baptiste Biot found out about a phenomenon called "optical activity." [10] Louis Pasteur, a famous student of Biot's, made a series of observations that led him to suggest that the optical activity of some substances is caused by their molecular asymmetry, which makes nonsuperimposable mirror-images. In 1848, Pasteur grew two different kinds of crystals from the racemic sodium ammonium salt of tartaric acid. He was the first person to separate enantiomeric crystals by hand. [11] In fact Pasteur laid the foundations of stereochemistry and chirality.

In 1874, Jacobus Henricus van 't Hoff came up with the idea of an asymmetric carbon atom. He said that all optically active carbon compounds have an asymmetric carbon atom. [12] In the same year, Joseph Achille Le Bel only used asymmetry arguments and talked about the asymmetry of the molecules as a whole instead of the asymmetry of each carbon atom. [13] So, Le Bel's idea could be seen as the general theory of stereoisomerism, while van 't Hoff's could be seen as a special case (restricted to tetrahedral carbon).

Soon, scientists started to look into what chiral compounds meant for living things. In 1903, Cushny was the first person to show that enantiomers of a chiral molecule have different biological effects. [14] Lord Kelvin used the word "chiral" for the first time in 1904. [15]

Chirality: terminology/descriptors

This is to give an overview of the evolving chirality nomenclature system commonly employed to distinguish enantiomers of a chiral drug. In the beginning, enantiomers were distinguished based on their ability to rotate the plane of plane-polarized light. The enantiomer that rotates the plane-polarized light to the right is named "dextro-rotatory", abbreviated as "dextro" or "d" and the counterpart as "levo" or "l". A racemic mixture is denoted as "(±)", "rac", or "dl". Now the d/l system of naming based on optical rotation is falling into disuse.

Later, the Fischer convention [16] [17] was introduced to specify the configuration of a stereogenic center and uses the symbols D and L. The use of capital letters is to differentiate from the "d" / "l" notation (optical descriptor) described earlier. In this system, the enantiomers are named with reference to D- and L-glyceraldehyde which is taken as the standard for comparison. The structure of the chiral molecule should be represented in the Fischer projection formula. If the hydroxyl group attached to the highest chiral carbon is on the right-hand side it is referred to as D-series and if on the left-hand side it is called L-series. This nomenclature system has also become obsolete. But D-/L-system of naming is still employed to designate the configuration of amino acids and sugars. In general the D/L system of nomenclature is superseded by the Cahn-Ingold-Prelog (CIP) rule to describe the configuration of a stereogenic/chiral center.

In the CIP or R/S convention, or sequence rule, the configuration, spatial arrangements of ligands/substituents around a chiral center, is labeled as either "R" or "S". [18] [2] This convention is now almost worldwide in use and become a part of the IUPAC (International Union of Pure and Applied Chemistry) rules of nomenclature. In this approach: identify the chiral center, label the four atoms directly attached to the stereogenic center in question, assign priorities according to the sequence rule ( from 1 to 4), rotate the molecule until the lowest priority (number 4) substituent is away from the observer/viewer, draw a curve from number 1 to number 2 to number 3 substituent. If the curve is clockwise, the chiral center is of R-absolute configuration, "R" (Latin, rectus = right). If the curve is counterclockwise, the chiral center is of S-absolute configuration, "S" (Latin, sinister = left). Refer to figure, the Cahn-Ingold-Prelog rule.

The Cahn-Ingold-Prelog rule Cahn-Ingold-Prelog convention.png
The Cahn-Ingold-Prelog rule

An overview of the nomenclature system is presented in the table below.

Chirality descriptor

(used as prefix)

DescriptionComments
(+)- / dextro-/ d-Optical rotation signs; does not reflect the configurationd; Obsolete terms/falling in disuse
(-)- / levo-Optical rotation signs; does not reflect the configurationl; Obsolete terms/falling in disuse
(±)- / rac- /dl-Racemate or racemic mixture is an equimolar (1:1) mixture of enantiomers; corresponds to the enantiomeric excess of 0%.dl; Obsolete terms/falling in disuse
D-Relative configuration with respect to D-glyceraldehyde; referred to as Fischer convention-
L-Relative configuration with respect to L-glyceraldehyde; referred to as Fischer convention-
R-Latin: rectus = right; absolute configuration as per Cahn-Ingold-Prelog rule/sequence rule-
S-Latin: sinister = left; absolute configuration as per Cahn-Ingold-Prelog rule/Sequence rule-

Racemic drugs

For many years scientists in drug development have been blind to the three-dimensional consequences of stereochemistry, chiefly due to the lack of technology for making enantioselective investigations. Besides the thalidomide tragedy, another event that raised the importance of issues of stereochemistry in drug research and development was the publication of a manuscript in 1984 entitled, "Stereochemistry, a basis of sophisticated nonsense in pharmacokinetics and clinical pharmacology" by Ariëns. [19] This article, and the series of articles that followed, criticized the practice of conducting pharmacokinetic and pharmacodynamic studies on racemic drugs and ignoring the separate contributions of the individual enantiomers. [20] [21] [22] [23] [24] [25] These papers have served to crystallize some of the important issues surrounding racemic drugs and stimulated much discussion in industry, government and academia.

Chiral pharmacology

As a result of these criticisms and the renewed awareness of the three-dimensional effects of drug action fueled by the exponential explosion of chiral technology emerged the new area "stereo-pharmacology". A more specific term is "chiral pharmacology", a phrase popularized by John Caldwell. [26] This field has grown itself into a specialized discipline concerned with the three-dimensional aspects of drug action and disposition. This approach essentially views each version of the chiral twins as separate chemical species. To express the pharmacological activities of each of the chiral twins two technical terms have been coined, eutomer and distomer. [27] The member of the chiral twin that has greater physiological activity is referred to as the eutomer and the other one with lesser activity is referred to as distomer. It is generally understood that this reference is necessarily to a single activity being studied. The eutomer for one effect may well be the distomer when another is studied. The eutomer/distomer ratio is called the eudysmic ratio. [28]

Bio-environment and chiral twins

The behavior of the chiral twins depends mainly on the nature of the environment (achiral/chiral) in which they are present. An achiral environment does not differentiate the molecular twins whereas a chiral environment does distinguish the left-handed version from the right-handed version. Human body, a classic bio-environment, is inherently handed as it is filled with chiral discriminators like amino acids, enzymes, carbohydrates, lipids, nucleic acids, etc. Hence when a racemic therapeutic gets exposed to biological system the component enantiomers will be acted upon stereoselectively. [29] For drugs, chiral discrimination can take place either in the pharmacokinetic or pharmacodynamic phase.

Chiral discrimination

Easson and Stedman [30] (1933) advanced a drug-receptor interaction model to account for the differential pharmacodynamic activity between enantiomeric pairs. In this model the more active enantiomer (the eutomer) take part in a minimum of three simultaneous intermolecular interactions with the receptor surface (good fit), Figure. A., where as the less active enantiomer (distomer) interacts at two sites only (bad fit), Figure B. [Refer image for Figure: Easson-Stedman model]. Thus the "fit" of the individual enantiomers to the receptor site differs, as does the energy of interaction. This is a simplistic model but used to explain the biological discrimination between enantiomeric pairs.

Easson-Stedman model Easson-Stedman Model.png
Easson-Stedman model

In reality the drug-receptor interaction is not that simple, but this view of such complex phenomenon has provided major insights into the mechanism of action of drugs.

Pharmacodynamic considerations

Racemic drugs are not drug combinations in the accepted sense of two or more co-formulated therapeutic agents, but combinations of isomeric substances whose pharmacological activity may reside predominantly in one specific enantiomeric form. In case of stereoselectivity in action only one of the components in the racemic mixture is truly active. The other isomer, the distomer, should be regarded as impurity or isomeric ballast, [31] a term coined by Ariëns, not contributing to the effects aimed at. In contrast to the pharmacokinetic properties of an enantiomeric pair, differences in pharmacodynamic activity tend to be more obvious. There is a wide spectrum of possibilities of distomer actions, many of which are confirmed experimentally. [32] [33] [34] Selected examples of the distomer actions (viz. equipotent, less active, inactive, antagonistic, chiral inversion) are presented in the table below.

Chiral drugStereogenic

center(s)

Therapeutic actionEutomerDistomerDistomer actionReference
promethazine 1Antihistaminic(R)-/(S)--Equipotent [35]
Salbutamol 1Bronchodilator(R)-(S)-Less active; no serious side-effects [5]
Propranolol 1Antihypertensive(S)-(R)-Inactive; half placebo [36] [37]
Indacrinone 1Diuretic(R)-(S)-Antagonizes side effect of the eutomer [38]
Propoxyphene 2Analgesic; (Dexpropoxyphene)(2R),(3S)-(2S),(3R)-Independent therapeutic value [39]
Antitussive; (Levopropoxyphene)(2S),(3R)-(2R),(3S)-
Ibuprofen 1Anti-inflammatory(S)-(R)- Chiral inversion (unidirectional; [(R)- to (S)-] [40]

Drug toxicity

Since there is a frequent large pharmacokinetic and pharmacodynamic differences between enantiomers of a chiral drug it is not surprising that enantiomers may result in stereoselective toxicity. They can reside in the pharmacologically active enantiomer (eutomer) or in the inactive one (distomer). [41] [42] [43] The toxicologic differences between enantiomers of have also been demonstrated. The following are examples of some of the chiral drugs where their toxic/undesirable side-effects dwell almost in the distomer. This would seem to be a clear cut case of going for a chiral switch.

Penicillamine

Penicillamine enantiomers Penicillamine enantiomers.png
Penicillamine enantiomers

Penicillamine is a chiral drug with one chiral center and exists as a pair of enantiomers. (S)-penicillamine is the eutomer with the desired antiarthritic activity while the (R)-penicillamine is extremely toxic. [44]

Ketamine

Ketamine enantiomers Ketamine enantiomers.png
Ketamine enantiomers

Ketamine is a widely used anaesthetic agent. It is a chiral molecule that is administered as a racemate. Studies show that (S)-(+)-ketamine is the active anaesthetic and the undesired side-effects (hallucination and agitation) reside in the distomer, (R)-(-)-ketamine. [45] [46] [47]

Dopa

Dopa enantiomers Dopa enantiomers.png
Dopa enantiomers

The initial use of racemic dopa for the treatment of Parkinson's disease resulted in a number of adverse effects viz. nausea, vomiting, anorexia, involuntary movements and granulocytopenia. The use of L-dopa [the (S)-enantiomer] resulted in reducing the required dose, and adverse effects. The granulocytopenia was not observed with the single enantiomer. [48] [49] [50]

Ethambutol

Ethambutol enantiomers Ethambutol enantiomers.png
Ethambutol enantiomers

The antitubercular agent Ethambutol contains two constitutionally symmetrical stereogenic centers in its structure and exists in three stereoisomeric forms. An enantiomeric pair (S,S)- and (R,R)-ethambutol, along with the achiral stereoisomer called meso-form, it holds a diastereomeric relationship with the optically active stereoisomers. The activity of the drug resides in the (S,S)-enantiomer which is 500 and 12 fold more potent than the (R,R)-ethambutol and the meso-form. The drug s initially introduced for clinical use as the racemate and was changed to the (S,S)-enantiomer, as a result of optic neuritis leading to blindness. Toxicity is related to both dose and duration of treatment. All the three stereoisomers were almost equipotent with respect to side effects. Hence the use of S,S)-enantiomer greatly enhanced the risk/benefit ratio. [51] [52]

Thalidomide

Thalidomide enantiomers Thalidomide enantiomers.png
Thalidomide enantiomers

Thalidomide is a classical example highlighting the alleged role of chirality in drug toxicity. Thalidomide was a racemic therapeutic and prescribed to pregnant women to control nausea and vomiting. 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. [53] [54] [55] Later studies established that under biological conditions the (R)-thalidomide, good partner, undergoes an in vivo metabolic inversion to the (S)-thalidomide, evil partner and vice versa. It is a bidirectional chiral inversion. Hence the argument that the thalidomide tragedy could have been avoided by using a single enantiomer is ambiguous and pointless. [56] [57] [58]

The salient features are presented in the table below.

Chiral drugs: Adverse effects residing in the distomer
Chiral drugChiral centersClinical effects
Eutomer; ActivityDistomer; Activity
Penicillamine1(S)-; Antiarthritic(R)-; Mutagen
Ketamine1(S)-; Anesthetic(R)-; Hallucinogen
Dopa1(S)-; Antiparkinson(R)-; Granulocytopenia
Ethambutol2(S,S)-; Tuberculostatic(R,R)-; and meso- form; Blindness
Thalidomide1(R)-; Sedative(S)-; Teratogenic

Unichiral drugs

Unichiral indicates configurationally homogeneous substance (i.e. made up of chiral molecules of one and the same configuration). Other commonly used synonyms are enantiopure drugs and enantiomerically pure drugs. Monochiral drugs has also been suggested as another synonym. [59] Professor Eliel, Wilen, and Gal expressed their deep concern over the misuse of the term "homochiral" in articles to denote enantiomerically pure drugs, which is incorrect. [60] [61] [62] Homochiral means objects or molecules of the same handedness. Hence should be used only for comparison of two or more objects of like "chirality". For instance, left hands of different individuals, or say R-naproxen and R-ibuprofen.

Globally drug companies and regulatory agencies have an inclination towards the development of unichiral drugs as a consequence of the increased understanding of the differing biological properties of individual enantiomers of a racemic therapeutics. Most of these unichiral drugs are the consequence of chiral switch approach. The table below list selected unichiral drugs used in drug therapy.

Unichiral drugs employed in drug therapy
Unichiral drugsDrug class/ Type of medicationTherapeutic area
Esomeprazole Proton pump inhibitorGastroenterology
S-pantoprazoleProton pump inhibitor
DexrabiprazoleProton pump inhibitor
Levosalbutamol BronchodilatorPulmonology
Levocetrizine Antihistaminic
Levofloxacin AntibacterialInfectious diseases
S-penicillamineRheumatoid ArthritisRheumatology / Pain/ Inflammation
S-etodolacNSAID
Dexketoprofen NSAID
S-ketamine AnestheticAnesthesiology
Levobupivacaine Local anesthetic
Levothyroxine Anti-hypothyroidismEndocrinology
Levodopa Anti-ParkinsonNeuropsychiatry
S-amlodipine Antianginal / AntihypertensiveCardiology
S-metoprololAntihypertensive
S-atenololAntihypertensive

A company may go in for developing a racemic drug against an enantiomer by providing adequate reasoning. The rationale why a company might pursue developing racemic drugs [63] [64] [65] could include expensive separation of enantiomers, eutomer racemizes in solution (e.g. oxazepam), [66] activities of the enantiomeric pair are different but supplementary, distomer is inactive, but separation is exorbitant. Insignificant/low toxicity of the distomer, high therapeutic index, mutually beneficial, pharmacological activities of both the enantiomers, and if the development of an enantiomer takes huge amount of time for a drug of emergency need e.g., cancer, AIDS, etc.

Chiral purity

Chiral purity is a measure of the purity of a chiral drug. Other synonyms employed include enantiomeric excess, enantiomer purity, enantiomeric purity, and optical purity. Optical purity is an obsolete term since today most of the chiral purity measurements are done using chromatographic techniques (not based on optical principles). Enantiomeric excess tells the extent (in %) to which the chiral substance contains one enantiomer over the other. For a racemic drug the enantiomeric excess will be 0%. There are number of chiral analysis tools such as polarimetry, NMR spectroscopy with the use of chiral shift reagents, chiral GC (gas chromatography), chiral HPLC (high performance liquid chromatography), chiral TLC (thin-layer chromatography) [67] and other chiral chromatographic techniques, that are employed to evaluate chiral purity. [68] Assessing the purity of a unichiral drug or enantiopure drug is of great importance from a drug safety and efficacy perspective.

See also

Related Research Articles

<span class="mw-page-title-main">Stereochemistry</span> Subdiscipline of chemistry

Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which by definition have the same molecular formula and sequence of bonded atoms (constitution), but differ in the geometric positioning of the atoms in space. For this reason, it is also known as 3D chemistry—the prefix "stereo-" means "three-dimensionality".

In chemistry, a racemic mixture or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates.

<span class="mw-page-title-main">Enantiomer</span> Stereoisomers which are non-superposable mirror images of each other

In chemistry, an enantiomer – also called optical isomer, antipode, or optical antipode – is one of two stereoisomers that are non-superposable onto their own mirror image. Enantiomers are much like one's right and left hands, when looking at the same face, they cannot be superposed onto each other. No amount of reorientation in three spatial dimensions will allow the four unique groups on the chiral carbon to line up exactly. The number of stereoisomers a molecule has can be determined by the number of chiral carbons it has. Stereoisomers include both enantiomers and diastereomers.

In chemistry, racemization is a conversion, by heat or by chemical reaction, of an optically active compound into a racemic form. This creates a 1:1 molar ratio of enantiomers and is referred to as a racemic mixture. Plus and minus forms are called Dextrorotation and levorotation. The D and L enantiomers are present in equal quantities, the resulting sample is described as a racemic mixture or a racemate. Racemization can proceed through a number of different mechanisms, and it has particular significance in pharmacology as different enantiomers may have different pharmaceutical effects.

<span class="mw-page-title-main">Stereocenter</span> Atom which is the focus of stereoisomerism in a molecule

In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups creates a new stereoisomer. Stereocenters are also referred to as stereogenic centers.

<span class="mw-page-title-main">Chirality (chemistry)</span> Geometric property of some molecules and ions

In chemistry, a molecule or ion is called chiral if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality. The terms are derived from Ancient Greek χείρ (cheir) 'hand'; which is the canonical example of an object with this property.

<span class="mw-page-title-main">Enantioselective synthesis</span> Chemical reaction(s) which favor one chiral isomer over another

Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction in which one or more new elements of chirality are formed in a substrate molecule and which produces the stereoisomeric products in unequal amounts."

In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during a non-stereospecific creation of a new stereocenter or during a non-stereospecific transformation of a pre-existing one. The selectivity arises from differences in steric and electronic effects in the mechanistic pathways leading to the different products. Stereoselectivity can vary in degree but it can never be total since the activation energy difference between the two pathways is finite: both products are at least possible and merely differ in amount. However, in favorable cases, the minor stereoisomer may not be detectable by the analytic methods used.

Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their own mirror images, so they are chiral. In biology, 19 of the 20 natural amino acids are homochiral, being L-chiral (left-handed), while sugars are D-chiral (right-handed). Homochirality can also refer to enantiopure substances in which all the constituents are the same enantiomer, but some sources discourage this use of the term.

<span class="mw-page-title-main">Atropisomer</span>

Atropisomers are stereoisomers arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual conformers. They occur naturally and are important in pharmaceutical design. When the substituents are achiral, these conformers are enantiomers (atropoenantiomers), showing axial chirality; otherwise they are diastereomers (atropodiastereomers).

The molecular configuration of a molecule is the permanent geometry that results from the spatial arrangement of its bonds. The ability of the same set of atoms to form two or more molecules with different configurations is stereoisomerism. This is distinct from constitutional isomerism which arises from atoms being connected in a different order. Conformers which arise from single bond rotations, if not isolatable as atropisomers, do not count as distinct molecular configurations as the spatial connectivity of bonds is identical.

<span class="mw-page-title-main">Chiral derivatizing agent</span> Reagent for converting a chemical compound to a chiral derivative

A chiral derivatizing agent (CDA) also known as a chiral resolving reagent, is a chiral auxiliary used to convert a mixture of enantiomers into diastereomers in order to analyze the quantities of each enantiomer present within the mix. Analysis can be conducted by spectroscopy or by chromatography. The use of chiral derivatizing agents has declined with the popularization of chiral HPLC. Besides analysis, chiral derivatization is also used for chiral resolution, the actual physical separation of the enantiomers.

<span class="mw-page-title-main">Dexibuprofen</span> Chemical compound

Dexibuprofen is a nonsteroidal anti-inflammatory drug (NSAID).

An enantiopure drug is a pharmaceutical that is available in one specific enantiomeric form. Most biological molecules are present in only one of many chiral forms, so different enantiomers of a chiral drug molecule bind differently to target receptors. Chirality can be observed when the geometric properties of an object is not superimposable with its mirror image. Two forms of a molecule are formed from a chiral carbon, these two forms are called enantiomers. 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. The desired enantiomer is known as an eutomer while the undesired enantiomer is known as the distomer. When equal amounts of both enantiomers are found in a mixture, the mixture is known as a racemic mixture. If a mixture for a drug does not have a 1:1 ratio of its enantiomers it is a candidate for an enantiopure drug. Advances in industrial chemical processes have made it economical for pharmaceutical manufacturers to take drugs that were originally marketed as a racemic mixture and market the individual enantiomers, either by specifically manufacturing the desired enantiomer or by resolving a racemic mixture. On a case-by-case basis, the U.S. Food and Drug Administration (FDA) has allowed single enantiomers of certain drugs to be marketed under a different name than the racemic mixture. Also case-by-case, the United States Patent Office has granted patents for single enantiomers of certain drugs. The regulatory review for marketing approval and for patenting is independent, and differs country by country.

The eudysmic ratio represents the difference in pharmacologic activity between the two enantiomers of a drug. In most cases where a chiral compound is biologically active, one enantiomer is more active than the other. The eudysmic ratio is the ratio of activity between the two. A eudysmic ratio significantly differing from 1 means that they are statistically different in activity. Eudisimic ratio (ER) reflects the degree of enantioselectivity of the biological systems. For example, (S)-propranolol meaning that (S)-propranolol is 130 times more active as its (R)-enantiomer.

<span class="mw-page-title-main">Levomethadone</span> Synthetic opioid

Levomethadone, sold under the brand name L-Polamidon among others, is a synthetic opioid analgesic and antitussive which is marketed in Europe and is used for pain management and in opioid maintenance therapy. In addition to being used as a pharmaceutical drug itself, levomethadone is the main therapeutic component of methadone.

<span class="mw-page-title-main">Indacrinone</span> Chemical compound

Indacrinone is a loop diuretic. It can be used in patients of gout with hypertension as an antihypertensive because it decreases reabsorption of uric acid, while other diuretics increase it.

A chiral switch is a chiral drug that has already approved as racemate but has been re-developed as a single enantiomer. The term chiral switching was introduced by Agranat and Caner in 1999 to describe the development of single enantiomers from racemate drugs. For example, levofloxacin is a chiral switch of racemic ofloxacin. The essential principle of a chiral switch is that there is a change in the status of chirality. In general, the term chiral switch is preferred over racemic switch because the switch is usually happening from a racemic drug to the corresponding single enantiomer(s). It is important to understand that chiral switches are treated as a selection invention. A selection invention is an invention that selects a group of new members from a previously known class on the basis of superior properties. To express the pharmacological activities of each of the chiral twins of a racemic drug two technical terms have been coined eutomer and distomer. The member of the chiral twin that has greater physiological activity is referred to as the eutomer and the other one with lesser activity is referred to as distomer. The eutomer/distomer ratio is called the eudisimic ratio and reflects the degree of enantioselectivity of the biological activity.

Chiral inversion is the process of conversion of one enantiomer of a chiral molecule to its mirror-image version with no other change in the molecule.

Chiral analysis refers to the quantification of component enantiomers of racemic drug substances or pharmaceutical compounds. Other synonyms commonly used include enantiomer analysis, enantiomeric analysis, and enantioselective analysis. Chiral analysis includes all analytical procedures focused on the characterization of the properties of chiral drugs. Chiral analysis is usually performed with chiral separation methods where the enantiomers are separated on an analytical scale and simultaneously assayed for each enantiomer.

References

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