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Putting chirality to work: the strategy of chiral switches

Key Points

  • Chiral switches are chiral drugs that have already been claimed, approved and marketed as racemates or as mixtures of stereoisomers, but have since been redeveloped as single enantiomers. The essential criterion of a chiral switch is a change in the status of chirality. There are still a significant proportion of racemic drugs among the recently approved new molecular entities.

  • The patentability of single enantiomers in a chiral switch is an extreme case of a selection patent. The novelty of a single enantiomer is not negated by the prior-art disclosure of its racemate. The strategy of enantiomeric pairs of patents of single enantiomers — E1 and E2 — in a chiral switch consists of two patents claiming simultaneously that E1 and E2 are pharmacologically superior to the racemic drug E1,2. This strategy has been questioned.

  • According to the US FDA, single enantiomers in chiral switches are not new molecular entities, and are therefore barred from five-years exclusivity. Such new products are treated as new derivatives of existing drugs or new formulations, on a case-by-case basis.

  • Despite the regulatory acceptance of 'bridging strategies' from racemate to single enantiomer, only a few successful switches have emerged from this route.

  • Successful chiral switches emerge from racemic drugs that have efficacy and/or safety that can be enhanced, leading to significantly superior single enantiomers that are patentable and compete effectively with cheaper generic versions of the racemates.

  • Chiral switches are also eligible in cases in which the mechanisms of action of the single-enantiomer drugs involve achiral intermediates — for example, esomeprazole magnesium — and/or racemization.

  • Paradoxically, a chiral switch can result in the increased use of the racemate, this being a feature of the failure of the chiral switch of fenfluramine to dexfenfluramine and the 'fen–phen' fiasco.

  • The timing of chiral switches of blockbuster drugs is crucial. The new single enantiomer should be launched ideally before the expiration of the patents that cover the racemate, with extended exclusivity and before the incursions of the respective generic drugs.


Most of the new drugs reaching the market today are single enantiomers, rather than the racemic mixtures that dominated up to ten years ago. Many of the new single-enantiomer drugs were developed as such, but there are also important examples of new single-enantiomer drugs derived from 'chiral switches' of established racemates. Indeed, a well-timed chiral switch can offer enhanced therapy and further profitability as a 'line extension' of a major racemic drug with patents that are expiring.

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Figure 1
Figure 2: Study of Praying Hands, by Albrecht Dürer (1508).
Figure 3: Annual distribution of worldwide approved drugs according to chirality character (1989–2000).
Figure 4
Figure 5
Figure 6: PPI drugs and achiral intermediates in the 'omeprazole cycle'.
Figure 7: Ab initio calculated conformations of (S,P)-omeprazole and (S,M)-omeprazole.
Figure 8


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We thank S. K. Branch (Medicines Control Agency, London, UK), R. Perry (Gill, Jenning & Every, London, UK), S. P. Miller (Office of New Drug Products, FDA, Rockville, USA), P. Lindberg (AstraZeneca, Mulndal, Sweden), B. G. Larsson (AstraZeneca, Sodertalje, Sweden), J. Senn-Bilfinger (ALTANA Pharma, Konstanz, Germany), J. P. Leeds (Eli Lilly and Company, Indianapolis, USA) and J. W. Jaroszewski (Royal Danish School of Pharmacy, Copenhagen, Denmark) for enlightening discussions. I.A. became interested in chiral switches in 1996 as a Fellow of the Institute for Advanced Studies at The Hebrew University of Jerusalem, where he directed a research group on Chirality of Drugs and Chiral Recognition: New Challenges.

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The geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-superimposable on its mirror image; such an object has no symmetry elements of the second kind (a mirror plane, σ = S1; a centre of inversion, i = S2; or a rotation–reflection axis, S2n). If an object is superposable on its mirror image, it is described as being achiral.


The spatial arrangement of the atoms affording distinction between stereoisomers, which can be interconverted by rotations about formally single bonds. A conformer is one of a set of stereoisomers, each of which is characterized by a conformation that corresponds to a distinct potential energy minimum.


In the context of stereochemistry, the term is restricted to the arrangements of atoms of a molecular entity in space that distinguishes stereoisomers, the isomerism between which is not due to conformational differences. The absolute configuration is the spatial arrangement of the atoms of a chiral molecular entity (or group) and its stereochemical description — for example, R or S (for chiral centres) and M or P (for chiral axes).


An equimolar mixture of a pair of enantiomers. It does not have optical activity. The chemical name or formula of a racemate is distinguished from those of the enantiomers by the prefix (±)- or rac- (or racem-), or by the symbols RS and SR.


A grant by the state of exclusive rights for a limited time (in most jurisdictions 20 years from filing date) in respect of a new and useful invention. The patentable invention must be new, it must involve an inventive step and it must be capable of industrial application.


(Chirality centre). An atom that holds a set of ligands in a spatial arrangement, which is not superimposable on its mirror image. A chiral centre is, therefore, a generalized extension of the concept of the asymmetric carbon atom to central atoms of any element.


Configurational descriptors for carbohydrates and α-amino acids.


The preferential formation in a chemical reaction of one stereoisomer over another. When the stereoisomers are enantiomers or diastereomers, the phenomenon is known as enantioselectivity or diastereoselectivity, respectively.


The part of a patent specification that defines the scope of protection.


Diastereomers that have the opposite configuration at only one of two or more tetrahedral stereogenic centres that are present in the respective molecular entities.


The basic conditions of patentability, which an application must meet if granted, are that the invention must be novel, contain an inventive step, be capable of industrial application and not be in one of several exluded fields.


The essential condition for patentability that what is claimed is new.


An invention is taken to involve an inventive step if it is not obvious to a person skilled in the art, having regard to any matter that forms part of the state of the art, but not including matter from a patent application with an earlier priority date that is published later then the priority date of the invention (European Patent Convention, Section 3). The state of the art is the total information in the relevant field known to the hypothetical person skilled in the art.


A ground of invalidity of a patent, if the description does not allow the skilled reader to work the invention.


The description of the identity and connectivity (and corresponding bond multiplicities) of the atoms in a molecular entity (omitting any distinction that arises from their spatial arrangement).


The date on which an invention was first disclosed to a patent office in a patent application or in an earlier application from which it validly claims priority.


An area of the law that concerns legal rights that are associated with creative effort or commercial reputation and goodwill.


Capable of being preformed by the average skilled person in possession of the prior art.


(Chirality axis). An axis about which a set of ligands is held so that it results in a spatial arrangement that is not superimposable on its mirror image.


A sample of material that can rotate the plane of polarization of a beam of transmitted plane-polarized light is said to have optical activity (or to be optically active). This optical rotation is the classical distinguishing characteristic (which is sufficient but not necessary) of systems that contain unequal amounts of corresponding enantiomers. An enantiomer that causes rotation in a clockwise direction (when viewed in the direction that faces the incoming light beam) under specified conditions is called dextrorotatory and its chemical name or formula is designated by the prefix (+)-; one causing rotation in the opposite sense is laevorotatory and is designated by the prefix (−)-. Materials that have optical activity also have other chiroptic phenomena.


All public knowledge before the priority date that could be relevant to the novelty or unobviousness of an invention.

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Agranat, I., Caner, H. & Caldwell, J. Putting chirality to work: the strategy of chiral switches. Nat Rev Drug Discov 1, 753–768 (2002).

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