Another interesting case is that of Citalopram (racemate, Celexa) and escitalopram (enantiomer, Lexapro). In the future, one would not be srprised if chiral switch becomes main weapon to maintain the monopoly in the pharmaceutical market. Provigil (racemate) and Nuvigil (enantiomer), dextromethorphan, PLAVIX ((S)-clopidogrel are already on their way to follow the suit.
Chrial switches have highlighted the importance of chiral molecules. In fact, the focus of whole drug industry has been shifted to chiral molecules. For stastical consdieration, the nine out of first ten block bluster molecules in the year 2006 were chiral in nature.
A review by John Caldwell of the University of Liverpool and organic chemists Israel Agranat and Hava Caner at the Hebrew University of Jerusalem indicates that since 1990, the proportion of single-enantiomer drugs among approved new chemical entities worldwide has been consistently greater than that of racemates [Nat. Rev. Drug Discovery, 1, 753 (2002)]. Chiral effects in pharmaceuticals are being exploited in several ways: as mentioned above by chiral switching, by discovering distinct therapeutic uses for enantiomers of chiral drugs, and by creating non-racemic mixtures of enantiomers.
Probably, keeping in mind the importance of Chiral molecules, the US Senate on May 09, 2007 passed FDARA (Food and Drug Administration Revitalization Act)
The main feature of this act, is that, it provides an opportunity, to elect 5-year exclusivity under certain circumstances for enantiomers.
According to provision of this Act, Specifically, FDARA § 264, if an application is submitted under [§ 505(b)] for a non-racemic drug containing as an active ingredient a single enantiomer that is contained in a racemic drug approved in another application under [§ 505(b)], the applicant may, in the application for such non-racemic drug, elect to have the single enantiomer not be considered the same active ingredient as that contained in the approved racemic drug . . . .
Thus, if a single enantiomer is not considered to be the same active ingredient as that contained in the approved racemic drug, FDA may consider it to be a new chemical entity eligible for 5-year exclusivity. However, such an election of 5-year exclusivity can only be made if:
- (A) (i) the single enantiomer has not been previously approved except in the approved racemic drug; and (ii) the application submitted under [§ 505(b)] for such non-racemic drug –
- includes full reports of new clinical investigations (other than bioavailability studies) – (aa) necessary for the approval of the application under subsections (c) and (d); and (bb) conducted or sponsored by the applicant; and
- does not rely on any investigations that are part of an application submitted under [§ 505(b)] for approval of the approved racemic drug; and
- (B) the application submitted under [§ 505(b)] for such non-racemic drug is not submitted for approval of a condition of use— (i) in a therapeutic category [(as identified in the list referenced at42. USC § 1860D-4(b)(3)(C)(ii)
In addition to these requirements, which essentially necessitate the submission of a “full” 505(b)(1) NDA, FDARA § 264 also includes two significant limitations that may offset the incentive for electing 5-year exclusivity.
- First, FDA may not approve a single enantiomer of a previously approved racemate granted 5-year exclusivity for any condition of use in the therapeutic category in which the racemic drug has been approved “[u]ntil the date that is 10 years after the date of approval of a non-racemic drug described in [proposed FDC Act § 505(t)(1)].”
- Second, “the labeling of a non-racemic drug described in [proposed FDC Act § 505(t)(1)] and with respect to which the applicant has made the election provided for by such paragraph shall include a statement that the non-racemic drug is not approved, and has not been shown to be safe and effective, for any condition of use of the racemic drug.”
Thus, a mere reading of this act suggest that USFDA wants pharmaceutical companies to develop the single enantiomers for new therapeutic indication and not for the same therapeutic indication, in which the chiral drug is approved.
Therfore, whether the implemenation of FDARA would limit the strategy of Chiral swithes and enhance the use of single enantiomer for new therapeutic indications need to be seen.