It is well established that an enantiomerically pure compound exhibiting advantageous properties not present in its isomer or its corresponding racemic mixture, can be patented even if its corresponding racemic mixtures are known in the art (See e.g., In re May, 574 F.2d 1082 at 1090; Sanofi-Synthelabo v. Apotex, Inc. 550 F.3d 1075-1084; Forest Laboratories, Inc. v. Ivax Pharmaceuticals, Inc., 501 F.3d 1263 (Fed. Cir. 2007) at 1268-69, and Sanofi-Synthelabo v. Apotex, Inc. 550 F.3d 1075 at 1085). But without identification of an exact enantiomeric structure, does a showing of physical possession of an advantageous enantiomer fulfill the written description requirement? 

Stereoisomers are molecules that share the same chemical formula and sequence of atoms but differ in their three-dimensional configuration. Enantiomers are stereoisomers that are mirror images of each other. A racemic mixture comprises two enantiomers of the same molecule in equal parts. Enantiomerically pure substances are the active ingredients in drugs such Lipitor®, Plavix®, Nexium®, Azilect®, and Nuvigil®. 

Given the strategic importance of “picture claims” (see e.g., here for more detail: Picture Claims as an Effective Patent Strategy: Top 10 Reasons to Precisely Tailor Your Patent Claim | Mintz), innovators bringing enantiomerically pure drugs to market often strategically pursue patent claims reciting the precise enantiomeric structure of their drug. Many patent applications, however, are filed without a solid identification of the chemical structure of a potentially advantageous enantiomer. Rather, it is common practice to disclose compounds in a more general fashion, such as:

Compound 1 = [Structure of enantiomer A] or [Structure of enantiomer B] 

Compound 2 = [Structure of enantiomer A] or [Structure of enantiomer B] 

Chiral chromatography retention times are often provided as basis for an assignment of the conformation of chemical structures at a later time, e.g., during prosecution, using post-filing data or expert declarations to match the retention times or other characterization data to the correct stereochemical structures. This all begs the question: is a disclosure demonstrating physical possession of an active enantiomer along with the possibility of a later identification of its stereochemical structure enough to provide written description for a picture claim reciting its chemical structure? 

Written description requires that possession of an invention is demonstrated within the four corners of the specification. “Actual possession or reduction to practice outside of the specification is not enough” Ariad, 598 F.3d at 1352. Moreover “[f]or claims to a chemical compound, an application satisfies the written description requirement if it details ‘relevant identifying characteristics’ such that the compound can be distinguished from other compounds.” In Re Wallach, 378 F. 3d 1330, 1333, 1335 (Fed. Cir. 2004). 

Thus, if an enantiomeric compound is distinguished from the prior art based on structural features other than its stereochemistry, a specification demonstrating that a racemic mixture was made and that enantiomers were separated may be sufficient to show possession of each enantiomer. 

However, if relevant prior art exists, for example, the corresponding racemic mixture is known, the “relevant identifying characteristic” distinguishing the lead enantiomer from the prior art compounds would be its stereochemistry. In such cases, if an inventor does not, in fact, know the chemical structure of the advantageous enantiomer, it may be difficult to argue that the inventor “possessed” that enantiomer, even if there was a physical possession of the advantageous compound. Providing data that could be used to identify the chemical structure of the active enantiomer may not remedy the lack of possession, as arguably those data would exist outside the four corners of the specification.

With this understanding, it may be possible to obviate the need for a structural assignment by drafting claims describing the compound in other ways. For example, drafting claims identifying the claimed compound by its chromatographic retention time, rather than its chemical structure, would not require structural assignment because the “relevant identifying characteristic” would be the retention time. However, a company wishing to obtain a picture claim reciting the chemical structure of their enantiomeric drug is best advised to provide full characterization of any compound that may turn out to be a lead, including its absolute stereochemistry.