Pharmaceutical sales constitute a $600-billion-per-year global industry. Less well-known is that more than half of the drugs listed in the U.S. Pharmacopoeia contain a class of compounds known as chiral molecules as the active pharmaceutical ingredient. Chiral molecules have special chemical and pharmacological properties that raise questions as to their patentability. When chiral molecules are synthesized in the laboratory, two distinct mirror-image molecules are formed called “enantiomers.” Although each enantiomer may have different levels of therapeutic activity and toxicity, technical challenges to separating the enantiomers caused most early chiral drugs to be sold as mixtures of the two molecules, or racemic mixtures.
As patents on these racemic mixtures expire and separation techniques improve, drugs formerly sold as racemic mixtures are now being marketed as single-enantiomer versions. For example, Nexium® is the single-enantiomer version of Prilosec,® and Clarinex® is the single-enantiomer version of Claritin®. In fact, all five of the top-selling drugs in the world are single-enantiomer drugs, generating total revenues of $35 billion per year. Switching from a racemic mixture to an enantiomer – called a “racemic switch” in industry parlance – can potentially allow a company to extend product life by securing a patent for an additional twenty years of protection. Whether the disclosure of the racemate in the prior art renders obvious (and thus unpatentable) each individual enantiomer is therefore a timely and important question. This article provides an overview of chiral drugs in the pharmaceutical industry, reviews early case law on the patentability of enantiomers, and addresses the patentability of single-enantiomer drugs in the context of modern chemical obviousness jurisprudence.