Fed. Cir. Clarifies Written Description for Chemical Genus Claims
The Federal Circuit held in Exelixis v. MSN that disclosing shared structural features—such as chemical name, formula, and crystalline nature—is sufficient to satisfy the written description requirement for a patent genus claim.
The U.S. Court of Appeals for the Federal Circuit affirmed the validity of several Exelixis patents covering a genus of crystalline forms of the cancer drug cabozantinib malate, rejecting a written description challenge from generic manufacturer MSN Laboratories. In 'Exelixis, Inc. v. MSN Laboratories', the court held the patents satisfied 35 U.S.C. § 112(a) because the specification disclosed structural features common to all members of the genus—specifically, the chemical name, formula, and crystalline character of the salt. This decision is significant for patent holders in the chemical and pharmaceutical arts because it reinforces the "common structural features" pathway for supporting genus claims, a vital tool for securing broad patent protection. It provides a favorable contrast to the Supreme Court's restrictive analysis of functionally-defined genus claims in 'Amgen v. Sanofi', clarifying that claims anchored to specific chemical structures face a different, and potentially more lenient, standard. Counsel should assess both prosecution and litigation strategies in light of this distinction, emphasizing structural definitions where possible to avoid the high bar set for functional claiming.