Federal Circuit Affirms Written Description Support, Vacates Mooted Non-Obviousness Finding

In Exelixis, Inc., v. MSN Lab’ys Private LTD., No. 2025-1236 (Fed. Cir. Aug. 31, 2026), the Federal Circuit affirmed the district court’s validity findings regarding a group of asserted patents (the “Salt Patents”) and dismissed MSN’s appeal and vacated the underlying non-obviousness finding regarding the ’349 Patent.

Exelixis sued MSN for infringement of patents related to its kidney, liver, and differentiated thyroid cancer drug, Cabometyx®. In a bench trial, the district court found sufficient written description support for the asserted claims of the Salt Patents, to which MSN conceded infringement. The district court then found that the ’349 Patent was neither infringed nor invalid for obviousness. MSN appealed, challenging both validity findings. Exelixis cross-appealed as to the ’349 Patent noninfringement finding but later dismissed its cross-appeal, rendering this judgment of noninfringement final.

On appeal, the Federal Circuit affirmed the district court’s validity finding regarding the Salt Patents. It held that the district court properly found that disclosure of the claimed salt’s chemical name, formula, and crystalline structure provided adequate written description support as an identification of structural features possessed by members of the claimed genus.

Further, the Court found Exelixis’ dismissal of its cross-appeal mooted MSN’s challenge to the ’349 Patent validity finding and granted MSN’s motion to dismiss. The Federal Circuit then vacated the district court’s non-obviousness finding to avoid the “unfairness” of MSN’s inability to oppose this adverse judgment due to the prevailing party’s unilateral action—Exelixis’ dismissal of its cross-appeal.