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Novartis markets and sells a combination therapy of valsartan and sacubitril under the brand name Entresto® for the treatment of various forms of heart failure. MSN submitted an Abbreviated New Drug Application seeking approval to market and sell a generic version of Entresto®. Novartis sued MSN for infringing claims 1-4 of U.S. Patent No. 8,101,659 (the ’659 patent), claim 1 of which is directed to:

core/paragraph

1. A pharmaceutical composition comprising:
(i) the AT 1-antagonist valsartan or a pharmaceutically acceptable salt thereof;
(ii) the NEP inhibitor [sacubitril] or a pharmaceutically acceptable salt thereof; and
(iii) a pharmaceutically acceptable carrier;
wherein said (i) AT 1-antagonist valsartan or pharmaceutically acceptable salt thereof and said (ii) NEP inhibitor [sacubitril] or pharmaceutically acceptable salt thereof are administered in combination in about a 1:1 ratio.

core/freeform

During claim construction, the U.S. District Court for the District of Delaware “construed the asserted claims to cover valsartan and sacubitril as a physical combination and as a complex.” In re Entresto, 125 F.4th 1090, 1096 (Fed. Cir. 2025). MSN argued that the claims lacked enablement and written description because the patent does not disclose complexes of valsartan and sacubitril. Novartis argued that the ’659 patent did not need to enable or describe complexes of valsartan and sacubitril because such a complex “was an after-arising invention that need not have been enabled or described.” (Id. at 1096-97).

The district court held that the claims were enabled but lacked written description. According to the district court, because complexes of valsartan and sacubitril were unknown as of the patent’s priority date, the patent need not enable such complexes. But the court stated that “the facts that helped [Novartis] with respect to enablement proved fatal for written description.” (Id. at 1097). The district court reasoned that, because the complexes were unknown, the patentee could not have had possession of the complexes at the time of filing the application.

The U.S. Court of Appeals for the Federal Circuit did not agree with the district court’s written description analysis. The Federal Circuit opined that the district court’s error in interpreting the claim construction “led [the court] astray in evaluating written description.” (Id. at 1098). The Federal Circuit noted that, by stating that the claims were “construed to cover complexes of valsartan and sacubitril,” the district court erroneously conflated the distinct issues of patentability and infringement. (Id. at 1098; emphasis in original). The Federal Circuit found that the “complex – not discovered until four years after the priority date of the ’659 patent – is not what is claimed” (id.), and that the patent adequately discloses the administration of valsartan and sacubitril in combination as a physical mixture. In reaching its conclusion, the Federal Circuit stated that “[t]he later-discovered valsartan-sacubitril complexes, which arguably may have improved upon the ‘basic’ or ‘underlying’ invention claimed in the ’659 patent, cannot be used to ‘reach back’ and invalidate the asserted claims.” (Id. at 1100). The Federal Circuit reversed the district court’s conclusion of lack of written description and held that the district court did not err in ruling that MSN failed to establish that the claims lacked enablement.

A Later-Discovered Improvement to an Invention Cannot Be Used To Reach Back and Invalidate an Earlier-Filed Patent

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