Myrbetriq® (mirabegron extended-release tablets)
Case Name: Astellas Pharma Inc. v. Ascent Pharms., Inc., Civ. No. 23-cv-486, 2026 WL 657768 (D. Del. Mar. 6, 2026) (Bataillon, J.)
Drug Product and Patent(s)-in-Suit: Myrbetriq® (mirabegron extended-release tablets); U.S. Patent Nos. 10,842,780 (“the ’780 patent”), 11,707,451 (“the ’451 patent”), 12,059,409 (“the ’409 patent”), and 12,097,189 (“the ’189 patent”)
Nature of the Case and Issue(s) Presented: Myrbetriq is indicated for the treatment of overactive bladder (OAB). The patents-in-suit claim compositions containing mirabegron and their use in treating OAB. The ‘780 and ‘409 patents contain limitations requiring that the “drug dissolution rate from the pharmaceutical composition is 39% or less after 1.5 hours and at least 75% after 7 hours.” The ‘451 and ‘189 patents each contain limitations requiring a “reduced food effect” in comparison to an “immediate release formulation.”
In March 2023, Ascent filed an ANDA for mirabegron. Astellas sued, alleging infringement of the patents-in-suit. Ascent alleged non-infringement of the aforementioned claim limitations and also raised invalidity defenses on the basis of unpatentable subject matter, anticipation, obviousness, and lack of written description, as well as a judicial estoppel defense based on Astellas’ Orange Book listing for Myrbetriq. With respect to unpatentable subject matter, Ascent argued that the claimed extended-release profile merely mirrored the small intestine’s natural transit time using “conventional” sustained-release methods. With respect to anticipation, Ascent relied on U.S. Patent No. 6,699,503 (“the ’503 patent”) and Astellas’ Orange Book listing of the ’503 patent for Myrbetriq. The ’503 patent disclosed a non-specific platform for sustained-release formulations including a drug, an additive, and a hydrogel-forming polymer. Relatedly, Ascent argued that Astellas was estopped from contesting that the ’503 patent disclosed mirabegron on the basis of its Orange Book listing of the ’503 patent for Myrbetriq. In support of its obviousness defense, Ascent relied on the ’503 patent in combination with a variety of other references. Finally, Ascent argued that the ’451 and ’189 patents failed to show that Astellas was in possession of an invention relating to the mirabegron free base as opposed to its dihydrochloride salt. After a bench trial, the court foundt that Ascent’s ANDA product infringed all the asserted claims and that Ascent had not met its burden of proving invalidity.
Why Astellas Prevailed: Starting with infringement, the court was unconvinced by Ascent’s argument that its ANDA product’s extended-release characteristics or its immediate-release comparator differed meaningfully from Myrbetriq, and noted that “an ANDA applicant should not be permitted to liken their product to the claimed composition to support their bid for FDA approval, yet avoid the consequences of such a comparison for purposes of infringement.”
The court then rejected Ascent’s Section 101 defense, finding that the claimed compositions are “directed to nonnatural compositions created using a nonnatural drug and nonnatural excipients,” and are therefore not directed towards ineligible subject matter and are not invalid. The court similarly found that the method claims are directed to a “specific method of treatment for specific patients using a specific compound at specific doses to achieve a specific outcome,” and are therefore also not directed towards ineligible subject matter.
The court next rejected Ascent’s anticipation defense, finding that the ‘503 patent did not disclose mirabegron or any dosage thereof, a method of treating an overactive bladder, or a “reduced food effect” relative to an immediate-release formula. The court was unconvinced by Ascent’s argument that a POSA would read the ’503 patent as disclosing mirabegron or a method of treating an overactive bladder. The court also found that Astellas’ Orange Book listing of the ’503 patent for Myrbetriq was not prior art because the listing post-dated the priority dates of each of the patents-in-suit. The court also rejected Ascent’s related estoppel argument, finding that none of the elements of judicial estoppel were met, particularly because Astellas’ Orange Book listing of the ’503 patent for Myrbetriq “does not ask the NDA applicant whether the listed patent enables or discloses how to make and use the entirety of Myrbetriq®, nor does any question on the form require Astellas to assess the validity of the ’503 patent prior to submitting the form.” The Court also noted that “the Orange Book filing is not binding between agencies (FDA and Patent Office).”
In addition, the court rejected Ascent’s obviousness arguments. After noting that only one of Ascent’s references disclosed mirabegron at all—and that even this reference failed to disclose extended-release formulations of mirabegron—the court found that there would be no motivation to combine Ascent’s references to arrive at a sustained-release formulation of mirabegron or a formulation of mirabegron with reduced food effect as of the priority date of the patents-in-suit. Specifically, a POSA would not have been motivated to combine the prior art references because a POSA “would not have known that immediate release mirabegron formulations exhibited a food effect” and would have believed that “a sustained-release formulation was not necessarily needed” given mirabegron’s half-life and absorption characteristics. The court also found that a POSA would not have a reasonable expectation of success in combining the prior art to arrive at a sustained-release formulation of mirabegron, particularly because food-effects are highly drug-specific and because the food effect of mirabegron were not characterized in any of Ascent’s references. An analysis of secondary considerations further established the non-obviousness of the patents-in-suit, particularly unexpected results concerning the reduced food effects from Astellas’ extended-release formulation and industry praise.
Last, the court rejected Ascent’s written description argument, finding that the ’451 and ’189 patents did, in fact, disclose the mirabegron free base when their specifications explained that their examples were conducted with “Compound A” and identified the chemical formula of mirabegron free base as Compound A.
