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Prosecution History Estoppel Bars Amgen’s Doctrine-of-Equivalents Infringement Claim Against Neulasta (Pegfilgrastim) Biosimilar Maker Coherus

Amgen Inc. v. Coherus Biosciences Inc., No. 2018-1993, Slip op. at 6 (Fed. Cir. July 29, 2019) stems from a Biologics Price Competition and Innovation Act action brought by Amgen against Coherus seeking FDA approval to market a biosimilar version of Amgen’s pegfilgrastim product Neulasta.

In Amgen, the Federal Circuit affirmed the district court’s dismissal of Amgen’s complaint for failure to state a claim, holding that Amgen’s infringement claim under doctrine of equivalents (“DOE”) was barred by prosecution history estoppel.  In so holding, the Court found that separate arguments made to distinguish the prior art during prosecution created separate estoppels.

The Technology at Issue

Recombinant therapeutic proteins are a class of biologics that are manufactured inside living cells.  Before a protein can be therapeutically useful, it must first be purified from contaminants.  One method involves using hydrophobic interaction chromatography (“HIC”).  An HIC column separates proteins from impurities based on the hydrophobic interactions between the proteins and the hydrophobic column. 

The first step involves loading the column with a buffer solution containing a mixture of the desired protein and associated impurities.  That solution contains salt, which exposes the hydrophobic moieties of the proteins such that the proteins can attach onto the hydrophobic column.  The impurities are then washed out, and the desired proteins are eluted by pouring a buffer solution that has a lower salt concentration through the column.

Amgen’s U.S. Patent No. 8,273,707 claims to solve an issue that arises during the loading step of the purification process.  It has been known that only a finite amount of protein can bind to the matrix.  Therefore, too much protein loading could cause loss of protein even before eluting.  The ’707 patent claims are directed to a process that increases the column’s loading capacity—also referred to as increasing “dynamic capacity.”  Slip op. at 3.  The claims purport to do so by requiring a “salt combination chosen from one of three pairs: citrate and sulfate, citrate and acetate, or sulfate and acetate” in the loading buffer.  Id. at 4.

The Federal Circuit Affirms Prosecution History Estoppel

Amgen sued Coherus for allegedly infringing the ’707 patent based on Coherus’ abbreviated Biologic License Application (“aBLA”).  Coherus’ aBLA revealed that its manufacturing process contains several chromatography steps used to purify pegfilgrastim.  One of the steps involves a chromatography buffer containing a salt combination, but not one of the specific combinations recited in Amgen’s claims.  This led Amgen to assert infringement under DOE.

The district court dismissed the complaint for failure to state a claim.  The lower court found that during prosecution, Amgen distinguished a prior art reference, U.S. Patent No. 5,231,178 (“Holtz”), by arguing that Holtz did not disclose “one of the particular, recited combinations of salts” and, in doing so, surrendered claim scope over other salt combinations.  Id. at 7 (emphasis in original).

Specifically, during prosecution, the examiner had rejected the pending claims on the basis that Holtz disclosed several salts for improving hydrophobic interactions between a protein and the column matrix.  Id. at 4.  In response, Amgen argued that: (i) “the pending claims recite a particular combination of salts,” (ii) “[n]o combination of salts [are] taught [by Holtz], nor [are] the particular combinations of salts recited in the pending claims taught [by Holtz],” (iii) Holtz does not teach dynamic capacity at all, and (iv) “[u]se of this particular combination of salts greatly improves the cost-effectiveness of commercial manufacturing.”  Id. at 4-5 (emphases in original).

Based on this prosecution record, the district court held that Amgen’s infringement under DOE was barred by a “clear and unmistakable surrender of claim scope by the patentee.”  Id. at 7.  Amgen appealed.

In this appeal, the Federal Circuit affirmed, holding that “argument-based prosecution history estoppel applies here because Amgen clearly and unmistakably surrendered unclaimed salt combinations during prosecution.”  Id. at 9.  The Court found that Amgen’s responses and inventor declaration during prosecution did not mention any salt combinations other than those claimed.  The Court also highlighted that Amgen had “emphasized ‘particular’ and referred to its particular salts three times in the span of two pages.”  Id. 

The Court disagreed with Amgen’s argument that it distinguished Holtz on the basis that Holtz failed to disclose increasing dynamic capacity or any salt combinations at all.  Amgen further pointed out that the arguments made in its last response prior to allowance did not contain the argument that Holtz failed to disclose the claimed particular salt combinations.

Citing PODS, Inc. v. Porta Stor, Inc., 484 F.3d 1359, 1367 (Fed. Cir. 2007), the Court held that “while Amgen did assert multiple reasons for why Holtz is distinguishable, our precedent instructs that estoppel can attach to each argument.”  Id. at 11.  The Court found that Amgen relied on separate arguments for distinguishing Holtz.  And one of the arguments was that the claimed particular salt combinations were not taught or suggested in Holtz.  According to the Court, the fact that this argument was not included in the last response that led to allowance “does not mean … that Amgen’s prior statements are erased.”  Id. at 12.

The Court held that prosecution history estoppel applies, and therefore Amgen is barred from asserting infringement under DOE.

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