Celltech Chiroscience Ltd. v Medimmune Inc.

Under US patent law, prosecution history estoppel is assessed by comparing the literal scope of original and amended claims, element by element. A narrowing amendment presumptively surrenders the territory between them, subject to recognised exceptions. Where alternative claim possibilities are narrowed so that a residue at a particular position becomes mandatory, equivalents of non-donor residues at that position may be surrendered, even if the accused product would have infringed the original claim. Argument estoppel may independently arise from an unmistakable assertion in the public prosecution file. The majority held that reliance on the doctrine of equivalents was barred.

Celltech licensed its prospective US Adair Patent to MedImmune. MedImmune sold SYNAGIS and denied liability for royalties because the antibody contained threonine at position 23 rather than the donor serine, although Celltech alleged that the two were equivalent.

During prosecution before the USPTO, Celltech amended its claim following objections based on prior art. Jacob J held that Celltech was barred by Argument Estoppel but not Amendment Estoppel, and dismissed the claim. Both parties appealed. The central issues were whether either form of prosecution history estoppel prevented reliance on the US doctrine of equivalents.

Disposition. The Vice-Chancellor and Longmore LJ formed the majority. They allowed MedImmune’s cross-appeal and dismissed Celltech’s appeal, leaving undisturbed the order dismissing Celltech’s action. Arden LJ dissented on both issues.

1. Applicable principles. Applying US law, the majority treated the doctrine of equivalents as operating element by element, not on the invention as a whole, following Graver Tank & Mfg. Co. v Linde Air Products Co. (339 US 605 (1950)) and Warner-Jenkinson Co. Inc. v Hilton Davis Chemical Co. (520 US 17 (1997)). The majority applied the flexible, rebuttable approach in Festo 2 (122 S.Ct.1831 (2002)), rather than the bright-line approach previously adopted in Festo 1 (187 F.3d 1381).
2. Amendment Estoppel. The relevant inquiry was a literal comparison between the original claim in its widest scope and the amended claim, concentrating on the particular element affected. The fact that SYNAGIS might have infringed both claims did not answer that question. The amendment, made in response to the prior-art objection under Patent Trademark and Copyright Law of the US §102(b), removed alternative possibilities and made a donor residue at position 23 compulsory. The majority held that the surrendered territory therefore included non-donor residues and their equivalents at that position. The presumption was not rebutted. The amendment’s rationale was not merely tangential, and it was irrelevant that the amendment might have gone further than strictly necessary.
3. Argument Estoppel. The prosecution file contained an objective and unmistakable representation that position 23 had to contain a donor residue and not an acceptor residue. The patent’s silence about conservative substitutions and the evidence relied on by Celltech did not displace that conclusion. The majority therefore held that Argument Estoppel independently barred reliance on equivalence.
4. Adjournment. The application to set aside the order, proceed in tandem with the German proceedings, or await further US litigation was refused. It was made too late and, without MedImmune’s consent, the court should not set aside an order in its favour without hearing its argument.
5. Dissent. Arden LJ considered that the element requiring a donor residue at position 23 was unchanged by the amendment, so its equivalents were not surrendered. She also considered that the prosecution statements were insufficiently unmistakable, particularly in light of the patent’s teaching that donor and acceptor residues could sometimes be identical.

The cross-appeal was allowed. Celltech’s appeal was dismissed with costs, including £60,000 payable on an interim basis.

• Court of Appeal (Civil Division) — In this judgment, the majority allowed MedImmune’s cross-appeal and dismissed Celltech’s appeal, leaving the dismissal of the action undisturbed: [2003] EWCA Civ 1008.
• Chancery Division (Patents Court) — Jacob J held that Argument Estoppel applied but Amendment Estoppel did not, and dismissed Celltech’s claim. Both parties appealed.