In a rehearing decision issued by a Delegated Rehearing Panel specially convened by the US Patent & Trademark Office (PTO) Director, the Patent Trial & Appeal Board vacated a prior panel decision denying institution, modified the claim construction to account for a “clear and unmistakable” prosecution history disclaimer, and remanded for further proceedings consistent with the modified claim construction. SynAffix B.V. v. Hangzhou DAC Biotech Co., Ltd., No. IPR2022-01531, Paper 23 (PTAB Mar. 4, 2024) (Kim, Acting Deputy Chief APJ; Gongola, Vice Chief APJ; Worth, Acting Senior Lead APJ).

SynAffix filed a petition for inter partes review (IPR) of certain patent claims owned by Hangzhou DAC Biotech. The claims at issue are directed to a hydrophilic linker of formula (I), reproduced below.

A Board panel issued a prior decision denying institution. In the prior decision, the Board noted that it was undisputed that moieties Y or Z enable connection of the linker to a cell-binding agent (Y) or a cytotoxic drug (Z), and that Q and T are one sulfone, sulfoxide or phosphinate. That said, the central issue was whether Y or Z must act independently or whether they can act in concert with an adjoined Q or T compound. The Board concluded that Y and Z must act independently to enable their respective chemical reactions without assistance from an adjoining Q or T compound, because during prosecution the patent owner disclaimed embodiments in which Y or Z was adjoined to a sulfone compound with no additional adjacent sulfone, sulfoxide or phosphinate to overcome a rejection based on a prior art reference (Lees). Thus, the Board denied institution.

SynAffix subsequently sought review by the Director. The Director issued an order delegating director review to a rehearing panel. The rehearing panel reviewed whether the prior decision misapprehended or overlooked any issue raised in the request for director review. In particular, the rehearing panel considered whether, when the hydrophilic linker contains a vinylsulfone group, one of Y and Z may be understood to be a vinyl group with one of Q and T as a sulfone group (i.e., the vinyl group and the sulfone group mapped to separate letters), or instead whether one of Y and Z must be a vinylsulfone group (i.e., the vinyl group and the sulfone group mapped to the same letter).

While the rehearing panel agreed with the first version of the prosecution history disclaimer in the prior decision, it rejected a second under which two sulfone groups would not fall within the scope of the claim. First, the rehearing panel accepted the Board’s prior finding that the patent owner disclaimed embodiments in which Y or Z was adjoined to a sulfone compound with no additional sulfone, sulfoxide or phosphinate. However, the conclusion (in the prior decision) that the challenged independent claim excludes compounds in which the sole terminal group is vinyl sulfone without one or more adjacent Q and T group(s) (i.e., that the claim requires both a Q and T group and a vinylsulfone group) was erroneous as it misapprehended or overlooked several portions of the prosecution history.

The rehearing panel concluded that when the patent owner amended the challenged independent claim to distinguish Lees, both the patent owner and the examiner understood that when there is a terminal vinylsulfone, Y (or Z) is mapped to vinyl, and Q (or T) is mapped to sulfone, there must be at least one additional phosphinate, sulfonyl or sulfoxide group as Q (or T) and prosecution disclaimer attached. However, as to the second version of prosecution estoppel relied on in the prior decision, the rehearing panel, after an exhaustive review of the relevant prosecution history, explained “consistent with the Examiner’s and the Applicant’s mapping at the time of the claim amendment, when the hydrophilic linker contains a vinylsulfone group, we determine that the vinyl group should be mapped to Y or Z, and the sulfone group should be mapped to Q or T. We [therefore] determine that such mapping satisfies the ‘enables reaction’ and ‘enables linkage’ requirements of [the] claim [].”

The case was remanded to the Board for reconsideration in view of the new claim construction.