Xencor, Inc. v. Merus N.V.

This case, IPR2025-00604, involves an Inter Partes Review (IPR) before the Patent Trial and Appeal Board (PTAB) of the U.S. Patent and Trademark Office, where Xencor, Inc. challenges the validity of U.S. Patent 9,358,286, owned by Merus N.V. Xencor, Inc. is a clinical-stage biopharmaceutical company based in California, specializing in engineered antibodies for cancer and autoimmune diseases through its proprietary XmAb platform, acting as both a technology licensor and a developer of its own drug candidates. Merus N.V., a clinical-stage oncology company headquartered in the Netherlands, develops innovative multispecific antibody therapeutics using its proprietary Biclonics® and Triclonics® platforms to target multiple pathways simultaneously for cancer treatment.

The patent at issue, U.S. Patent 9,358,286, broadly claims "methods and means for the production of Ig-like molecules," and more specifically, discloses methods for producing heterodimeric antibodies. This invention focuses on introducing specific amino acid substitutions in the CH3 domain of each heavy chain to promote the formation of the desired heterodimer over unwanted homodimers through electrostatic interactions. In a related patent infringement lawsuit, Merus N.V. v. Xencor, Inc. (Case No. 1:24-cv-00913) filed in the District of Delaware, Merus accused Xencor's XmAb bispecific platform and its portfolio of antibodies of infringing the '286 patent, among others. The district court, however, granted Xencor's motion to dismiss the claims related to the '286 patent, ruling that Xencor's accused activities fell within the Hatch-Waxman safe harbor.

This IPR is pending before a three-member panel of administrative judges at the PTAB, a venue established by the America Invents Act (AIA) to offer a faster and more cost-effective alternative to district court litigation for challenging patent validity based on prior art. The case is particularly notable because the IPR was instituted following a referral from the Acting Director of the USPTO, Coke Morgan Stewart, who found a "material error" by the examiner during the patent's original prosecution. The Director determined that the examiner had overlooked relevant prior art, specifically a reference named Lazar, which was cited in a later Information Disclosure Statement (IDS) for a continuation of the challenged patent. This decision underscored that a showing of material error by the Office could overcome arguments for discretionary denial of an IPR, highlighting the public interest in correcting errors in patent examination, even for older patents or those involved in ongoing litigation.

Key Legal Developments and Outcome in Merus N.V. v. Xencor, Inc. Litigation

The patent litigation involves U.S. Patent No. 9,358,286, initially asserted in a district court infringement case by Merus N.V. against Xencor, Inc., and subsequently challenged in an Inter Partes Review (IPR) by Xencor, Inc. at the Patent Trial and Appeal Board (PTAB).

Chronological Legal Developments:

• August 5, 2024 – Complaint Filed: Merus N.V. initiated a patent infringement lawsuit against Xencor, Inc. in the U.S. District Court for the District of Delaware, case number 1:24-cv-00913. The complaint alleged infringement of U.S. Patent Nos. 9,358,286, 9,944,695, and 11,926,859.
• October 10, 2024 – Motion to Dismiss Filed: Xencor, Inc. filed a motion to dismiss the complaint under Rule 12(b)(6) of the Federal Rules of Civil Procedure. Xencor argued that its accused activities, related to the development of antibody technologies, were protected by the Hatch-Waxman safe harbor provision (35 U.S.C. § 271(e)(1)), as they were "reasonably related to the development and submission of information under a Federal law which regulates the manufacture, use, or sale of drugs."
• February 11, 2025 – IPR Petition Filed: Xencor, Inc. filed an Inter Partes Review petition (IPR2025-00604) with the PTAB, challenging claims 1-2, 7-24, 26-28, and 30-32 of U.S. Patent No. 9,358,286. The petition asserted grounds of unpatentability based on anticipation by the "Lazar" prior art and obviousness over "Lazar" in view of "Arathoon."
• July 17, 2025 – Director's Decision on Discretionary Denial: The Acting Director of the USPTO, Coke Morgan Stewart, issued a decision denying Merus N.V.'s request for discretionary denial of institution for IPR2025-00604 (and a companion IPR, IPR2025-00605, for U.S. Patent No. 11,926,859). The Director determined that Xencor had provided persuasive evidence of a "material error" by the examiner during the prosecution of the '286 patent. Specifically, the examiner had overlooked the "Lazar" reference, which was cited in an Information Disclosure Statement (IDS) and taught the central inventive concept of the '286 patent: replacing neutral amino acids with positively and negatively charged amino acids in the CH3 domain. The Director also noted that the related district court proceeding had not scheduled a trial date and a motion to dismiss was pending, suggesting a final written decision in the IPR was likely to issue before any district court trial.
• September 26, 2025 – IPR Institution: Following the Acting Director's referral, the Patent Trial and Appeal Board (PTAB) instituted IPR2025-00604, agreeing to review the patentability of the challenged claims of U.S. Patent No. 9,358,286.
• September 30, 2025 – District Court Dismissal: The U.S. District Court for the District of Delaware granted Xencor's motion to dismiss the patent infringement lawsuit. The court held that Xencor's alleged activities fell within the safe harbor provision of 35 U.S.C. § 271(e)(1) because they were reasonably related to the development and submission of information for FDA regulatory approval. The court, however, granted Merus N.V. leave to amend its complaint.
• March 19, 2026 – IPR Deposition: A cross-examination deposition of Brian J. Sutton, Ph.D., an expert witness, was scheduled and agreed upon by the parties for both IPR2025-00604 and IPR2025-00605.

Outcome and Present Posture:

As of today, July 10, 2026:

• District Court Litigation: The district court infringement lawsuit (Merus N.V. v. Xencor, Inc., No. 1:24-cv-00913) was dismissed, though Merus N.V. was granted leave to amend its complaint. The current status of any amended complaint is not immediately available.
• PTAB IPR: IPR2025-00604 is active and was instituted following a referral from the Acting Director of the USPTO, who identified a material error by the examiner during the original prosecution of U.S. Patent No. 9,358,286. Discovery, including depositions, has taken place. The IPR remains pending, with a final written decision expected in due course.

Counsel of Record for Xencor, Inc.

Xencor, Inc., the petitioner in IPR2025-00604, is represented by attorneys from Paul Hastings LLP.

The identified counsel are:

• Naveen Modi

  • Role: Lead Counsel
  • Firm: Paul Hastings LLP, Washington, D.C. office.
  • Experience Note: Mr. Modi is a prominent patent litigator known for his work in inter partes reviews (IPRs) before the Patent Trial and Appeal Board (PTAB). He is frequently listed as counsel in significant patent disputes.

• Daniel Zeilberger

  • Role: Partner, Litigation Department
  • Firm: Paul Hastings LLP, Washington, D.C. office
  • Experience Note: Mr. Zeilberger is a partner in the litigation department at Paul Hastings LLP, involved in the IPR proceedings related to this case.

• Eric W. Dittmann

  • Role: Counsel (indicated by inclusion in correspondence as CC)
  • Firm: Paul Hastings LLP (inferred from inclusion with other Paul Hastings attorneys)
  • Experience Note: Mr. Dittmann is listed in correspondence related to the IPRs, suggesting involvement in the case strategy.

• Isaac S. Ashkenazi

  • Role: Counsel (indicated by inclusion in correspondence as CC)
  • Firm: Paul Hastings LLP (inferred from inclusion with other Paul Hastings attorneys)
  • Experience Note: Mr. Ashkenazi is included in the communication regarding the IPRs, indicating his participation in the legal team.

Ashley Mays-Williams, Michael Wolfe, Ryan Meuth, and Amanda Taylor were also included in correspondence related to the IPRs. However, their specific roles and firm affiliations were not definitively established as Paul Hastings LLP from the available search results, though their inclusion in the CC list of an email from a Paul Hastings partner suggests they are part of the broader legal team.

Merus N.V. is represented by a team of attorneys from several law firms in both the parallel district court litigation (Merus N.V. v. Xencor, Inc., Case No. 1:24-cv-00913) and the IPR2025-00604 proceeding.

Counsel for Merus N.V. (Patent Owner):

• Elizabeth J. Holland (Lead Counsel)

  • Firm: White & Case LLP
  • Office Location: New York, NY (likely, based on common patent litigation practice)
  • Note: White & Case has a strong intellectual property practice.

• Daniel P. Margolis (Pro Hac Vice)

  • Firm: White & Case LLP
  • Office Location: New York, NY (likely)
  • Note: As pro hac vice counsel, Mr. Margolis is likely from a firm with a significant patent litigation presence, and White & Case is known for this.

• Peter J. Armenio (Pro Hac Vice)

  • Firm: Cahill Gordon & Reindel LLP
  • Office Location: New York, NY (likely, given the firm's location)
  • Note: Mr. Armenio is listed as pro hac vice in the district court case as well, indicating a consistent role in representing Merus. Cahill Gordon & Reindel has experience in intellectual property and complex litigation.

• Colleen Tracy James (Pro Hac Vice)

  • Firm: Cahill Gordon & Reindel LLP
  • Office Location: New York, NY (likely)
  • Note: Ms. James is also listed as pro hac vice in the district court case.

• Jessica M. Stookey (Pro Hac Vice)

  • Firm: Cahill Gordon & Reindel LLP
  • Office Location: New York, NY (likely)
  • Note: Ms. Stookey is listed as pro hac vice in the district court case.

• Camille H. Mangiaratti (Pro Hac Vice)

  • Firm: Cahill Gordon & Reindel LLP
  • Office Location: New York, NY (likely)
  • Note: Ms. Mangiaratti is listed as pro hac vice in the district court case.

• Jack B. Blumenfeld (Local Counsel)

  • Firm: Morris, Nichols, Arsht & Tunnell LLP
  • Office Location: Wilmington, DE
  • Note: Morris, Nichols, Arsht & Tunnell is a prominent Delaware firm frequently serving as local counsel in District of Delaware patent cases.

• Jeremy A. Tigan (Local Counsel)

  • Firm: Morris, Nichols, Arsht & Tunnell LLP
  • Office Location: Wilmington, DE
  • Note: Also from Morris, Nichols, Arsht & Tunnell, a common local counsel firm in Delaware.

It's important to note that while some attorneys are listed as "Lead Attorney" in the district court docket, the roles for the IPR are inferred based on their appearance and firm's general practice in such proceedings. For the PTAB IPR, the "Counsel for Patent Owner" role is typically led by attorneys from the primary patent litigation firm, with other counsel potentially assisting in various capacities. The listed email addresses for Elizabeth J. Holland and Daniel P. Margolis in a May 15, 2026, Certificate of Service for a related IPR (IPR2025-00605, which is consolidated for oral argument with IPR2025-00604) confirm their involvement in the PTAB proceedings for Merus.