DCT

1:24-cv-00913

Merus NV v. Xencor Inc

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Key Events
Amended Complaint

I. Executive Summary and Procedural Information

  • Parties & Counsel:
  • Case Identification: 1:24-cv-00913, D. Del., 11/11/2025
  • Venue Allegations: Venue is alleged to be proper in the District of Delaware because Defendant Xencor, Inc. is a Delaware corporation.
  • Core Dispute: Plaintiff alleges that Defendant's use of the "RenLite mouse" to produce antibodies and its "XmAb platform" for antibody engineering infringe patents related to common light chain (CLC) and heterodimerization (HD) technologies.
  • Technical Context: The dispute is set in the biotherapeutics field, where technologies for efficiently producing complex molecules like bispecific antibodies are critical for developing novel treatments.
  • Key Procedural History: The complaint alleges that Defendant was aware of Plaintiff's patents prior to the suit, citing Defendant’s Form 10-K SEC filings from 2022 and 2023. It further alleges Plaintiff provided Defendant with written notice of infringement of the HD patents on May 15, 2024, which was followed by an intellectual property meeting between the parties on July 2, 2024.

Case Timeline

Date Event
2008-06-27 U.S. Patent No. 9,944,695 Priority Date
2012-04-20 U.S. Patent Nos. 9,358,286, 11,926,859, and 12,123,043 Priority Date
2016-06-07 U.S. Patent No. 9,358,286 Issue Date
2018-04-17 U.S. Patent No. 9,944,695 Issue Date
2020-10-27 Defendant allegedly licensed the "RenLite mouse"
2024-03-12 U.S. Patent No. 11926859 Issue Date
2024-05-15 Plaintiff allegedly sent written notice of HD Patents to Defendant
2024-07-02 Plaintiff and Defendant allegedly held an IP meeting
2024-10-22 U.S. Patent No. 12123043 Issue Date
2025-11-11 Complaint Filing Date

II. Technology and Patent(s)-in-Suit Analysis

U.S. Patent No. 9,944,695 - "Antibody Producing Non-Human Mammals"

  • Patent Identification: U.S. Patent No. 9,944,695, issued on April 17, 2018 (the "'695 Patent").

The Invention Explained

  • Problem Addressed: The patent's background section describes the challenge of producing a diverse repertoire of human-like antibodies in transgenic animals, particularly the difficulty of ensuring that various human heavy chains properly pair with available light chains to form functional antibodies (’695 Patent, col. 1:40-60).
  • The Patented Solution: The invention provides a method using a genetically engineered non-human mammal, such as a mouse, whose genome contains a pre-rearranged human immunoglobulin light chain gene (’695 Patent, col. 2:5-15). This "common light chain" is expressed by the animal's B cells and is capable of pairing with a wide variety of different, endogenously produced heavy chains, simplifying the subsequent process of identifying and assembling functional antibodies, including bispecific antibodies (’695 Patent, Abstract; col. 2:16-29).
  • Technical Importance: This approach addresses the "light chain pairing problem" in bispecific antibody production, whereby expressing two different antibodies in one cell can lead to mis-pairing of heavy and light chains; using a common light chain for both specificities simplifies assembly and manufacturing.

Key Claims at a Glance

  • The complaint asserts independent claim 3 (Compl. ¶42-43).
  • The essential elements of Claim 3, a method of obtaining an antibody, include:
    • Isolating nucleic acid encoding a rearranged immunoglobulin heavy chain variable region from a B cell.
    • The B cell produces an antibody comprising a rearranged human light chain variable region (encoded by fused human V/J gene segments) and a murine constant region.
    • Expressing the isolated heavy chain nucleic acid in a host cell together with nucleic acid encoding the rearranged human light chain.
    • Obtaining an antibody that binds to the antigen.
    • The B cell is obtained by immunizing a transgenic mouse whose genome comprises a specific transgene that includes fused human V/J gene segments, lacks a particular enhancer (MoExi), and is linked to a murine constant region.
    • The transgenic mouse, in response to the antigen, produces mature B cells secreting antibodies with the common light chain paired with a diversity of heavy chains.
  • The complaint does not explicitly reserve the right to assert dependent claims for the ’695 Patent.

U.S. Patent No. 9,358,286 - "Methods And Means For The Production Of Ig-Like Molecules"

  • Patent Identification: U.S. Patent No. 9,358,286, issued on June 7, 2016 (the "'286 Patent").

The Invention Explained

  • Problem Addressed: The patent addresses the challenge of producing bispecific (heterodimeric) antibodies. When two different antibody heavy chains are co-expressed in a single cell, they can randomly pair to form two undesired homodimers in addition to the desired heterodimer, creating purification and yield challenges (’286 Patent, col. 3:35-54).
  • The Patented Solution: The invention provides a recombinant host cell engineered to favor the production of heterodimers. This is achieved by introducing specific amino acid substitutions into the CH3 domains of the two different heavy chains. These substitutions create electrostatic interactions (a "charge pair") that promote the pairing of the two different heavy chains with each other while simultaneously disfavoring the pairing of two identical heavy chains through electrostatic repulsion (’286 Patent, Abstract; col. 4:46-54).
  • Technical Importance: This "electrostatic steering" technology increases the yield and purity of desired bispecific antibodies from a single-cell production system, which is a significant improvement for manufacturing therapeutic antibodies.

Key Claims at a Glance

  • The complaint asserts independent claim 26 (Compl. ¶60-61).
  • The essential elements of Claim 26, a recombinant host cell, include:
    • A recombinant host cell comprising nucleic acid sequences.
    • The nucleic acid sequences encode at least a 1st and 2nd antibody heavy chain.
    • The CH3 domain of the 1st antibody heavy chain comprises at least one substitution of a neutral amino acid residue by a positively charged amino acid residue.
    • The CH3 domain of the 2nd antibody heavy chain comprises at least one substitution of a neutral amino acid residue by a negatively charged amino acid residue.
  • The complaint does not explicitly reserve the right to assert dependent claims for the ’286 Patent.

Multi-Patent Capsules

  • Patent Identification: U.S. Patent No. 11,926,859, titled “Methods And Means For The Production Of Ig-Like Molecules,” issued March 12, 2024 (the “'859 Patent”).

  • Technology Synopsis: Belonging to the same family as the ’286 Patent, this patent is directed to the resulting antibody product itself. It claims a heterodimeric antibody comprising first and second human CH3 domains with specific charged amino acid residues at designated positions (364 and 368), which promote the preferential pairing of two different heavy chains (Compl. ¶24, ¶75; ’859 Patent, Abstract).

  • Asserted Claims: Independent claim 1 is asserted (Compl. ¶74-75).

  • Accused Features: The complaint alleges that Defendant's "XmAb platform" is a heterodimeric antibody that contains the claimed amino acid modifications (Compl. ¶74, ¶77).

  • Patent Identification: U.S. Patent No. 12,123,043, titled “Methods And Means For The Production Of Ig-Like Molecules,” issued October 22, 2024 (the “'043 Patent”).

  • Technology Synopsis: Also in the same family as the ’286 and ’859 Patents, this patent is directed to a method of manufacturing. It claims a method of making a host cell for heterodimeric antibody production by introducing into the cell nucleic acids that encode heavy chains containing specific charged amino acid residues (lysine at 364, aspartic acid at 368) (Compl. ¶24, ¶88; ’043 Patent, Abstract).

  • Asserted Claims: Independent claim 25 is asserted (Compl. ¶87-88).

  • Accused Features: The complaint alleges that Defendant's process for making the host cells that produce its XmAb platform infringes the claimed method (Compl. ¶89-91).

III. The Accused Instrumentality

Product Identification

The complaint identifies two primary accused instrumentalities: (1) Defendant's use of the "RenLite mouse," which it licenses from Biocytogen, and (2) Defendant's "XmAb platform" (Compl. ¶18, ¶29).

Functionality and Market Context

  • The complaint alleges that Defendant uses the RenLite mouse to generate hybrid human-murine antibodies that possess a "common light chain" (CLC) (Compl. ¶21-22). The RenLite mouse is described as a transgenic animal engineered with a fixed human light chain gene, which allows it to produce antibodies with diverse heavy chains that all pair with the same light chain (Compl. ¶49, ¶53). A diagram from Biocytogen's website illustrates the genetic construct of the RenLite mouse's kappa light chain, showing a rearranged human 'Single Vk/Jk DNA' and a humanized common light chain with a murine constant region (Ck) (Compl. p. 11).
  • The complaint characterizes the XmAb platform as an "antibody building block" that facilitates the creation of heterodimeric antibodies (Compl. ¶33-34). Its functionality is allegedly based on specific amino acid modifications made to the CH3 domains of antibody heavy chains, which promote the preferential pairing of two different heavy chains to form a heterodimer (Compl. ¶30-32). Defendant allegedly markets this technology as "plug-and-play" for antibody development (Compl. ¶33).

IV. Analysis of Infringement Allegations

U.S. Patent No. 9,944,695 Infringement Allegations

Claim Element (from Independent Claim 3) Alleged Infringing Functionality Complaint Citation Patent Citation
A method of obtaining an antibody that binds to an antigen, the method comprising (a) isolating nucleic acid encoding a rearranged immunoglobulin heavy chain variable region from a B cell... Defendant is alleged to use a process that involves isolating nucleic acids from B cells obtained from the RenLite mouse. ¶44 col. 2:5-10
...that produces an antibody that comprises a rearranged human light chain immunoglobulin variable region encoded by fused human V/J gene segments and comprises a murine constant region... The RenLite mouse allegedly produces antibodies with a rearranged human light chain variable region (from fused V/J gene segments) and a murine constant region. A diagram from an AACR poster depicts the humanized mouse heavy chain pairing with the light chain (Compl. p. 12). ¶45, ¶46 col. 2:16-29
...paired with a heavy chain variable region encoded by a rearranged and somatically hypermutated VH gene that encodes said antibody; The B cells from the RenLite mouse allegedly pair the common light chain with a diverse and somatically hypermutated heavy chain variable region. ¶47 col. 2:30-33
(b) expressing said nucleic acid encoding said rearranged immunoglobulin heavy chain variable region in a host cell together with nucleic acid encoding at least a rearranged human light chain immunoglobulin variable region encoded by fused human V/J gene segments; Biocytogen's process, allegedly used by Defendant, is described as expressing the isolated heavy chain nucleic acids together with the common light chain nucleic acids in a host cell to obtain antibodies. ¶44 col. 2:54-58
...wherein said B cell is obtained by immunizing a transgenic mouse with the antigen... The RenLite mouse is allegedly immunized with antigens as part of the process to generate antibodies. ¶48 col. 2:34-36
...wherein the genome of the transgenic mouse comprises a transgene comprising a human immunoglobulin light chain germline V gene segment fused to a human immunoglobulin light chain germline J gene segment... The RenLite mouse genome allegedly contains a transgene with a fused human V/J gene segment encoding the common light chain variable region. ¶49 col. 2:36-42
...wherein said transgene lacks the intronic light chain enhancer MoExi... The complaint alleges that the RenLite mouse transgene lacks the specific intronic light chain enhancer MoExi, citing analysis of a disclosed nucleic acid sequence (SEQ ID NO: 35). ¶51 col. 2:44-46
...wherein the transgene comprises a murine light chain constant region gene segment or is operatively linked to an endogenous mouse light chain constant region gene segment; The RenLite mouse transgene is allegedly linked to an endogenous mouse light chain constant region, which is retained for normal B cell development. ¶52 col. 2:47-50
  • Identified Points of Contention:
    • Scope Questions: A central question may be whether the accused "RenLite mouse" technology, developed by a third party, meets every positive and negative limitation of the asserted method claim. For example, claim 3 requires that the transgene "lacks the intronic light chain enhancer MoExi." The infringement analysis may turn on whether the evidence presented in the complaint is sufficient to prove this specific absence.
    • Technical Questions: The claim requires that the B cell is obtained from a "transgenic mouse," which then produces antibodies paired with a "diversity of clonally unrelated" heavy chains. The litigation may explore the exact genetic makeup of the RenLite mouse and the degree of heavy chain diversity it actually produces to determine if it aligns with the claimed method.

U.S. Patent No. 9,358,286 Infringement Allegations

Claim Element (from Independent Claim 26) Alleged Infringing Functionality Complaint Citation Patent Citation
A recombinant host cell comprising nucleic acid sequences encoding at least a 1st and 2nd antibody heavy chain, Defendant's XmAb platform is allegedly generated by making and using a recombinant host cell that produces two different heavy chains. ¶65, ¶66 col. 4:10-14
wherein the CH3 domain of said 1st antibody heavy chain comprises at least one substitution of a neutral amino acid residue by a positively charged amino acid residue, The first heavy chain of the XmAb platform allegedly has an S364K substitution, which changes the charge of the amino acid at that position from neutral (Serine) to positive (Lysine). ¶64 col. 5:40-45
and wherein the CH3 domain of said 2nd antibody heavy chain comprises at least one substitution of a neutral amino acid residue by a negatively charged amino acid residue. The second heavy chain of the XmAb platform allegedly has an L368D substitution, which changes the charge of the amino acid at that position from neutral (Leucine) to negative (Aspartic acid). ¶64 col. 5:40-45
  • Identified Points of Contention:
    • Scope Questions: The infringement analysis may focus on whether the specific S364K and L368D substitutions alleged in the XmAb platform fall within the scope of the claim's broader "neutral to positive" and "neutral to negative" language. The definitions of "neutral," "positively charged," and "negatively charged" could become a point of contention.
    • Technical Questions: A key evidentiary question may be what proof Plaintiff can provide that Defendant actually makes and uses a "recombinant host cell" with nucleic acids encoding these exact substitutions. The complaint cites a scientific paper and a patent application by Defendant’s affiliates or employees, and the strength of this evidence may be scrutinized.

V. Key Claim Terms for Construction

For U.S. Patent No. 9,944,695

  • The Term: "lacks the intronic light chain enhancer MoExi" (from Claim 3)
  • Context and Importance: This is a negative limitation, meaning infringement requires proving the absence of a specific genetic element. Practitioners may focus on this term because the parties will likely dispute the precise definition of the "MoExi" enhancer and what evidence is sufficient to prove that the accused RenLite mouse "lacks" it.
  • Intrinsic Evidence for Interpretation:
    • Evidence for a Broader Interpretation: The patent does not appear to provide an explicit definition of MoExi by sequence, which may suggest that the term should be given its plain and ordinary meaning as understood by a person of ordinary skill in the art at the time of the invention.
    • Evidence for a Narrower Interpretation: The patent repeatedly refers to this specific enhancer in the context of achieving desired expression levels and avoiding silencing (’695 Patent, col. 5:35-45). A party could argue that "lacks" means more than mere physical absence and requires a functional lack of the enhancer's activity.

For U.S. Patent No. 9,358,286

  • The Term: "neutral amino acid residue" (from Claim 26)
  • Context and Importance: The infringement theory relies on Serine (S) and Leucine (L) being classified as "neutral." Practitioners may focus on this term because while the biochemical classifications are standard, the patent's own disclosure provides the primary basis for interpretation in claim construction, and any ambiguity could create a non-infringement argument.
  • Intrinsic Evidence for Interpretation:
    • Evidence for a Broader Interpretation: A party could argue that the term should encompass any amino acid without a net positive or negative charge under physiological conditions, consistent with its plain meaning in biochemistry.
    • Evidence for a Narrower Interpretation: The specification provides exemplary lists of amino acids, categorizing them by charge. A party could argue that the term "neutral amino acid residue" should be limited to the specific examples provided in the patent's disclosure, which may support a narrower definition than all biochemically neutral residues (’286 Patent, col. 13:39-49).

VI. Other Allegations

  • Indirect Infringement: The complaint does not contain separate counts for indirect infringement. However, it alleges facts that may support a future claim of inducement, stating that Defendant engages in "partnered" making and using of the claimed recombinant host cell, which may imply encouraging or instructing others to infringe (Compl. ¶62, ¶76, ¶89).
  • Willful Infringement: The complaint alleges willful infringement of the HD Patents (’286, ’859, and ’043 Patents) (Compl. ¶71, ¶84, ¶100). The basis for this allegation includes alleged pre-suit knowledge, evidenced by Defendant’s own 10-K SEC filings that purportedly reference Plaintiff’s patents, and alleged post-suit knowledge stemming from a notice letter sent by Plaintiff on May 15, 2024, and a subsequent meeting on July 2, 2024 (Compl. ¶36, ¶38).

VII. Analyst’s Conclusion: Key Questions for the Case

  • A core issue will be one of evidentiary proof: can Plaintiff demonstrate, to a degree sufficient for infringement, that the accused "RenLite mouse" and "XmAb platform" technologies precisely practice every element of the asserted claims? This may be particularly challenging for the detailed method steps of the ’695 Patent and for proving the exact genetic contents of the host cell claimed in the ’286 Patent, as the complaint relies heavily on third-party and public-facing documents.
  • A second key question will be one of technical and legal equivalency: for the ’695 patent, does the genetic makeup of the RenLite mouse—specifically its alleged lack of the "MoExi" enhancer—map directly onto the specific negative limitations required by claim 3? For the ’286 patent, does the S364K/L368D modification pair in the XmAb platform function in a way that is covered by the claim language describing substitutions of neutral residues for charged ones?