One Antibody, One Claim, One Hundred Million Dollars: The Power of Strategic Means-Plus-Function Claiming in the Life Sciences
In the world of patent law, a single claim can be worth a fortune. A real-life example of this can be found in the ongoing IP dispute between Xencor, Inc. and Alexion Pharmaceuticals, Inc., where Xencor’s recently issued patent, which issued with just one claim, underlies Xencor’s expectations for 100-120 million dollars in additional US royalty payments from Alexion.
The Drug at the Center of the Dispute – Ravulizumab
To understand the stakes, it helps to understand the drug at the center of this dispute. Ravulizumab is a monoclonal antibody that targets a human protein called C5, which is a key part of the body’s complement system. Ravulizumab is used to treat paroxysmal nocturnal hemoglobinuria (PNH), a rare and serious disease in which the complement system attacks and destroys the body’s own red blood cells. The loss of red blood cells typically manifests as anemia, thrombosis, chronic kidney disease, hemoglobinuria and bone marrow failure. PNH is a devastating condition, and for patients living with it, effective treatment is life-changing.
Ravulizumab was developed from an earlier antibody called eculizumab, marketed as Soliris®, which the FDA approved to treat PNH in 2007. These two antibodies have nearly identical amino acid sequences; the sequence for ravulizumab’s heavy chain differs in only four amino acid positions from the heavy chain of eculizumab, as indicated below in Figure 1.
In the variable heavy chain complementarity determining regions of ravulizumab, histidine (H) was used to replace tyrosine (Y) in CDR1 and to replace serine (S) in CDR2 (Figure 1, top arrows). These substitutions weaken the affinity of ravulizumab for C5 by about 36-fold at pH of 6.0, which is the typical pH inside an endosome. By increasing the proportion of ravulizumab-C5 complexes that dissociate within endosomes, this results in lysosomal degradation of C5 and the recycling of unbound ravulizumab back into the vascular compartment.
The other two amino acid substitutions, which were made in the Fc region of ravulizumab (shown in Figure 1), involved the substitution of a leucine (L) for methionine (M) at position 428, and the substitution of a serine (S) for an asparagine (N) at position 434, which sits within the CH3 domain. This “LS substitution” increases the affinity of ravulizumab for the neonatal Fc receptor (FcRn) inside endosomes by about 10-fold. By remaining bound to the FcRn for longer, ravulizumab avoids being degraded, which extends its half-life in the human body, as shown below in Figure 2.
Eculizumab has a half-life of about 11 days in the human body and must be administered to patients with PNH intravenously every two weeks. In contrast, ravulizumab has a half-life of just over 49 days in the human body and is administered intravenously every eight weeks. For patients managing a chronic and serious disease like PNH, that reduction in treatment burden represents a meaningful improvement in their quality of life.
Xencor’s Xtend® Platform and the Race Against Patent Expiration
Xencor developed a technology platform (the “Xtend® platform”) based on introducing the LS amino acid substitutions into the Fc domain of therapeutic antibodies. Xencor patented its Xtend® platform and licensed it broadly, collecting royalties from companies that incorporated the LS substitutions into their own therapeutic antibodies. Alexion, the company that makes ravulizumab, is among those paying royalties to Xencor. Xencor enjoyed significant licensing revenue from its Xtend platform as the years passed; however, the foundational patents on the Xtend® platform would begin to expire in 2025.
At least five years before their patents would expire, Xencor sought to extend its monopoly over the use of LS substitutions. It knew that it needed to file new claims with the USPTO that would: (1) encompass ravulizumab, (2) pass USPTO scrutiny, and (3) survive likely legal challenge. This was no small task, especially considering that the specification in Xencor’s available patent family was, by any measure, thin with respect to its description of the anti-C5 antibody that would eventually become ravulizumab.
A Landscape Transformed by Amgen and Juno
To appreciate why Xencor’s path forward was so difficult, it is also important to understand the patent landscape that had developed in the years leading up to this dispute. Two major court decisions had made obtaining broad functional genus claims in the life sciences significantly harder.
The first was Amgen Inc. v. Sanofi, a unanimous 2023 decision from the United States Supreme Court. That case involved Amgen’s patent on antibodies that lower cholesterol by binding to the protein PCSK9 at specific residues and blocking its interaction with the low-density lipoprotein receptor. Amgen had disclosed 26 specific antibodies that met the claim limitations but had written claims that potentially covered millions of antibodies that had never been made or tested. The Supreme Court held that Amgen’s claims were not enabled, emphasizing that scientists would need to rely on random trial and error strategies to identify which antibodies in that vast space would actually work. The message to US patent practitioners was clear – the more a patent applicant claims, the broader a monopoly it demands, the more it must enable.
The second was Juno Therapeutics, Inc. v. Kite Pharma, Inc., a Federal Circuit decision from 2021. This case concerned US Patent No. 7,446,190 (“the ‘190 patent”) which was owned by Memorial Sloan Kettering Cancer Center (“MSKCC”) and licensed to Juno Therapeutics. The claims of the ‘190 patent were generally directed to chimeric T-cell receptors comprising single chain variable fragment (scFv) antibodies that could recognize and interact with CD19 for purposes of treating cancer.
Juno and MSKCC sued Kite Pharma, alleging that it infringed several claims of the ‘190 patent by making and selling Yescarta® in the United States. Despite the fact that a jury had unanimously awarded Juno $585,000,000 in damages and a 27.6% running royalty on Kite’s net revenues from the sale of Yescarta®, the Federal Circuit found that the claims of the ‘190 patent were invalid because the specification did not adequately describe the broad genus of scFvs that the claims encompassed. The court noted that while single-chain variable fragments were generally known, for even the narrowest claims at issue, the realm of possible CD19-specific scFvs was “vast” and the number of known CD19-specific scFvs was “small (five at most).” Because the patent specification provided “no details about which scFvs bind to CD19 in a way that distinguishes them from scFvs that do not”, the Federal Circuit held that no reasonable jury could have found that the ‘190 patent satisfied the written description requirement.
Together, Amgen and Juno created a landscape in which broad functional genus claims faced a very narrow and difficult path to allowance. The more an applicant claimed by function, the more disclosure was required to support that functional breadth. For Xencor, with a specification that barely mentioned the anti-C5 antibody the claims needed to cover, a conventional genus claim was not a viable path forward.
An Unconventional Strategy
This is where means plus function claiming entered the picture. Means plus function claiming, codified in 35 U.S.C. § 112(f), has been part of United States patent law since 1952. It allows a claim element to be expressed as a means for performing a specified function without reciting the structure that performs that function. Under the statute, such a claim is construed to cover the corresponding structure described in the specification and equivalents thereof.
The critical distinction between means plus function claiming and conventional functional genus claiming is this: under a means plus function framework, the applicant does not need to describe or enable all possible equivalents. Only one corresponding structure needs to be adequately described in the specification. The equivalents to that one corresponding structure are provided by operation of the statue. That is a fundamentally different standard from the one applied to genus claims under Amgen and Juno, where a broad functional claim must be supported by a disclosure that spans the full scope of the claimed genus.
In early 2020, Xencor decided to try using a means-plus-function claiming strategy to see if it could get a claim allowed that would cover ravulizumab. Xencor began this process by taking one of its pending patent applications (USSN 16/803,690) and cancelling its 7 originally filed claims. Then Xencor submitted just two claims (claim 8 and claim 9) for examination. The text of those claims is shown below in Figure 3.
Claim 8 was a Jepson-style method of treatment claim, which is another claiming strategy that is rarely used in the life sciences. Due to space constraints, we will keep discussion of claim 8 to a minimum, so that we can focus on the means-plus-function claim in this story, which is claim 9.
As shown in Figure 3 above, claim 9 was a method of treatment claim which contained a means plus function claim limitation. It was directed to a “method of treating a patient by administering an anti-C5 antibody comprising: a) means for binding human C5 protein” and “an Fc domain comprising amino acid substitutions M428L/N434S”, wherein that antibody has “increased in vivo half-life” as compared to that antibody without the LS substitutions.
The corresponding structure in Xencor’s specification for the “means for binding human C5 protein” limitation was, essentially, a single reference to an anti-C5 antibody known as 5G1.1. Figure 4 below shows the single paragraph from Xencor’s specification ([0133]) that describes a structure capable of performing the recited function of binding human C5 protein.
As far as the recited function of “binding human C5 protein”, the phrase “anti-complement (C5) antibodies such as 5G1.1” was the sole description provided. There was no sequence information for 5G1.1 provided. No CDR definitions. No binding affinity data. No epitope mapping. No humanization details. The word eculizumab does not appear in Xencor’s specification.
A Five-Year Battle
What followed was a five-year prosecution battle that wound its way through multiple examiner rejections, three Patent Trial and Appeal Board decisions, a first-ever Appeals Review Panel decision and two trips to the Federal Circuit. At every stage, the key question was the same. Could naming a single antibody in the specification, without describing it in any structural detail, serve as adequate corresponding structure for a means plus function limitation?
The Examiner said no. On appeal, the Patent Trial and Appeal Board (“PTAB”) also said no – twice. According to the PTAB, “the single mention of one species in the Specification coupled with a limited number of species in the prior art” did not sufficiently describe the claimed genus of anti-C5 antibodies as required under 35 U.S.C. §112(a).
But then something unprecedented happened. After the PTAB’s final written decision rejecting claims 8 and 9, Xencor appealed the decision to the Federal Circuit. After Xencor had filed its Opening Brief, the USPTO Director moved to remand the case to the USPTO in order to convene an Appeals Review Panel to “clarify the USPTO’s position on the proper analysis of ‘Jepson-format and means-plus-function claims in the field of biotechnology, and particularly in the antibody art’ and ‘to issue a revised decision.'” The Federal Circuit granted the motion, and the USPTO convened its first ever Appeals Review Panel.
The Appeals Review Panel (ARP) sided with Xencor on the means plus function issue. It held that 5G1.1, construed to include both the original mouse monoclonal antibody and eculizumab as its humanized counterpart, was adequate corresponding structure because both were well known in the prior art with publicly available sequences. The ARP stated its conclusion on this issue in very clear and certain terms:
The Board, in part, based its written description and indefiniteness rejections on the fact that the Specification did not describe equivalents of 5G1.1. We disagree with the Board that the Specification must disclose or describe the equivalents of the corresponding structure, in this case 5G1.1, for a mean-plus-function claim limitation under 35 U.S.C. § 112 ¶ 1 (written description) and ¶ 2 (definiteness).
This was a direct and significant statement that means plus function claims in the life sciences operate differently from typical genus claims. For a functional genus claim under 35 U.S.C. § 112(f), a single well-known corresponding structure can be sufficient to satisfy the written description and definiteness requirements of the statute.
The ARP did, however, decide that the preamble language directed to treating a patient in claim 9 was limiting, because other language in the claim referencing increased in vivo half-life placed the claim in a therapeutic context. Because Xencor’s specification provided no working examples of treating any patient for any disease with the described antibody, the Board’s rejection of claim 9 as lacking written description support under 35 U.S.C. §112 ¶ 1 was affirmed. That written description rejection was affirmed by the Federal Circuit in March 2025, although the Federal Circuit did not need to reach the means plus function issue directly.
Xencor regrouped. On May 23, 2025, it filed a new application (USSN 19/218,154) with a single revised claim, this time directed to an antibody rather than a method of treating a patient. The preamble issue was gone. So too was the functional language about increased in vivo half-life. The claim was stripped down to its essentials. An antibody comprising a means for binding human C5 and an Fc domain comprising the LS substitutions. That claim was allowed and issued in December 2025 as US Patent No. 12,492,253 (“the ‘253 patent”)
Xencor promptly issued a press release on December 9, 2025, announcing the issuance of the ‘253 patent and noting that this patent would expire in December of 2028, approximately three years later than their previous “latest-to-expire” U.S. patent that covered their Xtend platform. Xencor also announced that it anticipated receiving low-single-digit royalties on net sales of Ultomiris “in the range of $100 million to $120 million in aggregate for the extended patent term through 2028.”
A few months later, Alexion notified Xencor that it would cease United States royalty payments and disputed any further royalty obligations for Ultomiris®; if this dispute is not settled, the ensuing litigation seems likely to produce yet another important precedent on the use of means plus function claiming in biotechnology.
What This Means for Patent Practitioners
The Xencor story carries several important lessons for patent practitioners working in the life sciences and chemical sciences.
First, means plus function claiming is a legitimate and distinct claiming tool, not a workaround. It operates under its own statutory framework with its own requirements, and it is not subject to the same disclosure standards that apply to conventional genus claims under Amgen and Juno. The Appeals Review Panel’s decision made that clear, and practitioners should feel confident asserting that framework when examiners push back.
Second, a single well-known corresponding structure can be sufficient. Xencor’s specification was remarkably thin, yet the Appeals Review Panel found that the mere naming of a publicly known antibody with publicly available sequences met the corresponding structure requirement. This does not mean practitioners should rely on minimal disclosure as a strategy, but it does confirm that means plus function claims can provide meaningful coverage even when the specification is not exhaustive.
Third, drafting the entire claim with Section 112 in mind is essential. The preamble was what brought Xencor’s original claim 9 down at both the PTAB and the Federal Circuit. Practitioners should carefully consider whether preamble language is necessary and whether it introduces limitations that cannot be supported by the specification. Treating a patient may seem like a natural and harmless preamble for a method claim, but as the Xencor litigation demonstrated, it can carry significant consequences when the specification lacks working examples of actual treatment.
Fourth, this type of claiming is most powerful when it is deliberate and targeted. Xencor filed an application with only two claims. That was not an accident. It was a focused, litigation-informed strategy designed to simplify prosecution, minimize examiner objections, and zero in on the specific protection Xencor needed. Not every practitioner will be able to take that approach, but the lesson is that means plus function claiming works best when it is intentional and carefully integrated into a broader portfolio strategy.
Finally, the Xencor story is a reminder that the value of a well-drafted patent claim can be almost impossible to overstate. A single claim, built on a single corresponding structure, supported by what amounts to a single line of disclosure, now stands between Xencor and nine figures in royalties. The outcome of the dispute with Alexion remains to be seen. But the fact that we are even having this conversation is a testament to what strategic patent prosecution can accomplish.
If you would like more information about how to strategically incorporate means-plus-function claims into your existing patent applications, contact me via email at acavazos@slwip.com. If you enjoyed reading this article, comment “MPF-2026” and I’ll send you a copy of the detailed PowerPoint slide deck “Strategic Use of Means-Plus-Function Claiming”.
References
1 This article represents the opinion of its author, Alexandra Cavazos, JD, PhD. It does not represent the opinion of Schwegman Lundberg & Woessner. This article is informational and does not constitute legal advice or form an attorney-client relationship.
2 US Pat. No. 12,492,253 (hereafter, “Xencor’s ‘253 patent”).
3 Claim 1 of Xencor’s ‘253 patent reads as follows: “A monoclonal antibody comprising: (a) a means for binding human C5; and (b) an Fc domain comprising amino acid substitutions M428L/N434S as compared to a human Fc polypeptide, wherein numbering is according to EU numbering.”
4 https://www.xencor.com/news-releases/news-release-details/xencor-announces-change-to-ultomirisr-royalty-revenue-forecast ; last accessed April 29, 2026.
5 Apostolidou, E. et al., Paroxysmal Nocturnal Hemoglobinuria: Unraveling Its Molecular Pathogenesis and Advancing Targeted Therapeutic Strategies, Diseases, Vol. 13, Issue 9, pp. 1-17 (2025).
6 Id. at pp. 1-2.
7 Id. at p. 2.
8 Ladwig, P.M. and Willrich, M.A.V., Ravulizumab: Characterization and quantitation of a new C5 inhibitor using isotype specific affinity purification and high-resolution mass spectrometry, Journal of Mass Spectrometry and Advances in the Clinical Lab, Vol. 21: 10-18 (2021); https://doi.org/10.1016/j.jmsacl.2021.08.002.
9 Vu, T. et al., Ravulizumab in Myasthenia Gravis: A Review of the Current Evidence, Neuropsychiatric Disease and Treatment, Vol. 19: 2639-2655 (2023) at p. 2643 (Figure 2(a) and legend).
10 Id. (Figure 2(b)) and legend); see also Stern, R.M. et al., Ravulizumab: a novel C5 inhibitor for the treatment of paroxysmal nocturnal hemoglobinuria, Therapeutic Advances in Hematology, Vol. 10: 1-11 (2019), at p. 7. Although each pair of amino acid changes provides a benefit, Xencor asserted that the effects resulting from the LS substitutions are separate from the benefits provided by the pH-dependent binding to C5 conferred by the 2 histidine substitutions. See Xencor’s Remarks filed on Feb. 27, 2020, in USSN 16/803690, pp. 3-4 (noting that addition of M428L/N434S substitutions to the ravulizumab prototype which already had the 2 histidine mutations in the CDRs resulted in an “additional doubling of the half-life”).
11 Id. at p. 5.
12 Id. at p. 2 (Figure 1: Comparative Dosing Schedules for Eculizumab and Ravulizumab).
13 Amgen Inc. v. Sanofi, 143 S. Ct. 1243 (2023).
14 Id. at 1254.
15 Id. at 1256.
16 Juno Therapeutics, Inc. v. Kite Pharma, Inc., 10 F. 4th 1330 (Fed. Cir. 2021), cert. denied, 143 S. Ct. 402 (2022), reh’g denied, No. 21-1566, 2023 WL 124509 (Jan. 9, 2023).
17 The patent at issue was US Patent No. 7,446,190: “Nucleic acids encoding chimeric T cell receptors”, which was originally filed by the Memorial Sloan Kettering Cancer Center and licensed to Juno Therapeutics, Inc.
18 Juno Therapeutics, 10 F.4th at 1333-1334.
19 Yescarta® (axicabtagene ciloleucel) is a prescription CAR-T cell immunotherapy that works by modifying a patient’s own t-cells to recognize and attack cancer cells that express the CD19 protein. See https://www.yescarta.com.
20 Juno Therapeutics, Inc. et al v. Kite Pharma, Inc., No. 2:2017cv07639 – Document 728 (C.D. Cal. 2020), available at https://law.justia.com/cases/federal/district-courts/california/cacdce/2:2017cv07639/691790/728/. Last accessed on May 1, 2026.
21 Juno Therapeutics, 10 F. 4th at 1330.
22 Id.
23 35 U.S.C. § 112(f) reads as follows: “ELEMENT IN CLAIM FOR A COMBINATION.—An element in a claim for a combination may be expressed as a means or step for performing a specified function without the recital of structure, material, or acts in support thereof, and such claim shall be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.”
24 A Jepson claim has a preamble that recites what is “conventional or known,” followed by recitation of that “which the applicant considers as the new or improved portion.” 37 C.F.R. §1.75(e). A Jepson claim is also called an “improvement” claim. In claim 8, the preamble serves as an admission that a method of treating a patient with “an anti-C5 antibody with an Fc domain” was known in the prior art, and the body of the claim recites the improvement in which the Fc domain comprises “amino acid substitutions M428L/N434S as compared to a human Fc polypeptide.” This improvement provides the antibody with “increased in vivo half-life as compared to said antibody without said substitutions.”
25 The concept of blockading C5 to inhibit inflammation goes back over 35 years to the description of BB5.1, an anti-C5 blocking monoclonal antibody raised in C5-deficient mice. It was tested in “a long list of diverse model diseases” and its resulting efficacy and safety played a key role in providing “proof of concept” for C5 blockade and subsequent generation of human-specific blocking anti-C5 monoclonal antibodies (mAbs) including eculizumab. In 1996, “a human C5 blocking mAb 5G1.1 was described that was modified, humanized and finally marketed as eculizumab in 2002.” Zelek, W.M. et al., Characterizing the original anti-C5 function-blocking antibody, BB5.1, for species specificity, mode of action and interactions with C5, Immunology, Vol. 161: 103-113 at 104 (2020); see also Thomas, T.C. et al., Inhibition of complement activity by humanized antig-C5 antibody and single-chain Fv, Molecular Immunology, Vol. 33: 1389-1401 (1996); Adis International Limited, Eculizumab: 5G1.1, h5G1.1, long-acting anti-C5 monoclonal antibody 5G1-1, long-acting monoclonal antibody 5G1.1, Drugs in R&D, Vol. 8(1): 61-68 (2007); Kaplan, M., Eculizumab (Alexion), Current Opinion and Investigation Drugs, Vol. 3: 1017-1023 (2002).
26 The Examiner issued the first final rejection of claims 8 and 9 on Oct. 9, 2020. According to the Examiner, “[t]he recitation of the Jepson format and means-plus-function format do not limit compliance with 112 written description.” Final Office Action mailed 10/09/2020 in USSN 16/803690, p. 4. According to the Examiner, Xencor’s reliance upon “anti-C5 antibodies such as 5G1.1… to identify a particular structure that correlates to [a] ‘means for binding human C5′” was insufficient because Xencor still needed to show they were “in possession of the genus of anti-C5 antibodies broadly encompassed by the claimed invention.” Id. at pp. 8, 9. Without a disclosure of a representative number of anti-C5 antibodies with the claimed LS mutations, and without any disclosure of the use of such antibodies as medical treatments, the claims lacked sufficient written description support. Id. at 10. Very similar arguments were presented in the Final Office Action mailed on March 26, 2021, from which Xencor appealed to the PTAB. Due to space constraints, this article does not address the obviousness-type double patenting rejections against claims 8 and 9, except to note they eventually were reversed by the PTAB or the Appeals Review Panel.
27 In its initial decision the PTAB affirmed the Examiner’s rejections of claim 8 and claim 9 as lacking written description under 35 USC §112(a) on the grounds that the specification inadequately disclosed the antibody structure that binds to the C5 protein. The PTAB also put forth a new basis for rejection by finding that claim 9 was indefinite under 35 USC §112(b) because this lack of disclosure of antibody structure left a POSA unable to “know and understand what structure corresponds to the means limitation” and thus unable to perceive the metes and bounds of the invention. Ex Parte Chamberlain, Appeal 2022-001944 (PTAB Initial Decision – December 19, 2022), at pp. 28-29. This initial PTAB decision was vacated by the PTAB on December 28, 2022, to correct several errors, and a new decision was issued on January 10, 2023. See Ex Parte Chamberlain, Appeal 2022-001944 (PTAB Second Decision – January 10, 2023), at pp. 1-2 and 8.
28 Id. at pp. 22-23. Xencor’s request for a rehearing before the PTAB was denied. Ex Parte Chamberlain, Appeal No. 2022-001944 (PTAB Denial of Rehearing – June 1, 2023) at p. 14 (“In sum, we do not agree with Appellant that a different standard for compliance with the written description requirement should be applied to an antibody claim simply because the claim is written in means-plus-function format.”)
29 In re Xencor Inc., No. 23-2048 (Fed. Cir. Jan. 23, 2024) at pp. 4-5 (describing procedural history).
30 The Appeals Review Panel in Ex Parte Chamberlain consisted of USPTO Director Kathi Vidal, Commissioner for Patents Vaishali Udupa, and Chief Administrative Patent Judge Scott Boalick.
31 Ex Parte Chamberlain, Appeal No. 2022-001944 (ARP Decision – May 21, 2024), pp. 32-33 (“We find that a person of ordinary skill in the art would have known the structure of 5G1.1 based on the teachings in the prior art, and thus the ‘means for binding human C5 protein’ is adequately described in the Specification.”). Interestingly, the ARP held that the corresponding structure was not the full monoclonal antibody 5G1.1, which had an antigen-binding region and an unmodified Fc region, because that would result in a an antibody with two Fc regions in the context of the other limitation in claim 9 (for a modified Fc region with LS substitutions).
32 Id. at p. 35.
33 Id. at pp. 38 (“We note that claim 9 is narrower than claim 8 because the ‘means for binding human C5 protein’ in claim 9 limits the claim to 5G1.1, i.e., the original mouse monoclonal antibody and eculizumab, and equivalents thereof, as discussed above, rather than encompassing all anti-C5 antibodies. Regardless, … the Specification fails to provide adequate written description to support a ‘method of treating a patient’ with the recited antibody.”)
34 See https://investors.xencor.com/news-releases/news-release-details/xencor-announces-extension-us-patent-term-certain-xtendtm, last accessed on May 7, 2026.