{"id":39549,"date":"2026-09-04T09:23:00","date_gmt":"2026-09-04T13:23:00","guid":{"rendered":"https:\/\/www.drugpatentwatch.com\/blog\/?p=39549"},"modified":"2026-08-27T09:54:00","modified_gmt":"2026-08-27T13:54:00","slug":"zero-for-two-why-no-antibody-drug-conjugate-has-ever-faced-a-biosimilar","status":"publish","type":"post","link":"https:\/\/www.drugpatentwatch.com\/blog\/zero-for-two-why-no-antibody-drug-conjugate-has-ever-faced-a-biosimilar\/","title":{"rendered":"Zero for Two: Why No Antibody-Drug Conjugate Has Ever Faced a Biosimilar"},"content":{"rendered":"\n
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Kadcyla’s twelve-year reference exclusivity period ran out in February 2025. Adcetris lost its exclusivity years before that. Neither drug has a biosimilar on the market, in late-stage trials, or even filed with the FDA. That is not an accident of timing. It is the result of a drug class that was built, structurally, to resist the abbreviated approval pathway that turned trastuzumab, adalimumab, and bevacizumab into commodity biologics.<\/p>\n\n\n\n

Antibody-drug conjugates, or ADCs, are now a roughly $10-billion-a-year oncology category built from three moving parts: an antibody, a cytotoxic payload, and a chemical linker holding them together. That third part is the problem. It is also the part the “old biosimilar playbook,” written for single-chain monoclonal antibodies, never had to account for. This piece works through why, using the patent record, the regulatory guidance, and the one nonclinical biosimilarity study anyone has actually published on an ADC.<\/p>\n\n\n\n

The Short Answer<\/strong><\/h2>\n\n\n\n

A biosimilar has to match its reference product closely enough that regulators find no clinically meaningful difference. For a plain monoclonal antibody, that means matching one molecule’s amino acid sequence, glycosylation, and higher-order structure. For an ADC, a biosimilar developer has to match three molecules and the chemistry that joins them: the antibody’s structure, the small-molecule payload’s identity, and the linker’s conjugation site, stability, and drug-to-antibody ratio (DAR) distribution. Get the antibody right and the conjugation wrong, and the product is not biosimilar. Miss the DAR distribution, and the safety profile can shift even if every underlying molecule is identical to the reference.<\/p>\n\n\n\n

Layered on top of that analytical burden is a patent structure that does not resemble a normal biologic’s. ADC intellectual property is frequently split across three or more owners: the company that engineered the antibody, the company that licensed the linker chemistry, and the company that supplies or licenses the payload. Seagen alone has licensed its linker-payload platform to Astellas (Padcev), Genmab (Tivdak), and, according to Seagen’s own infringement claims, to Daiichi Sankyo (Enhertu). ImmunoGen licensed its ADC technology to Vertex in a 2022 multi-target deal worth up to $337 million in milestones on top of a $15 million upfront payment [17]. That licensing structure means a biosimilar developer targeting one ADC may need freedom to operate against patents held by two or three unrelated companies, not one.<\/p>\n\n\n\n

What Counts as an Antibody-Drug Conjugate<\/strong><\/h2>\n\n\n\n

An ADC links a monoclonal antibody to a cytotoxic small molecule through a chemical linker, so the antibody can carry the toxin directly to antigen-expressing tumor cells and spare healthy tissue [1]. Three design choices define the resulting molecule, and each one is a distinct axis of possible variation between an originator ADC and any proposed copy.<\/p>\n\n\n\n

The antibody, the linker, and the payload<\/h3>\n\n\n\n

The antibody component targets a tumor-associated antigen: HER2 for Kadcyla and Enhertu, CD30 for Adcetris, Nectin-4 for Padcev, Trop-2 for Trodelvy. The payload is a cytotoxin too potent to give systemically on its own, typically a tubulin inhibitor (auristatins like MMAE and MMAF, or maytansinoids like DM1 and DM4) or a topoisomerase-1 inhibitor (deruxtecan, SN-38). The linker determines when and where the payload releases.<\/p>\n\n\n\n

Drug-to-antibody ratio: the hidden fourth variable<\/h3>\n\n\n\n

DAR is the average number of payload molecules attached per antibody. It is not a fixed number for a given ADC; it is a distribution. Kadcyla has an average DAR of 3.5, Enhertu runs at approximately 8, Padcev at roughly 4 [42][44]. A biosimilar candidate has to match not just the average DAR but the shape of that distribution, because a batch skewed toward higher-DAR species can be more toxic and a batch skewed lower can be less effective, even at an identical average.<\/p>\n\n\n\n

Cleavable versus non-cleavable linkers<\/h4>\n\n\n\n

Kadcyla uses a non-cleavable thioether linker (SMCC), so the payload only releases after the entire antibody is degraded inside the cell, which limits the “bystander effect” on neighboring antigen-negative cells [42][44]. Enhertu, Padcev, and Adcetris use protease-cleavable linkers designed to be cut by enzymes inside the lysosome, which produces more bystander killing but adds a stability variable: how much payload leaks into circulation before the ADC ever reaches the target cell.<\/p>\n\n\n\n

Site-specific versus stochastic conjugation<\/h4>\n\n\n\n

Older ADCs, including Kadcyla, use stochastic conjugation, attaching the payload at whatever surface lysines or cysteines are chemically available, which produces a heterogeneous mixture of DAR species in every batch [7]. Newer platforms increasingly use site-specific conjugation to narrow that distribution. A biosimilar of a stochastically conjugated originator has to reproduce that same heterogeneity, not eliminate it, or the “similarity” claim breaks down.<\/p>\n\n\n\n

The Findings That Matter<\/strong><\/h2>\n\n\n\n
    \n
  • No biosimilar ADC has been approved by the FDA or the European Medicines Agency, as of this writing [1].<\/li>\n\n\n\n
  • Kadcyla’s twelve-year BPCIA reference exclusivity, running from its February 22, 2013 approval, expired in February 2025 [8][36]. No biosimilar application has been made public.<\/li>\n\n\n\n
  • Adcetris was approved August 19, 2011; its reference exclusivity lapsed years ago. No biosimilar has been filed [99][104].<\/li>\n\n\n\n
  • The only published nonclinical biosimilarity data for any ADC, a December 2025 study of Zhejiang Hisun’s HS630 against Kadcyla, is still at the animal-pharmacokinetics stage, not human trials [87].<\/li>\n\n\n\n
  • Seagen won a $41.8 million jury verdict against Daiichi Sankyo and AstraZeneca over Enhertu’s linker patent in April 2022, then lost that patent entirely to a Federal Circuit written-description ruling on December 2, 2025 [11][21][29].<\/li>\n\n\n\n
  • India’s Drugs Controller General approved Zydus Cadila’s Ujvira, a “similar biologic” of Kadcyla, in 2021, the only regulator anywhere to have cleared a Kadcyla copy [1].<\/li>\n\n\n\n
  • ImmunoGen licensed its ADC linker-payload platform to Vertex for up to $337 million in 2022, on top of licenses already running to other originator companies, illustrating how ADC intellectual property routinely sits across three or more corporate owners for a single molecule [110].<\/li>\n<\/ul>\n\n\n\n

    Twenty-Five Years of FDA-Approved ADCs: A Timeline<\/strong><\/h2>\n\n\n\n

    The following table compiles FDA-approved ADCs with confirmed target, payload, linker, and approval data drawn from FDA records, company filings, and peer-reviewed pharmacology literature.<\/p>\n\n\n\n

    ADC<\/th>Trade Name<\/th>Target<\/th>Payload<\/th>Linker Type<\/th>Developer<\/th>US Approval<\/th>Source<\/th><\/tr><\/thead>
    Gemtuzumab ozogamicin<\/td>Mylotarg<\/td>CD33<\/td>Calicheamicin<\/td>Acid-labile hydrazone<\/td>Wyeth\/Pfizer<\/td>2000 (withdrawn 2010; re-approved 2017)<\/td>[1][46]<\/td><\/tr>
    Brentuximab vedotin<\/td>Adcetris<\/td>CD30<\/td>MMAE<\/td>Protease-cleavable<\/td>Seagen\/Takeda<\/td>2011<\/td>[1]<\/td><\/tr>
    Trastuzumab emtansine (T-DM1)<\/td>Kadcyla<\/td>HER2<\/td>DM1<\/td>Non-cleavable (SMCC)<\/td>Genentech\/Roche<\/td>2013<\/td>[36][38]<\/td><\/tr>
    Inotuzumab ozogamicin<\/td>Besponsa<\/td>CD22<\/td>Calicheamicin<\/td>Acid-labile hydrazone<\/td>Pfizer\/Wyeth<\/td>2017<\/td>[49]<\/td><\/tr>
    Polatuzumab vedotin<\/td>Polivy<\/td>CD79b<\/td>MMAE<\/td>Protease-cleavable<\/td>Genentech<\/td>2019<\/td>[49]<\/td><\/tr>
    Enfortumab vedotin<\/td>Padcev<\/td>Nectin-4<\/td>MMAE<\/td>Protease-cleavable<\/td>Astellas\/Seagen<\/td>2019<\/td>[42][44]<\/td><\/tr>
    Trastuzumab deruxtecan (T-DXd)<\/td>Enhertu<\/td>HER2<\/td>Deruxtecan<\/td>Cleavable tetrapeptide (GGFG)<\/td>Daiichi Sankyo\/AstraZeneca<\/td>2019<\/td>[16][44]<\/td><\/tr>
    Sacituzumab govitecan<\/td>Trodelvy<\/td>Trop-2<\/td>SN-38<\/td>Cleavable<\/td>Immunomedics\/Gilead<\/td>2020<\/td>[42]<\/td><\/tr>
    Belantamab mafodotin<\/td>Blenrep<\/td>BCMA<\/td>MMAF<\/td>Non-cleavable maleimidocaproyl<\/td>GSK<\/td>2020 (withdrawn 2023; re-approved Oct. 2025)<\/td>[51][54][56]<\/td><\/tr>
    Loncastuximab tesirine<\/td>Zynlonta<\/td>CD19<\/td>PBD dimer<\/td>Protease-cleavable<\/td>ADC Therapeutics<\/td>2021<\/td>[48]<\/td><\/tr>
    Tisotumab vedotin<\/td>Tivdak<\/td>Tissue factor<\/td>MMAE<\/td>Protease-cleavable<\/td>Genmab\/Seagen<\/td>2021<\/td>[42]<\/td><\/tr>
    Mirvetuximab soravtansine<\/td>Elahere<\/td>FR\u03b1<\/td>DM4<\/td>Cleavable sulfo-SPDB<\/td>ImmunoGen\/AbbVie<\/td>2022<\/td>[109]<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Mylotarg: the first ADC, and the first ADC withdrawal<\/h3>\n\n\n\n

    Mylotarg was the first ADC ever approved, in 2000, under an accelerated pathway for CD33-positive acute myeloid leukemia [1]. Its sponsor voluntarily withdrew it in 2010 after a confirmatory trial showed a higher rate of fatal toxicity in the treatment arm than in chemotherapy alone. It came back in 2017 at a lower, fractionated dose. That is the first instance of a pattern that matters more than it might seem: an ADC’s regulatory identity is not fixed the way most biologics’ identities are.<\/p>\n\n\n\n

    Blenrep repeats the pattern<\/h3>\n\n\n\n

    Blenrep received accelerated approval in August 2020, was withdrawn in March 2023 after the confirmatory DREAMM-3 trial missed its primary endpoint, and returned to the US market in October 2025 on the strength of the DREAMM-7 trial, now paired with bortezomib and dexamethasone as a second-line regimen instead of monotherapy [51][54][56]. A reference product that can be withdrawn, redesigned around a new combination regimen, and reapproved is a moving target for any biosimilar sponsor trying to define what, exactly, it needs to match.<\/p>\n\n\n\n

    Why the Small-Molecule and Simple-Antibody Playbook Doesn’t Transfer<\/strong><\/h2>\n\n\n\n

    The biosimilar pathway that produced dozens of approved copies of trastuzumab, adalimumab, bevacizumab, and rituximab was built around a single-protein comparison problem. A sponsor characterizes the reference product’s primary sequence, glycosylation pattern, charge variants, and higher-order structure, runs analytical similarity studies, and, where needed, a comparative pharmacokinetic and immunogenicity study in a sensitive population. FDA’s own 2026 guidance on monoclonal antibody biosimilars describes exactly this evidence chain [9].<\/p>\n\n\n\n

    The conjugation step adds a dimension nothing else in biosimilar regulation has to handle<\/h3>\n\n\n\n

    An ADC biosimilar sponsor has to run every one of those antibody-level studies and then layer on a second, largely separate analytical program for the conjugate itself: DAR determination, mapping of conjugation sites, detection of reaction by-products and free linker, and confirmation that the conjugation chemistry has not altered the antibody’s own folding or aggregation behavior [1]. As the GaBI Journal’s 2023 review of ADC biosimilar characterization put it plainly, alteration of higher-order structure between the parent antibody and the finished ADC is expected and is not itself disqualifying, but it has to be characterized and explained batch to batch, using orthogonal techniques including circular dichroism, FT-IR, fluorescence spectroscopy, NMR, and a combination of sedimentation velocity analytical ultracentrifugation and size-exclusion chromatography with multi-angle light scattering for aggregation [1].<\/p>\n\n\n\n

    \n

    “The question is though, are ADCs a class of drug that may be almost immune to biosimilar competition given their complexity? Undoubtedly the development challenges are far greater which may limit the number of biosimilar sponsors willing to take them on.” [1]<\/p>\n<\/blockquote>\n\n\n\n

    What FDA Actually Requires: The Analytical Similarity Problem<\/strong><\/h2>\n\n\n\n

    FDA’s biosimilarity standard, wherever it applies, follows the same statistical logic: comparative pharmacokinetic endpoints such as Cmax and AUC need a 90% confidence interval for the geometric mean ratio between test and reference product falling within 80% to 125% [87]. That standard was designed for a single analyte. An ADC generates at least three analytes that plausibly need separate biosimilarity assessment: total antibody (conjugated plus unconjugated), intact ADC, and released free payload.<\/p>\n\n\n\n

    Case study: HS630, the only published ADC biosimilarity data set<\/h3>\n\n\n\n

    The clearest evidence of how this plays out in practice comes from a December 2025 Frontiers in Pharmacology paper on HS630, a proposed Kadcyla biosimilar developed by Zhejiang Hisun Pharmaceutical [87]. Researchers ran surface plasmon resonance binding studies, tumor-bearing mouse pharmacokinetics, and cynomolgus monkey pharmacokinetics and immunogenicity, comparing HS630 against reference Kadcyla across all three analytes.<\/p>\n\n\n\n

    The results were mixed in a way that illustrates the underlying problem. In tumor-bearing mice, both the total-antibody and intact-ADC geometric mean ratios for Cmax and AUC fell inside the FDA\/WHO\/NMPA 80% to 125% biosimilarity window. In cynomolgus monkeys, the intact-ADC analyte cleared the window too, with a Cmax ratio of 109.15% and an AUC ratio of 97.51%. But the total-antibody analyte in monkeys came in at an AUC geometric mean ratio of 136.16%, which the study’s own authors describe as only “basically” meeting the criteria, since it sits above the upper bound [87]. No anti-drug antibodies were detected in either group, and the paper’s authors acknowledge the animal numbers were too small to rule out sex-specific or individual variability, and recommend close attention during any future clinical development [87].<\/p>\n\n\n\n

    That is the practical shape of the ADC biosimilarity problem: a sponsor can hit the target on two of three required analytes and land at the edge of the acceptable range on the third, in the same study, using the same drug substance. A conventional monoclonal antibody biosimilar only has one analyte to miss.<\/p>\n\n\n\n

    Zydus Cadila’s Ujvira: what India’s approval does and does not establish<\/h3>\n\n\n\n

    The one jurisdiction that has cleared a copy of an ADC is India, where the Drugs Controller General approved Zydus Cadila’s Ujvira, described as a “similar biological” of Kadcyla, in 2021 for HER2-positive breast cancer [1]. India’s biosimilar framework does not require the same comparative clinical evidence package FDA and EMA use, and neither FDA nor EMA has recognized Ujvira or granted it any form of approval [1]. Ujvira demonstrates that an ADC copy can be manufactured and marketed somewhere. It does not demonstrate that one can clear the analytical and clinical bar FDA and EMA apply.<\/p>\n\n\n\n

    The Patent Layer Nobody Talks About: Linker and Conjugation IP<\/strong><\/h2>\n\n\n\n

    A generic small-molecule drug typically has one dominant composition-of-matter patent and a handful of secondary patents, all usually owned by one company. An ADC routinely has patents held by the company that made the antibody, patents held by the company that invented the linker chemistry, and patents held by the company that supplies or licenses the payload, because ADC linker-payload technology is itself a licensable platform business. Seagen licensed its auristatin linker-payload technology to Astellas for Padcev and to Genmab and Pfizer for Tivdak, and separately alleged that Daiichi Sankyo’s Enhertu infringed the same family of patents [11][18].<\/p>\n\n\n\n

    Case study: Seagen v. Daiichi Sankyo and the ‘039 patent<\/h3>\n\n\n\n

    Seagen sued Daiichi Sankyo in October 2020, alleging that Enhertu’s tetrapeptide GGFG linker infringed Seagen’s US Patent 10,808,039, which the company said covered a class of protease-cleavable linkers it had pioneered [12]. This dispute is worth tracing in full because it shows exactly how ADC patent litigation differs from small-molecule patent litigation: the fight was never about the antibody (trastuzumab was off-patent) or the payload (deruxtecan was Daiichi Sankyo’s own chemistry). It was entirely about the linker.<\/p>\n\n\n\n

    The 2022 jury verdict<\/h4>\n\n\n\n

    In April 2022, a jury in the Eastern District of Texas found that Daiichi Sankyo willfully infringed the ‘039 patent and awarded Seagen $41.8 million in damages covering the period from October 2020 through March 2022 [15]. The court later denied Daiichi Sankyo’s post-trial motions and, in October 2023, entered an amended judgment adding an 8% ongoing royalty on US Enhertu sales through the patent’s original expiration date [28].<\/p>\n\n\n\n

    The January 2024 PTAB invalidation<\/h4>\n\n\n\n

    Running in parallel, Daiichi Sankyo and AstraZeneca had petitioned the USPTO for post-grant review of the ‘039 patent in December 2020 and January 2021, arguing it lacked adequate written description and enablement [28]. In January 2024, the Patent Trial and Appeal Board agreed, issuing a final written decision invalidating all challenged claims [19][26].<\/p>\n\n\n\n

    The December 2025 Federal Circuit reversal<\/h4>\n\n\n\n

    Seagen appealed the PTAB decision, and Daiichi Sankyo and AstraZeneca separately appealed the jury verdict. On December 2, 2025, the Federal Circuit ruled on both appeals the same day. It reversed the district court’s denial of judgment as a matter of law, holding that no reasonable jury could find the ‘039 patent’s claims adequately supported by written description or enablement, and vacated the entire infringement judgment and damages award. In a companion decision, it dismissed Seagen’s appeal of the PTAB’s invalidation as moot, since the same claims were now independently invalid on appeal [4][5][21][22]. The deadlines to seek further review passed on March 2, 2026, closing the case for good [21].<\/p>\n\n\n\n

    The Federal Circuit’s reasoning is itself instructive for anyone doing ADC freedom-to-operate work. The ‘039 patent, filed in 2019, claimed priority back to a 2004 application that disclosed linkers only at the level of a broad chemical genus. Enhertu’s actual GGFG tetrapeptide linker was never exemplified in that 2004 filing. The court held that a genus-level disclosure does not support a claim to one specific, non-exemplified species within that genus, which is exactly the kind of claiming pattern common in early ADC linker patents filed before any company had settled on a specific commercial linker chemistry [16][27][29].<\/p>\n\n\n\n

    What the ‘039 saga establishes for the biosimilar question<\/h3>\n\n\n\n

    The ‘039 dispute was between two originator companies fighting over which one owns rights to a class of linker chemistry, not a biosimilar dispute at all. But it demonstrates why a biosimilar sponsor targeting any cleavable-linker ADC has to clear a patent landscape that may include foundational linker patents unrelated to the reference sponsor, foundational payload patents from a third company, and the reference sponsor’s own antibody and formulation patents, before it can even begin the analytical work described above.<\/p>\n\n\n\n

    BPCIA Mechanics for ADCs: Exclusivity, the Patent Dance, and Why Expiring Isn’t the Same as Solved<\/strong><\/h2>\n\n\n\n

    ADCs are licensed as biologics under Section 351(a) of the Public Health Service Act, which means they qualify for the same twelve-year reference product exclusivity period the Biologics Price Competition and Innovation Act grants every other biologic, running from first licensure [71][74]. A 351(k) biosimilar applicant cannot get FDA approval until that exclusivity period runs out, and once a biosimilar application is accepted, both sides go through the BPCIA’s “patent dance,” an information-exchange process under 42 U.S.C. \u00a7262(l) in which the biosimilar applicant shares its application with the reference sponsor, the reference sponsor lists patents it believes are infringed within 60 days, and the two sides work through claim-by-claim invalidity and infringement contentions before any commercial launch [76].<\/p>\n\n\n\n

    Kadcyla and Adcetris show the limit of exclusivity as a predictor<\/h3>\n\n\n\n

    Kadcyla’s twelve-year exclusivity clock, running from its February 22, 2013 approval, expired in February 2025 [36][38]. Adcetris, approved August 19, 2011, has been outside its exclusivity period for years. Under the small-molecule Hatch-Waxman playbook, both of these facts would ordinarily draw a first-filer generic within a year or two of exclusivity lapsing. Under the ADC playbook, exclusivity expiring has not produced a single public biosimilar filing at either the FDA or EMA for either drug. The regulatory exclusivity clock and the technical and commercial viability of a biosimilar candidate are two separate questions, and for ADCs they appear to be almost entirely decoupled.<\/p>\n\n\n\n

    FDA’s 2025 to 2026 Regulatory Push, and What It Still Doesn’t Cover<\/strong><\/h2>\n\n\n\n

    FDA has been active on ADC-adjacent guidance recently, without yet issuing guidance specific to ADC biosimilars.<\/p>\n\n\n\n

    The March 2024 ADC clinical pharmacology guidance<\/h3>\n\n\n\n

    In March 2024, FDA finalized “Clinical Pharmacology Considerations for Antibody-Drug Conjugates,” covering bioanalytical methods, dosing strategy, exposure-response analysis, and immunogenicity assessment for new ADCs [92][93]. This guidance addresses how to develop a novel ADC. It does not address how to establish biosimilarity to an already-approved one.<\/p>\n\n\n\n

    The December 2025 streamlined nonclinical safety draft guidance<\/h3>\n\n\n\n

    In December 2025, FDA issued a draft guidance, “Monoclonal Antibodies: Streamlined Nonclinical Safety Studies,” aimed at reducing unnecessary animal testing, particularly in non-human primates, for monospecific antibody programs [94][95]. Because it is scoped to monospecific antibodies, it is unclear how directly it will extend to the conjugated form of those same antibodies, where the safety profile is driven as much by the payload and linker as by the antibody itself.<\/p>\n\n\n\n

    What’s still missing<\/h3>\n\n\n\n

    As of this writing, neither FDA nor EMA has published biosimilar-specific guidance addressing DAR-distribution comparability, conjugation-site mapping standards, or how to weight discordant results across the multiple analytes an ADC biosimilar study generates, the exact problem the HS630 data set ran into. Industry commentary has flagged FDA’s broader biosimilar-simplification push, including an October 2025 draft guidance aimed at reducing comparative efficacy study requirements for biosimilars generally, but that guidance is not ADC-specific either [88][89].<\/p>\n\n\n\n

    A Four-Type Taxonomy of ADC Biosimilar Barriers<\/strong><\/h2>\n\n\n\n

    Drawing on the regulatory record and the patent history above, ADC biosimilar barriers sort into four distinct types. Distinguishing them matters because they call for different mitigation strategies and resolve on different timelines.<\/p>\n\n\n\n

    1. Structural-heterogeneity barrier<\/h3>\n\n\n\n

    The DAR distribution, conjugation-site pattern, and higher-order-structure changes introduced by conjugation chemistry, which the HS630 data show can produce a passing result on two analytes and a borderline result on a third within the same study [87].<\/p>\n\n\n\n

    2. Multi-party intellectual property barrier<\/h3>\n\n\n\n

    Linker and payload technology licensed from a platform company distinct from the antibody’s originator, illustrated by Seagen’s licensing relationships with Astellas, Genmab, and Pfizer, and by ImmunoGen’s 2022 platform license to Vertex [110].<\/p>\n\n\n\n

    3. Reference-product-instability barrier<\/h3>\n\n\n\n

    A reference product whose approved label, dosing regimen, or even market availability has changed after initial approval, as with Mylotarg’s 2010 withdrawal and 2017 reapproval at a different dose, and Blenrep’s 2023 withdrawal and 2025 reapproval in a different combination regimen [1][51][56].<\/p>\n\n\n\n

    4. Manufacturing-conjugation barrier<\/h3>\n\n\n\n

    The need to reproduce not just a molecule but a specific, validated conjugation process, since the same antibody and the same linker chemistry run through different conjugation conditions can still yield different DAR distributions, a variable a small-molecule generic manufacturer never has to control for.<\/p>\n\n\n\n

    What This Means for Brand Manufacturers<\/strong><\/h2>\n\n\n\n

    The absence of biosimilar competition on Kadcyla and Adcetris despite years of lapsed exclusivity is not a permanent guarantee. The HS630 data show a domestic Chinese manufacturer already has a nonclinical package assembled for the single easiest target in the class, the oldest ADC with the simplest non-cleavable linker chemistry. Brand teams tracking loss-of-exclusivity risk on ADC franchises should treat linker and conjugation patents, not just antibody composition-of-matter patents, as the assets worth defending, since the Seagen v. Daiichi Sankyo history shows that is where ADC patent litigation actually concentrates. DrugPatentWatch’s patent and exclusivity data can help portfolio teams map which of an ADC’s patent families cover the antibody, which cover the linker, and which cover the payload, since those three categories rarely share an expiration date or, in many cases, an owner.<\/p>\n\n\n\n

    What This Means for Would-Be Biosimilar Developers<\/strong><\/h2>\n\n\n\n

    A sponsor targeting an ADC biosimilar needs a freedom-to-operate analysis that spans at least three separate patent families and, frequently, three separate corporate counterparties, before committing to the multi-year, multi-analyte characterization program the HS630 study illustrates. The most tractable early targets are likely to be the oldest ADCs with the simplest, non-cleavable linker chemistry and single-owner patent portfolios, which is consistent with both Kadcyla drawing the only two biosimilar-track efforts identified in this piece, Zydus Cadila’s Ujvira and Zhejiang Hisun’s HS630, and neither one clearing FDA or EMA review yet.<\/p>\n\n\n\n

    What This Means for Payers and Health Systems<\/strong><\/h2>\n\n\n\n

    Health systems modeling future savings from ADC loss of exclusivity should not apply small-molecule or simple-biologic erosion curves to this class. A twelve-year exclusivity expiration date is a necessary condition for biosimilar entry, not a predictor of when it will happen. Given that Kadcyla and Adcetris have gone a combined multiple years past exclusivity expiration with zero biosimilar approvals anywhere in the US or EU, payer budget models built on standard biosimilar-entry timelines are likely to overstate near-term savings on ADC spend.<\/p>\n\n\n\n

    Methodology<\/strong><\/h2>\n\n\n\n

    This analysis draws on FDA approval letters and guidance documents, USPTO and Federal Circuit filings and opinions, company SEC filings and press releases, and peer-reviewed pharmacology literature, all cited inline. The FDA-approved ADC table reflects drugs with a confirmed FDA approval date and payload\/linker\/target data corroborated across at least one peer-reviewed source and one regulatory or company source. Where sources disagreed on DAR values, the range is presented rather than a single figure. The taxonomy in the barriers section is original analysis built from the patent and regulatory record described above, not an established regulatory classification.<\/p>\n\n\n\n

    Key Takeaways<\/strong><\/h2>\n\n\n\n
      \n
    • No biosimilar ADC has been approved by FDA or EMA, despite Kadcyla’s and Adcetris’s exclusivity periods having expired years ago [1][8][36].<\/li>\n\n\n\n
    • The only published ADC biosimilarity data set, HS630 versus Kadcyla, met the standard biosimilarity range on two of three tested analytes and landed at the edge of the range on the third, in the same nonclinical study [87].<\/li>\n\n\n\n
    • ADC patent litigation concentrates on linker chemistry, not antibody composition, as shown by the five-year Seagen v. Daiichi Sankyo fight over Enhertu’s linker patent, which ended in the patent’s invalidation on written-description grounds [4][21][29].<\/li>\n\n\n\n
    • ADC intellectual property is routinely split across the antibody originator and one or more linker\/payload licensors, illustrated by Seagen’s and ImmunoGen’s platform licensing businesses [110].<\/li>\n\n\n\n
    • Reference ADCs can be withdrawn and later reapproved under a different regimen, as both Mylotarg and Blenrep have been, complicating what a biosimilar sponsor is even trying to match [1][51][56].<\/li>\n\n\n\n
    • India’s approval of Zydus Cadila’s Ujvira demonstrates an ADC copy can be manufactured, not that one can clear FDA or EMA’s analytical and clinical bar [1].<\/li>\n<\/ul>\n\n\n\n

      FAQ<\/strong><\/h2>\n\n\n\n

      Has the FDA ever approved a biosimilar antibody-drug conjugate?<\/h3>\n\n\n\n

      No. As of this writing, neither the FDA nor the European Medicines Agency has approved a biosimilar version of any ADC [1].<\/p>\n\n\n\n

      Why hasn’t a Kadcyla biosimilar been approved, given its exclusivity expired in 2025?<\/h3>\n\n\n\n

      BPCIA exclusivity expiring only removes the regulatory block on FDA approving a biosimilar application; it does not mean a technically and clinically sufficient application exists yet. The only public nonclinical data package for a proposed Kadcyla biosimilar, Zhejiang Hisun’s HS630, was published in December 2025 and is still at the animal-study stage, not human clinical trials [36][87].<\/p>\n\n\n\n

      What makes ADC biosimilar development harder than a standard monoclonal antibody biosimilar?<\/h3>\n\n\n\n

      An ADC biosimilar sponsor has to demonstrate similarity across three linked components (antibody, linker, payload) and their conjugation chemistry, including drug-to-antibody ratio distribution and conjugation-site pattern, on top of every characterization study a plain antibody biosimilar already requires [1].<\/p>\n\n\n\n

      What is drug-to-antibody ratio (DAR) and why does it matter for biosimilarity?<\/h3>\n\n\n\n

      DAR is the average number of cytotoxic payload molecules attached to each antibody in an ADC batch. It is a distribution, not a single value, and a biosimilar candidate has to match both the average and the shape of that distribution, since skewed DAR can change either toxicity or efficacy [42][44].<\/p>\n\n\n\n

      Who owns the intellectual property behind an ADC’s linker technology?<\/h3>\n\n\n\n

      It varies by molecule and is frequently not the same company that owns the antibody patents. Seagen has licensed its auristatin linker-payload platform to multiple companies including Astellas and Genmab, and separately sued Daiichi Sankyo over Enhertu’s linker, despite AstraZeneca and Daiichi Sankyo owning the Enhertu antibody and payload patents themselves [11][18].<\/p>\n\n\n\n

      What happened in the Seagen v. Daiichi Sankyo Enhertu patent case?<\/h3>\n\n\n\n

      Seagen won a $41.8 million jury verdict in April 2022 over Enhertu’s linker patent, plus an 8% running royalty. The USPTO invalidated the patent in January 2024. The Federal Circuit then reversed the jury verdict outright on December 2, 2025, holding the patent’s claims lacked adequate written description, and dismissed Seagen’s PTAB appeal as moot. The dispute concluded in March 2026 [4][15][19][21].<\/p>\n\n\n\n

      Is there any approved biosimilar version of an ADC anywhere in the world?<\/h3>\n\n\n\n

      India’s Drugs Controller General approved Zydus Cadila’s Ujvira, described as a “similar biological” of Kadcyla, in 2021. It has not been recognized by FDA or EMA, whose biosimilar frameworks require a more extensive comparative evidence package [1].<\/p>\n\n\n\n

      Does an ADC’s BPCIA exclusivity period work the same way as for other biologics?<\/h3>\n\n\n\n

      Yes. ADCs are licensed under Section 351(a) of the Public Health Service Act and receive the same twelve-year reference product exclusivity as any other biologic under the BPCIA [71][74].<\/p>\n\n\n\n

      Can an ADC be withdrawn from the market and later reapproved?<\/h3>\n\n\n\n

      Yes, and it has happened twice. Mylotarg was withdrawn in 2010 after a confirmatory trial showed excess fatal toxicity and reapproved in 2017 at a lower dose. Belantamab mafodotin (Blenrep) was withdrawn in March 2023 after its confirmatory trial missed its endpoint and reapproved in October 2025 in a new combination regimen [1][51][54][56].<\/p>\n\n\n\n

      What should brand teams and biosimilar developers track to anticipate ADC biosimilar competition?<\/h3>\n\n\n\n

      Linker and conjugation-chemistry patents, not just antibody composition-of-matter patents, since ADC patent litigation has concentrated there, along with any published nonclinical or clinical biosimilarity data on the specific ADC in question, since publications like the December 2025 HS630 study are the earliest public signal of real biosimilar development activity. DrugPatentWatch tracks patent and exclusivity data across these separate ADC patent families.<\/p>\n\n\n\n

      References<\/h2>\n\n\n\n
        \n
      1. Easton, R. L. (2023). Biosimilar antibody drug conjugates: considerations of higher order structure and aggregation. Generics and Biosimilars Initiative Journal<\/em>, 12(3), 108-112. https:\/\/gabi-journal.net\/biosimilar-antibody-drug-conjugates-considerations-of-higher-order-structure-and-aggregation.html<\/li>\n\n\n\n
      2. Fierce Pharma. (2022, April 15). Seagen scores $41.8M in Enhertu patent case against Daiichi Sankyo. https:\/\/www.fiercepharma.com\/pharma\/seagen-awarded-418-million-royalties-enhertu-patent-case-against-daiichi-sankyo<\/li>\n\n\n\n
      3. Precision Medicine Online. (2022, April 11). Jury in Enhertu Patent Infringement Case Orders Daiichi Sankyo to Pay $41.8M in Damages. https:\/\/www.precisionmedicineonline.com\/business-news\/jury-enhertu-patent-infringement-case-orders-daiichi-sankyo-pay-418m-damages<\/li>\n\n\n\n
      4. Daiichi Sankyo. (2025, December 3). Federal Circuit Court of Appeals Vacates Infringement Judgment and Damages Award in Dispute between Daiichi Sankyo and Seagen. https:\/\/daiichisankyo.us\/press-releases\/-\/article\/federal-circuit-court-of-appeals-vacates-infringement-judgment-and-damages-award-in-dispute-between-daiichi-sankyo-and-seagen<\/li>\n\n\n\n
      5. Fed Circuit Blog. (2025, December 2). Opinions & Orders – December 2, 2025. https:\/\/fedcircuitblog.com\/2025\/12\/02\/opinions-orders-december-2-2025\/<\/li>\n\n\n\n
      6. Daiichi Sankyo. (2026, March 10). Patent Dispute Between Daiichi Sankyo and Seagen has Concluded. https:\/\/daiichisankyo.us\/press-releases\/-\/article\/patent-dispute-between-daiichi-sankyo-and-seagen-has-concluded<\/li>\n\n\n\n
      7. McKee, C., Chapman, C., & Bayley, C. (2023). SDE-100 a stochastic cysteine linked vedotin ADC: assessing comparability of higher order structure using multiple orthogonal analytical approaches [Poster]. 13th World ADC, London.<\/li>\n\n\n\n
      8. SEER Cancer.gov. Ado-trastuzumab emtansine (Kadcyla). https:\/\/seer.cancer.gov\/seertools\/seerrx\/rx\/53c44af3102c1290262dbdea<\/li>\n\n\n\n
      9. Crystal Biosolutions. (2026, March 10). What Does the FDA Require to Approve a Monoclonal Antibody Biosimilar in 2026? https:\/\/www.crystalbiosolutions.com\/news-and-events\/what-does-the-fda-require-to-approve-a-monoclonal-antibody-biosimilar-in-2026<\/li>\n\n\n\n
      10. PatSnap. (2026, June 5). Daiichi Sankyo & AstraZeneca v. Seagen | Enhertu ADC Patent Litigation. https:\/\/www.patsnap.com\/resources\/blog\/litigation\/daiichi-sankyo-astrazeneca-v-seagen-enhertu-adc-patent-litigation-patsnap\/<\/li>\n\n\n\n
      11. Precision Medicine Online. (2022, April 11). [cross-reference: Seagen v. Daiichi Sankyo complaint details]. https:\/\/www.precisionmedicineonline.com\/business-news\/jury-enhertu-patent-infringement-case-orders-daiichi-sankyo-pay-418m-damages<\/li>\n\n\n\n
      12. Precision Medicine Online. (2022, April 11). Jury in Enhertu Patent Infringement Case Orders Daiichi Sankyo to Pay $41.8M in Damages. https:\/\/www.precisionmedicineonline.com\/business-news\/jury-enhertu-patent-infringement-case-orders-daiichi-sankyo-pay-418m-damages<\/li>\n\n\n\n
      13. BioPharma Dive. (2022, August 12). Seagen loses arbitration case to Daiichi Sankyo over cancer drug technology. https:\/\/www.biopharmadive.com\/news\/seagen-daiichi-sankyo-arbitration-ruling-adc\/629620\/<\/li>\n\n\n\n
      14. Petrie-Flom Center, Harvard Law School. New Resource: BPCIA Legislative History Documents. https:\/\/petrieflom.law.harvard.edu\/2016\/07\/08\/new-resource-bpcia-legislative-history-documents\/<\/li>\n\n\n\n
      15. Seagen Inc. Form 10-Q, FY2023 (Q1). U.S. Securities and Exchange Commission. https:\/\/www.sec.gov\/Archives\/edgar\/data\/1060736\/000106073623000032\/sgen-20230331.htm<\/li>\n\n\n\n
      16. Polsinelli. (2026, January 14). Federal Circuit Strikes Life Sciences Patent Over Insufficient Disclosure of Particular Species. https:\/\/www.polsinelli.com\/publications\/federal-circuit-life-sciences-patent-disclosure<\/li>\n\n\n\n
      17. BusinessWire. (2023, March 1). ImmunoGen Reports Recent Progress and 2022 Financial Results. https:\/\/www.businesswire.com\/news\/home\/20230301005398\/en<\/li>\n\n\n\n
      18. BioSpace. (2024, January 17). USPTO Rules Against Seagen in ADC Patent Battle with Daiichi Sankyo. https:\/\/www.biospace.com\/uspto-rules-against-seagen-in-adc-patent-battle-with-daiichi-sankyo<\/li>\n\n\n\n
      19. BioSpace. (2024, January 17). [same as above].<\/li>\n\n\n\n
      20. Seagen Inc. Form 10-Q, FY2023 (Q3). U.S. Securities and Exchange Commission. https:\/\/www.sec.gov\/Archives\/edgar\/data\/1060736\/000106073623000047\/sgen-20230930.htm<\/li>\n\n\n\n
      21. Daiichi Sankyo. (2026, March 10). Patent Dispute Between Daiichi Sankyo and Seagen has Concluded. https:\/\/daiichisankyo.us\/press-releases\/-\/article\/patent-dispute-between-daiichi-sankyo-and-seagen-has-concluded<\/li>\n\n\n\n
      22. Daiichi Sankyo. (2025, December 3). Federal Circuit Court of Appeals Vacates Infringement Judgment and Damages Award. https:\/\/www.daiichisankyo.com\/files\/news\/pressrelease\/pdf\/202512\/20251203_E.pdf<\/li>\n\n\n\n
      23. MarketScreener. (2026, March 10). Daiichi Sankyo: Patent Dispute Between Daiichi Sankyo and Seagen has Concluded. https:\/\/www.marketscreener.com\/news\/daiichi-sankyo-patent-dispute-between-daiichi-sankyo-and-seagen-has-concluded-ce7e5fdedd8bf125<\/li>\n\n\n\n
      24. PatSnap. (2026, June 5). [cross-reference]. https:\/\/www.patsnap.com\/resources\/blog\/litigation\/daiichi-sankyo-astrazeneca-v-seagen-enhertu-adc-patent-litigation-patsnap\/<\/li>\n\n\n\n
      25. Daiichi Sankyo. (2025, December 3). Press release PDF. https:\/\/www.daiichisankyo.com\/files\/news\/pressrelease\/pdf\/202512\/20251203_E.pdf<\/li>\n\n\n\n
      26. Fed Circuit Blog. (2025, December 2). Opinions & Orders – December 2, 2025. https:\/\/fedcircuitblog.com\/2025\/12\/02\/opinions-orders-december-2-2025\/<\/li>\n\n\n\n
      27. Polsinelli. (2026, January 14). Federal Circuit Strikes Life Sciences Patent Over Insufficient Disclosure of Particular Species. https:\/\/www.polsinelli.com\/publications\/federal-circuit-life-sciences-patent-disclosure<\/li>\n\n\n\n
      28. AstraZeneca PLC. Form 6-K, FY2026. U.S. Securities and Exchange Commission. https:\/\/www.sec.gov\/Archives\/edgar\/data\/901832\/000165495426001073\/a3234s.htm<\/li>\n\n\n\n
      29. IPWatchdog. (2025, December 2). CAFC Kills Cancer Treatment Patent Claims Due to Lack of Written Description, Enablement. https:\/\/ipwatchdog.com\/2025\/12\/02\/cafc-kills-cancer-treatment-patent-claims-lack-written-description-enablement\/<\/li>\n\n\n\n
      30. U.S. Food and Drug Administration. FDA approves ado-trastuzumab emtansine for early breast cancer. https:\/\/www.fda.gov\/drugs\/resources-information-approved-drugs\/fda-approves-ado-trastuzumab-emtansine-early-breast-cancer<\/li>\n\n\n\n
      31. Kondrashov, A., Sapkota, S., Sharma, A., Riano, I., Kurzrock, R., & Adashek, J. J. (2023). Antibody-Drug Conjugates in Solid Tumor Oncology: An Effectiveness Payday with a Targeted Payload. Pharmaceutics<\/em>, 15(8), 2160.<\/li>\n\n\n\n
      32. U.S. Food and Drug Administration. FDA grants accelerated approval to belantamab mafodotin-blmf for multiple myeloma. https:\/\/www.fda.gov\/drugs\/resources-information-approved-drugs\/fda-granted-accelerated-approval-belantamab-mafodotin-blmf-multiple-myeloma<\/li>\n\n\n\n
      33. U.S. Food and Drug Administration. FDA approves belantamab mafodotin-blmf for relapsed or refractory multiple myeloma. https:\/\/www.fda.gov\/drugs\/resources-information-approved-drugs\/fda-approves-belantamab-mafodotin-blmf-relapsed-or-refractory-multiple-myeloma<\/li>\n\n\n\n
      34. Seeking Alpha. (2025, October 23). GSK wins FDA approval of Blenrep for multiple myeloma. https:\/\/seekingalpha.com\/news\/4507994-gsk-wins-fda-approval-blenrep-multiple-myeloma<\/li>\n\n\n\n
      35. Jiang, H., & Che, J. (2025). Nonclinical evaluation of HS630, a proposed biosimilar of trastuzumab emtansine: affinity, pharmacokinetics, and immunogenicity. Frontiers in Pharmacology<\/em>, 16, 1698727. https:\/\/doi.org\/10.3389\/fphar.2025.1698727<\/li>\n\n\n\n
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      43. BusinessWire. (2023, March 1). ImmunoGen Reports Recent Progress and 2022 Financial Results (Vertex license agreement). https:\/\/www.businesswire.com\/news\/home\/20230301005398\/en<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"

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