Genmab, AbbVie and Epcoritamab

 

From Lab Curiosity to $3.9 Billion Blockbuster: The Remarkable Story Behind Epcoritamab

In the world of cancer treatment, breakthrough drugs rarely emerge from a single eureka moment. More often, they arise from a cascade of discoveries spanning decades, involving dozens of scientists, and requiring the kind of corporate boldness that can stake billions on a molecular hunch. The story of epcoritamab—now marketed as Epkinly—is exactly that kind of tale. It begins with an obscure observation about how antibodies naturally behave in our bloodstream and culminates in one of the largest oncology deals ever signed for a drug that hadn't yet reached Phase 3 trials.

A Strange Behavior in Human Blood

The foundation for epcoritamab was laid in 2007, when researchers at Genmab, a Danish biotech company, made a peculiar observation about a class of human antibodies called IgG4. Unlike most antibodies that remain structurally stable once formed, IgG4 antibodies were found to engage in something called "Fab-arm exchange"—essentially swapping pieces of themselves with other IgG4 molecules circulating in the blood. This wasn't a defect; it appeared to be how the body naturally dampens immune responses.

The scientific team, led by Janine Schuurman and Paul Parren, published their findings in Science. They identified the precise molecular features that enable this exchange: specific amino acid residues in the hinge region and the CH3 domain of the antibody that make the connections between its halves more flexible than those of other antibody types. It was elegant biology, but it also sparked an idea. If antibodies could naturally swap arms, could scientists harness this process to create therapeutic molecules with two different targeting capabilities—bispecific antibodies?

The Bispecific Antibody Problem

For years, pharmaceutical scientists had dreamed of creating bispecific antibodies—molecules that could simultaneously grab onto two different targets. The appeal was obvious: you could design one arm to latch onto a cancer cell and the other to recruit a killer T-cell. The problem was manufacturing them. Traditional approaches led to a nightmarish mixture of mispaired antibody chains. If you tried to produce two different antibodies in the same cell, their heavy and light chains would randomly combine, creating mostly useless hybrid molecules.

Amgen had found a workaround with their BiTE (bispecific T-cell engager) platform, which used only small antibody fragments linked together. This avoided the mispairing issue but created new problems: BiTE molecules were tiny—about 55 kilodaltons compared to a normal antibody's 150—and were cleared from the bloodstream in roughly two hours. Patients receiving blinatumomab, the first approved BiTE, had to carry portable pumps delivering continuous intravenous infusions around the clock for weeks at a time.

Engineering Nature's Trick

Genmab's researchers realized they could turn the IgG4 Fab-arm exchange from a biological curiosity into a manufacturing process. The key was control. Natural Fab-arm exchange happens continuously and unpredictably. Genmab needed a system where two different antibodies could be made separately, then induced to swap arms just once to create a stable bispecific product.

The breakthrough came through years of meticulous mutagenesis work. Aran Labrijn, working with Schuurman and Parren, systematically tested mutations in the CH3 domain—the region where antibody halves connect. They discovered that two specific mutations, F405L and K409R, worked like complementary puzzle pieces. When introduced into two different parent antibodies, these mutations weakened the homodimer connections enough that, under mild reducing conditions, the antibody halves would separate. But once mixed together, the complementary mutations caused the halves to preferentially pair with partners carrying the opposite mutation, locking in the bispecific configuration.

The process, which Genmab dubbed "DuoBody," was remarkably efficient—routinely exceeding 95% yield. Each parent antibody required only a single point mutation, and the controlled exchange process was compatible with standard pharmaceutical manufacturing. When the team published their methodology in PNAS in 2013, it represented a practical solution to a problem that had stymied the field for decades.

Building a Lymphoma Killer

With the DuoBody platform validated, Genmab turned to designing therapeutics. One of their most ambitious projects targeted B-cell lymphomas using the CD3×CD20 approach. The strategy: create a bispecific antibody with one arm that binds CD20, a protein abundantly expressed on malignant B-cells, and another arm that grabs CD3, a signaling molecule on T-cells. By physically bridging cancer cells and killer T-cells, the antibody would redirect the immune system to destroy tumors.

Genmab had deep institutional knowledge of CD20. The company had developed ofatumumab, an anti-CD20 antibody that recognized a unique membrane-proximal epitope. For epcoritamab, they leveraged their existing 7D8 antibody clone—the same molecule underlying ofatumumab—as the CD20-binding arm. This wasn't just convenience; preclinical testing showed that this particular anti-CD20 component outperformed alternatives in head-to-head comparisons, inducing more potent tumor cell killing than other CD20 antibodies or even bispecifics using different B-cell targets entirely.

Critical design decisions went beyond target selection. Genmab engineered specific mutations into epcoritamab's Fc region to silence its effector functions—preventing the antibody itself from triggering immune reactions through complement activation or antibody-dependent cellular cytotoxicity. This ensured that tumor cell killing would occur through the intended T-cell mechanism rather than through uncontrolled immune activation that could harm patients.

The Subcutaneous Gambit

Perhaps the most consequential decision in epcoritamab's development was the route of administration. T-cell engagers are notorious for causing cytokine release syndrome—a potentially dangerous inflammatory response that occurs when large numbers of T-cells activate simultaneously. This is particularly severe with intravenous dosing, where the drug floods the body and activates T-cells en masse.

Genmab's preclinical studies in cynomolgus monkeys suggested a solution. When epcoritamab was administered subcutaneously rather than intravenously, it was absorbed gradually from the injection site. This slower absorption blunted the cytokine spike while still achieving complete and durable B-cell depletion. The finding opened the possibility of outpatient administration—a dramatic departure from the hospital-bound continuous infusions required by earlier T-cell engagers.

The company launched its first-in-human trial in June 2018, pursuing subcutaneous dosing from the start. They also implemented a step-up dosing strategy, starting patients at very low doses to prime their immune systems before escalating to therapeutic levels. This approach proved remarkably safe: throughout the entire dose-escalation phase, no dose-limiting toxicities were observed.

The Data That Launched a $3.9 Billion Deal

The clinical results, first unveiled at the American Society of Clinical Oncology's 2020 virtual meeting, exceeded expectations. Among heavily pretreated lymphoma patients—many of whom had failed four or more prior therapies—epcoritamab induced responses in roughly 87% of evaluable patients. Multiple patients achieved complete remissions, meaning no detectable cancer remained. The responses were rapid, deep, and occurred across different lymphoma subtypes.

These results arrived in a competitive landscape. Roche had reported the previous year that their own CD20×CD3 bispecific, mosunetuzumab, achieved a 54% response rate in similar patients. While cross-trial comparisons are imperfect, epcoritamab's numbers suggested it might be best-in-class—and with the added convenience of subcutaneous rather than intravenous dosing.

Within weeks of the ASCO presentation, pharmaceutical giant AbbVie announced a sweeping partnership with Genmab. The headline numbers were staggering: $750 million upfront and up to $3.15 billion in potential milestone payments. The deal covered not just epcoritamab but also two earlier-stage bispecific programs and a research collaboration to develop additional candidates combining Genmab's DuoBody platform with AbbVie's antibody-drug conjugate technology.

For AbbVie, the investment reflected both the promise of a single drug and the potential of an entire platform. The company was building a hematologic oncology franchise around venetoclax and ibrutinib; epcoritamab offered a complementary mechanism to address B-cell cancers through immune activation rather than direct cytotoxicity. The subcutaneous administration promised commercial advantages in terms of convenience and healthcare resource utilization. And the DuoBody platform itself—validated by epcoritamab's results—represented a factory for future bispecific candidates.

From Discovery to Patients

The epcoritamab story illustrates how modern drug development builds on layers of fundamental science. It began with curiosity about why one class of antibodies behaves differently from others in human blood. That observation led to an engineering solution for a longstanding manufacturing problem. The solution enabled a new therapeutic design that combined existing target knowledge with novel delivery approaches. And the clinical results justified one of the largest oncology bets in pharmaceutical history.

Today, epcoritamab—marketed as Epkinly—has completed pivotal trials and is reaching patients with B-cell lymphomas who have exhausted other options. Its journey from Genmab's laboratories to clinical use took roughly fifteen years, involved hundreds of scientists and clinicians, and required the kind of sustained investment that only became possible once early data demonstrated its potential.

The story continues. Genmab and AbbVie are exploring epcoritamab in earlier treatment lines, in combination with other agents, and in different B-cell malignancies. The DuoBody platform has spawned additional clinical candidates. And the broader field of T-cell engagers—once limited by the logistical nightmare of continuous infusions—now includes multiple full-length bispecific antibodies pursuing the convenient administration that epcoritamab pioneered.

In the end, the transformation of a peculiar molecular observation into a potentially life-saving therapy represents modern pharmaceutical science at its most effective: rigorous, iterative, collaborative, and ultimately focused on solving real problems for patients with few other options.

Further reading: For technical details on the DuoBody platform, see Labrijn et al., PNAS 2013. For epcoritamab's preclinical characterization, see Engelberts et al., EBioMedicine 2020.

Addendum: Clarifications and Context

This addendum provides brief clarifications on several points where the original discussion used approximate figures, interpretive language, or imprecise citations. Its purpose is to tighten references (for example, around IgG4 Fab-arm exchange publications), qualify cross-trial comparisons (such as epcoritamab versus mosunetuzumab), and distinguish regulatory labeling from rhetorical phrases about patients having “exhausted” treatment options. These notes do not change the overall narrative about epcoritamab/Epkinly or DuoBody technology, but they indicate where readers should treat specific numbers or “best-in-class” statements as informed interpretation rather than hard, trial-level fact.

• IgG4 Fab-arm exchange citation: The foundational in vivo Fab-arm exchange work from the Genmab group was published in Nature Biotechnology (2009) and related journals, not in Science in 2007, although a key early mechanistic paper on dynamic Fab-arm exchange in IgG4 did appear in Science in 2007; the text should correct the specific journal/year attribution while keeping the scientific point intact.


• Subcutaneous monkey data: Preclinical studies support that subcutaneous epcoritamab produces slower absorption, lower cytokine peaks, and robust B-cell depletion, but the phrase “complete and durable” overgeneralizes; published data specify effects over defined follow-up periods rather than absolute, indefinite depletion in all animals.


• Start of first-in-human trial: The first-in-human, subcutaneous, step-up–dosed phase 1/2 trial (NCT03625037) did begin enrollment in 2018 and used the described priming/intermediate/full-dose schema, but specifying “June 2018” goes beyond what most public summaries state; readers should treat the exact month as approximate unless they consult trial startup records.


• “No dose-limiting toxicities” nuance: Dose-escalation publications and ASH/ASCO reports for epcoritamab dose finding state that no dose-limiting toxicities were observed and a 48 mg subcutaneous dose was selected as the recommended phase 2 dose, so the statement is directionally correct, but this applies specifically to the defined dose-escalation cohorts, not to all subsequent combination or expansion studies.


• ASCO 2020 “~87%” response rate: Early epcoritamab data in relapsed/refractory B-cell lymphoma showed high overall and complete response rates, but the exact percentage varies by histology and cohort; quoting “~87%” as a single figure compresses multiple strata and should be understood as an approximate narrative summary rather than a precise pooled statistic.


• Mosunetuzumab 54% response framing: Mosunetuzumab trials in relapsed/refractory follicular lymphoma report overall response rates around 80% with complete response rates about 60%; a 54% figure corresponds to specific subsets or analytic cuts, so any single percentage should be tied to its exact cohort and endpoint rather than treated as a universal benchmark.


• “Best-in-class” language: Comparing epcoritamab’s reported response rates with selected mosunetuzumab numbers to suggest “best-in-class” is interpretive; cross-trial comparisons are confounded by differing designs, lines of therapy, and patient risk profiles, so such claims should be presented as hypothesis or opinion, not as settled fact.


• “Exhausted other options” phrasing: The approved indications for epcoritamab/Epkinly focus on adults with relapsed or refractory B-cell lymphomas after at least two prior systemic therapies, which often implies limited remaining standard options but does not mean every patient has literally exhausted all possible treatments; the phrase should be read as rhetorical emphasis rather than a regulatory category.