Antibody-Drug Conjugates: Why Big Pharma Is Betting Billions on the Hardest Drug to Build

Prepared by Richstorm.co

Key Takeaways

• Antibody-drug conjugates (ADCs) combine an antibody, a chemical linker, and a toxic payload, making them far more complex to build than a typical protein-based drug.

• That complexity has not stopped pharmaceutical companies from spending tens of billions of dollars on ADC acquisitions and licensing deals since 2023.

• Japan's Daiichi Sankyo currently holds the strongest position in the field, reinforced by multibillion-dollar partnerships with AstraZeneca and Merck.

• A newer design called a bispecific ADC, meant to stop tumors from evading treatment, has produced the world's first approved drug of its kind, developed in China and licensed to Bristol Myers Squibb.

A Guided Missile for Cancer Cells

Picture a regular chemotherapy drug as a bomb dropped from high altitude. It kills cancer cells, but it also damages a lot of healthy tissue on the way down, which is why chemotherapy is so hard on the body.

An antibody-drug conjugate works more like a guided missile. It starts with an antibody, a protein trained to recognize one specific marker on the surface of a cancer cell. That antibody carries a much more powerful, more toxic payload than chemotherapy could ever use safely on its own, connected by a chemical linker. The idea is simple: let the antibody do the aiming, so the toxic payload only goes off once it reaches the cancer cell, rather than spreading through the entire body.

When it works, this combination hits the tumor harder while sparing more of the healthy tissue around it. That is the entire value proposition of the modality, and it is why ADCs have become one of the most closely watched categories in oncology.

Why This Is One of the Hardest Drugs to Build

Building an ADC means building three different things at once and getting all three to work together. The antibody has to find the right target. The payload has to be strong enough to kill a cancer cell but stable enough not to leak out early. The linker has to hold everything together in the bloodstream, then release the payload at exactly the right moment once it reaches the tumor.

Each of those pieces is manufactured differently. The antibody is a biologic, grown in living cells. The payload and linker are made using much more traditional chemistry, often involving compounds so toxic that only a limited number of manufacturing facilities in the world are equipped to handle them safely. Then all of it has to be joined together in a separate step, and the leftover unattached antibody and excess toxin have to be filtered back out before the drug is pure enough to give to a patient.

A standard antibody drug is essentially one manufacturing process. An ADC is three manufacturing processes stitched into one, which is a meaningful part of why the modality carries a higher cost and a higher failure rate than most other protein-based drugs.

Why Pharma Keeps Betting Billions Anyway

The short answer is that when an ADC works, it can work extremely well, and the underlying chemistry can often be reused across many different drugs. A company that develops one successful linker-and-payload combination is not just building one drug. It is building a platform that can be attached to different antibodies aimed at different cancers, which is a much more valuable and durable asset than a single approved product.

The dollar figures reflect that thinking. Pfizer paid roughly $43 billion to acquire Seagen and its ADC portfolio in 2023. AbbVie paid roughly $10 billion for ImmunoGen the same year. Merck committed up to $22 billion for rights to a handful of Daiichi Sankyo's ADC candidates. These are not small bets on unproven science. They are large companies concluding it is cheaper to buy their way into a working platform than to build one from scratch.

The Current Leader: Daiichi Sankyo's DXd Platform

If there is a clear leader in ADCs today, it is Japan's Daiichi Sankyo, largely on the strength of its DXd technology. The clearest evidence is Enhertu, a Daiichi Sankyo and AstraZeneca drug that has generated close to $5 billion in annual sales and has shown meaningfully better results than older-generation ADCs in head-to-head studies for HER2-positive breast cancer.

What makes this more than a one-drug success story is that Daiichi Sankyo has reused the same underlying chemistry across several other cancer drugs aimed at completely different targets. That reuse is exactly the kind of platform value described above, and it is why two of the largest pharmaceutical companies in the world, AstraZeneca and Merck, have each paid billions of dollars to license pieces of it rather than compete against it directly.

The Next Wave: Bispecific ADCs and a Milestone From China

The biggest weakness of a standard ADC is that cancer can adapt. If a tumor stops producing the one marker the antibody was designed to find, the drug can no longer locate the cell, and the tumor can grow back resistant to a treatment that used to work.

A newer design, called a bispecific ADC, is built to close that gap by training the antibody to recognize two different markers instead of one. Even if a tumor loses or hides one marker, the drug can still find the cancer cell using the other. It is a more difficult and more expensive drug to build, but the payoff, if the science holds up, is a treatment that stays effective for longer and can be used against a wider range of tumors.

This is no longer just a theory. A bispecific ADC called izalontamab brengitecan, developed by China's Sichuan Biokin Pharmaceutical and its subsidiary SystImmune, recently became the first bispecific ADC of any kind to receive regulatory approval, clearing China's national drug regulator for a hard-to-treat form of nasopharyngeal cancer. It has also shown positive late-stage results in triple-negative breast cancer and esophageal cancer, and it holds a Breakthrough Therapy Designation from the U.S. Food and Drug Administration for a form of lung cancer. Bristol Myers Squibb has licensed the rights to develop and sell the drug outside China, the same kind of platform-licensing arrangement that built Daiichi Sankyo's position in the field.

China has become one of the most active sources of new ADC science generally, with several companies advancing bispecific and other next-generation ADC designs through clinical trials. For global pharmaceutical companies, that innovation has become a meaningful sourcing ground for new technology, mirrored by a wave of licensing deals between Western pharma and China-based biotech companies over the past year.

The Investor Takeaway

The story of ADCs is really a story about who owns the underlying chemistry, not who owns any single drug. Daiichi Sankyo built the strongest position in the current generation of ADCs by developing a linker-and-payload platform that keeps working across different cancer targets, then licensing it out for billions of dollars. Bispecific ADCs look like the next stage of that same story, with China-based biotech companies currently leading the science and global pharmaceutical companies, from Bristol Myers Squibb to AstraZeneca to Merck, providing the capital and commercial reach to bring it to patients worldwide.

For an industry built on precision, the biggest wins so far have gone to the companies that figured out how to make one good chemistry platform work more than once.