FDA Approves Bristol Myers’ Zenbexus, the First of a New Class of Myeloma Drugs

The FDA has approved Zenbexus (iberdomide) from Bristol Myers Squibb — the first drug in a new class called CELMoDs — for multiple myeloma.

Granted accelerated approval on August 13, 2026, it is cleared in a three-drug combination with daratumumab (plus hyaluronidase) and dexamethasone — abbreviated ZDd — for adults with at least one prior line of therapy, meaning it can be used as early as first relapse.

What a molecular glue does

CELMoD stands for cereblon E3 ligase modulator, and the mechanism is worth understanding because it differs fundamentally from how most drugs work.

Conventional drugs inhibit: they bind a protein and block its function, and the effect lasts only while the drug is bound. That requires the target to have a binding pocket a small molecule can occupy, which many disease-driving proteins do not.

A molecular glue instead redirects the cell’s disposal machinery. Cells continuously tag unwanted proteins for destruction using enzymes called E3 ligases, and a glue binds one of those ligases in a way that changes which proteins it recognises — causing it to tag and destroy a protein it would normally ignore.

The consequence is degradation rather than inhibition. The protein is eliminated rather than temporarily blocked, which means the effect outlasts the drug’s presence and works on targets with no druggable pocket.

The accidental discovery behind it

CELMoDs build on the mechanism of older drugs including lenalidomide, and that lineage has a remarkable history.

Those drugs derive from thalidomide, notorious for causing birth defects when prescribed in pregnancy in the 1950s and 60s, and later found to be effective in myeloma. For decades nobody knew why it worked, and the mechanism — binding cereblon and redirecting protein degradation — was only established relatively recently.

That explanation converted an empirical observation into a designable principle. CELMoDs are the result: molecules engineered from the outset to exploit the mechanism rather than stumbled upon, and designed to be more potent than their predecessors.

Why the combination matters

The ZDd regimen pairs Zenbexus with daratumumab, which targets CD38 on myeloma cells, and dexamethasone.

Myeloma is treated with combinations almost universally, because the disease reliably develops resistance to single agents. Attacking through several mechanisms simultaneously delays that, and the standard approach uses three or four drugs from different classes.

Building a new agent onto an established backbone is also the practical route to adoption: clinicians already use daratumumab-based regimens, so adding a component is a smaller change than replacing a strategy.

Why first relapse is the significant part of the label

Approval after at least one prior line is a broad indication, and it matters more than it might appear.

New myeloma drugs typically enter in heavily pretreated patients — those who have exhausted several regimens — where the unmet need is greatest and the regulatory bar is most achievable. Moving earlier takes years and additional trials.

Being available at first relapse means treating patients who are healthier, whose disease has accumulated fewer resistance mechanisms, and who have more remaining treatment options afterwards. Drugs generally work better earlier, and this label allows that from the outset.

Why myeloma keeps absorbing new drugs

Multiple myeloma remains incurable, and patients typically cycle through many treatments as the disease returns.

That pattern creates persistent demand for new mechanisms. A patient may go through five or more lines of therapy over years, each providing a remission that eventually ends, and each new class extends the sequence rather than replacing it.

It also means drugs with genuinely different mechanisms are worth more than incremental improvements on existing ones, since cross-resistance is the limiting factor.

What accelerated approval implies

Continued approval may hinge on confirmatory data, which is standard for this pathway and worth noting rather than alarming.

Targeted protein degradation as a broader field

CELMoDs belong to a wider effort that has become one of the more active areas in drug discovery, and the class distinction is worth understanding.

Molecular glues like these are small molecules that fit into the interface between an E3 ligase and a target protein, effectively creating an interaction that would not otherwise occur. They are compact and drug-like, and they were mostly discovered by accident — the thalidomide derivatives being the canonical example.

The alternative approach uses bifunctional degraders: larger molecules with one end binding the target and the other binding an E3 ligase, physically tethering them together. These can be designed rationally for a chosen target, and their size makes them harder to formulate and deliver.

Both aim at the same prize — reaching the large fraction of disease-relevant proteins that lack a pocket a conventional inhibitor could occupy. A first approval in either category validates the mechanism for regulators and investors, which is part of why this approval matters beyond myeloma.

BMS has more CELMoDs behind it: mezigdomide is under FDA review with a decision expected in 2027. A company with a class-first approval and a second candidate following is in a strong position — the class validation transfers, and prescriber familiarity with the first eases adoption of the second. Regulatory news, not medical advice.