Bristol Myers Squibb has secured accelerated approval from the United States Food and Drug Administration for Zenbexus, making iberdomide the first therapy from the company’s CELMoD class to reach the US market. The oral drug was approved as part of a three-drug regimen with subcutaneous daratumumab and hyaluronidase-fihj plus dexamethasone for adults with multiple myeloma who have received at least one previous line of treatment that included a proteasome inhibitor and an immunomodulatory agent. The decision places the regimen within the early relapse setting rather than reserving it for patients who have exhausted several advanced therapies. It also introduces a new regulatory precedent because the accelerated approval was based on minimal residual disease-negative complete response rather than mature progression-free survival data.
The supporting EXCALIBER-RRMM Phase 3 trial found that 41% of patients receiving the iberdomide combination achieved an MRD-negative complete response, compared with 21% of those treated with daratumumab, bortezomib and dexamethasone. That difference was statistically significant, but the study remains underway to determine whether the deeper responses translate into longer periods without disease progression. Consequently, the approval is both a clinical milestone and an unfinished regulatory test. Zenbexus can now enter practice, but its continued approval may depend on verification of clinical benefit as the survival-related endpoints mature.
What exactly did the FDA approve for relapsed multiple myeloma?
The approved population consists of adults with multiple myeloma who have received at least one prior line of therapy containing both a proteasome inhibitor and an immunomodulatory agent. Zenbexus is not approved as a stand-alone medicine. It must be administered with daratumumab and hyaluronidase-fihj, the subcutaneous formulation marketed as Darzalex Faspro, and dexamethasone.
The recommended Zenbexus dose is 1 mg orally once daily, with or without food, on days one through 21 of each 28-day cycle. Subcutaneous daratumumab and hyaluronidase-fihj is given at 1,800 mg on days one, eight, 15 and 22 during the first two cycles, on days one and 15 during cycles three through six, and on day one from the seventh cycle onward. Dexamethasone is administered orally at either 20 mg or 40 mg on days one, eight, 15 and 22.
Bristol Myers Squibb has described the regimen as an option beginning at first relapse. That description is consistent with the trial’s inclusion of patients who had received one or two previous treatment lines, but the approved label includes an important treatment-history requirement. A patient must already have been exposed to both a proteasome inhibitor and an immunomodulatory medicine. First relapse alone does not automatically establish eligibility if the previous regimen lacked either class.
The trial population also does not represent every patient encountered in relapsed multiple myeloma practice. EXCALIBER-RRMM excluded people whose disease was refractory to previous anti-CD38 antibody treatment and those with disease refractory to bortezomib. Those exclusions matter because daratumumab exposure has moved earlier in the treatment sequence, and an increasing number of patients may relapse after receiving an anti-CD38-containing frontline regimen.
Why does the first CELMoD approval matter scientifically?
Iberdomide belongs to a class commonly described as cereblon E3 ligase modulators, or CELMoDs. These oral compounds bind to cereblon, a component of the cell’s protein disposal machinery, and alter the proteins selected for degradation. Iberdomide promotes the removal of transcription factors including Ikaros and Aiolos, which are involved in myeloma-cell survival and immune regulation.
The result is intended to combine direct activity against malignant plasma cells with stimulation of immune responses. This approach makes iberdomide a logical combination partner for daratumumab, an antibody directed against CD38 on myeloma cells.
Zenbexus represents a new approved class, but it should not be portrayed as biologically disconnected from earlier immunomodulatory drugs. Lenalidomide and pomalidomide also act through cereblon and have been central to multiple myeloma treatment. CELMoD compounds were designed to bind cereblon more efficiently and induce faster or more extensive degradation of selected proteins. The distinction is therefore an evolution of a validated biological mechanism rather than the discovery of an entirely unrelated pathway.
That scientific continuity may help clinicians understand how the drug fits into established combination strategies. At the same time, it raises practical questions about activity after prior immunomodulatory treatment, the influence of cereblon-related resistance mechanisms and whether individual CELMoD compounds can retain efficacy as patients receive more complex frontline regimens.
The approval also strengthens Bristol Myers Squibb’s broader targeted protein degradation platform. Another CELMoD, mezigdomide, is being developed for multiple myeloma and has produced positive Phase 3 progression-free survival results in combination with carfilzomib and dexamethasone. Zenbexus establishes that the platform can support an approved product, but clinical performance across the class will still need to be assessed compound by compound.
How persuasive are the EXCALIBER-RRMM efficacy results?
EXCALIBER-RRMM is a randomized, multicenter, open-label Phase 3 study with a two-stage design. The first stage evaluated multiple iberdomide dose levels and selected the 1 mg dose for further development. Across both stages, 939 patients were randomized.
The primary efficacy analysis supporting accelerated approval involved the first 420 patients assigned to the selected iberdomide dose or the control regimen. Of those, 207 received iberdomide with daratumumab and dexamethasone, while 213 received daratumumab, bortezomib and dexamethasone.
At a median follow-up of 16 months, 41% of patients in the iberdomide group achieved an MRD-negative complete response at any time. The corresponding rate was 21% in the control group. The 20-percentage-point absolute difference was statistically significant, with a p-value below 0.0001.
The result indicates that the Zenbexus regimen produced substantially more deep responses than the bortezomib-containing comparator. MRD testing can detect malignant cells at levels far below those identifiable through conventional clinical response assessments. An MRD-negative complete response therefore represents a more stringent measure than complete response alone.
However, MRD negativity does not establish that every cancer cell has been eliminated. It also does not, by itself, reveal how long the response will last. Patients can become MRD-positive again, and the relationship between an early deep response and long-term benefit may vary according to disease risk, treatment type, testing method and the timing of assessment.
The comparison also requires clinical context. Daratumumab and bortezomib with dexamethasone is an established regimen, but current treatment choices at first relapse extend well beyond that single comparator. Physicians may select carfilzomib-based combinations, pomalidomide-containing regimens, other antibody combinations or advanced immune therapies depending on previous exposure, refractory status, cytogenetic risk, transplant history, frailty and access.
Zenbexus has therefore defeated a relevant active control on the approval endpoint. It has not yet demonstrated superiority over every major contemporary relapse strategy.
Why is the FDA’s use of MRD a regulatory turning point?
Zenbexus is the first treatment approved for relapsed or refractory multiple myeloma on the basis of MRD-negative complete response. That makes the decision relevant beyond iberdomide or Bristol Myers Squibb.
Progression-free survival events can take years to accumulate in multiple myeloma, particularly as combination treatments become more effective. Waiting for mature progression data may delay access to medicines that have already produced unusually deep responses. A validated early marker could shorten development timelines while still requiring manufacturers to demonstrate that the initial biological effect leads to a meaningful clinical outcome.
EXCALIBER-RRMM was structured around that approach. MRD-negative complete response and progression-free survival are dual primary endpoints. The MRD analysis supported accelerated approval, while the continuing progression-free survival assessment is expected to address whether the regimen delays disease worsening or death.
The design is more informative than relying on an uncontrolled response rate from a small, late-line study. Patients were randomized against an active regimen, and the confirmatory endpoint is being collected within the same Phase 3 program. Even so, accelerated approval transfers part of the evidentiary burden into the post-approval period.
If progression-free survival ultimately supports the MRD result, Zenbexus could reinforce the use of deep molecular response as an expedited development tool in myeloma. If the survival outcome is weak or inconsistent, regulators may need to reassess how MRD thresholds, assessment timing and response durability should be applied.
The remaining EXCALIBER-RRMM data will therefore influence more than the conversion of one product from accelerated to traditional approval. They may affect endpoint selection and trial planning across the next generation of multiple myeloma programs.
What safety burden accompanies the Zenbexus combination?
The depth of response comes with substantial hematologic and infectious toxicity. In the Zenbexus safety population, all-grade neutropenia occurred in 90.2% of patients. Grade 3 neutropenia was reported in 30.9%, and Grade 4 neutropenia occurred in 53.4%. Febrile neutropenia affected 5.4%.
Infections, including opportunistic infections, were reported in 78.9% of patients. Grade 3 and Grade 4 infections occurred in 35.8% and 3.4%, respectively, while fatal infections were recorded in 2%. Serious infections affected 40% of patients.
Pneumonia was reported in 34% of the iberdomide group compared with 17% of the control arm. Hypogammaglobulinemia occurred in 24% and 12%, respectively. These differences are clinically relevant because multiple myeloma itself, prior treatments and daratumumab can already compromise immune function.
Overall, 58.3% of Zenbexus-treated patients experienced a serious adverse reaction. Fatal adverse reactions occurred in 4.9%, although that figure includes events with varied potential relationships to treatment and underlying disease. Treatment discontinuation due to adverse reactions was reported in 7.8%.
The prescribing information carries boxed warnings for embryo-fetal toxicity and serious venous and arterial thromboembolism. Venous thromboembolic events occurred in 6.4% of patients despite mandatory prophylaxis in the trial. Deep vein thrombosis was reported in 3.4% and pulmonary embolism in 1.5%. Arterial events occurred in 3.4%, including myocardial infarction in 2% and stroke in 1.5%.
Thromboprophylaxis is recommended, with the specific approach determined by individual risk factors. Clinical use will also require complete blood count monitoring, infection surveillance, possible growth-factor support, dose interruptions and consideration of preventive anti-infective medicines.
Second primary malignancies occurred in 6.9% of patients in the Zenbexus group and 4.9% of those receiving the control regimen at the 16-month median follow-up. Longer observation will be needed to interpret that difference.
Because of the embryo-fetal risk, Zenbexus is available only through a restricted Risk Evaluation and Mitigation Strategy. Prescribers must be certified, pharmacies must meet distribution requirements, and patients must be enrolled and comply with pregnancy testing and contraception provisions.
Where could Zenbexus fit in the crowded multiple myeloma sequence?
The most immediate opportunity is among patients relapsing after a proteasome inhibitor and immunomodulatory agent who remain suitable for daratumumab. The regimen may be particularly attractive when clinicians want to retain a familiar antibody and steroid backbone while replacing bortezomib with an oral protein degrader.
Its place becomes less certain after prior anti-CD38 treatment. Daratumumab is increasingly used in newly diagnosed disease, including transplant-eligible and transplant-ineligible settings. Patients whose disease progresses while receiving daratumumab may not resemble the EXCALIBER-RRMM population, because anti-CD38-refractory disease was excluded.
That limitation does not prohibit treatment under the label, but it reduces the strength of evidence available for a clinically important group. Subgroup analyses examining previous daratumumab exposure, lenalidomide-refractory disease, high-risk cytogenetics, age and renal function will be important for sequencing decisions.
Zenbexus also arrives as BCMA-directed CAR-T therapies and bispecific antibodies move earlier in multiple myeloma care. Those treatments can produce deep responses but bring different burdens, including specialized administration, cytokine release syndrome, neurologic toxicity, infection risk and, for autologous CAR-T therapy, manufacturing time.
The Zenbexus regimen offers an oral core drug and uses a widely adopted subcutaneous antibody. It does not require cell collection or hospitalization solely for administration. However, it is not a low-monitoring regimen. Severe neutropenia, infection risk, thrombosis precautions and REMS obligations could create considerable workload for both specialist centers and community practices.
Will dosing, price and access affect early adoption?
Zenbexus is taken at home for three weeks of every four-week cycle, but the regimen still requires repeated clinic visits for daratumumab injections, particularly during the first six cycles. Laboratory monitoring and supportive care may add further appointments.
Bristol Myers Squibb has set a US list price of approximately $29,500 for a 28-day Zenbexus cycle. That figure covers iberdomide alone, not the complete three-drug regimen, thrombosis prophylaxis, infection management, laboratory monitoring or other supportive care. Actual patient and payer costs will vary according to insurance coverage, discounts and assistance arrangements.
The accelerated approval status may also influence reimbursement review. Payers will be assessing a statistically strong MRD result while progression-free survival remains immature. The active-controlled Phase 3 design provides more evidence than many accelerated approvals have at launch, but uncertainty around long-term benefit has not disappeared.
Strong or moderate CYP3A inhibitors and inducers can alter iberdomide exposure and should generally be avoided or managed through dose adjustment. Patients with severe renal impairment who are not receiving dialysis may also require a reduced dosing schedule. These considerations add another medication-management layer for a population that commonly has comorbidities and extensive treatment histories.
What evidence could determine the long-term value of Zenbexus?
The central question is whether the 41% MRD-negative complete response rate leads to a meaningful progression-free survival advantage. Duration of MRD negativity will also matter. A transient negative result may have different clinical significance from molecular remission sustained across repeated assessments.
Overall survival, quality of life, treatment discontinuation, cumulative infection burden and time to the next therapy will help define the regimen’s full risk-benefit profile. Detailed results are also needed to determine whether the efficacy advantage is consistent across patient subgroups rather than driven by those with more treatment-sensitive disease.
Real-world experience will test the regimen in patients underrepresented or excluded from EXCALIBER-RRMM, particularly those previously treated with daratumumab, older adults with frailty and people with complex infection or thrombosis risks. Serious adverse reactions in the trial became more frequent with age, reaching 74% among patients aged 75 years or older. That observation may be important when selecting a regimen for the older population commonly affected by multiple myeloma.
Zenbexus has already achieved two meaningful milestones. It is the first FDA-approved CELMoD and the first relapsed multiple myeloma therapy cleared using MRD-negative complete response as the accelerated approval endpoint. Whether those milestones translate into a durable treatment advance will depend on the progression-free survival result, the persistence of deep responses and the ability of clinical practices to manage the regimen’s considerable safety burden.
