BMY Zenbexus Doubles MRD-Negative CR in EXCALIBER Data

Bristol Myers Squibb presented the first full readout of the registrational Phase 3 EXCALIBER-RRMM trial at the International Myeloma Society meeting in Glasgow, deepening the case for Zenbexus while leaving the biggest question unanswered. The data were delivered as a late-breaker and published simultaneously in The Lancet Oncology, the first peer-reviewed look at the full dataset behind the drug’s approval.

The study randomized 939 patients with relapsed or refractory multiple myeloma to Zenbexus (iberdomide) plus daratumumab and dexamethasone (ZDd), or daratumumab, bortezomib, and dexamethasone (DVd). In the 420-patient MRD analysis cohort, the minimal residual disease-negative complete response rate was 41.1% versus 20.7% (p<0.0001), roughly doubling the comparator. Overall response was 88.9% versus 76.1%, and complete response or better nearly doubled to 45.9% versus 24.9%. The readout lands in a crowded field, where daratumumab-based regimens, bispecific antibodies, and CAR-T therapies all compete for the same relapsed patients.

Zenbexus is Bristol Myers’ first CELMoD, a cereblon E3 ligase modulator and the designated successor to the aging Revlimid franchise. The oral drug tags disease-driving proteins for degradation, and it won accelerated approval on August 13 as the first multiple myeloma therapy cleared on an MRD-negative CR endpoint. The timing matters: Revlimid faces generic erosion after patent expiry, and roughly 36,000 Americans are diagnosed with multiple myeloma each year, most of whom eventually relapse. Bristol Myers has a second CELMoD, mezigdomide, with its own PDUFA in May 2027, making Zenbexus one leg of a broader protein-degradation push.

The safety profile is the trade investors have to weigh. Grade 3/4 neutropenia hit 84.3% of ZDd patients versus 11.3% on DVd, and Grade 3/4 infections reached 39.2% versus 21.6%, with fatal infections at 2.0% versus 1.5%. In exchange, peripheral sensory neuropathy, the hallmark burden of the bortezomib arm, fell sharply to 12.3% versus 42.6%. For community oncologists the calculus is more myelosuppression requiring G-CSF support versus far less nerve damage to manage.

The open question is progression-free survival, the trial’s second dual primary endpoint. That analysis is still running in a larger confirmatory cohort of about 800 patients, so the conversion from accelerated to full approval rests on data months away. Responses held up across subgroups, including patients refractory to lenalidomide, but deep MRD responses only matter commercially if they translate into longer survival.

What to watch next: the PFS readout. If it confirms a survival edge, Zenbexus becomes a durable Revlimid-successor franchise; if not, it stays a niche play built on response depth. Bristol Myers closed at $62.86 ahead of the presentation.

What It Means for the Trade

The full EXCALIBER-RRMM readout tightens the Zenbexus thesis without settling it. The Lancet Oncology publication and late-breaker podium give the drug scientific credibility in a crowded relapsed-refractory market, and the MRD-negative CR doubling (41.1% versus 20.7%) is the deepest-response argument in its class segment — but the PFS question, the endpoint that converts response depth into commercial durability, is still running in the ~800-patient confirmatory cohort. Until that reads out, the stock trades on belief that depth predicts duration, which myeloma history says is a reasonable but not certain bet.

Strategically, this is a Revlimid-successor trade with a two-engine structure: Zenbexus (PDUFA-serviced since its August 13 accelerated approval) plus mezigdomide, which carries its own PDUFA in May 2027. BMY at $62.86 is pricing Revlimid’s patent-cliff erosion; the CELMoDs are the offsetting growth story. The safety trade-off laid out in the full dataset — 84.3% Grade 3/4 neutropenia and 39.2% Grade 3/4 infections on ZDd versus far less peripheral neuropathy than the bortezomib arm — will shape the prescribing debate. Community oncologists weighing G-CSF-managed myelosuppression against nerve damage have historically tolerated the former better in relapsed patients, but fatal infections at 2.0% keep the question live.

Mechanism and Pipeline Context

Iberdomide is a cereblon E3 ligase modulator — the CELMoD class — that recruits disease-driving proteins to the cell’s own disposal machinery. Cereblon modulation is the mechanism behind Revlimid and Pomalyst, so Bristol Myers is upgrading its own franchise chemistry: iberdomide is a more potent degrader of the same targets (Ikaros and Aiolos) with effects that extend beyond immunomodulation into direct anti-myeloma activity. The ZDd regimen pairs it with daratumumab (the CD38 antibody backbone of modern relapsed myeloma) and dexamethasone, and it won accelerated approval as the first myeloma therapy cleared on an MRD-negative CR endpoint — a regulatory milestone in itself, since it legitimizes response depth as an approval basis.

The competitive set defines the stakes: daratumumab-based regimens, bispecific antibodies, and CAR-T all chase the same relapsed patients, and roughly 36,000 Americans are diagnosed with myeloma each year, nearly all of whom eventually relapse. An oral, once-daily, chemotherapy-free backbone that slots into existing clinic workflows is the convenience edge over the infused and cell-based options.

Risks and What Could Go Wrong

1. The PFS readout disappoints — MRD-negative depth that fails to translate into longer time-to-progression undermines the entire conversion thesis and the accelerated approval with it.
2. Infection burden limits real-world adoption — 39.2% Grade 3/4 infections and 2.0% fatal infections are numbers safety monitors will watch in the confirmatory cohort.
3. Class cannibalization — mezigdomide’s own May 2027 PDUFA could crowd Zenbexus’s launch if the second CELMoD profiles as strictly stronger.
4. Competing modalities compress the window — bispecifics and CAR-T are moving earlier in the treatment sequence, potentially shrinking the relapsed niche before Zenbexus peaks.

Source: Bristol Myers Squibb EXCALIBER-RRMM press release and EXCALIBER-RRMM on ClinicalTrials.gov (NCT04975997)