Zenas BioPharma Stock Halved as Drug Trial Win Falls Short
Shares plunge over 56% as investors signal that positive data for its autoimmune drug obexelimab is not strong enough to challenge Amgen's rival treatment.
Shares of Zenas BioPharma (NASDAQ: ZBIO) were more than halved in Tuesday trading, a stark rebuke from investors after the company announced positive results from a late-stage trial for its autoimmune disease drug, obexelimab.
The stock plummeted by more than 56% to close at $15.04, wiping out over $2 billion in market capitalization. The sell-off occurred despite Zenas reporting that its Phase 3 INDIGO study for obexelimab in treating Immunoglobulin G4-Related Disease (IgG4-RD) had met its primary endpoint.
According to the company's announcement, obexelimab demonstrated a 56% reduction in the risk of disease flare-ups compared to a placebo over a 52-week period. The trial also successfully met all key secondary endpoints with a favorable safety profile.
However, the market's brutal reaction suggests investors are looking past the headline success and focusing on the competitive landscape. The central issue is the comparison to Uplizna (inebilizumab), a rival treatment from pharmaceutical giant Amgen, which was approved for IgG4-RD in April 2025. In its pivotal trial, Uplizna demonstrated an 87% reduction in flare risk, setting a high bar for efficacy in this rare, chronic autoimmune condition.
Investors and analysts were quick to draw comparisons, concluding that obexelimab’s 56% risk reduction, while statistically significant, is not compelling enough to effectively compete against a deeply entrenched and more effective therapy. Uplizna is marketed by Amgen, a company with vast resources and an established presence in immunology.
"While Zenas delivered a clinical trial success, the market has delivered a commercial reality check," said a healthcare analyst at a London-based fund. "In a competitive market, being better than placebo isn't enough. You have to be competitive with the standard of care, and this data makes that a very difficult argument."
The trading session reflected this sentiment unequivocally. ZBIO's trading volume surged to more than 15 times its daily average as institutional investors recalibrated their valuation of the company's lead asset. The stock, which had traded as high as $44.60 in the past year, fell sharply from its previous close of $34.50.
IgG4-RD is a chronic, immune-mediated fibro-inflammatory disease that can affect multiple organs, including the pancreas, bile ducts, and salivary glands. While Zenas has highlighted that obexelimab has a different mechanism of action—targeting B cells without depleting them—and offers the convenience of a subcutaneous injection versus Uplizna's intravenous infusion, these advantages were clearly insufficient to outweigh the efficacy gap in the eyes of the market.
Looking forward, Zenas BioPharma is expected to proceed with regulatory filings based on the positive Phase 3 results. However, the company now faces a significant uphill battle. Even if approved, securing market access and convincing physicians and payers to adopt obexelimab over a more effective, established alternative will be a formidable commercial challenge. The stock's precipitous fall indicates that Wall Street is pricing in a low probability of significant market penetration, placing the future of Zenas's lead drug in a precarious position.