Zenas BioPharma reports sustained flare control in IgG4-RD extension

Zenas BioPharma has presented updated data from the open-label extension of its Phase 3 INDIGO trial, showing that obexelimab maintained strong flare protection in patients with IgG4-related disease (IgG4-RD) over a longer follow-up period. The data were disclosed at the 6th International Symposium on IgG4-Related Disease, held in Cambridge, Massachusetts from 8 to 10 October 2026.

Participants who completed INDIGO's 52-week randomised, controlled period were eligible to continue into the open-label extension on 250 mg obexelimab administered subcutaneously once weekly. The 12-month probability of remaining flare-free was 0.93 based on investigator- and adjudication committee-confirmed flares requiring rescue therapy, and 0.87 on investigator-determined events alone, with five patients still pending adjudication at the data cut-off. Median cumulative glucocorticoid rescue dose in the extension was zero milligrams, and adverse event rates were comparable to or lower than those recorded during the controlled phase.

Steroid toxicity and vaccine data

A secondary analysis using the Glucocorticoid Toxicity Index (GTI) showed that obexelimab significantly reduced cumulative steroid-associated harm versus placebo at week 52, with a GTI Cumulative Worsening Score difference of -11.4 (p=0.021). Fewer patients receiving obexelimab crossed clinical thresholds for cumulative glucocorticoid toxicity than those on placebo, a finding that could carry weight for payers and prescribers concerned about the long-term harms of corticosteroid dependence in a chronically relapsing condition.

The company also released initial data from an ongoing vaccine-response sub-study embedded in the open-label extension. During a planned treatment-free window averaging 34.6 days, six of seven participants (86%) achieved seroimmunity to influenza and four of four (100%) achieved seroimmunity to COVID-19 following vaccination. No disease flares occurred during that window. Separately, participants maintained pre-existing markers of immunity following long-term obexelimab exposure, at rates of 71% for influenza and 50% for COVID-19. The sub-study sample sizes remain small, and these figures should be read accordingly, but the findings address a practical concern around B-cell inhibition and vaccine responsiveness that has dogged this drug class.

Market context and competitive backdrop

Obexelimab's bifunctional mechanism targets both CD19 and the inhibitory receptor FcγRIIb, suppressing B-cell activity without depleting the cells entirely. That distinguishes it from anti-CD20 agents such as rituximab, which have been used off-label in IgG4-RD and carry a more aggressive B-cell depletion profile. An estimated 25,000 people in the United States carry an IgG4-RD diagnosis, making it a relatively rare condition; however, the unmet need is genuine given the disease's propensity for organ fibrosis and its relapsing course.

The IgG4-RD field has few approved therapies, which gives regulatory precedent limited guidance on endpoints and comparators. Zenas is also advancing obexelimab in systemic lupus erythematosus, with topline results from the Phase 2 SunStone trial expected in the fourth quarter of 2026. A positive SLE readout would substantially broaden the commercial and clinical narrative for the asset and provide a second indication anchor for any future regulatory submission strategy.

Lonnie Moulder, founder and chief executive of Zenas, said the new findings "reinforce the potential role of obexelimab in long-term disease management, including helping patients remain flare-free, reducing steroid exposure, and potentially supporting vaccine responses following a treatment-free window."

Investors will be focused on two near-term catalysts: the SunStone SLE readout and any signal from Zenas on a regulatory filing pathway for obexelimab in IgG4-RD, where the Phase 3 INDIGO data are accumulating but no submission date has yet been disclosed.