Merck's Tulisokibart Succeeds in Phase 3 Ulcerative Colitis Trial

Merck's Tulisokibart Meets Primary Goal in Phase 3 Ulcerative Colitis Study

Merck & Co.'s experimental drug, tulisokibart, has successfully met its primary goal in a Phase 3 study for moderately to severely active ulcerative colitis. Acquired through the 2023 buyout of Prometheus Biosciences, tulisokibart is a TL1A inhibitor, representing a new class of inflammatory disease medicines. Merck plans to share these positive results with regulatory authorities and present them at an upcoming scientific meeting. A second late-stage study for the drug is currently ongoing. This development is strategically important for Merck, aiming to boost future revenue as its cancer immunotherapy Keytruda approaches patent expiration.

• Merck's experimental drug, tulisokibart, achieved its primary endpoint in a Phase 3 clinical trial for patients with moderately to severely active ulcerative colitis. This success marks a significant milestone for the drug, which was central to Merck's 2023 acquisition of Prometheus Biosciences, validating the strategic investment in inflammatory disease medicines.
• Tulisokibart belongs to a novel class of inflammatory disease treatments targeting the TL1A protein. Following these positive results, Merck intends to engage with regulatory authorities to discuss the findings and present the data at an upcoming scientific conference. A second late-stage study for the drug is currently in progress, indicating ongoing development.
• The successful development of tulisokibart is strategically vital for Merck, as it represents a key asset to bolster future revenue streams. This is particularly important as the company prepares for the eventual patent expiration of its blockbuster cancer immunotherapy, Keytruda, positioning tulisokibart as a potential successor in its portfolio.

Tulisokibart's Phase 3 Triumph in Ulcerative Colitis

The ASTRO trial (NCT05528510) evaluated subcutaneous guselkumab — an IL-23p19 inhibitor — in adults with moderately to severely active ulcerative colitis (modified Mayo score 5–9) across 153 sites in 25 countries. In this double-blind, placebo-controlled Phase 3 trial, 418 patients were randomized to guselkumab 400 mg induction followed by either 100 mg every 8 weeks or 200 mg every 4 weeks, versus placebo. Clinical remission at week 12 was achieved by 28% of guselkumab-treated patients versus 6% on placebo (adjusted treatment difference: 21 percentage points, 95% CI 14–28; p<0.0001), with week 24 remission rates of 35–36% across both active dosing arms compared to 9% with placebo. The safety profile was favorable: serious adverse events occurred in only 4% of guselkumab-treated patients versus 12% in the placebo group, with no treatment-related deaths and no new safety signals identified.

The LUCENT-3 open-label extension evaluated long-term outcomes of mirikizumab — an anti-IL-23p19 antibody — through week 212 of continuous treatment. Among week 52 maintenance remitters, 77.7% maintained clinical remission, 81.3% achieved endoscopic remission, and 66.0% achieved histologic-endoscopic mucosal remission at week 212 (observed cases). Disease clearance rates — defined as concurrent symptomatic, endoscopic, and histologic remission — progressed from 16.0% at week 12 to 36.4% at week 52 and 51.3% at week 104 across the LUCENT Phase 3 program (NCT03518086, NCT03524092, NCT03519945). Importantly, early disease clearance at week 12 was associated with significantly better long-term clinical outcomes, and efficacy results were generally consistent across biologic-naïve and biologic-experienced subgroups. Over 160 weeks of open-label extension treatment, no new safety signals emerged.

The TUSCANY-2 trial (NCT04090411) assessed afimkibart, a novel subcutaneous TL1A-directed antibody, in a Phase 2b, multicentre, double-blind, randomised, placebo-controlled design across 114 centres in 23 countries. Among 245 treated patients with moderately to severely active UC, the primary endpoint of clinical remission by total Mayo score at week 14 was not met for any dose (26%, 23%, and 24% for the 50 mg, 150 mg, and 450 mg groups, respectively, versus 12% for placebo; all p>0.05). However, clinical remission assessed by modified Mayo score at week 14 showed clinically meaningful risk differences of 18.2%, 23.4%, and 20.2% for the three doses versus placebo. Treatment-emergent adverse events occurred in 48% of patients during induction and 59% during maintenance, with the most common events being nausea, urinary tract infection, anaemia, and fatigue. No deaths were reported, and the overall benefit-risk profile was considered favorable based on secondary endpoint data, supporting further investigation of this mechanism.

Unlocking TL1A Inhibition's Potential Beyond Ulcerative Colitis

Beyond ulcerative colitis, tulisokibart's anti-TL1A mechanism of action is being investigated across two additional indications: Crohn's disease (CD) and hidradenitis suppurativa (HS). These programmes reflect the broad inflammatory and fibrotic pathophysiology driven by the TL1A/DR3 axis across both gastrointestinal and dermatological disease contexts.

Navigating the Evolving Treatment Landscape for Ulcerative Colitis

The treatment landscape for moderately to severely active ulcerative colitis (UC) has undergone substantial transformation over the past five years, driven by the approval of novel therapeutic classes and evolving clinical frameworks for treatment targets. Beyond the established anti-tumor necrosis factor (TNF) agents and vedolizumab, the period has seen regulatory approval and trial validation of several new modalities: ozanimod, a sphingosine-1-phosphate (S1P) receptor modulator, demonstrated significant superiority over placebo across clinical response, corticosteroid-free remission, endoscopic improvement, and mucosal healing in the pivotal phase 3 True North study, with durable efficacy observed through approximately three years of continuous treatment in the open-label extension. Upadacitinib, a selective JAK1 inhibitor, has demonstrated rapid and deep clinical, endoscopic, and histological remissions in UC patients. The IL-23p19 inhibitor guselkumab achieved clinical remission at week 12 in 28% of patients versus 6% on placebo in the phase 3 ASTRO trial, with maintenance remission rates of 35–36% at week 24 across both dosing regimens and an adverse event profile comparable to placebo. Anti-TL1A agents have also shown promising endoscopic and histological outcomes in phase 2 trials, with phase 3 programs ongoing. Concurrently, the STRIDE II guidelines have formalised clinical and endoscopic remission as co-primary therapeutic targets, with histological remission and the composite concept of disease clearance — encompassing clinical, endoscopic, and histological remission — increasingly recognised as aspirational endpoints associated with improved long-term outcomes and health-related quality of life.

Despite this expanding armamentarium, real-world data reveal significant gaps between trial efficacy and routine clinical practice. A large observational study of 6,726 patients with a mean follow-up of 61.5 months found that advanced therapy persistence rates at three years were below 40% across all treatment periods, falling to under 10% when restricted to patients who did not require dose escalation or concomitant corticosteroids. Approximately 27–34% of patients required dose escalation, 40% switched advanced therapies, and 57% discontinued advanced therapy without initiating a subsequent agent. Vedolizumab utilisation increased over time in parallel with a reduction in anti-TNF prescribing, consistent with claims database analyses spanning 2012–2023. An Italian population-based study recorded an increase in advanced therapy use from 11.9% to 19.3% between 2018 and 2023, while treatment costs declined by approximately 21%, largely attributable to the uptake of anti-TNF biosimilars. Network meta-analyses incorporating 17 randomised controlled trials found no statistically significant efficacy differences among biological therapies and tofacitinib in TNF inhibitor-naïve patients, though in TNF inhibitor-exposed populations, only tofacitinib demonstrated significant superiority over placebo for induction, and only tofacitinib and vedolizumab for maintenance.

Several unresolved clinical and strategic questions continue to challenge optimal positioning of available therapies. Head-to-head comparative trial data remain limited; the GARDENIA phase 3 study, which compared etrolizumab against infliximab as an active comparator, found no statistical superiority for either agent in achieving the composite endpoint of clinical response at week 10 and remission at week 54. Early versus late biologic intervention has not demonstrated statistically significant differences in steroid-free remission or clinical remission rates at weeks 14 or 52, though a numerically higher mucosal healing rate was observed in the early intervention group at both timepoints. The optimal role of therapeutic drug monitoring, combination therapy with immunomodulators, and the management of acute severe UC — including perioperative biologic use — remain areas of uncertainty. The IBSEN III inception cohort (2017–2019) confirmed that biologic therapy utilisation at one year reached 14.5%, with anaemia and systemic corticosteroid initiation at diagnosis identified as predictors of early biologic escalation. Collectively, these data underscore the need for durable, well-tolerated treatment options and robust, prospective comparative studies to guide sequencing decisions in a rapidly evolving therapeutic environment.

TL1A Inhibition: A New Era for IBD Treatment

The recent positive Phase 3 results for tulisokibart in moderately to severely active ulcerative colitis represent a pivotal moment for both patients and the pharmaceutical industry. This experimental drug, a TL1A inhibitor, is poised to introduce a novel therapeutic class to the inflammatory bowel disease (IBD) landscape. Studies indicate that TL1A plays a critical role in driving both inflammation and fibrosis, a debilitating complication often refractory to current treatments. By targeting this pathway, tulisokibart offers a dual mechanism of action, addressing a significant unmet medical need.

Clinical trials have consistently shown tulisokibart's potential. Earlier Phase 2 studies demonstrated an acceptable safety profile and statistically significant endoscopic improvement in ulcerative colitis, alongside evidence of reduced inflammatory T-cell cytokines and fibrosis pathways. More recently, a Phase 2 trial highlighted a significantly higher percentage of patients achieving clinical remission with tulisokibart compared to placebo, with adverse events being mostly mild to moderate. This robust clinical profile, coupled with the potential for a precision medicine approach through a genetic-based diagnostic test, could allow for more targeted and effective patient selection.

However, the path forward is not without its challenges. The emergence of anti-drug antibodies in a notable proportion of patients, while not yet shown to impact safety or drug levels, warrants careful long-term monitoring for potential effects on efficacy. Furthermore, the competitive landscape for TL1A inhibitors is rapidly evolving, with several other agents in various stages of development, including other monoclonal antibodies and even bispecific approaches. This growing competition could lead to market fragmentation. Despite these considerations, the successful Phase 3 readout positions tulisokibart as a significant asset, not only offering a new hope for IBD patients but also strategically bolstering the acquiring company's immunology portfolio as it navigates the impending patent expiration of its key oncology product.