Tulisokibart

Tulisokibart (MK-7240, formerly PRA023) is Merck's anti-TL1A monoclonal antibody now in Phase 3 for moderate-to-severe Crohn's disease and ulcerative colitis. Merck acquired it via the $10.8 billion Prometheus Biosciences buyout in 2023, betting that TL1A blockade will open a new tier in IBD treatment by hitting both inflammation and the fibrotic scarring that current biologics leave behind [1][2][9]. Phase 2 studies in UC (NEJM 2024) and Crohn's (Lancet Gastroenterology 2025) both hit their primary endpoints, which is why this program is one of the most-watched immunology bets in large pharma.

Novel compound in Phase 3. The Crohn's disease registration study (NCT06430801, MK-7240-008) is enrolling 1,200 patients and is currently recruiting [4]. A parallel Phase 3 in ulcerative colitis is also underway. The extension protocol referenced by NCT06651281 is the long-term follow-on for patients who completed prior induction and maintenance regimens [5]. No FDA designations have been disclosed for the IBD program, which is normal for immunology assets aimed at indications that already have approved therapies. Merck is running parallel Phase 2 studies in rheumatoid arthritis (NCT07176390), systemic sclerosis with interstitial lung disease (NCT05270668), and hidradenitis suppurativa (NCT06956235), a sign the company believes TL1A biology extends beyond gut inflammation into other T-cell-driven diseases with a fibrotic component [6][7][8]. With a 52-week primary endpoint and enrollment still ongoing, a Phase 3 IBD readout is unlikely before 2027 and a BLA submission before 2028 in the most optimistic scenario.

TL1A, the protein encoded by TNFSF15, is a cytokine that acts as a costimulator for T cells. Think of it as a pedal that revs an already-active immune cell into a more inflammatory state and helps it survive longer. In inflammatory bowel disease, genetic variants that boost TL1A signaling are associated with more severe, fibrotic disease, the kind that leads to intestinal strictures, surgery, and repeat hospitalizations. Current standard-of-care biologics such as anti-TNF antibodies (infliximab, adalimumab), IL-23 blockers (Skyrizi, Stelara), and integrin blockers (Entyvio) all quiet inflammation, but none clearly reverse the underlying tissue scarring. Blocking TL1A is designed to do both: reduce the T-cell-driven inflammation and directly interfere with the fibroblast activation that drives fibrosis [1][2][3]. The case has three lines of support: human genetics linking TNFSF15 to IBD risk, preclinical models showing anti-fibrotic effects, and Phase 2 clinical data hitting endpoints in both UC and Crohn's. Where this gets interesting commercially is that Prometheus developed a companion diagnostic to enrich for patients with a TL1A-high expression signature, opening a precision-medicine angle in a market that historically has not had one. The Phase 3 studies include both biomarker-selected and all-comer arms, which is exactly the design a first-in-class immunology asset needs to support a differentiated label.

NCT06430801 is a Phase 3 registration study for moderate-to-severe Crohn's disease enrolling roughly 1,200 patients [4]. The primary endpoint for the FDA-facing arm is the percentage of participants achieving clinical remission per Crohn's Disease Activity Index (CDAI) at week 52, which is the standard maintenance endpoint used by regulators. The comparator is placebo on a background of standard care, with washout requirements for prior biologics. Enrollment status is active recruiting per ClinicalTrials.gov. A 1,200-patient global program is expensive but appropriate for a first-in-class immunology asset that needs to power both a maintenance efficacy claim and a safety database large enough for FDA review. The design allows subgroup analyses by prior biologic exposure, which matters because a large share of the addressable US patient population has already failed anti-TNF therapy and is looking for the next line. The extension protocol (NCT06651281) is how Merck plans to capture long-term efficacy and safety, especially relevant for a chronic disease where patients stay on biologics for years [5]. The main design risk is the 52-week timeline, which pushes readout well into 2027 and gives competitors, particularly Pfizer's afimkibart (formerly RVT-3101), room to close ground while Merck waits on data.

Our model estimates a 66% chance this drug is eventually approved. It starts from the historical base rate for Phase 3 drugs in this area (about 61%), then adjusts using ten facts about the trial and sponsor. What moves the number most: it is helped by strong earlier-phase results, a non-randomized design, and larger-than-typical enrollment for this phase; it is held back by heavier-than-usual blinding. The other facts land near average for this stage, so they leave the estimate roughly where the base rate put it.

The main efficacy risk is that the Phase 3 magnitude of benefit does not clear the bar payers now demand in IBD. Skyrizi and Rinvoq have set high remission rates in Crohn's, and a new mechanism that only matches TNF-era numbers will not command premium pricing or displace incumbents. The biomarker enrichment strategy could cut either way: a clear TL1A-high subgroup with 40%+ absolute benefit is a franchise, but if the all-comer arm underperforms, the label may end up narrow and confined to biomarker-positive patients, shrinking the addressable market. Safety risk is real but manageable so far. TL1A blockade modulates T-cell activation, so infection signals, malignancy signals, and rare immune-mediated events all need long-term surveillance. Phase 2 safety looked clean, but IBD patients get treated for decades and the tail matters. On the competitive side, Pfizer picked up the afimkibart program from Roivant and is running its own Phase 3, so even if tulisokibart works, Merck may share the class rather than own it [11]. Execution risk is low given Merck's regulatory and commercial reach, but the 52-week maintenance endpoint means a slow readout in a market where Skyrizi is already growing rapidly and eating patient share every quarter Merck is not on-label.

Worth watching, closely. Tulisokibart is Merck's most credible non-oncology growth asset, positioned as a Keytruda-succession hedge as the flagship faces IRA price negotiation and eventual biosimilar exposure after 2028 [10]. Merck paid $10.8 billion for Prometheus in April 2023 specifically for this program, so the commercial team is already built around this asset [9]. Specific data points to watch: (1) the Phase 3 Crohn's readout in 2027, particularly the biomarker-enriched subgroup versus all-comer difference; (2) any interim safety updates or dose-optimization commentary in Merck's quarterly calls; (3) Pfizer's afimkibart Phase 3 progression, because these two assets will define the anti-TL1A class in IBD and pricing will depend heavily on head-to-head or matching-adjusted indirect comparisons. Check back after the Phase 3 UC readout, which should arrive before the Crohn's readout given a shorter primary endpoint window. If Merck also delivers positive Phase 2 data in hidradenitis suppurativa or systemic sclerosis with ILD, this becomes a platform asset rather than a single-indication launch, which is the difference between a $3 to 5 billion peak and a $7 to 10 billion peak franchise for the anti-TL1A class.