NX-13, NLRX1, and the limits of an agonist with almost no human genetic support in ulcerative colitis

What was tried

AbbVie ran a randomized, double-blind, placebo-controlled Phase 2 induction study, named NEXUS, of the oral agent NX-13 in adults with moderate to severe ulcerative colitis (NCT05785715). Participants received NX-13 at 250 mg or 750 mg once daily or matching placebo, with a long-term extension. The primary endpoint was the change from baseline in the Modified Mayo Score at Week 12. Actual enrollment reached 81 participants. The study started in April 2023 and recorded a primary completion date of May 2025. ClinicalTrials.gov lists the study as terminated, with the posted reason "Sponsor Decision_Not Safety Related" (NCT05785715).

NX-13 is a first-in-class oral agonist of NLRX1, a mitochondrially localized member of the NOD-like receptor family. AbbVie obtained the molecule by acquiring Landos Biopharma in March 2024, where NX-13 was the lead asset. The Phase 2 readout was scheduled before the acquisition closed, which makes the later termination a useful test case for how a large sponsor treats an inherited asset built on a thin human evidence base.

The biological hypothesis

NLRX1 sits at an unusual point in innate immune signaling. Rather than driving inflammation, it restrains it. Preclinical work described NX-13 as acting through NLRX1 to lower oxidative and inflammatory signaling in CD4+ T cells and to support epithelial repair, a bimodal effect that fit the two problems of ulcerative colitis: an overactive mucosal immune response and a damaged barrier (Leber et al., 2019). Reviews of NLRX1 across this period framed it as a metabolic checkpoint that tunes mitochondrial reactive oxygen species, NF-kB activity, and type I interferon responses (Liu et al., 2022, and Pickering and Booty, 2021).

The clinical rationale rested on a Phase 1b study in active ulcerative colitis, which reported acceptable safety and early symptomatic improvement over four weeks of dosing (Verstockt et al., 2024). The appeal was an oral, gut-relevant mechanism that did not suppress immunity in the broad way that existing biologics do.

What actually happened

The Phase 2 program was stopped after the primary completion date, and the long-term extension was closed. AbbVie recorded the termination as a sponsor decision unrelated to safety. No positive Phase 2 efficacy readout for NX-13 in ulcerative colitis has been released publicly, despite topline results having been planned for late 2024 at the time of the Landos acquisition.

The Open Targets Platform places the NLRX1 to ulcerative colitis association at an overall score of 0.0111, with the only non-zero contribution coming from literature text mining at 0.0913 and no genetic association signal. By contrast, validated inflammatory bowel disease targets carry strong genetic association evidence. NX-13 also has no marketed adverse event footprint, since it never reached approval, so FAERS returns no disproportionality signal.

Failure mechanism, best guess

The honest reading is that this was a portfolio decision, and the posted reason confirms it was not driven by a safety finding. The more useful question is why a sponsor would walk away from its acquired lead asset rather than push to Phase 3.

The structural weakness was the evidence pyramid under the target. NLRX1 has compelling mouse pharmacology and a coherent mechanistic story, but it has essentially no human genetic support for ulcerative colitis (Open Targets overall score 0.0111). Targets that lack human genetic validation carry a lower historical probability of clinical success, and the gap between a clean mouse colitis model and a heterogeneous human disease is where many immunology programs fail. When the Phase 1b signal was a short symptomatic improvement rather than a durable endoscopic effect, the program entered Phase 2 with a fragile prior. A sponsor decision to stop without releasing efficacy data is most consistent with a Week 12 Modified Mayo result that did not clear the bar for a competitive ulcerative colitis asset, in a field now anchored by oral agents with positive Phase 3 data. This is an inference from the absence of a positive disclosure and the non-safety framing, not a stated efficacy failure.

How to prevent this next time

Two quantitative tools would have sharpened the go or no-go decision before Phase 2 committed.

First, a Bayesian predictive probability of success at the Phase 1b to Phase 2 boundary. The predictive probability integrates the chance of a positive Phase 2 over the posterior for the treatment effect implied by the small Phase 1b:

PP(success)=∫θ​P(final success∣θ,interim data)⋅π(θ∣interim data)dθ

A four-week symptomatic signal in a single-arm style cohort produces a wide posterior on the true induction effect, which pulls the predictive probability toward the historical base rate rather than the optimistic point estimate.

Second, an explicit power calculation tied to the realized design. With 81 participants split across three arms, roughly 27 per arm, a two-sided test at alpha 0.05 has about 84 percent power to detect a between-arm difference of 0.8 standard deviations on the Modified Mayo Score but only about 45 percent power for a more realistic 0.5 standard deviation difference. A program betting on a moderate effect was underpowered to distinguish a real but modest benefit from noise.

Layered on these, a historical base-rate adjustment matters most when human genetic support is absent. Trials that use biomarker-based or genetically anchored patient selection have shown higher success probabilities than unselected trials (Wong, Siah, and Lo, 2019). NLRX1 offered no validated patient-selection biomarker, so the program could not buy back the prior it was missing. The single highest leverage change would have been gating the Phase 2 commitment on a Bayesian predictive probability that explicitly down-weighted the mouse-heavy, genetics-light evidence base, rather than on the encouraging but uncontrolled four-week Phase 1b symptom data.

What this means for similar programs

Immunology targets that restrain inflammation are attractive because they promise to dampen disease without broad immunosuppression. The NLRX1 case shows the cost of that attraction when human validation is thin. A target with an Open Targets ulcerative colitis score near 0.01 is effectively unvalidated in humans, and acquiring such an asset transfers that risk to the buyer. Programs in this position should treat positive animal data as a hypothesis generator, not as a substitute for human genetics, and should design Phase 2 trials powered for the modest effect sizes that unvalidated targets typically deliver.

Open questions

Did the NEXUS Phase 2 miss on the Modified Mayo Score, or was the decision purely a post-acquisition portfolio reshuffle independent of the data? Will AbbVie or any party publish the Week 12 results so the field can separate target failure from molecule failure? And is there a definable subgroup, for example a mucosal metabolic or epithelial-repair signature, in which NLRX1 agonism still has a credible path, given that the mechanism itself was never disproven?

Sources

1. • ClinicalTrials.gov, NCT05785715, study design, arms, Modified Mayo Score primary endpoint, enrollment of 81, primary completion May 2025, and termination reason "Sponsor Decision_Not Safety Related." https://clinicaltrials.gov/study/NCT05785715 - Verstockt B, Vermeire S, Peyrin-Biroulet L, et al. The Safety, Tolerability, Pharmacokinetics, and Clinical Efficacy of the NLRX1 agonist NX-13 in Active Ulcerative Colitis: Results of a Phase 1b Study. Journal of Crohn's and Colitis.

2. - Leber A, Hontecillas R, Zoccoli-Rodriguez V, et al. Activation of NLRX1 by NX-13 Alleviates Inflammatory Bowel Disease through Immunometabolic Mechanisms in CD4+ T Cells. Journal of Immunology.

3. - Liu M, Liu K, Cheng D, et al. The regulatory role of NLRX1 in innate immunity and human disease. Cytokine.

4. - Pickering RJ, Booty LM. NLR in eXile: Emerging roles of NLRX1 in immunity and human disease. Immunology.

5. - Open Targets Platform, NLRX1 (ENSG00000160703) to ulcerative colitis (EFO_0000729) association, overall score 0.0111, literature component 0.0913. https://platform.opentargets.org/target/ENSG00000160703 - AbbVie. AbbVie to Acquire Landos Biopharma, Further Strengthening its Portfolio in Inflammatory and Autoimmune Diseases. March 25, 2024. https://news.abbvie.com/2024-03-25-AbbVie-to-Acquire-Landos-Biopharma,-Further-Strengthening-its-Portfolio-in-Inflammatory-and-Autoimmune-Diseases - Wong CH, Siah KW, Lo AW. Estimation of clinical trial success rates and related parameters. Biostatistics. 2019;20(2):273-286.