Allakos is a clinical-stage biopharmaceutical company whose pipeline has been built almost entirely around one biological target: Siglec-8, a receptor found on eosinophils and mast cells. Its lead drug candidate, lirentelimab (originally known as AK002), is a monoclonal antibody designed to deplete eosinophils and quiet mast cell activity. Early trial results were striking enough to generate enormous excitement, but later-stage programs have delivered a more complicated story, with dramatic tissue-level improvements that have not always translated into the symptom relief that regulators and patients need to see.
What Siglec-8 Is and Why Allakos Targeted It
Siglec-8 belongs to a family of cell-surface receptors that recognize sugar molecules on other cells. What makes it attractive as a drug target is its selectivity: it shows up almost exclusively on two cell types that drive allergic and eosinophilic diseases, namely eosinophils and mast cells. When an antibody like lirentelimab binds to Siglec-8 on eosinophils, it triggers those cells to self-destruct through a process called apoptosis. On mast cells, the effect is different: instead of killing them, Siglec-8 engagement dials down the release of inflammatory mediators without destroying the cell itself.1PubMed Central. Siglec-8 on human eosinophils and mast cells, and Siglec-F on murine eosinophils, are functionally related inhibitory receptors
This dual mechanism was a key selling point for Allakos. Other drugs in the eosinophilic disease space tend to go after a single pathway. Anti-IL-5 therapies like mepolizumab and benralizumab, for example, reduce circulating eosinophils effectively but do not directly address mast cells. Lirentelimab’s ability to hit both cell types was hypothesized to matter in gastrointestinal eosinophilic disorders, where tissue-resident eosinophils and mast cells both contribute to inflammation and symptoms. Preclinical work in transgenic mice confirmed that a Siglec-8 antibody could reduce both eosinophils and mast cells in stomach tissue affected by eosinophilic gastroenteritis, and that Siglec-8 was selectively expressed on those cells even in diseased tissue.2The Journal of Clinical Investigation. Siglec-8 antibody reduces eosinophils and mast cells in a transgenic mouse model of eosinophilic gastroenteritis
The ENIGMA Phase 2 Trial and Its Promising Early Signal
Lirentelimab’s first significant clinical readout came from a Phase 2 trial called ENIGMA, which enrolled patients with eosinophilic gastritis and eosinophilic duodenitis. These are rare conditions in which eosinophils pile up in the stomach or upper small intestine lining, causing pain, nausea, bloating, and early satiety. There were no approved drugs for these diseases at the time, and treatment options were limited to dietary restrictions or off-label steroid use.
The results, published in the New England Journal of Medicine, were remarkable on the tissue level. Patients receiving lirentelimab saw their gastrointestinal eosinophil counts drop by about 86% on average, compared with a 9% increase in the placebo group. Around 63% of treated patients met the definition of histologic response, versus just 5% on placebo. The drug also showed a statistically significant reduction in total symptom scores: symptoms fell by roughly 48% in the treatment group compared to 22% with placebo.3PubMed Central. Anti-Siglec-8 Antibody for Eosinophilic Gastritis and Duodenitis
This was a strong set of data for a Phase 2 trial: the tissue changes were overwhelming, and the symptom improvement, while more modest, was statistically meaningful. The challenge came in replicating this in larger, more rigorous studies. Allakos advanced lirentelimab into Phase 2/3 trials (ENIGMA 2) for both eosinophilic gastritis and eosinophilic duodenitis, setting the stage for what the company hoped would be a path to regulatory approval.
What Happened in Later-Stage Eosinophilic GI Trials
In late 2021, Allakos announced that the ENIGMA 2 studies had failed to meet their primary endpoints. While the drug continued to produce tissue-level eosinophil depletion, the improvement in patient-reported symptoms did not reach statistical significance. For a company that had staked its near-term future on these programs, the miss was devastating. The stock price collapsed, and Allakos was forced to restructure, cutting staff and refocusing its resources.
The disconnect between what the biopsies showed and what patients reported was not unique to Allakos. Eosinophilic gastrointestinal diseases are notoriously difficult to study because the link between tissue eosinophil burden and day-to-day symptoms is loose and poorly understood. Patients with high eosinophil counts can feel relatively well, while others with modest tissue infiltration can be miserable. On top of that, symptom measurement tools for these rare diseases were still being developed and validated during the trial period. Instruments like the SAGED questionnaire were created specifically for this purpose and went through rigorous content validation, but they were new and untested at scale.4PubMed Central. Development and content validation of a symptom assessment for eosinophilic gastritis and eosinophilic gastroenteritis in adults and adolescents Whether the symptom failure reflected a true lack of clinical benefit or simply inadequate tools for measuring benefit in these rare conditions remains debated.
The Eosinophilic Esophagitis Program
Allakos also ran lirentelimab through a Phase 2/3 trial in eosinophilic esophagitis, a related condition in which eosinophils infiltrate the lining of the esophagus and cause difficulty swallowing, food impaction, and chest pain. The trial used co-primary endpoints: a histologic response measuring eosinophil counts in esophageal biopsies and a change in the Dysphagia Symptom Questionnaire, a daily diary of swallowing difficulty.
Once again, the histologic results were impressive. By week 24, roughly 88% to 92% of patients in the treatment groups achieved histologic response, compared with about 11% of those on placebo. Adolescents responded particularly well, with 94% in both dose groups hitting the tissue endpoint. But the symptom scores told a less convincing story. The reduction in dysphagia scores did not differ significantly from placebo in the overall population. In adolescents, there was a trend toward improvement, but it did not cross the threshold for statistical significance either.
This pattern, in which the tissue responds dramatically to treatment but patients do not report meaningfully different symptoms, has been a persistent frustration across the eosinophilic disease space. It is not limited to lirentelimab. A separate large trial of benralizumab in eosinophilic esophagitis produced nearly identical results: about 87% of treated patients achieved histologic response versus roughly 7% on placebo, but there was no significant improvement in dysphagia symptoms.5New England Journal of Medicine. Benralizumab for Eosinophilic Esophagitis The consistency of this pattern across different drugs with different mechanisms suggests the problem may lie partly in how eosinophilic esophagitis symptoms are measured and partly in the biology of the disease itself, where tissue remodeling and fibrosis may persist even after eosinophils are cleared.
The Histology-Symptom Gap as a Field-Wide Problem
Understanding why tissue eosinophil clearance does not reliably produce symptom relief matters for interpreting not just Allakos’s pipeline but the entire drug development landscape in eosinophilic diseases. There are several working theories. One is that by the time patients enroll in trials, years of chronic inflammation have already caused structural damage, such as esophageal narrowing or fibrotic changes in the stomach wall, that lowering eosinophil counts alone cannot reverse. Another is that mast cells, nerve sensitization, and other inflammatory players contribute to symptoms independently of eosinophil levels, so eliminating eosinophils addresses only part of the problem.
A third factor is measurement. Symptom questionnaires in eosinophilic gastrointestinal diseases are still relatively new. Developing a patient-reported outcome measure for a rare disease is slow work: researchers must interview patients to identify which symptoms matter, confirm that the wording captures the actual patient experience, and validate that the scoring system is sensitive enough to detect change. Work on instruments like the EG/EGE-SQ has established content validity through qualitative research, but the field is still learning how well these tools perform in large interventional trials.6American Journal of Gastroenterology. 1247 Development of a Patient Reported Outcome (PRO) Questionnaire to Assess the Symptoms of Eosinophilic Gastritis and Gastroenteritis (EG/EGE-SQ©) A high placebo response rate in symptom diaries compounds the problem, as patients who know they might be getting an active drug often report feeling better regardless.
For Allakos, this gap has been both an explanation and a problem. It is scientifically plausible that lirentelimab is doing something biologically meaningful that current symptom tools cannot fully capture. But regulators generally require evidence that a drug makes patients feel or function better, not just that it changes a biomarker. Until the field develops clearer ways to bridge the tissue-symptom divide, drugs like lirentelimab will face this hurdle.
Beyond the GI Tract: Chronic Urticaria
Allakos explored lirentelimab in chronic spontaneous urticaria, a condition marked by unpredictable hives and itching that does not respond to standard antihistamines. Mast cells are the central driver here, making Siglec-8’s inhibitory effect on mast cell activation a logical fit. An open-label proof-of-concept study tested lirentelimab in patients who had failed antihistamines, including some who had also failed omalizumab, the only biologic approved for the condition at the time.
In patients who had never tried omalizumab, disease activity scores dropped by about 73% by week 22, and around three-quarters met the definition of clinical response. The results were less dramatic in omalizumab-refractory patients, with a roughly 47% reduction in disease activity and a response rate closer to 45%.7PubMed. An open-label, proof-of-concept study of lirentelimab for antihistamine-resistant chronic spontaneous and inducible urticaria These numbers are encouraging, especially in a population that has already failed available treatments. But as an open-label study without a placebo group, the results need to be interpreted cautiously. Chronic urticaria has notoriously high placebo response rates, and without blinding, expectation effects can inflate the apparent benefit.
Allergic Conjunctivitis and Other Explored Indications
Lirentelimab has also been studied in severe and chronic allergic conjunctivitis, where mast cells in the eye drive inflammation, tearing, itching, and swelling. The safety profile in these studies was consistent with what had been observed elsewhere: the most frequently reported side effects were mild to moderate infusion-related reactions, meaning symptoms like flushing, headache, or mild allergic responses during or shortly after the intravenous infusion itself.8PubMed. Lirentelimab for severe and chronic forms of allergic conjunctivitis Across trials and indications, lirentelimab has not raised major safety red flags, which is notable for a drug that depletes an entire immune cell population. The tolerability profile has been one of the more consistent positives in Allakos’s clinical story.
Interest in lirentelimab for mast cell activation syndromes and indolent systemic mastocytosis has also emerged. These are conditions in which mast cells accumulate or become hyperactive, causing episodes of flushing, low blood pressure, abdominal cramping, and anaphylaxis. Early signals have been described as promising, though the competitive landscape has shifted since tyrosine-kinase inhibitors like avapritinib received FDA approval for indolent systemic mastocytosis, establishing a new standard of care.9PubMed. Mast Cell-Targeting Therapies in Mast Cell Activation Syndromes Lirentelimab would need to show a clear differentiated benefit, whether through better symptom control, a different safety profile, or activity in patients who do not respond to kinase inhibitors, to carve out a role in that space.
How Lirentelimab Compares to Other Approaches
The eosinophilic and mast cell disease space has become considerably more crowded since Allakos first entered clinical development. Several classes of drugs are either approved or in late-stage trials for overlapping indications. Anti-IL-5 agents like mepolizumab and benralizumab reduce blood and tissue eosinophils effectively but do not target mast cells. Dupilumab, which blocks IL-4 and IL-13 signaling, received FDA approval for eosinophilic esophagitis and has shown both histologic and symptomatic benefit, giving it an advantage that Allakos has struggled to match. For mast cell-driven diseases, avapritinib and midostaurin work through a completely different mechanism, directly inhibiting the KIT receptor that drives mast cell survival and proliferation.
Lirentelimab’s theoretical advantage is that dual eosinophil-mast cell mechanism: it is the only clinical-stage anti-Siglec-8 antibody, targeting a receptor that is uniquely co-expressed on both problematic cell types.10The Journal of Clinical Investigation. Siglec-8 antibody reduces eosinophils and mast cells in a transgenic mouse model of eosinophilic gastroenteritis Whether this dual mechanism actually leads to better clinical outcomes than targeting just one cell type is the central unresolved question. The clinical data so far suggest that eosinophil depletion is not sufficient on its own to control symptoms in many patients, and mast cell inhibition, while mechanistically appealing, has not yet produced the symptom endpoints needed for approval in the studied indications.
Biopsy Challenges in Rare Eosinophilic Diseases
One underappreciated aspect of Allakos’s clinical development is the difficulty of simply diagnosing and monitoring the diseases lirentelimab is designed to treat. Eosinophilic gastritis, for example, can be patchy: eosinophils may cluster in one part of the stomach while adjacent areas look normal. If a biopsy needle misses the inflamed patch, the disease goes undetected. Work using screening data from the lirentelimab trial program explored how many biopsies are needed to reliably detect eosinophilic gastritis or duodenitis and found that optimizing biopsy protocols could substantially improve detection rates.11PubMed Central. Determination of Biopsy Yield That Optimally Detects Eosinophilic Gastritis and/or Duodenitis in a Randomized Trial of Lirentelimab
This matters for pipeline interpretation because it means that clinical trials in these diseases are inherently noisy. A patient classified as a non-responder on histology might simply have had biopsies taken from the wrong spot. Conversely, a patient whose biopsies show improvement may still have patches of active disease elsewhere. This sampling variability adds uncertainty to every endpoint, both histologic and symptomatic, and makes it harder to draw clean conclusions about drug efficacy.
Emerging Science Around the Siglec Family
While Allakos’s clinical setbacks have slowed its commercial trajectory, the underlying biology of Siglec receptors continues to generate research interest. Siglec-8 is one member of a broader family, and other Siglecs are being studied for potential roles in immune regulation and disease. Siglec-6, for instance, is expressed primarily on mast cells and has been shown to function as an inhibitory receptor. Engaging Siglec-6 on mast cells significantly reduced IgE-driven degranulation and production of certain inflammatory cytokines in laboratory studies, raising the possibility that it could serve as an alternative or complementary target in mast cell-driven conditions.12PubMed Central. Functional Inhibitory Siglec-6 Is Upregulated in Human Colorectal Cancer-Associated Mast Cells
Recent structural biology work has revealed that Siglec-6 binds its targets in a different way from most other Siglec family members. While most Siglecs rely on a conserved amino acid to grab onto sugar molecules, Siglec-6 supplements that interaction by binding to the surrounding cell membrane itself, using specific residues that interact with the phospholipids around the target sugar.13bioRxiv. Glycolipid recognition and binding by Siglec-6 hinges on interactions with the cell membrane This kind of mechanistic insight could eventually inform the design of more potent or selective antibodies against Siglec family members. For Allakos, any future expansion of the pipeline beyond Siglec-8 would likely involve navigating the distinct binding and signaling properties of these related but biologically distinct receptors.
Whether Allakos or another company ultimately translates the Siglec-8 mechanism into an approved therapy may depend less on the biology, which remains compelling, than on the field’s ability to solve the practical problems of symptom measurement, patient selection, and trial design in rare eosinophilic diseases. The tissue-level efficacy of lirentelimab has been demonstrated repeatedly. The question that has defined and constrained the Allakos pipeline from the beginning is whether clearing the cells and quieting the mast cells can be shown, with the tools currently available, to make patients feel meaningfully better.