Eli Lilly and Company (NYSE: LLY) has reported new exploratory Phase 3b findings showing broader immune and metabolic biomarker changes when its psoriasis treatment Taltz (ixekizumab) is administered alongside obesity medicine Zepbound (tirzepatide), compared with Taltz alone. Presented at the 2026 Fall Clinical Dermatology Conference in Las Vegas and announced on October 9, the prespecified analysis of the TOGETHER-PsO trial identified changes in 482 circulating proteins and 467 gene expression measures with the combination at Week 36, compared with 140 proteins and 16 genes in the Taltz monotherapy group.
The results provide additional biological context for earlier findings demonstrating that the two medicines, when administered together, improved complete skin clearance and weight-related outcomes in adults with moderate-to-severe plaque psoriasis and obesity or overweight with at least one additional weight-related condition. The newly reported changes emerged as early as Week 12 and included inflammatory pathways associated with neutrophils, immune cells involved in inflammatory responses. Lilly’s analysis suggests that changes in certain neutrophil-associated markers may partly explain the additional improvement observed in psoriasis severity.
The announcement does not represent a new primary efficacy result, regulatory approval or proof that tirzepatide directly modifies psoriasis independently of its effects on weight and metabolism. The principal clinical findings from TOGETHER-PsO were reported earlier in 2026, with additional one-year outcomes released in August. The October presentation instead investigates the biological processes accompanying those clinical changes, addressing a question that could influence future research into the relationship between obesity, systemic inflammation and immune-mediated skin disease.
For dermatologists and pharmaceutical developers, the significance extends beyond comparing two treatment regimens. Psoriasis and obesity frequently coexist, and excess adiposity is associated with inflammatory abnormalities that may influence disease severity and treatment response. Understanding whether metabolic treatment can contribute to better control of immune-mediated disease could support more integrated management strategies, although the current findings remain exploratory and cannot establish the mechanism responsible for the observed clinical differences.
What do the 482 protein and 467 gene expression changes reveal about psoriasis and obesity?
The new TOGETHER-PsO analysis examined circulating proteins and gene expression in blood samples to investigate the biological responses associated with treatment. At Week 36, patients receiving Taltz and Zepbound showed 482 differentially expressed proteins compared with 140 among those receiving Taltz alone. The corresponding analysis identified 467 differentially expressed genes with combination treatment versus 16 with monotherapy, suggesting a broader measurable biological response when obesity treatment was added.
Differences were already apparent at Week 12, indicating that the observed molecular changes were not confined to the end of the primary treatment assessment period. Lilly reported that both treatment groups demonstrated biomarker patterns consistent with the known pharmacological activity of their respective medicines. However, the combination was associated with changes across a larger number of immune-related, psoriasis-associated and metabolic markers.
The distinction between the number of affected biomarkers and the clinical importance of those changes is essential. A larger number of differentially expressed proteins or genes does not automatically indicate greater therapeutic benefit, more effective immune regulation or improved long-term outcomes. Such findings depend on the statistical thresholds, biological pathways, sampling methods and individual changes within the dataset, none of which can be fully evaluated from the numerical counts alone.
The new analysis is therefore most useful as evidence that the combination produces a more extensive molecular response than Taltz monotherapy within the study conditions. Determining which changes are directly related to improvements in psoriasis, which reflect weight reduction and which represent broader metabolic effects will require further research. The findings provide a basis for investigating immunometabolic pathways rather than identifying a fully established mechanism of treatment benefit.
How could changes in neutrophil-associated inflammation help explain better psoriasis clearance?
One of the more interesting findings involves neutrophils, a class of immune cells that participate in inflammatory responses and contribute to the development of psoriasis lesions. Neutrophil activity is associated with several inflammatory pathways implicated in the disease, including processes connected with interleukin-17 signalling. The new analysis identified greater reductions in certain markers of inflammatory immune activity among patients receiving both medicines.
Lilly reported that changes in a subset of neutrophil-associated markers statistically mediated part of the additional improvement in Psoriasis Area and Severity Index scores observed with combination treatment. This suggests that the measured inflammatory changes may help account for differences in clinical responses between the groups. However, statistical mediation analysis does not independently establish that modifying those markers caused the improvement, particularly when multiple biological changes occur simultaneously.
The pharmacological mechanisms of the two medicines offer a plausible explanation for why their effects might differ from those of Taltz alone. Taltz is a monoclonal antibody that selectively inhibits interleukin-17A, a cytokine involved in psoriasis-associated inflammation. Zepbound activates glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptors, primarily supporting appetite regulation, reduced energy intake and weight management.
Obesity is associated with altered adipose tissue signalling, metabolic dysfunction and chronic low-grade inflammation, all of which may influence immune-mediated disorders. Reducing excess body weight could change inflammatory activity indirectly, while other metabolic or cellular effects of tirzepatide may also contribute. The current study cannot establish how much of the additional psoriasis improvement resulted from weight loss rather than other pharmacological consequences of treatment.
James G. Krueger, professor and laboratory head at The Rockefeller University, described the findings as evidence warranting further investigation of the relationship between immune and metabolic biology. His interpretation focused on the differences in inflammatory pathways observed when both conditions were treated concurrently. The results strengthen the rationale for mechanistic research without proving that tirzepatide has a direct, weight-independent anti-psoriasis effect.
How did Taltz and Zepbound perform on skin clearance and weight loss in the Phase 3b trial?
The new molecular findings follow previously reported efficacy results from TOGETHER-PsO, a randomized Phase 3b study involving 274 adults with moderate-to-severe plaque psoriasis and obesity or overweight with at least one weight-related comorbidity. Participants were assigned in a 1:1 ratio to receive Taltz with Zepbound or Taltz alone, with both groups receiving counselling on reduced-calorie diets and increased physical activity. The study used an open-label treatment design with masked assessors evaluating psoriasis outcomes.
At Week 36, the trial achieved its prespecified primary endpoint, which required participants to attain both complete skin clearance, measured by PASI 100, and at least 10% body weight reduction. This combined outcome occurred in 27.1% of patients receiving both medicines compared with 5.8% receiving Taltz alone. The absolute treatment difference was approximately 21.2 percentage points, with statistical significance at p less than 0.001.
The skin clearance results are particularly relevant when evaluating whether the combination offers benefits beyond weight management. PASI 100 was achieved by 40.6% of participants receiving both medicines compared with 29.0% receiving Taltz alone, a statistically significant difference with p equal to 0.04. This represents an absolute improvement of 11.6 percentage points, although the relatively modest statistical margin warrants appropriate interpretation.
Weight reduction was substantially more common in the combination group. At least 10% body weight loss occurred in 69.2% of participants receiving Taltz and Zepbound, compared with 9.1% of those receiving Taltz alone. These findings establish that adding tirzepatide produced a substantially greater weight-management effect in the studied population, alongside improvements in complete skin clearance.
The Week 36 results have also been reported in a peer-reviewed randomized clinical trial publication in JAMA Dermatology, providing a stronger evidentiary foundation than a company announcement alone. Nevertheless, the results do not establish whether adding tirzepatide to a different psoriasis biologic would produce comparable skin benefits. Nor do they demonstrate that the combination is superior to every alternative psoriasis treatment strategy.
Did the skin and metabolic improvements persist through 52 weeks of treatment?
Eli Lilly’s August 31, 2026 update provided additional information on the durability of the clinical findings. At Week 52, 30.6% of participants receiving Taltz and Zepbound achieved both complete skin clearance and at least 10% weight loss, compared with 4.4% receiving Taltz alone. Complete skin clearance was maintained in 40.5% of the combination group versus 29.1% of the monotherapy group.
The company also reported sustained or additional improvements in several exploratory metabolic measures, including body mass index, blood pressure, glucose, glycated haemoglobin, triglycerides and total cholesterol. High-sensitivity C-reactive protein, a circulating marker associated with systemic inflammation, also showed deeper improvement with the combination. These findings provide clinical context for the broader immune and metabolic changes described in the October analysis.
However, the Week 52 analyses were prespecified exploratory assessments without multiplicity control, and comparisons between time points were descriptive rather than confirmatory statistical tests. The findings support the durability of previously demonstrated effects but should not be presented as additional independently confirmed primary endpoint successes. Longer follow-up would also be needed to determine whether the metabolic changes translate into fewer cardiovascular events or other long-term health benefits.
The company separately reported sustained improvements in its TOGETHER-PsA study involving adults with psoriatic arthritis and obesity or overweight. That investigation evaluated different disease activity endpoints and should not be combined with the psoriasis findings as though they came from a single patient population. Together, the studies provide evidence supporting further investigation of concurrent immune and metabolic treatment, while their clinical conclusions remain specific to their respective designs.
What safety considerations remain when combining an IL-17 inhibitor with tirzepatide?
The potential benefits of combination treatment must be assessed alongside the safety considerations associated with administering two medicines with different mechanisms and adverse-event profiles. Taltz acts on the immune system and carries warnings involving serious infections, hypersensitivity reactions and the development or worsening of inflammatory bowel disease. Zepbound has a boxed warning concerning thyroid C-cell tumours observed in rodents, along with important precautions involving gastrointestinal events, pancreatitis, gallbladder disease and dehydration-related kidney injury.
The peer-reviewed Week 36 TOGETHER-PsO findings reported treatment-emergent adverse events in 71.0% of participants receiving both medicines compared with 66.4% receiving Taltz alone. Serious adverse events occurred in 3.6% and 6.0% of the respective treatment groups, while adverse events leading to treatment discontinuation were reported in 3.6% and 1.5%. These results were generally consistent with the established profiles of the individual medicines, but the differences should not be interpreted as evidence that combination treatment reduces serious adverse-event risk.
Gastrointestinal events were notably more frequent when tirzepatide was included. Nausea occurred in 24.6% of the combination group compared with 2.2% receiving Taltz alone, while diarrhoea was reported in 14.5% versus 1.5%. Constipation and vomiting were also more common among participants receiving both treatments.
Lilly’s subsequent Week 52 disclosure stated that no new safety concerns had been identified and that the observed adverse-event patterns remained consistent with known drug profiles. This provides additional reassurance within the observed trial period, although it does not establish the absence of rare or longer-term risks. The October biomarker presentation itself did not provide a new comprehensive safety assessment.
Does the new evidence change the FDA-approved indications for Taltz or Zepbound?
Both medicines are already commercially available in the United States, but their existing approvals cover different conditions. Taltz is approved for moderate-to-severe plaque psoriasis in patients aged six years and older who are candidates for systemic therapy or phototherapy, alongside specified adult indications including psoriatic arthritis and axial spondyloarthritis. Zepbound is approved for chronic weight management in eligible adults and for moderate-to-severe obstructive sleep apnea in adults with obesity.
The TOGETHER-PsO findings do not represent FDA authorization of a new fixed-dose combination, a new psoriasis indication for Zepbound or a change to the approved Taltz label. The study evaluated concurrent administration of two existing medicines in adults meeting defined psoriasis and weight-related eligibility criteria. Whether additional regulatory discussions or labeling changes will follow remains dependent on decisions by the company and the relevant authorities.
The distinction also matters for reimbursement and clinical implementation. Coverage for psoriasis treatment and obesity medicines can involve separate eligibility requirements, prior authorization criteria and payer policies. Patients who meet the trial’s clinical characteristics may not necessarily have insurance coverage for both medicines, and the costs of combined treatment could influence adoption even where clinical use is appropriate.
From a commercial perspective, Eli Lilly is investigating how two established therapeutic franchises could address overlapping conditions rather than advancing an entirely new molecular asset. The broader opportunity concerns identifying patient populations in which treating metabolic disease alongside immune-mediated inflammation provides additional measurable benefit. However, the October announcement did not include revenue projections, a regulatory submission timeline or evidence that the biomarker results had changed commercial prescribing patterns.
What are the main limitations of the new TOGETHER-PsO biomarker findings?
Although the parent study used randomization, the latest molecular analysis was exploratory, and the disclosed summary does not provide sufficient detail to fully assess every analytical decision. Important considerations include the number of participants contributing evaluable blood samples, the criteria used to identify differentially expressed proteins and genes, and the statistical methods applied to large numbers of molecular measurements. These details are particularly important when interpreting large differences in biomarker counts.
Another limitation is that the study examined circulating biomarkers rather than directly demonstrating the same molecular changes within psoriasis lesions. Blood measurements can provide valuable information about systemic inflammatory activity, but they do not necessarily reveal the precise biological processes operating in affected skin. Establishing how the observed changes relate to tissue-level responses would require additional mechanistic investigation.
The trial’s population also influences generalizability. Participants had a mean baseline body mass index of approximately 39.2, indicating substantial obesity on average, and all had moderate-to-severe plaque psoriasis. The findings therefore do not establish that comparable benefits would occur among patients with normal body weight, milder disease or different metabolic characteristics.
The open-label design introduces another consideration because participants and treating clinicians knew which regimen had been assigned. Masking the psoriasis assessors helped reduce some potential measurement bias, while the randomized design strengthened the clinical comparison. Nevertheless, the molecular findings cannot independently separate the effects of weight reduction, medication exposure and changes in systemic inflammation.
Could the results influence future immunometabolic treatment strategies beyond psoriasis?
The relationship between obesity and immune-mediated disease has become an increasingly important area of research because metabolic dysfunction can coexist with conditions involving chronic inflammation. Investigators are examining whether treatments developed primarily for weight management may influence outcomes in other disease areas through direct or indirect biological pathways. TOGETHER-PsO contributes to this research by combining clinical efficacy measurements with molecular assessments in a randomized population.
The findings may encourage further investigations involving patients with psoriasis who have metabolic conditions that complicate disease management. They may also inform studies examining whether different degrees of weight reduction correspond with particular inflammatory changes or improvements in response to biologic therapy. Such research could help distinguish patients who benefit primarily from weight loss from those who might experience additional immune-related effects.
However, the current results cannot be generalized to other incretin medicines, alternative biologics or unrelated inflammatory diseases. Tirzepatide’s dual GIP and GLP-1 receptor activity differs from treatments that target GLP-1 alone, while immune-mediated diseases vary considerably in their underlying mechanisms. Any broader clinical application would require evidence appropriate to the specific medicine, combination and patient population.
For Eli Lilly, the next scientific priority will be to establish whether the molecular changes identified in TOGETHER-PsO can be reproduced and linked more convincingly to meaningful clinical outcomes. Detailed analyses of the neutrophil-associated pathways, the relationship between weight reduction and skin clearance, and the durability of treatment effects could help explain why the combination performed differently from Taltz alone.
The October 2026 findings add a mechanistic dimension to a programme that has already demonstrated statistically significant improvements in its Week 36 clinical endpoints. Yet the number of altered proteins and genes is not itself a measure of treatment success, and statistical associations with inflammatory pathways do not establish causal mechanisms. The most important contribution of the new analysis is its evidence that concurrent treatment of psoriasis and obesity is accompanied by broader measurable biological changes, creating a clearer scientific basis for investigating how metabolic and immune pathways interact in patients living with both conditions.
