GLP-1 Agonists: From the Scale to the Joint in Arthritis

Observed each October 12, World Arthritis Day has historically been a moment to spotlight unmet need in inflammatory and degenerative joint disease. In 2026, one of the most-discussed developments in that space did not originate in rheumatology at all. Glucagon-like peptide-1 (GLP-1) receptor agonists and dual incretin agents — developed for type 2 diabetes and then obesity — have begun to reshape conversations in the arthritis clinic, as a growing body of data suggests their benefit in psoriatic and other inflammatory arthritides may extend beyond weight loss alone.

The clearest signal to date comes from the TOGETHER-PsA trial, the first controlled study to pair an incretin therapy with a biologic in psoriatic arthritis (PsA). The open-label phase 3b study randomized 271 adults with active PsA and obesity, or overweight with at least one weight-related comorbidity (mean body mass index, 37.6), to the interleukin (IL)-17A inhibitor ixekizumab plus the dual glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 receptor agonist tirzepatide, or to ixekizumab alone; all participants received diet and exercise counseling.^¹ At week 36, 31.7% of the combination group met the primary composite endpoint of an American College of Rheumatology 50% (ACR50) response plus at least 10% weight reduction, versus 0.8% with ixekizumab monotherapy (P < .001).^² By 52 weeks, that composite was met by 39.2% versus 1.7%.^³

A Signal Beyond the Scale

The composite primary endpoint builds weight loss into its definition, so superiority there is unsurprising. The more scrutinized finding is the joint-specific secondary endpoint, which excludes weight: ACR50 was achieved by 33.5% of the combination arm versus 20.4% with ixekizumab alone (P = .020), with separation between groups observed as early as week 4.^¹ That early divergence — before substantial weight loss could accrue — is what has drawn investigators' attention.

Joseph F. Merola, MD, MMSc, professor and chair of dermatology at UT Southwestern Medical Center in Dallas, Texas, who presented TOGETHER-PsA data, framed the result as a possible mechanistic departure from the expected weight-loss story.

"What was exciting to see with Together PSA is that we do see a substantial bump in efficacy in clinically meaningful joint endpoints," Merola said. "It's beyond just the weight loss piece or even the joint piece, but we're seeing that synergy."

He was careful not to overstate the mechanism. "I don't want to over-read, but we saw very early separation, before there was much weight loss in the study, which might even posit that might we be impacting some of the systemic inflammation, some of the adipocyte-cytokine milieu that drives potentially inflammation in skin and joints," Merola said. The rationale rests on the recognition that adipose tissue is not inert: obese adipose tissue produces adipokines and pro-inflammatory cytokines, and obesity is both a risk factor for and a comorbidity of psoriatic disease, with estimates that upwards of 70% to 80% of patients with psoriatic disease meet overweight or obese criteria.

Philip J. Mease, MD, MACR, director of rheumatology research at Swedish Medical Center/Providence and clinical professor at the University of Washington in Seattle, and a TOGETHER-PsA co-author, has been developing educational programming on obesity in psoriatic disease through the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA), which he previously led. He situated the combination result within that pathophysiology.

"One of the things that we've been seeing more and more is with the addition of the GLP-1 receptor agonist that patients are coming back and saying, you know, I'm feeling better overall, my disease process is better," Mease said. "We're getting a hint that is to be will be proven with further studies that the GLP-1 receptor agonist group, particularly tirzepatide, can be immunomodulatory in and of themselves in addition to the other treatments that we're giving to these patients."

Reading the Secondary Signal With Caution

The enthusiasm is tempered by the structure of the evidence. TOGETHER-PsA is open-label, and the headline composite endpoint pairs a joint response with a weight threshold that only the incretin arm could realistically achieve — a design that all but guarantees separation on the primary measure. The joint-only ACR50 difference is a secondary endpoint, and Merola himself noted that the magnitude of weight loss alone would be expected to improve patient-reported outcomes and function regardless of any direct anti-inflammatory effect. The mechanistic question of whether tirzepatide acts on synovial inflammation independent of weight remains unresolved and, by the investigators' own framing, awaits dedicated mechanistic study.

Eric M. Ruderman, MD, professor of medicine at Northwestern University Feinberg School of Medicine in Chicago, Illinois, offered the more measured vantage, placing GLP-1 agents in the context of a field still defining their role.

"That's the next big frontier, is the GLP-1," Ruderman said, pointing to the ongoing combination trial in PsA pairing ixekizumab with tirzepatide. He noted the practical reality already reshaping clinics: "A third of the people walk into my office are taking a GLP-1 antagonist. And many of them tell me that they feel better."

Like Merola and Mease, Ruderman suspects the effect is not purely attributable to weight.

"There's something there, and it's not just weight loss, I don't think. And we'll learn more about that," he said. But he positioned the drugs as adjuncts rather than disease-modifying antirheumatic therapies. "They're not going to be primary therapy, but as an adjunct to whatever else we're doing, I think they're going to have tremendous impact."

On the broader search for whom these drugs help most, Ruderman was blunt about how early the field is: much of the past year's work in PsA and axial spondyloarthritis, he noted, has focused on simply defining the difficult-to-treat and treatment-refractory populations before trials can be built around them.

Safety, Function, and Open Questions

On tolerability, the TOGETHER-PsA safety profile was consistent with the known profiles of both agents, with gastrointestinal adverse events attributable to tirzepatide; discontinuation for GI events was low, at 2.9% in the combination group versus 0.8% with ixekizumab alone.^¹ Merola flagged this as clinically reassuring: "This is not something where patients are running for the hills because of GI side effects."

Merola also highlighted a less expected secondary finding — improvement in the mental component score of the SF-36, a measure he noted is often static across PsA programs. He cautioned that the observation needs further analysis but tied it to the broader theme of treating skin, joints, and weight together. A separate concern raised in the wider discussion of GLP-1 use, and relevant to an arthritis population, is the loss of lean muscle mass accompanying rapid weight reduction — an issue that newer adjunctive agents aimed at preserving muscle may eventually address, but which currently argues for pairing these drugs with resistance training and dietary support that many rheumatology practices are not yet structured to provide.

A recurring, unsettled question is who should prescribe. "Should we, as dermatologists, as rheumatologists, be the ones prescribing these drugs?" Merola asked, suggesting that at minimum clinicians should discuss the option with patients and then decide whether to own the prescription or co-manage with primary care or endocrinology. "As long as we get the patient to the right place, I think that that's very reasonable."

For now, the 3 perspectives converge on a cautious optimism: the TOGETHER-PsA data represent the strongest controlled signal yet that incretin therapy adds value in inflammatory arthritis, the early separation hints at a mechanism beyond weight, and the definitive mechanistic and long-term outcome data are still to come. As Mease put it, the field is "getting a hint" of something it expects larger studies to confirm.

Mease’s disclosures include AbbVie, Amgen, BMS, Eli Lilly, Johnson & Johnson, Novartis, Pfizer, and others. Merola’s disclosures include Amgen, Astra-Zeneca, Biogen, Boehringer Ingelheim, Bristol-Myers Squibb, Abbvie, Dermavant, Eli Lilly, Moonlake, Novartis, Janssen, Oruka, UCB, Sanofi, Regeneron, Sun Pharma, Galderma, Biogen, Pfizer, and others. Ruderman’s disclosures include AbbVie, BMS, Janssen, Merck, Novartis, NS Pharma, Takeda, Treg, UCB, and others.

References

1. Combination therapy improves outcomes in psoriatic arthritis with obesity. Practical Dermatology. Accessed October 12, 2026. https://practicaldermatology.com/news/combination-therapy-improves-outcomes-in-psoriatic-arthritis-with-obesity/2486455/

2. Lilly's Taltz (ixekizumab) and Zepbound (tirzepatide) used together delivered superior efficacy in first-of-its-kind phase 3b trial for adults with active psoriatic arthritis and obesity or overweight. News release. Eli Lilly and Company. January 8, 2026. Accessed October 12, 2026. https://investor.lilly.com/news-releases/news-release-details/lillys-taltz-ixekizumab-and-zepbound-tirzepatide-used-together

3. New phase 3b data on Lilly's Taltz (ixekizumab) and Zepbound (tirzepatide) used together showed improved and durable efficacy at one year in adults with psoriatic disease and obesity. News release. Eli Lilly and Company. August 31, 2026. Accessed October 12, 2026. https://investor.lilly.com/news-releases/news-release-details/new-phase-3b-data-lillys-taltz-ixekizumab-and-zepbound