Eli Lilly announced 52-week results on 31 August 2026 from two Phase 3b trials testing whether adding tirzepatide (Zepbound) to the psoriasis biologic ixekizumab (Taltz) works better than the biologic alone in adults who have psoriatic disease and obesity. It did, on both trials’ primary outcomes and by wide margins. But the primary outcomes were built so that the combination was always going to win them, and the more interesting numbers are the secondary ones.
What the trials reported
TOGETHER-PsO (NCT06588283, 281 participants) enrolled adults with moderate-to-severe plaque psoriasis; TOGETHER-PsA (NCT06588296, 279 participants) enrolled adults with active psoriatic arthritis. Both required obesity, or overweight plus at least one weight-related condition; mean baseline BMI was 39.2 and 37.6 respectively. Both are registered as randomised, multicentre, open-label studies, which Lilly describes as assessor-blinded. Each compared the two drugs together against ixekizumab alone.
In TOGETHER-PsO the primary outcome combined complete skin clearance (PASI 100) with at least 10% weight loss: 30.6% on the combination versus 4.4% on ixekizumab alone at week 52. In TOGETHER-PsA the primary outcome combined a 50% reduction in disease activity (ACR50) with at least 10% weight loss: 39.2% versus 1.7%. Both composites include a weight-loss threshold that a psoriasis biologic has no mechanism to reach, so the comparison is close to foregone. The disease-activity endpoints on their own are the ones that test something: PASI 100 was 40.5% versus 29.1%, an 11.4-point gap, and ACR50 was 43.7% versus 15.7%, a 28-point gap. Put another way, on the arthritis measure roughly one additional patient in four reached ACR50 who would not have on the biologic alone; on the skin measure it was closer to one in nine. Lilly also reported deeper reductions in hs-CRP and sustained or improved BMI, blood pressure, glucose, HbA1c, triglycerides and total cholesterol. Adverse events reported in at least 5% of the combination arm were nausea, diarrhoea, constipation, injection-site reactions, vomiting, dizziness and headache.
What it changes, and what it does not
For now, not what anyone can do at a pharmacy counter. Tirzepatide is not approved for psoriasis or psoriatic arthritis, and nothing reported here changes its label. What these trials add is evidence for a claim that has been argued from association rather than tested: that treating the metabolic side of immune-mediated disease improves the immune side too. The detail Lilly highlights in support of that is the timing — in the arthritis trial, the disease activity separation appeared by week 4, before clinically meaningful weight loss had occurred. If that holds up in the full publication, it points at something other than weight as the mechanism, which would be a genuinely new argument for this class of drug rather than a restatement of the case for losing weight.
Almost all of the load-bearing detail. This is a company press release; detailed results have not been published or peer reviewed, and Lilly says they will be presented at future medical meetings. The week-52 analyses are described as pre-specified but exploratory, without multiplicity control, and comparisons between timepoints as descriptive and not statistically tested — so the week-52 numbers are weaker evidence than the week-36 primary endpoint they follow. The studies are open-label, with no placebo arm and no tirzepatide-alone arm, which means they cannot separate a drug effect from the effect of losing weight, or from knowing which treatment you are on. Around 280 participants per trial is small for detecting anything but large differences. No safety signal has emerged, but a year in 560 people would not be expected to surface one.
Sources: Eli Lilly and Company, “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,” 31 August 2026. Lilly investor news release · ClinicalTrials.gov NCT06588283 (TOGETHER-PsO) · ClinicalTrials.gov NCT06588296 (TOGETHER-PsA)
