Eli Lilly's Multi-Indication Strategy: Expanding the GLP-1 TAM via Joint Pain and Psoriatic Arthritis Trials

Eli Lilly is aggressively executing a multi-indication strategy to expand the total addressable market (TAM) of its incretin therapeutics beyond general obesity and diabetes. By targeting highly prevalent, high-burden inflammatory and musculoskeletal conditions—specifically knee osteoarthritis pain and psoriatic arthritis—Lilly is positioning its drugs as essential clinical treatments rather than lifestyle or cosmetic weight-loss choices.

1. Retatrutide in Knee Osteoarthritis (TRIUMPH-4)

On December 11, 2025, Lilly announced positive topline results from its Phase 3 TRIUMPH-4 clinical trial. The global registration trial evaluated retatrutide (9 mg and 12 mg once-weekly), its triple hormone receptor agonist, in adults with obesity/overweight and knee osteoarthritis (without diabetes).

• Massive Pain Relief: Retatrutide met all primary and key secondary endpoints, reducing knee pain on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain scale by up to an average of 4.5 points (75.8%) at 68 weeks.
• Pain-Free Outcomes: In a post-hoc analysis, 14.1% of patients on the 9 mg dose and 12.0% on the 12 mg dose were completely free of knee pain at 68 weeks, compared to just 4.2% on placebo.
• Weight Loss: Participants taking retatrutide 12 mg lost an average of 28.7% of their body weight (71.2 lbs) at 68 weeks.
• Clinical Meaning: Obesity is a primary driver of knee osteoarthritis due to mechanical loading and systemic inflammation. By proving that retatrutide can arrest pain and potentially delay or prevent total joint replacements, Lilly opens up a massive musculoskeletal market.

2. Zepbound and Taltz Combination in Psoriatic Arthritis (TOGETHER-PsA)

On January 8, 2026, Lilly announced positive topline results from its novel TOGETHER-PsA open-label Phase 3b trial. The study evaluated the concomitant use of Lilly's IL-17A biologic Taltz (ixekizumab) and GIP/GLP-1 receptor agonist Zepbound (tirzepatide) in adults with active psoriatic arthritis (PsA) and obesity or overweight.

• Dual-Targeting Efficacy: The trial met its primary endpoint, with 31.7% of patients on the Taltz plus Zepbound combination achieving both a 50% improvement in arthritis activity (ACR50) and $\ge 10%$ weight reduction at 36 weeks, compared to just 0.8% of patients on Taltz monotherapy.
• Enhanced Joint Control: Concomitant use delivered a 64% relative increase in the proportion of patients achieving ACR50 compared to Taltz alone (33.5% vs. 20.4%).
• Clinical Meaning: Psoriatic arthritis is a complex inflammatory-metabolic disease where obesity worsens joint symptoms and reduces the efficacy of biologic therapies. This trial makes Taltz the first biologic for PsA with clinical data supporting a comprehensive, integrated treatment approach alongside an incretin therapy.

Investor Takeaway

Lilly's ability to demonstrate that its incretins act synergistically with biologics or serve as standalone therapies for structural joint conditions significantly expands the clinical utility and insurance reimbursement potential of these drugs. This strategy targets millions of patients who suffer from joint pain and systemic inflammatory disorders, representing a major TAM expansion that bypasses traditional obesity coverage barriers.