From Fibromyalgia to Depression: TNX-102 Sublingual Cyclobenzaprine Trials for MDD

Gregory Sullivan, MD, discussed the pharmacological rationale for TNX-102 SL (sublingual cyclobenzaprine) and the anticipated tolerability advantages of this approach in the ongoing HORIZON phase 2 trial for major depressive disorder (MDD).

Sullivan explained that cyclobenzaprine's receptor activity is dose-dependent: at higher oral doses, it also inhibits serotonin and norepinephrine reuptake, effects that TNX-102 SL's very low dose of 5.6 mg is specifically designed to avoid, concentrating activity on the 2 target receptors—5-HT2A and alpha-1 adrenergic—most relevant to sleep architecture restoration. The sublingual delivery route is central to this selectivity strategy. By bypassing hepatic first-pass metabolism, the formulation substantially raises the ratio of cyclobenzaprine to its active metabolite norcyclobenzaprine in systemic circulation. Norcyclobenzaprine is a potent norepinephrine reuptake inhibitor that is long-acting and prosympathetic—properties that would counteract the intended nighttime deep sleep-promoting mechanism and generate daytime adverse effects including somnolence, fatigue, dry mouth, and dizziness. Sullivan noted that with oral cyclobenzaprine, norcyclobenzaprine accumulates substantially and exerts off-target receptor activity around the clock, whereas the sublingual formulation is designed to confine receptor blockade largely to the sleep period.¹

Sullivan contrasted the anticipated tolerability profile of TNX-102 SL with that of conventional antidepressants, including SSRIs and SNRIs, citing as common concerns sexual dysfunction, weight gain, emotional blunting, apathy, cognitive impairment, sedation, and sleep disruption—effects he characterized as resulting from off-target activity rather than antidepressant mechanism. He noted that in fibromyalgia trials, TNX-102 SL was associated with enhanced rather than impaired sexual function in female participants relative to placebo.² Sullivan framed TNX-102 SL's target product profile as a first-line antidepressant offering comparable efficacy to existing agents but with a substantially cleaner daytime tolerability profile. He also cited supportive preliminary data from a post-traumatic stress disorder trial in which MADRS scores improved with TNX-102 SL, providing cross-diagnostic signal for antidepressant activity.

Dr Sullivan is chief medical officer at Tonix Pharmaceuticals.

References

1. Tonix Pharmaceuticals announces first patient enrolled in phase 2 HORIZON study of TNX-102 SL for the treatment of major depressive disorder. Press release. June 29, 2026. https://ir.tonixpharma.com/news-events/press-releases/detail/1634/tonix-pharmaceuticals-announces-first-patient-enrolled-in

2. Tonix Pharmaceuticals presents additional data highlighting the favorable tolerability and differentiated side effect profile of TNX-102 SL in second positive phase 3 clinical trial for the management of fibromyalgia. Press release. January 9, 2024. https://ir.tonixpharma.com/news-events/press-releases/detail/1446/tonix-pharmaceuticals-presents-additional-data-highlighting