The Skeletal Muscle Relaxer Cyclobenzaprine Is a Potent Non-Competitive Antagonist of Histamine H1 Receptors

Cyclobenzaprine is a tricyclic dimethylpropanamine skeletal muscle relaxant, which is used clinically to decrease muscle spasm and hypercontractility, as well as acute musculoskeletal pain. Although the absolute mechanism of action of cyclobenzaprine remains elusive, it is known to mediate its effects centrally via inhibition of tonic somatic motor function, likely through modulation of noradrenergic and serotonergic systems. While cyclobenzaprine is effective as a muscle relaxant, greater than 30% of patients experience drowsiness and sedative-hypnotic effects, yet the mechanisms that cause this adverse effect are also undescribed. Based on this common adverse effect profile and the structural similarity of cyclobenzaprine to tricyclic antidepressants, as well as ethanolamine first-generation antihistamines, we hypothesized that cyclobenzaprine facilitates sedative effects via off-target antagonism of central histamine H1 receptors (H1Rs). Here, for the first time, we present data that demonstrate that cyclobenzaprine exhibits low nanomolar affinity for the cloned human H1R, as well as that expressed in both rat and mouse brain. Using saturation radioligand binding, we also demonstrate that cyclobenzaprine binds to the H1R in a noncompetitive manner. Similarly, functional assays measuring both Ca⁺² influx and novel TRUPATH G-protein subunit bioluminescence resonance energy transfer biosensors reveal that cyclobenzaprine also blocks histamine-mediated H1R functional activity in a noncompetitive manner, whereas the classical H1R antagonist diphenhydramine does so competitively. Given that cyclobenzaprine readily crosses the blood-brain barrier and its muscle relaxant effects occur centrally, our data suggest that off-target central antagonism of H1R by cyclobenzaprine facilitates the significant sedative effect of this agent seen in patients. SIGNIFICANCE STATEMENT: Cyclobenzaprine, a clinically used muscle relaxant that is strongly linked to sedation, demonstrates high-affinity noncompetitive antagonism at the histamine H1 receptor. This effect likely modulates the high degree of sedation that patients experience.