Comparative Pharmacological Profiling of Psychotherapeutic Drugs Reveals a Functional Taxonomy Based on Direct Inhibition of Smooth Muscle Excitability

Background: Autonomic side effects are a major determinant of tolerability for many psychotherapeutic drugs. While often attributed to receptor-mediated mechanisms, the potential contribution of direct modulation of smooth muscle excitability remains poorly characterized at a comparative pharmacological level. Methods: A systematic comparative pharmacological profiling of a broad panel of psychotherapeutic drugs (antidepressants, antipsychotics, and anxiolytics) was conducted using a standardized ex vivo model. Potassium chloride (KCl, 105 mM) was used to induce depolarization-dependent contraction in three isolated smooth muscle preparations (rat uterus, rat vas deferens, and guinea-pig ileum). Inhibitory potency (IC50), dose-dependency, and tissue consistency were integrated to define functional inhibitory profiles. Results: Psychotherapeutic drugs exhibited marked heterogeneity in their ability to inhibit K+-induced smooth muscle contraction. Integrative analysis stratified compounds into four distinct functional profiles: (i) High Inhibitory Liability (e.g., nortriptyline, paroxetine), characterized by low micromolar IC50 values and dose-dependent inhibition across multiple tissues; (ii) Non-Selective Inhibition (e.g., flunarizine, cinnarizine), showing consistent but dose-independent inhibition; (iii) Tissue-Dependent Inhibition (e.g., risperidone, reboxetine); and (iv) Minimal Inhibition (e.g., moclobemide). Agents classified within the High Inhibitory Liability profile correspond to drugs known to carry a higher clinical burden of autonomic adverse effects. Conclusions: This study reveals a previously underrecognized pharmacodynamic dimension of psychotherapeutic drugs and establishes a comparative functional taxonomy based on their direct, non-receptor-mediated inhibition of smooth muscle excitability. The identified profiles provide a mechanism-informed framework for contextualizing autonomic side-effect liability and may support improved safety evaluation in psychotherapeutic drug development.