Novel anxiolytic-antidepressant combinations TF-1 and TF-2 and their intervention mechanisms in a mouse model of comorbid anxiety and depression (CAD)

Background and Objective To address the high mortality, slow onset, and suboptimal efficacy of current treatments for comorbid anxiety and depression (CAD), this study developed and systematically evaluated a novel combination formulation. Methods The anxiolytic tandospirone (TDS) was combined with fluoxetine (FLU), escitalopram, or milnacipran for cell-based screening. The optimal combination was selected for pharmacokinetic studies in rats, followed by a 49-day pharmacodynamic evaluation and mechanistic investigation in a CAD mouse model. Results Two optimal TDS-FLU combinations, TF-1 (TDS:FLU = 1.2:1,w/w) and TF-2 (TDS:FLU = 2.4:1,w/w), were obtained, both exhibiting good neuroprotective effects. Pharmacokinetic results showed that TF-1 and TF-2 significantly increased drug exposure and prolonged half-life compared with monotherapies of TDS and FLU. Pharmacodynamically, TF-1 and TF-2 exhibited superior anxiolytic-antidepressant efficacy compared with monotherapies after 28-day and 49-day interventions. At day 28, both formulations significantly increased open-field central-zone activity and sucrose preference, and reduced immobility time (all P < 0.05). Their therapeutic benefits were further augmented at day 49, with sustained and improved behavioral improvements upon long-term treatment. Mechanistic studies showed that both combinations elevated plasma and hippocampal 5-HT and BDNF levels, upregulated hippocampal 5-HT1AR and TrKB expression, and enhanced neuronal plasticity. Conclusion TF-1 and TF-2 were superior to single agents in pharmacokinetics and long-term CAD efficacy, acting via 5-HT modulation and BDNF-TrKB activation, supporting their development as CAD combination therapies.