Cytochrome P450 2D6 (CYP2D6) Inhibition by Bergamottin and Diosmetin as a Strategy to Enhance the Pharmacokinetics and Antidepressant Efficacy of Amoxapine

Amoxapine (AMX) is an FDA-approved tricyclic antidepressant used to treat major depressive disorders by modulating monoaminergic neurotransmitters, including serotonin, dopamine, and noradrenaline. However, its clinical efficacy is limited due to poor aqueous solubility, rapid first-pass metabolism, and extensive biotransformation by cytochrome P450 enzymes, particularly CYP2D6, leading to reduced systemic and brain availability. Bergamottin (BER) and diosmetin (DIO), natural furanocoumarin and flavonoid compounds, respectively, are known inhibitors of CYP3A4 and CYP2D6 and may enhance AMX pharmacokinetics through drug-herb interactions. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the quantification of amoxapine during pharmacokinetic and neurotransmitter studies. <i>In vitro</i> and <i>in vivo</i> drug-herb interactions of AMX with bergamottin and diosmetin, as well as their impact on brain biodistribution and neurotransmitter levels in mice, were evaluated. <i>In vitro</i> CYP2D6 inhibition assays demonstrated that BER and DIO significantly suppressed CYP2D6-mediated dextromethorphan metabolism in a concentration-dependent manner, while rat liver microsome studies showed markedly improved metabolic stability of AMX, with reduced intrinsic clearance and extended half-life in the presence of BER and DIO compared with AMX alone. <i>In vivo</i> pharmacokinetic evaluation revealed a 1.26-fold and 2.13-fold increase in <i>C</i> _max, along with enhanced bioavailability, when AMX was coadministered with bergamottin (BER) and diosmetin (DIO), respectively. Behavioral testing (forced swim test (FST), tail suspension test (TST) and open field test (OFT)) demonstrated significantly improved antidepressant-like responses with combination therapy versus AMX alone. Biodistribution studies confirmed increased AMX accumulation in the hippocampus and cortex, accompanied by elevated monoamine neurotransmitters, including serotonin, dopamine, norepinephrine, and epinephrine. These findings demonstrate that BER and DIO augment the pharmacokinetic and pharmacodynamic profiles of AMX through CYP2D6 inhibition and improved brain delivery, supporting their potential as natural bioenhancers to enhance antidepressant efficacy. This combination strategy represents a promising approach for developing superior AMX-based therapeutic regimens and warrants further clinical investigation.