Mechanistic Insights into the Cardiotoxic Effects of Dextromethorphan and Its Metabolite Dextrorphan in Zebrafish Using Network Toxicology and Molecular Dynamics

The high incidence of respiratory diseases has driven a substantial increase in the use of antitussive dextromethorphan (DXM), leading to the frequent detection of its primary human metabolite, dextrorphan (DXO), in aquatic environments. However, existing research has paid limited attention to the toxic effects of DXM and its metabolite, DXO, on fish. In this study, zebrafish embryos were exposed to DXM and DXO at concentrations ranging from 100 to 10,000 ng/L for 72 hours post-fertilization (hpf) to evaluate their impacts on cardiac function and elucidate underlying molecular mechanisms using network toxicology and molecular dynamics approaches. Bradycardia was observed at 48 hpf. By 72 hpf, key cardiac morphometric and functional indices, as well as blood flow, exhibited concentration-dependent suppression after exposure to DXM and DXO. Network toxicology analysis revealed that the beta2-adrenergic receptor (<i>adrb2a</i>) was the key common target in cardiomyocyte adrenergic signaling for both compounds. Further observations included significant downregulation of <i>adrb2a</i>, decreased levels of beta2-adrenergic receptor and protein kinase A proteins, and phenotypic rescue after coexposure to adrenaline. The binding energies of DXM and DXO to the beta2-adrenergic receptor were -23.45 and -25.76 kcal/mol, respectively. These findings confirmed that DXM and DXO impaired cardiomyocyte function via suppression of beta2-adrenergic receptor expression, leading to bradycardia. It is worth noting that DXO exhibited stronger cardiac toxicity than its parent compound DXM, and its ecological toxicity warrants special attention and further evaluation.