Mechanism of Off-Target Interactions and Toxicity of Tamoxifen and Its Metabolites

Tamoxifen is an estrogen modulator that acts to competitively inhibit the binding of endogenous estrogens. It is widely used for treatment of breast cancer; however, analogous with many antineoplastic agents, tamoxifen is associated with numerous adverse effects, most prominently nausea. We have identified several off-target receptors of tamoxifen and 22 of its metabolites that include histamine H1 and H3, and muscarinic M1, M4, and M5 subtypes, and dopamine D2 receptor. We have shown how they are associated with tamoxifen and its metabolites' toxicity through a comprehensive computational analysis of their interaction modes, which were also compared to that of the related endogenous substrates of each receptor. The results were further evaluated using available in vivo and in vitro data. The presented work provides foundational knowledge toward the determination of the precise mechanism of nausea induction, and in particular, interactions of tamoxifen and its metabolites with the receptors involved in that biomolecular pathway. This study can assist in predicting the potential undesired effects of the chemicals with common pharmacophores or similar fragments to that of tamoxifen and its metabolites and serve drug discovery research in developing more effective and tolerable tamoxifen analogues or chemotherapeutic agents.