Breast cancer remains the most prevalent malignancy among women globally, with hormone receptor-positive subtypes representing the majority of cases. Despite significant advances in treatment, resistance to Tamoxifen, a cornerstone therapy for estrogen receptor-positive (ER⁺) breast cancer, poses a critical challenge, affecting up to 50 % of patients. Emerging evidence suggests that dysregulated lipid metabolism plays a pivotal role in breast cancer progression and therapy resistance. This systematic review aims to explore the intricate relationship between lipid metabolism and Tamoxifen resistance, with a particular focus on the potential therapeutic synergy of combining Tamoxifen with lipid metabolism modulators, such as Quercetin. We systematically searched PubMed, Scopus, Web of Science, and the Cochrane Library for studies published between 2000 and 2023, examining the role of lipid metabolic pathways in breast cancer and the impact of Quercetin on enhancing Tamoxifen efficacy. The findings suggest that targeting key enzymes involved in lipid metabolism, including fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC), may impair cancer cell survival mechanisms and sensitize tumors to Tamoxifen. The combination of Tamoxifen and Quercetin appears to exhibit synergistic effects, enhancing apoptosis and reducing cell proliferation more effectively than either agent alone. This review highlights the potential of combining Tamoxifen with Quercetin as a therapeutic strategy to overcome Tamoxifen resistance, providing a rationale for further clinical trials to investigate this combination therapy.

Chemotherapy remains foundational to oncology, yet treatment response, resistance, and gastrointestinal toxicity vary widely between patients in ways that tumor stage, molecular subtype, and host pharmacogenomics do not fully explain. The gut microbiome contributes to this variability through two primary mechanisms: direct biotransformation and bioaccumulation of cytotoxic agents by microbial enzymes, and indirect modification of epithelial barrier integrity, inflammatory tone, and ecological resilience after cytotoxic injury. We propose precision pharmacomicrobiomic conditioning: the drug-specific, timing-anchored preservation or reshaping of microbial function before and during chemotherapy, which is distinguished from generic microbiome modulation by its linkage to a defined pharmacologic and toxicity profile. We operationalize this concept through two original constructs: five-archetype baseline risk classification stratifying patients by microbial enzymatic, inflammatory, ecological, and recovery phenotypes; and a three-phase clinical workflow specifying the assay, time point, and decision rule from pre-treatment stratification through adaptive longitudinal monitoring. These constructs are proposed strictly for prospective validation and are not presented as clinically validated decision tools. Furthermore, while dynamic functional and metabolic biomarkers may have higher predictive value than static, single bacterial taxa, this hypothesis requires rigorous confirmation in sufficiently powered, externally validated clinical cohorts. We conclude with the minimum reporting standards required to test these constructs: chemotherapy exposure metrics, toxicity grading versions, functional and strain-level profiling, and engraftment endpoints, and position microbiome profiling as an additional stratification layer integrated with pharmacogenomics and conventional clinical predictors, not as independent grounds for dose modification. Currently, no microbiome-derived biomarker has undergone prospective validation or received regulatory approval for standard chemotherapy stratification.