The present study provides a comprehensive view of the role of the UGT2B family, particularly UGT2B17, as a possible biomarker of resistance to neoadjuvant treatment in HER2-positive and HR-negative breast cancer. Through a sequential approach based on the analysis of gene expression using a microarray in the discovery phase, more than 6,200 differentially expressed genes were identified between patients with and without a response to treatment, which highlights the great molecular heterogeneity of HER2-positive breast cancer [19, 20]. The identification of altered metabolic pathways in treatment-resistant tumors has significant biological and clinical relevance since these molecular circuits represent functional nodes where the processes of detoxification, drug biotransformation and tumor metabolic reprogramming converge. Alterations in pathways such as xenobiotic metabolism and steroid hormone biosynthesis not only affect the intratumoral pharmacokinetics of chemotherapeutic agents but also modulate the cellular microenvironment, affecting proliferation, apoptosis and the ability to evade therapies. The overexpression of genes in the UGT2B family in this context could not only promote the inactivation of cytotoxic drugs by glucuronidation but also modulate the levels of steroids and bioactive metabolites, favoring resistance phenotypes [13, 21, 22]. Our analysis confirmed the existence of clearly differentiated gene expression profiles according to the therapeutic response, which reinforces the hypothesis that resistance to neoadjuvant therapy is determined by complex and multifactorial molecular mechanisms. Among the protein-coding genes, the UGT2B family (especially UGT2B17, UGT2B28 and UGT2B10) was upregulated in nonresponders, with UGT2B17 showing the highest discriminative value in the validation cohort (AUC 0.699, sensitivity 77.2%, specificity 64.5%) [19, 20].

UGT enzymes can contribute to drug resistance in cancer through glucuronidation, a biotransformation process that inactivates lipophilic compounds, including cytotoxic drugs, facilitating their elimination from the body. This mechanism is widely related to drug resistance in various types of cancer, both in the context of intrinsic and acquired resistance, including cytostatics and targeted therapies [13, 23,24,25]. The activity of UGTs converts drugs into more polar metabolites, which are better substrates for efflux transporters, contributing to multidrug resistance [24, 25].

UGT2B17 was significantly upregulated in patients who did not respond to neoadjuvant treatment compared with those who did, suggesting its involvement in the modulation of the therapeutic response. UGT2B17 is involved in the metabolism of both drugs and steroid hormones. UGT2B17 shows high interindividual variability in its expression and activity, as determined by genetic polymorphisms and variations in the number of copies, and is responsible for the glucuronidation of testosterone, dihydrotestosterone and numerous drugs, especially in the liver and intestine. The activity of UGT2B17 is correlated with its ability to metabolize androgens and drugs, which may influence the therapeutic response and pharmacokinetics of antineoplastic agents [26,27,28]. Specifically, UGT2B17 participates in the glucuronidation of drugs such as anthracyclines, which are used in standard neoadjuvant regimens. Upregulation of UGT2B17 could reduce the intracellular bioavailability of drugs and, therefore, compromise their cytotoxic effectiveness [29].

Likewise, tumor metabolic reprogramming could play a relevant role, since the overexpression of UGT2B17 can contribute to the inactivation of endogenous metabolites such as steroids, and bioactive lipids, altering the tumor microenvironment and activating signaling pathways associated with cell proliferation, survival and immune evasion. Notably, in our functional enrichment analysis, UGT2B17 and UGT2B28 participate in critical metabolic pathways such as xenobiotic metabolism and steroid hormone biosynthesis, processes previously associated with mechanisms of therapeutic resistance and tumor metabolic reprogramming [13, 32].

From a biological standpoint, UGT2B17 overexpression could contribute to reduced efficacy of anthracycline-based regimens by increasing glucuronidation of doxorubicin and related metabolites, thereby lowering intracellular exposure and facilitating efflux of more polar conjugates. In parallel, UGT2B17 may influence HER2-positive tumor biology by reshaping intratumoral steroid availability; glucuronidation of androgens could dampen AR signaling in AR-high tumors or, conversely, reflect an adaptive AR/FOXA1-driven transcriptional program previously described in breast cancer [30, 31].

With respect to UGT2B28 as a possible modulator, although it did not show the most powerful discrimination in our cohort, its overexpression in nonresponding patients and its demonstrated participation in the metabolism of steroids and xenobiotics suggest a possible complementary or modulatory role in the mechanisms of therapeutic resistance. UGT2B28 catalyzes the glucuronidation of steroid hormones such as estradiol, estrone, androsterone and other lipophilic compounds, facilitating their elimination and altering their intracellular levels [33]. Its expression has also been detected in breast tissue, and its regulation seems to be mediated by hormonal signals such as androgen receptors and growth factors (EGF, GR) [34].

Although UGT2B17 appears to be the predominant marker, the concomitant presence of UGT2B28 strengthens the notion that a UGT2B family is involved in drug resistance, increasing the robustness of these findings.

Although the findings of the present study offer a solid basis that suggests the involvement of UGT2B17 in resistance to neoadjuvant treatment, further in vitro and in vivo functional studies that directly evaluate the impact of UGT2B17 upregulation on the sensitivity to specific drugs are needed.

Notably, resistance to treatment is a multifactorial phenomenon. In addition to metabolic mechanisms mediated by UGTs, factors such as tumor heterogeneity, interactions with the microenvironment, and the presence of specific immunological signatures can also modulate the response therapy. In this context, the integration of UGT2B17 within multigene biomarker panels, which include molecular and immunological factors, could optimize patient stratification and favor a more personalized therapeutic approach.

Among the limitations of our study are the small size of the discovery cohort. The discovery cohort was necessarily limited because, after the initial samples had been processed, pertuzumab became incorporated into routine neoadjuvant regimens, and we prioritized an independent validation cohort treated in the contemporary setting. Notably, similar translational biomarker studies frequently include modest sample sizes due to tissue availability and assay requirements [35]. In addition, variability in the therapeutic regimens administered—especially the introduction of pertuzumab in the validation cohort—and the high prevalence of obesity in the validation cohort could have influenced both the observed gene expression and the response to treatment. Furthermore, the absence of functional studies limits the possibility of establishing a direct causal relationship between the upregulation of UGT2B17 and UGT2B28 and resistance to treatment. Although the transcriptomic findings and their validation support a robust association, only functional confirmation will make it possible to rule out indirect effects or epiphenomena. We also acknowledge that androgen receptor status and intrinsic molecular subtyping were not available; given the link between UGT2B17 and steroid/androgen metabolism, these unmeasured factors might also have influenced the observed association.

In conclusion, the overexpression of UGT2B17 in patients who do not respond to neoadjuvant therapy in HER2-positive breast cancer suggests that this gene could function as a biomarker of therapeutic resistance. Our findings underscore the importance of the integration of transcriptomic profiles to advance personalized medicine in HER2-positive breast cancer, allowing the identification of patients with a greater risk of resistance and optimizing therapeutic strategies. Although the upregulation of UGT2B17 is presented as the most consistent finding as a biomarker of resistance, the additional implication of UGT2B28 supports the existence of a UGT2B gene family with a possible modulatory role in drug resistance, which suggests that both genes could be incorporated together in predictive panels.