Hormone receptor-positive (HR+) and HER2-negative breast cancer accounts for approximately 70% of cases. In early stages, standard treatment includes surgery and endocrine therapy (ET), reducing the risk of late recurrence. However, over 30% of patients develop advanced disease, for which CDK4/6 inhibitors (CDKi) represent the standard treatment. Approximately 20% of patients show intrinsic resistance to combined therapy, and no reliable tools exist to predict treatment response. Therefore, managing patients with resistance to CDKi presents a significant clinical challenge. Preliminary studies from our group have shown increased STAT3 expression in circulating tumour cells from patients treated with CDKi who did not respond to first-line treatment (recurrence within 6 months by RECIST criteria). Therefore, we aim to demonstrate whether STAT3 is associated with the mechanisms underlying intrinsic resistance to CDKi in HR+/HER2- metastatic breast cancer.

In vitro assays were performed using luminal breast cancer cell models. STAT3 was activated with IL-6 and inhibited using C188-9, a STAT3 inhibitor. STAT3 modulation was confirmed by Western blot. MTT assays were performed to study cell viability and resistance to CDKi (Palbociclib). Transwell migration assays were conducted to assess functionality. Transcriptomic analysis by RNA sequencing was performed externally. Statistical significance was determined by the Mann-Whitney U test (p < 0.05) or Wald binomial test (|fc|≥2 and raw p-value<0.05).

IL-6-induced cell models showed enhanced STAT3 activation, as indicated by the STAT3/p-STAT3 ratio by Western blot. In contrast, inhibited cell models showed reduced phosphorylation levels in STAT3, confirming the inhibitory effect of C188-9. Thus, they are suitable models for studying the link between STAT3 activation and resistance to CDKi. The activation of STAT3 (via IL-6) induces resistance to CDKi, and its inhibition in induced cells increased the sensitivity to CDKi (p = 0.028), suggesting that STAT3 inhibition may reverse resistance to CDKi. Moreover, STAT3 activation associates with increased cell migration and enhanced viability. Differential gene expression analysis between cell lines with activated/inhibited STAT3 identified a set of genes that by Gene Ontology analysis revealed an enrichment in key pathways related to tumour progression and therapy resistance (PPI enrichment p-value: 0.00148) that need further validation.

Preliminary data suggest that STAT3 activation induces resistance to CDK inhibitors in HR+/HER2-cells, while its inhibition sensitises these cells to treatment. These findings highlight the potential of STAT3 as a response biomarker and a possible therapeutic target to counteract resistance to CDK inhibitors in this breast cancer subtype.