CDK4/6 inhibitors (CDK4/6i) are recommended in patients with estrogen receptor (ER)-positive, HER2-negative advanced breast cancer (ABC). Up to now, no prognostic biomarkers have been identified in this setting. We retrospectively analyzed the expression of progesterone receptor (PR) and Ki67, assessed by immunohistochemistry, in 71 ABC patients treated with CDK4/6i and analyzed the impact of these markers on progression-free survival (PFS). The majority of patients 63/71 (88.7%) received palbociclib, 4 (5.6%) received ribociclib, and 4 (5.6%) received abemaciclib. A higher median value of Ki67 was observed in cases undergoing second-line treatment (p = 0.047), whereas the luminal B subtype was more prevalent (p = 0.005). In the univariate analysis of the first-line setting, luminal A subtype showed a trend towards a correlation with a longer PFS (p = 0.053). A higher continuous Ki67 value led to a significantly shorter PFS. When the interaction between pathological characteristics and line of treatment was considered, luminal B subtype showed a significantly (p = 0.043) worse outcome (Hazard Ratio (HR) 2.84; 1.03–7.82 95% Confidence Interval (CI)). PFS in patients undergoing endocrine therapy plus CDK4/6i was inversely correlated with Ki67 expression but not with PR, suggesting that tumor proliferation has a greater impact on cell cycle inhibitors combined with endocrine therapy than PR expression.

Background: Adjuvant trastuzumab is the standard of care for HER2+ breast cancer (BC) patients. However, >50% of patients become resistant. This study aimed at the identification of the molecular factors associated with disease relapse and their further investigation as therapeutically exploitable targets. Methods: Analyses were conducted on formalin-fixed paraffin-embedded tissues of the primary tumors of relapsed (cases) and not relapsed (controls) HER2+ BC patients treated with adjuvant trastuzumab. The nCounter Human Breast Cancer Panel 360 was used. Logistic regression and partitioning around medoids were employed to identify the genes associated with disease recurrence. Cytotoxicity experiments using trastuzumab-resistant cell lines and a network pharmacology approach were carried out to investigate drug efficacy. Results: A total of 52 patients (26 relapsed and 26 not relapsed) were analyzed. We found that a higher expression of HDAC6 was significantly associated with an increased risk of recurrence, with an adjusted OR of 3.20 (95% CI 1.38–9.91, p = 0.016). Then, we investigated the cytotoxic activity of the selective HDAC6 inhibitor Nexturastat A (NextA) on HER2+ cell lines, which were both sensitive and trastuzumab-resistant. A sub-cytotoxic concentration of NextA, combined with trastuzumab, showed a synergistic effect on BC cell lines. Finally, using a network pharmacology approach, we identified HSP90AA1 as the putative molecular candidate responsible for the synergism observed in vitro. Conclusions: Our findings encourage the exploration of the role of HDAC6 as a prognostic factor and the combinatorial use of HDAC6 selective inhibitors combined with trastuzumab in HER2+ BC, in particular for those patients experiencing drug resistance.

Background/Objectives: Vacuum-assisted breast biopsy (VABB) is widely used for the diagnosis of nonpalpable breast cancer but is not considered definitive treatment because microscopic residual disease may persist within or around the biopsy cavity. The ARTHE phase I trial evaluated a cavity-targeted radionuclide strategy based on same-session sequential intralesional administration of avidin followed by [⁹⁰Y]Y-DOTA-biotin after VABB. Methods: Eighteen women with nonpalpable breast cancer measuring ≤15 mm and a skin-to-cavity distance ≥ 13 mm were treated in three sequential activity-escalation cohorts. The primary objective was to assess acute local and systemic safety, including dose-limiting toxicity. The protocol-specified co-primary objective was to evaluate preliminary antitumor activity, assessed as breast-only pathologic complete response (pCR) at surgery. Given the phase I single-arm design, pCR was analyzed descriptively. The protocol-specified secondary objective was patient-specific dosimetry. Additional exploratory assessments included post-injection biodistribution and integration with subsequent breast-conserving surgery. Results: No dose-limiting toxicities, grade ≥ 3 adverse events, clinically relevant hematologic toxicity, treatment discontinuations, hospitalizations, or treatment-related surgical delays were observed. Local toxicity was limited to grade 1 injection-site pain and/or erythema. Post-injection imaging consistently demonstrated focal intralesional localization without clinically relevant extra-lesional uptake. All patients underwent breast-conserving surgery 4–7 weeks after treatment. No residual tumor cellularity (RTC), from either invasive or in situ carcinoma, was identified in the breast surgical specimen in 5/18 patients (27.8%). However, because diagnostic VABB may have removed part or all of the malignant lesion, the absence of residual carcinoma at surgery cannot be attributed specifically to the radionuclide treatment. Conclusions: The intralesional avidin-mediated local trapping strategy using [⁹⁰Y]Y-DOTA-biotin after VABB was feasible, showed favorable acute and short-term tolerability, was associated with early focal localization on post-injection imaging, and was compatible with standard breast-conserving surgery. Controlled studies are warranted to determine the therapeutic contribution of this post-biopsy cavity-targeted strategy, optimize administered activity, and refine patient selection.