Purpose: To assess palbociclib in combination with trastuzumab with or without endocrine therapy in patients with HER2-positive advanced breast cancer.Patients and Methods: PATRICIA is a prospective, open-label, multicenter phase II trial. Patients had received 2–4 prior lines of anti-HER2–based regimens. Treatment consisted of palbociclib 200 mg daily for 2 weeks and 1 week off plus trastuzumab. The study was based on a Simon two-stage design comprising three cohorts: estrogen receptor (ER)-negative (cohort A), ER-positive (cohort B1), and ER-positive with letrozole (cohort B2). ER-positive patients were randomized to cohorts B1 or B2. Primary endpoint was progression-free survival rate at 6 months (PFS6). Secondary objectives included safety and evaluation of the PAM50 intrinsic subtypes.Results: Seventy-one patients were recruited (n = 15 in cohort A and 28 in each cohort B). The PFS6 rate in cohorts A, B1, and B2 was 33.3% (5/15), 42.8% (12/28), and 46.4% (13/28), respectively. Regarding safety, grade 1–2 and 3–4 toxicities occurred in 97.7% and 84.4% of patients, respectively. The most common grade 3–4 toxicities were neutropenia (66.4%) and thrombocytopenia (11.3%). Regarding PAM50, 59 (83.1%) tumors were profiled. Luminal disease defined by PAM50 was found independently associated with longer PFS compared with non-luminal disease (10.6 vs. 4.2 months median PFS; adjusted hazard ratio = 0.40; P = 0.003).Conclusions: Palbociclib in combination with trastuzumab is safe and exhibits promising survival outcomes in trastuzumab pretreated ER-positive/HER2-positive advanced breast cancer with a PAM50 Luminal A or B subtype. The enrollment was stopped prematurely, and a new randomized cohort was opened in this population.

Neoadjuvant standard-of-care for HER2-positive early-stage breast cancer is trastuzumab + pertuzumab with polychemotherapy; however, existing regimens have high toxicity burdens and suboptimal outcomes. DESTINY-Breast11 assessed efficacy and safety of neoadjuvant trastuzumab deruxtecan (T-DXd) alone or followed by paclitaxel + trastuzumab + pertuzumab (THP) versus dose-dense doxorubicin + cyclophosphamide (ddAC) followed by THP for high-risk (≥cT3cN0 or cT0-4cN1-3) HER2-positive disease. This open-label, phase III trial (147 sites, 18 countries) randomised adults 1 : 1 : 1 to T-DXd (×8 cycles), T-DXd-THP (4 + 4 cycles), or ddAC-THP (4 + 4 cycles). T-DXd-alone arm enrolment closed early following the Independent Data Monitoring Committee recommendation. The primary endpoint was pathological complete response (pCR; ypT0/is ypN0; intent-to-treat population). Secondary endpoints included event-free survival (EFS; intent-to-treat population) and safety (safety analysis set). Between 25 October 2021 and 12 March 2025, 286 (T-DXd), 321 (T-DXd-THP), and 320 (ddAC-THP) female patients were randomised. pCR rates were 43.0% (T-DXd, n = 123), 67.3% (T-DXd-THP, n = 216), and 56.3% (ddAC-THP, n = 180). T-DXd-THP versus ddAC-THP absolute pCR rate difference was 11.2% [95% confidence interval (CI), 4.0% to 18.3%, P = 0.003], with benefit in hormone receptor (HR)-positive [61.4% (n/N = 145/236) versus 52.3% (n/N = 123/235); difference in pCR (ΔpCR) 9.1% (95% CI 0.2% to 17.9%)] and HR-negative [83.1% (n/N = 69/83) versus 67.1% (n/N = 57/85); ΔpCR 16.1% (95% CI 3.0% to 28.8%)] subgroups. Median EFS (T-DXd-THP versus ddAC-THP, maturity 4.5%) hazard ratio was 0.56 (95% CI 0.26 to 1.17). Grade ≥3 adverse events (AE; T-DXd, 22.6% (n = 64); T-DXd-THP, 37.5% (n = 120); ddAC-THP, 55.8% (n = 174)], serious AE [T-DXd, 10.2% (n = 29); T-DXd-THP, 10.6% (n = 34); ddAC-THP, 20.2% (n = 63)], and all-grade left-ventricular dysfunction [T-DXd, 0.7% (n = 2); T-DXd-THP, 1.3% (n = 4); ddAC-THP, 6.1% (n = 19)] rates were lower for T-DXd and T-DXd-THP than ddAC-THP. All-grade adjudicated drug-related interstitial lung disease/pneumonitis rates were low and similar across arms [T-DXd, 4.9% (n = 14); T-DXd-THP, 4.4% (n = 14); ddAC-THP, 5.1% (n = 16)]. Three treatment-related deaths occurred [T-DXd-THP, 0.3% (n = 1); ddAC-THP, 0.6% (n = 2)]. Neoadjuvant T-DXd-THP demonstrated statistically significant and clinically meaningful pCR benefit and improved safety versus ddAC-THP.

T cell bispecific antibodies against an isoform of HER2 effectively target many HER2-expressing tumors but not normal tissues.

Trastuzumab-emtansine (T-DM1) is an antibody-drug conjugate (ADC) approved for the treatment of HER2 (human epidermal growth factor receptor 2)-positive breast cancer. T-DM1 consists of trastuzumab covalently linked to the cytotoxic maytansinoid DM1 via a non-cleavable linker. Despite its efficacy, primary or acquired resistance frequently develops, particularly in advanced stages of the disease. Second generation ADCs targeting HER2 are meant to supersede T-DM1 by using a cleavable linker and a more potent payload with a different mechanism of action. To determine the effect of one of these novel ADCs, SYD985, on tumors resistant to T-DM1, we developed several patient-derived models of resistance to T-DM1. Characterization of these models showed that previously described mechanisms—HER2 downmodulation, impairment of lysosomal function and upregulation of drug efflux pumps—account for the resistances observed, arguing that mechanisms of resistance to T-DM1 are limited, and most of them have already been described. Importantly, SYD985 was effective in these models, showing that the resistance to first generation ADCs can be overcome with an improved design.

Antibody–drug conjugates (ADC) are antineoplastic agents recently introduced into the antitumor arsenal. T-DM1, a trastuzumab-based ADC that relies on lysosomal processing to release the payload, is approved for HER2-positive breast cancer. Next-generation ADCs targeting HER2, such as [vic-]trastuzumab duocarmazine (SYD985), bear linkers cleavable by lysosomal proteases and membrane-permeable drugs, mediating a bystander effect by which neighboring antigen-negative cells are eliminated. Many antitumor therapies, like DNA-damaging agents or CDK4/6 inhibitors, can induce senescence, a cellular state characterized by stable cell-cycle arrest. Another hallmark of cellular senescence is the enlargement of the lysosomal compartment. Given the relevance of the lysosome to the mechanism of action of ADCs, we hypothesized that therapies that induce senescence would potentiate the efficacy of HER2-targeting ADCs. Treatment with the DNA-damaging agent doxorubicin and CDK4/6 inhibitor induced lysosomal enlargement and senescence in several breast cancer cell lines. While senescence-inducing drugs did not increase the cytotoxic effect of ADCs on target cells, the bystander effect was enhanced when HER2-negative cells were cocultured with HER2-low cells. Knockdown experiments demonstrated the importance of cathepsin B in the enhanced bystander effect, suggesting that cathepsin B mediates linker cleavage. In breast cancer patient-derived xenografts, a combination treatment of CDK4/6 inhibitor and SYD985 showed improved antitumor effects over either treatment alone. These data support the strategy of combining next-generation ADCs targeting HER2 with senescence-inducing therapies for tumors with heterogenous and low HER2 expression. Significance: Combining ADCs against HER2-positive breast cancers with therapies that induce cellular senescence may improve their therapeutic efficacy by facilitating a bystander effect against antigen-negative tumor cells.