Scientists Crack the Code of Pyrotinib Resistance in HER2-Positive Breast Cancer

When a targeted cancer drug stops working, the question that haunts oncologists is rarely whether the drug was good enough. It is what the tumor did to survive. In HER2-positive breast cancer, a subtype that accounts for roughly 15 to 20 percent of all breast cancer cases and is driven by overexpression of the HER2 receptor, the oral pan-ErbB tyrosine kinase inhibitor pyrotinib has delivered striking clinical results, including a sustained progression-free and overall survival benefit in the phase III PHILA trial. Yet many patients with advanced disease eventually progress, and the mechanisms that undermine pyrotinib-based therapy have remained frustratingly opaque. Now, a study published in Materials Today Bio offers one of the most detailed answers yet, mapping a self-reinforcing molecular circuit that keeps resistant cells alive and designing a smart nanoparticle to break it.

The research team, led by investigators at the First Affiliated Hospital of Nanchang University in China, began by forcing pyrotinib-sensitive SKBR3 breast cancer cells to endure gradually escalating doses of the drug for approximately eight months. The surviving cells, dubbed SKBR3-Res, were dramatically hardened against treatment: the concentration of pyrotinib needed to kill half of the cells jumped from 4.6 nanomolar in the parental line to over 150 nanomolar in the resistant clone. RNA sequencing of these paired cell populations revealed a single gene standing out above the noise. ASCL1, a transcription factor best known as the identity-defining driver of high-grade neuroendocrine lung cancers, was the most prominently upregulated gene in the resistant cells, and only ASCL1, among the differentially expressed candidates, was consistently associated with shorter overall survival in an independent HER2-positive cohort from the KM-Plotter database.

Crucially, the finding was not confined to laboratory glassware. In pretreatment tumor specimens from a small exploratory group of eleven patients treated with pyrotinib-based regimens, ASCL1 protein levels measured by immunohistochemistry were markedly higher in those whose disease progressed than in those who responded. And in a broader cohort of 146 HER2-positive breast cancer cases, high ASCL1 expression correlated with advanced TNM stage, positive lymph node status, and a significantly elevated risk of death. After statistical adjustment for TNM stage, patients with ASCL1-high tumors faced roughly a threefold increase in the hazard of death compared with those whose tumors expressed low levels. The authors are careful to frame the eleven-patient analysis as exploratory, given its size and single-center origin, but the prognostic signal across the larger cohort is difficult to dismiss.

To determine whether ASCL1 merely accompanies resistance or actively causes it, the researchers manipulated its expression both ways. Forcing ASCL1 into drug-sensitive cells drove the pyrotinib IC50 from under 5 nanomolar to about 137 nanomolar, boosted colony formation, DNA synthesis, migration, and invasion, and blunted the apoptosis that pyrotinib would normally trigger. Silencing ASCL1 in resistant cells did the opposite, pulling the IC50 back down to between roughly 27 and 39 nanomolar and restoring the drug’s ability to kill. In nude mice implanted with luciferase-labeled tumors, ASCL1-overexpressing xenografts grew relentlessly even under daily oral pyrotinib, while vector-control tumors shrank under the same regimen. Immunohistochemical staining of the excised tumors revealed the likely explanation: phosphorylated ERK, the activated form of the terminal kinase in the RAS-MAPK pathway, remained stubbornly high in ASCL1-overexpressing tumors despite pyrotinib treatment, while AKT signaling stayed flat.

That ERK observation pointed the investigators toward the second character in their story. Building on prior work showing that ASCL1 activates WNT11 in small-cell lung cancer, the team confirmed that ASCL1 binds directly to the WNT11 promoter, as demonstrated by chromatin immunoprecipitation and luciferase reporter assays, and that WNT11 protein was elevated in resistant cells and in clinical specimens. WNT11 is a secreted ligand of the non-canonical WNT family, and in breast cancer it is known to engage a receptor called ROR2 to drive invasive behavior. Immunofluorescence imaging of patient tumors showed increased membrane co-localization of WNT11 and ROR2 in pyrotinib-resistant samples compared with sensitive ones. Knocking down WNT11 partially reversed the ASCL1-driven rise in phospho-ERK and resensitized cells to pyrotinib; silencing ROR2 produced the same effect. AKT phosphorylation, notably, was untouched throughout, isolating the resistance mechanism to the MAPK arm of HER2 signaling.

But the loop had one more twist. If ASCL1 turns on WNT11, and WNT11 keeps ERK active, what keeps ASCL1 switched on? The answer, the researchers found, is ERK itself. ERK signaling promotes the phosphorylation of the transcription factor CREB1 at serine 133, and phosphorylated CREB1 was shown to occupy a specific binding motif in the ASCL1 promoter. Mutating that motif, changing the sequence 5-prime-TGAGGCCA-3-prime to 5-prime-TCTAAACA-3-prime, abolished CREB1-driven activation of the ASCL1 promoter in reporter assays. CREB1 knockdown lowered ASCL1, WNT11, and phospho-ERK levels and sensitized resistant cells to pyrotinib, effects partially rescued by re-expressing ASCL1. Conversely, treating cells with the MEK inhibitors U0126 or GSK1120212 suppressed CREB1 phosphorylation, ASCL1 promoter activity, and ASCL1 mRNA levels, while actinomycin D chase experiments showed that ASCL1 mRNA decay rates were unaffected. In other words, ERK sustains ASCL1 at the level of transcription, not message stability, closing a feed-forward circuit: ASCL1 drives WNT11, WNT11 engages ROR2 to sustain ERK, and ERK-dependent CREB1 activation maintains ASCL1.

Identifying such a loop is elegant biology, but it immediately raises a therapeutic problem. ASCL1 is a transcription factor, and transcription factors are notoriously difficult to inhibit with conventional small molecules because they lack the deep, druggable pockets that enzymes offer. The team’s solution was to skip small-molecule chemistry altogether and deliver genetic silencing material directly into tumor cells, packaged alongside the drug the cells are resisting. The result is a nanoparticle the authors call HApt-PEG-ZIF-8@siASCL1/Pyrotinib, a construct built from zeolitic imidazolate framework-8, a zinc-based metal-organic framework that self-assembles from zinc nitrate and 2-methylimidazole and can trap both a drug and short interfering RNA inside its crystalline lattice.

The engineering details are worth savoring. The particles, roughly polyhedral under the electron microscope with a hydrodynamic diameter of about 345 nanometers after surface modification, were first loaded with pyrotinib and siRNA against ASCL1, achieving encapsulation efficiencies of 78.5 and 74.8 percent respectively. They were then coated with polyethylene glycol to reduce nonspecific protein adsorption and improve circulation time, and finally decorated with the HB5 DNA aptamer, a short synthetic oligonucleotide that folds into a shape recognizing HER2 and thereby steering the particle toward HER2-positive cells. The beauty of ZIF-8 lies in its pH sensitivity: at physiological pH 7.4, only about 16 percent of pyrotinib and 21 percent of siASCL1 leaked out over 48 hours, but at endolysosomal pH 5.5, the acidic environment inside the cellular compartments that swallow nanoparticles, release climbed to 85 and 81 percent respectively. The particles were also benign to blood cells, inducing hemolysis below 1 percent, and conjugation of the aptamer reached an efficiency of 87.8 percent.

In cell culture, the targeted formulation outperformed every control. Uptake experiments showed that the aptamer-functionalized particles delivered visibly more fluorescently labeled siRNA into HER2-positive SKBR3-Res cells than into HER2-negative or HER2-low lines, and the dual-loaded, aptamer-targeted particles reduced ASCL1 mRNA by approximately 70 percent, suppressed phospho-CREB1 and phospho-ERK, and drove the pyrotinib IC50 back down to around 8 nanomolar, compared with 26 nanomolar for the non-targeted version and 102 nanomolar for drug-loaded particles without siRNA. In mice bearing resistant xenografts, the targeted nanoparticles accumulated about 1.56 times more siRNA in tumors than the PEGylated version without aptamer, and by day 28 the mean tumor volume in the targeted group was 92.6 cubic millimeters versus 454.4 cubic millimeters in the non-targeted group, a 79.6 percent reduction. Tumor staining confirmed the mechanism in action: ASCL1 and phospho-ERK fell, the proliferation marker Ki-67 dropped, and cleaved caspase-3, a signature of apoptosis, rose. Serum liver and kidney markers and organ histology showed no detectable toxicity, even at a 70-day endpoint in a separate biosafety cohort.

The study is not without caveats, which the authors acknowledge candidly. The xenografts were subcutaneous and lacked an intact immune system, so orthotopic, metastatic, and immunocompetent models will be needed before the platform approaches the clinic. Encapsulation metrics came from a single nanoparticle batch, and the chemical stability of pyrotinib across pH conditions was not directly measured. Still, the conceptual contribution is substantial. By showing that acquired pyrotinib resistance in HER2-positive breast cancer can rest on a lineage-associated transcriptional loop rather than the familiar litany of receptor mutations and bypass pathways, and by pairing that discovery with a tumor-targeted nanocarrier that simultaneously maintains HER2 blockade and dismantles the resistance circuit, the work offers a template for mechanism-guided combination therapy. It also hints that ROR2, already the target of antibody-drug conjugates in development, could become a more conventional druggable handle on the same loop. For patients whose tumors eventually outmaneuver even the best HER2 inhibitors, that dual insight, of what breaks and how to fix it, is precisely the kind of progress that turns resistance from a dead end into a design problem.

Subject of Research: Mechanisms of acquired pyrotinib resistance in HER2-positive breast cancer and nanoparticle-based co-delivery therapy

Article Title: Targeting an ASCL1–WNT11–ERK feed-forward loop to overcome pyrotinib resistance in HER2-positive breast cancer

Article References: Yin, F., Zhou, B., Li, Y., Liu, Z., Zhu, C., Wang, Y., Meng, C., Zhang, Z., Jiang, A., Liu, W., Liu, H., Mao, X., Tang, H., Fu, T., Fan, L., Gao, C., Yu, K., Zeng, Q., & Le, A. (2026). Targeting an ASCL1–WNT11–ERK feed-forward loop to overcome pyrotinib resistance in HER2-positive breast cancer. Materials Today Bio, 41, Article 103679. https://doi.org/10.1016/j.mtbio.2026.103679

Image Credits: AI Generated

DOI: Not provided

Keywords: HER2-positive breast cancer, pyrotinib, ASCL1, WNT11, ROR2, ERK signaling, CREB1, drug resistance, ZIF-8 nanoparticles, siRNA delivery, aptamer targeting, transcription factors