Ivermectin Modulates Growth Signals in Hormone-Resistant Tumors

Endocrine resistance is a major clinical challenge in ER-positive breast cancer, where tumors initially responsive to hormonal therapies such as tamoxifen eventually become refractory. This resistance is driven by multiple molecular mechanisms, including upregulation of HER2 signaling, activation of the TGF-β pathway, and crosstalk between estrogen receptor and growth factor signaling networks. Once resistance develops, treatment options become more limited and often involve switching to chemotherapy or targeted agents.

A 2026 study published in PLOS One investigated whether ivermectin—an antiparasitic drug with documented preclinical anticancer activity—could modulate these resistance pathways in ER-positive and endocrine-resistant breast cancer cells. The researchers focused on three key signaling axes: the estrogen receptor (ER) pathway, the HER2 pathway, and the TGF-β pathway, all of which are known to contribute to endocrine resistance and tumor progression. For readers already following ivermectin cancer research, this study adds a new mechanistic dimension specifically targeting resistance biology.

The findings suggest that ivermectin may have a dual capacity: preventing the emergence of endocrine resistance in sensitive tumors and enhancing the efficacy of anti-hormonal therapies in already resistant cases. However, it is important to stress that these data come from laboratory cell culture experiments, not human clinical trials. Patients should not alter their treatment plans without oncologist guidance. Those using a dosing calculator as part of their protocol planning should treat this research as background information, not as a basis for dose adjustment.

Table of Contents

• Background: Endocrine Resistance in Breast Cancer
• Study Design and Methods
• Key Results
• Clinical Implications and Limitations
• Frequently Asked Questions

Background: Endocrine Resistance in Breast Cancer

Approximately 70% of breast cancers are ER-positive, making endocrine therapy the cornerstone of treatment. Tamoxifen and aromatase inhibitors are first-line options, but resistance develops in a significant proportion of patients over time. The mechanisms are complex and interconnected:

• HER2 upregulation: HER2 signaling can bypass the ER pathway, activating downstream proliferation signals independently of estrogen.
• TGF-β pathway activation: TGF-β promotes epithelial-mesenchymal transition, immunosuppression, and drug resistance in the tumor microenvironment.
• ERK phosphorylation: Downstream of both HER2 and TGF-β, ERK activation drives cell proliferation and survival.
• SMAD2 phosphorylation: Phosphorylated SMAD2 (pSMAD2) is a key TGF-β effector that promotes tumor progression and resistance.

Any compound that can simultaneously suppress multiple resistance pathways without severe toxicity would be of considerable interest in oncology research. The PLOS One study tested whether ivermectin could do exactly this.

Study Design and Methods

The researchers used ER-positive breast cancer cell lines, including both tamoxifen-sensitive and endocrine-resistant variants. They measured:

• Estrogen-induced proliferation after ivermectin treatment
• ER and HER2 protein expression levels by Western blotting
• ERK phosphorylation as a downstream signaling readout
• SMAD2 phosphorylation (pSMAD2) and total SMAD4 levels in the TGF-β pathway

All experiments were conducted in controlled cell culture conditions. The study did not include animal models or human subjects.

Key Results

The study reported the following findings:

• Proliferation suppression: Ivermectin significantly suppressed estrogen-induced proliferation in both sensitive and resistant cell lines.
• ER downregulation: Ivermectin reduced ER protein expression, potentially weakening the estrogen-driven growth signal.
• HER2 suppression: HER2 expression was downregulated, which is particularly significant because HER2 upregulation is a well-documented mechanism of endocrine resistance.
• ERK inhibition: Phosphorylated ERK levels were reduced, indicating dampening of the downstream proliferation cascade.
• TGF-β pathway modulation: pSMAD2 was decreased, while SMAD4—a factor associated with favorable prognosis in endocrine-resistant settings—was maintained.

Together, these results suggest that ivermectin acts on multiple resistance nodes simultaneously, rather than targeting a single pathway. This polypharmacology is consistent with ivermectin's known mechanism of targeting multiple kinases and transporters in cancer cells.

Clinical Implications and Limitations

The most compelling implication is that ivermectin might be useful as an adjunct to endocrine therapy, either to delay resistance or to restore sensitivity in resistant tumors. However, several caveats apply:

• Cell culture only: No animal or human data were included. The translation rate from cell culture to clinical efficacy is historically low.
• Dose relevance: The concentrations used in cell culture may not be achievable or safe in humans.
• No combination data: The study did not test ivermectin alongside tamoxifen or other endocrine agents in combination.
• Tumor heterogeneity: Cell lines represent clonal populations. Real tumors contain diverse cell subpopulations that may respond differently.

Despite these limitations, the study adds a mechanistically coherent rationale for testing ivermectin in ER-positive breast cancer, particularly in the context of endocrine resistance. It also aligns with the broader body of preclinical ivermectin literature showing activity across multiple cancer types and signaling pathways.

Frequently Asked Questions

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References

1. PMID 42060658: Ivermectin inhibits ER, HER2, and TGF-β pathways in ER-positive and endocrine-resistant breast cancer cells. PLOS One, 2026.
2. Ivermectin and Cancer Immunotherapy: What the Research Shows — Sanare Lab long-read.
3. Ivermectin: What It Is, How It Works, and Why It's Used — Sanare Lab long-read.