Ivermectin & ER+ Breast Cancer: What the 2026 Research Actually Found
Plain-language breakdown of the 2026 ER-positive breast-cancer study showing ivermectin effects on ESR1/ERα, HER2, TGF-β signalling, and tamoxifen synergy
This page breaks down an April 2026 paper on ivermectin in ER-positive breast cancer.
The study is preclinical. It uses breast-cancer cells in the lab, not patients.
That still matters. It shows, in unusual detail, which pathways ivermectin hits in ER-positive, tamoxifen-resistant, and fulvestrant-resistant models.
What's covered
Part 1
Part 2: Ivermectin as an Immune “Ignition Switch” — And When, Even in ER+ It Can Backfire
Part 3: Members Questions from Parts 1 & 2 Answered
Why this paper matters
This paper helps answer a more specific question than older ivermectin papers.
It asks how ivermectin slows ER-positive breast-cancer cells, and whether that still holds when endocrine resistance appears.
The answer is yes, within this lab model.
This is not internet fluff.
It is a proper mechanistic paper from a breast-cancer pharmacology team at Chulalongkorn University, published in the peer-reviewed journal PLOS One.
The methods are standard, the statistics are appropriate, and the authors stay close to what their data actually show.
What the paper proves is narrower than many readers may want.
It shows how ivermectin can switch down ER, HER2, and TGF-β-linked pathways in cancer cells in a dish, including tamoxifen-resistant and fulvestrant-resistant models.
What it does not prove is that ivermectin, at any given dose, will control breast cancer in real patients.
That will need animal studies, pharmacology work, and clinical trials.
So this is a serious early paper worth paying attention to, but it is still one piece of the puzzle rather than a practice-changing clinical result.
The researchers show that ivermectin can:
suppress ERα biology at both the gene and protein level
reduce HER2 protein in standard and tamoxifen-resistant models
suppress pERK and pSMAD2, which matter in invasion and treatment escape
work synergistically with tamoxifen by formal combination-index testing
That combination result is especially important.
It suggests ivermectin is not just acting in parallel. It may strengthen endocrine pressure in some ER-positive settings.
This is not a dosing guide.
It does not show patient benefit, clinical safety, or achievable breast-tissue concentrations in humans.
What the researchers tested
The team used six ER-positive breast-cancer cell lines.
These included endocrine-sensitive, tamoxifen-resistant, and fulvestrant-resistant models.
MCF-7 and T-47D — endocrine-sensitive ER-positive cells
MCF-7/LCC2 and T-47D Tam1 — tamoxifen-resistant cells
MCF-7/LCC9 and T47D-182R1 — fulvestrant-resistant or dual-resistant cells
They also tested normal human fibroblast cells to check whether ivermectin looked more toxic to cancer cells than to non-cancer cells.
The main methods were:
MTT viability assays for growth inhibition
Western blotting for pathway proteins
qPCR for ESR1 mRNA
Chou-Talalay combination analysis for tamoxifen synergy
The main findings
Ivermectin reduced viability across sensitive and resistant ER-positive cells
The reported IC50 values were broadly similar across the six cancer cell lines.
Across the cancer-cell lines, the active range was roughly 9–13 µM, with most of the key growth-inhibition results clustering around that micromolar window.
That means tamoxifen resistance and fulvestrant resistance did not make these cells obviously less sensitive to ivermectin in this experiment.
This is one of the most important signals in the paper.
It suggests ivermectin may act through pressure points that sit outside the usual endocrine-resistance workaround.
Ivermectin looked more selective for cancer cells than for normal fibroblasts
Normal fibroblasts needed a much higher concentration for the same growth-inhibition effect.
That does not prove real-world safety.
It does suggest a useful early selectivity signal.
Ivermectin suppressed ESR1 mRNA and ERα protein
This is a central finding.
Ivermectin did not only reduce the ERα receptor protein.
It also reduced ESR1 mRNA, which is the message the cell uses to make more ERα.
That places part of the effect further upstream than simple receptor blockade.
In plain language, ivermectin appears to reduce the instruction to build the receptor, not only the receptor already present.
The paper also reports that the anti-proliferative effect stayed strong when estradiol was present.
That matters because ER-positive disease often remains biologically shaped by estrogen even during treatment.
Ivermectin reduced HER2 protein in key ER-positive models
The paper shows reduced HER2 protein in standard MCF-7 cells and tamoxifen-resistant MCF-7/LCC2 cells.
That is relevant because HER2 upregulation is one known endocrine-escape route in ER-positive disease.
The HER2 effect looked weaker in the fulvestrant-resistant MCF-7/LCC9 model.
The paper also notes that ivermectin did not significantly reduce pHER2, and it did not significantly change PI3K, AKT, or mTOR.
So the HER2 result is real, but not complete.
This is not a full shutdown of every downstream resistance pathway.
Ivermectin suppressed pERK and pSMAD2
This was one of the most consistent findings.
Across tested concentrations and cell lines, ivermectin reduced phosphorylated ERK and especially phosphorylated SMAD2.
That matters because this signalling axis links to:
treatment adaptation
invasion
epithelial-mesenchymal transition
metastatic behaviour
The paper also reports that SMAD4 was preserved.
That is a useful nuance.
It means ivermectin reduced damaging signalling through this pathway without reducing one of the pathway components often linked to tumour-suppressive behaviour.
Ivermectin and tamoxifen were synergistic
The study used formal combination-index analysis.
That is the standard way to test whether two drugs work better together than expected from simple add-on effects.
The reported CI values were below 1 in multiple settings.
That means the interaction was synergistic, not merely additive.
This was shown in:
MCF-7
MCF-7/LCC2
MCF-7/LCC9
In the tamoxifen-sensitive and tamoxifen-resistant lines, the combination reduced ERα and HER2 more strongly than either drug alone.
In the fulvestrant-resistant setting, the ERα result looked less clean.
That fits the paper's broader message that ivermectin may be especially relevant in tamoxifen resistance, while still retaining some anti-invasive value in more resistant states.
Pathway map
What this paper does not tell us
This is the key caution section.
It is cell-line research only.
It does not show patient outcomes.
It does not show what human dose would be needed.
It does not prove that the active lab concentrations are achievable in human breast tissue.
It does not make ivermectin a substitute for endocrine therapy.
The authors also acknowledge a translational gap.
The active concentrations in the paper sit above standard antiparasitic plasma levels typically reported in humans.
That means formulation, delivery, tissue distribution, or combination strategy may matter if this biology is ever moved toward clinical use.
Practical takeaways
If the issue is tamoxifen resistance
This is where the paper looks strongest.
The cells remained sensitive to ivermectin.
HER2 reduction, ER suppression, and tamoxifen synergy were all most convincing in the tamoxifen-sensitive and tamoxifen-resistant models.
If the issue is fulvestrant resistance
The signal is more mixed.
Some ER and HER2 effects looked weaker.
The more durable signal in that setting was the continued suppression of pSMAD2, which may still matter for invasion biology.
If the question is whether ivermectin replaces endocrine therapy
This paper supports the opposite conclusion.
It supports using endocrine therapy as the backbone and viewing ivermectin, at most, as a possible adjunctive research topic.
The tamoxifen data specifically point toward combination logic, not replacement logic.
If the question is whether low white blood cells remove this effect
The direct pathway effects in this paper do not depend on immune cells.
These experiments happened in isolated cancer-cell systems.
So the ER/HER2/pSMAD2 findings represent direct tumour-cell effects, not immune-mediated ones.
Common questions
Is this worth taking seriously if it is only a lab study?
Yes, with the right frame.
It is a mechanistic paper, not a clinical trial.
Mechanistic papers matter because they show whether a drug is hitting a coherent target network or producing only vague toxicity.
This paper shows a coherent target network.
Does this apply equally to all ER-positive disease?
No.
The paper itself suggests stronger relevance to tamoxifen-sensitive and tamoxifen-resistant settings than to fulvestrant-resistant ones.
That does not make the fulvestrant signal irrelevant.
It means the pattern is not uniform.
Does luminal A versus luminal B likely matter here?
Probably.
The paper includes models that broadly reflect different parts of the ER-positive spectrum.
The stronger anti-ER signal looks most intuitive in more classic ER-dominant biology.
The HER2, ERK, and resistance findings are especially relevant where endocrine escape is already active.
So this looks most useful as a resistance-biology paper, not as a claim that every luminal tumour behaves the same way.
Does this answer the human dosing question?
No.
That remains the biggest unresolved issue.
Standard human ivermectin exposure is generally below the 9–13 µM micromolar range used for many of the key findings in this study.
Whether breast-tissue distribution, alternative delivery, or combination design could narrow that gap remains unanswered here.
Does this prove benefit in ESR1-mutant disease?
No.
The paper shows ESR1 transcriptional suppression, which is interesting.
But it does not directly test classic ESR1 mutation models such as Y537S or D538G.
That means it gives a plausible reason to study ivermectin in that setting.
It does not yet prove that constitutively active mutant ER signalling would be adequately controlled.
Does this matter for HER2-low or HER2-enriched escape biology?
Potentially, yes.
The HER2 result is one reason this paper stands out.
But the result is incomplete because downstream PI3K/AKT/mTOR signalling did not significantly change.
So this is a meaningful clue, not a complete HER2-resistance solution.
If pHER2 did not significantly fall, does the HER2 result still matter?
Yes.
The more reliable signal here is the fall in total HER2 protein plus the downstream fall in pERK.
That pattern suggests functional weakening of the signalling network even without a strong standalone pHER2 result.
So the HER2 effect looks partial, but still biologically relevant.
Does this say anything about triple-negative breast cancer?
Not directly.
This paper is focused on ER-positive and endocrine-resistant ER-positive models.
Some ivermectin mechanisms discussed elsewhere may still matter in other subtypes.
That is outside what this paper itself proves.
Why does ivermectin seem stronger in tamoxifen resistance than in fulvestrant resistance?
The paper points toward different resistance biology.
Tamoxifen resistance often retains more ER/HER2/ERK dependency.
That leaves ivermectin with more of its apparent targets still in play.
Fulvestrant-resistant cells may rely less on that same network, or rely on it in a less suppressible way.
That fits the weaker ERα and HER2 signal in the more resistant models.
What does this paper suggest about pulsing ivermectin?
It suggests caution about assuming the effect is permanent.
One of the key findings in this paper is that ivermectin reduces ESR1 mRNA.
That matters because ESR1 mRNA is the message the cell uses to make more ERα receptor.
This looks like active suppression of receptor production, not permanent silencing of the gene.
So if ivermectin is stopped and tissue conditions still favour estrogen signalling, the pathway may be able to rebound.
That rebound question matters most in ER-positive disease because local estrogen pressure, inflammatory signalling, and resistance-pathway activity can all push cells back toward growth once the drug pressure is removed.
This is one reason the paper fits better with combination logic than a stop-start replacement idea.
The mechanistic message is that ivermectin may help suppress the pathway while it is present.
The paper does not show that short pulses create durable pathway shutdown after the drug is gone.
Does that mean endocrine therapy still matters if ivermectin is being discussed?
Yes.
This paper supports keeping standard endocrine treatment central.
The strongest logic is complementary pressure:
endocrine therapy keeps blocking the estrogen axis
ivermectin may add extra pressure on ESR1 / ERα, HER2, and ERK / SMAD2 signalling
the tamoxifen data suggest this may be especially relevant when used as an adjunct rather than as a substitute
Does this tell us anything about CDK4/6 inhibitor combinations?
Not directly.
The paper does not test palbociclib, ribociclib, or abemaciclib.
Still, there is a reasonable mechanistic argument for future study because ivermectin appears to reduce upstream growth signalling that feeds into cell-cycle progression.
That remains a research question, not a current treatment conclusion.
What about the low white blood cell question?
This paper supports a clear distinction.
The direct ER, HER2, ERK, and SMAD2 effects shown here do not require immune cells.
They happen inside the tumour cells themselves.
That does not answer every ivermectin question.
It does show that this Ivermectin ER+ specific mechanism set is not dependent on intact immune-cell participation.
What should readers do with this information?
Use it as a research-interpretation tool.
It supports three practical ideas:
keep endocrine therapy central rather than replacing it
take the tamoxifen combination signal seriously as a mechanistic clue
recognise that the main unresolved issue is still human pharmacology, not whether the pathway signal is interesting
How this fits with the broader ivermectin picture
This 2026 paper does not stand alone.
It builds on the same research group's earlier 2025 work in endocrine-resistant breast-cancer models.
That earlier paper focused more on Wnt signalling, EMT biology, and invasion-related behaviour.
The 2026 paper extends that map by showing that ivermectin also presses on ESR1/ERα, HER2, and the ERK/SMAD2 arm of the escape network.
Taken together, the two papers suggest ivermectin is acting on several converging pathways rather than on one isolated target.
That does not solve the clinical translation problem.
It does strengthen the argument that the breast-cancer signal is mechanistically coherent.
Broader pathway summary
ESR1 / ERα
Suppressed at mRNA and protein level
Directly reduces core estrogen-driven signalling
HER2
Total protein reduced, strongest in standard and tamoxifen-resistant models
Cuts into a common endocrine-escape route
pERK
Reduced across tested ER-positive models
Weakens proliferative and escape signalling downstream of HER2
pSMAD2
Consistently reduced
May limit invasion, EMT-related behaviour, and metastatic potential
SMAD4
Preserved
Suggests pathway suppression without loss of a potentially protective node
PI3K / AKT / mTOR
No significant change reported
Shows ivermectin is not covering the whole endocrine-resistance network
Cyclin D1
Reduced in standard ER-positive cells
Supports relevance to cell-cycle pressure and proliferation control
PAK-1
Reduced in tamoxifen-resistant cells
Adds another possible resistance-relevant signal in selected models
Wnt / EMT
Shown more clearly in the 2025 companion paper
Extends the anti-invasive story beyond the 2026 paper alone
Tamoxifen combination
Formal synergy with CI < 1
Supports adjunctive combination logic rather than replacement logic
The 2025 companion-paper context
The 2025 companion paper from the same group is important because it helps explain why the 2026 data look more convincing when viewed together.
The earlier work supports an anti-EMT and anti-invasion effect in endocrine-resistant cells.
The 2026 paper then adds upstream and midstream pathway detail:
ESR1 / ERα suppression
HER2 reduction in key models
pERK suppression
highly consistent pSMAD2 suppression
preserved SMAD4
formal tamoxifen synergy
That sequence matters.
The 2025 paper helps show that ivermectin is not only changing markers on a blot.
The 2026 paper helps explain which signalling routes may sit behind those phenotype-level effects.
Reference
Earlier 2025 paper from the same research group on Wnt signalling and EMT in endocrine-resistant breast-cancer cells. Included here for context because the 2026 paper extends that earlier mechanistic work.
Rujimongkon K, Adchariyasakulchai P, Boonyaratsewee C, Horpratraporn K, Ketchart W. Ivermectin inhibits ER, HER2, and TGF-β pathways in ER-positive and endocrine-resistant breast cancer cells. PLOS One. 2026 Apr 30;21(4):e0348260. DOI:
10.1371/journal.pone.0348260
Part 2
Beyond ER+: Ivermectin as an Immune “Ignition Switch” — And When It Can Backfire
The 2026 Chula paper shows ivermectin (IVM) acting directly inside ER+ breast cancer cells, turning down ER, HER2 and TGF‑β‑linked growth signals. But IVM also has a second personality that comes from other studies: it behaves like an immunological ignition switch in the tumour microenvironment.
IVM's second personality
What IVM can do to the immune system
Animal and lab studies in breast and other cancers suggest that IVM can:
Trigger immunogenic cell death (ICD) – a “loud” form of cell death that releases danger signals like ATP and HMGB1, and flips cancer cells from “invisible” to “noticeable” for the immune system.
Increase T‑cell entry into tumours and improve the ratio of attacking T cells to regulatory T cells (Tregs), especially when combined with immune checkpoint blockers (e.g. anti‑PD‑1).
Target immunosuppressive myeloid cells (myeloid‑derived suppressor cells and some tumour‑associated myeloid populations), which normally keep anti‑tumour immunity on a tight leash.
Induce autophagy (the cell’s self‑eating recycling program) through PAK1/AKT/mTOR signalling in several cancer models, including breast cancer and glioma.
In a person with a reasonably intact immune system, this can be a good thing: IVM can both damage the cancer cell directly and call in immune reinforcements to help finish the job.
Why context matters
When the “ignition switch” fires into an empty room, the effect can change.
All of the above assumes you have enough functional white blood cells (especially T cells) ready to respond.
In a more fragile situation — for example:
Markedly low white blood cell counts (post‑chemo, post-radiation, marrow suppression, infection)
Periods of strongly upregulated autophagy from fasting or other drugs that like Ivermectin will further boost autophagy e.g Loratadine boosts autophagy. Autophagy is running when dormant cells are present and most treatments ER+ patients use have been shown to create dormant cells. Read the full Autophagy and the Dormancy section of the site at some point.
A highly inflamed bone environment (e.g. acute fracture, high TGF‑β and RANKL, recent SBRT, active infection)
No aromatase inhibitor on board in ER+ disease
the same IVM mechanisms can, in theory, behave very differently.
Autophagy and dormant ER+ bone cells
Several studies show IVM can switch on autophagy via AKT/mTOR and PAK1‑related pathways. In glioma models, IVM‑induced autophagy actually protected tumour cells from dying unless an autophagy blocker (like hydroxychloroquine) was added — meaning the autophagy response became a survival tool rather than a kill switch.
Dormant ER+ breast cancer cells in bone are known to be unusually dependent on autophagy to stay alive in a low‑nutrient, stressful niche. Although this has not been tested directly with IVM in bone dormancy models, it is biologically plausible that:
In dormant ER+ bone cells that already rely on autophagy,
Adding IVM (which further induces autophagy), plus fasting or other autophagy‑boosting inputs,
Could tilt autophagy toward deepening survival and metabolic re‑awakening, rather than pushing the cells into death.
This is a hypothesis, not proven fact, but it is consistent with what has been seen in glioma and with what is known about autophagy’s role in ER+ bone dormancy.
It is the reason why I created the Autophagy — Cancer's Escape Route and Dormant Cancer Cells pages.
MDSC depletion without T-cell backup
Changing the “brakes” on dormancy is the core concern here.
In an intact system, depleting immunosuppressive myeloid cells (MDSCs and some myeloid populations) with IVM helps T cells attack the tumour. But those same myeloid cells also participate in a delicate dormancy network in bone marrow, along with M2‑type macrophages, osteoclasts, and stromal cells.
If you:
Use IVM to strip out immunosuppressive myeloid cells,
But have too few functional T cells (because of low WBC or lymphopenia),
you may temporarily create an “immunological vacuum”. The usual suppressive tone that helps keep disseminated tumour cells quiet is disrupted, but there are not enough effector T cells ready to take advantage of the opening. In bone, where dormancy is partly maintained by a balance of suppressive and activating signals from myeloid and stromal cells, suddenly disturbing that balance could, in theory, nudge dormant cells toward reactivation rather than controlled clearance.
Again, this is theoretical but biologically coherent: current IVM data show myeloid and T‑cell reshaping in favour of immunity when T cells are present. The dormancy‑awakening risk arises when those reinforcements are missing or severely blunted.
Putting it together
This can become a multi-vector “awakening” pattern in a stacked-risk scenario.
So outside the clear ER+ signalling advantages shown in the 2026 Chula paper, IVM sits at the crossroads of:
Direct danger signalling (ATP, HMGB1, ICD)
Autophagy induction (which can be protective in some tumours) but current research points to it needing inhibition in ER+ ones. See Autophagy Escape in ER-Positive Breast Cancer.
Myeloid and T‑cell reshaping in the tumour microenvironment
In a robust immune system with ongoing endocrine coverage and safe autophagy inhibition stratagies in place, this combined profile can be strongly anti‑tumour. In a stacked‑risk situation — low WBCs, high inflammation from something like a bone fracture/SBRT/infection, boosted autophagy, and no aromatase inhibitor or autophagy inhibitor in ER+ disease — the same levers could, in principle, act more like a multi‑vector dormancy‑awakening stimulus than a clean kill signal.
That doesn’t negate the ER+ benefits shown in the Chula work; it just underlines that context (immune status, inflammatory load, endocrine coverage, autophagy tone) matters enormously when you move from cell dishes into real, complex human biology.
Part 2 references
Ivermectin → ICD, ATP/HMGB1 release, “cold to hot” tumours, MDSC/T-cell effects
Zeng et al. Ivermectin converts cold tumors hot and synergizes with immune checkpoint therapy. Open-access mechanistic immunology work. https://pmc.ncbi.nlm.nih.gov/articles/PMC7925581/
Ivermectin induces autophagy via AKT/mTOR in glioma
Tang et al. Ivermectin induces autophagy-mediated cell death through the AKT/mTOR signaling pathway in glioma cells. In-vitro and in-vivo glioma study. https://pmc.ncbi.nlm.nih.gov/articles/PMC6900471/
Ivermectin, PAK1, and “non-protective” autophagy
Li et al. Ivermectin induces nonprotective autophagy by downregulating PAK1 and inhibiting Akt/mTOR pathway. Cancer-cell model. https://www.semanticscholar.org/paper/Ivermectin-induces-nonprotective-autophagy-by-PAK1-Li-Zhang/d8c86b8a962f9411d3c0a8f13d75dc9496df13ae
Review: Myeloid-derived suppressor cells (MDSCs) and their dual roles
Veglia et al. Myeloid-Derived Suppressor Cells: Not Only in Tumor Immunity. Frontiers in Immunology 2019. https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.01099/full
General review: MDSCs and antitumour strategies
Gabrilovich & Nagaraj. Antitumor effects of targeting myeloid-derived suppressive cells. Foundational review on MDSCs. https://pmc.ncbi.nlm.nih.gov/articles/PMC8798346/
Review: Dendritic cells and immunogenic cancer cell death (danger signals, HMGB1, ATP)
Ramos et al. Dendritic Cells and Immunogenic Cancer Cell Death. Frontiers in Immunology 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7151083/
Part 3
Members Questions from Parts 1 & 2 Answered
Q: If I'm taking ivermectin and doing intermittent fasting, is that bad for dormancy?
A: It’s not automatically “bad,” but it’s also not the simple win that social media makes it sound like. You’re layering several powerful levers on top of each other, and dormant ER+ cells sit right in the cross‑hairs of that biology.
Intermittent fasting creates a low‑nutrient state that activates autophagy – the cell’s recycling and energy‑conservation program. In fast‑growing cancer cells, this can add stress and, in some contexts, tip cells toward death. Fasting can also improve insulin sensitivity, reduce chronic inflammation, and prime parts of the immune system. All of that is genuinely helpful.
Dormant cells are a different story. Dormant ER+ breast‑cancer cells – the ones that have tucked themselves away into bone marrow and other niches and gone quiet – are not dividing fast. They survive by down‑shifting their metabolism and leaning heavily on autophagy as a survival tool. Autophagy is one of the reasons they can sit there for years without dying.
Ivermectin adds another layer. In several cancer models, including glioma and breast, ivermectin has been shown to trigger autophagy through PAK1/AKT/mTOR‑related pathways. In some of those models, ivermectin‑induced autophagy actually protects tumour cells unless an autophagy blocker (like hydroxychloroquine) is added. In other words, the autophagy that’s switched on is “protective autophagy,” not a kill switch.
So if you combine: dormant ER+ cells that already rely on autophagy fasting, which further increases autophagy ivermectin, which also increases autophagy
you can end up amplifying a survival mechanism in dormant cells rather than threatening it. That doesn’t mean they will necessarily wake up – it may keep them deeper in dormancy for a while – but it is not a straightforward “starve and kill” scenario.
Where this becomes more concerning is when you add an inflammatory or injury signal on top – an infection, a fracture, a major radiation exposure, even a big oxidative burst from something like IV vitamin C. Those kinds of events send “wake‑up” and “repair” signals into bone and other niches. If, at that same moment, dormant cells are metabolically well‑supported through autophagy (from fasting + ivermectin), they may be in better shape to respond to those wake‑up signals than to die from them.
This doesn’t mean “never fast” or “never use ivermectin.” It means: Think about what else is running in the background (endocrine therapy, autophagy blockers, inflammatory load) when you stack them. Consider whether some autophagy‑modulating support (for example, hydroxychloroquine in a trial setting, or polyphenols like EGCG/curcumin/berberine at meaningful doses) is part of the picture. Be extra cautious about combining heavy autophagy‑inducing inputs around the time of big inflammatory events if you know you have dormant ER+ disease.
Dormancy biology is not simple, but it is understandable. The key is to recognise autophagy as a survival pathway for dormant cells, not just a “detox” button, and to plan combinations with that in mind. The Cancer Cell Dormancy page is always found under D section of the Home page.
References
Ivermectin induces autophagy‑mediated cell death through the AKT/mTOR signaling pathway in glioma cells https://pmc.ncbi.nlm.nih.gov/articles/PMC6900471/
Ivermectin induces nonprotective autophagy by downregulating PAK1 and inhibiting Akt/mTOR pathway https://pubmed.ncbi.nlm.nih.gov/37741955/
Autophagy and Cancer Dormancy https://pmc.ncbi.nlm.nih.gov/articles/PMC8017298/
The Role of Intermittent Fasting in the Activation of Autophagy https://pmc.ncbi.nlm.nih.gov/articles/PMC12112746/
Intermittent Fasting and Cancer https://restorativemedicine.org/journal/intermittent-fasting-cancer/
Q: If ivermectin down-regulates ER while I'm on a CDK4/6 inhibitor and my blood counts are low, does that make my endocrine treatment and CDK4/6 drug work worse and push cancer into other pathways?
A: It’s a really important question, and the short answer is: current evidence does not support the idea that ivermectin’s ER‑lowering effect makes your endocrine or CDK4/6 drugs “work worse.” It does, however, add pressure on the system, and cancer will always look for other survival routes – that part of your instinct is right.
First, what CDK4/6 inhibitors are actually doing. They sit at a cell‑cycle checkpoint controlled by Cyclin D and CDK4/6. ER signalling is one input into that checkpoint, but so are HER2 and other growth pathways. CDK4/6 inhibitors don’t need ER to be high; they block the kinase activity at that checkpoint so that cells can’t easily move from “resting” to “dividing,” regardless of which growth signal is pushing them.
Endocrine therapy (tamoxifen, fulvestrant, aromatase inhibitors) is trying to reduce ER signalling directly or remove its fuel (estrogen). Ivermectin, based on the 2026 Chulalongkorn paper, further reduces ESR1 mRNA (the gene message for ERα) and ERα protein, and also reduces HER2 in key ER+ models. That means ivermectin is pressing down on the same growth axis your endocrine drugs are already targeting, not lifting it.
In the lab, when they combined ivermectin with tamoxifen’s active form (4‑OHT), they didn’t see antagonism; they saw synergy. The combination lowered ERα and HER2 more than either drug alone and produced a formal combination index less than 1 (the standard definition of synergy) in ER+ and tamoxifen‑resistant cells. If ivermectin’s ER down‑regulation made tamoxifen “work worse,” you would expect to see the opposite.
For fulvestrant‑resistant models, the ER signal was more complicated and less clean, which the authors openly discuss, but the data still don’t show ivermectin reversing fulvestrant’s effect or making CDK4/6 mechanisms fail.
On the immune side: CDK4/6 inhibitors do lower neutrophil counts in the blood, and that matters for infection risk. But low counts on a lab printout are not the whole immune story. Preclinical and early human data suggest CDK4/6 blockade can: make tumour cells display more pathway‑specific antigens on their surface, making them more visible to T cells, nudge CD8⁺ T cells toward a longer‑lived “memory” state instead of a short‑lived exhausted state and reduce some immune‑suppressive pressure in the tumour microenvironment
This is why CDK4/6 inhibitors are being actively combined with immunotherapies in trials. So “on CDK4/6 = no immune help” is an oversimplification.
The direct ER/HER2/TGF‑β effects of ivermectin that we’ve been talking about come from experiments in cancer cell lines with no immune system present at all. Those are cell‑intrinsic effects. That means those particular benefits don’t depend on your T‑cells being perfect – they still happen in the cancer cells themselves as long as the ivermectin is present.
What we don’t know yet, is what repeated medium‑to‑long stretches on ivermectin followed by medium‑to‑long stretches off might do to these pathways over time. In biology, when you keep forcing a pathway down and then suddenly lift the pressure, you often get rebound or compensatory up‑regulation – the system pushes back and overshoots. We don’t have human data on that with ivermectin in ER+ disease, but the general pattern is common enough in endocrine and growth signalling to treat stop–start use with caution rather than assuming it’s neutral.
Where your concern is absolutely valid is that any strong pressure on ER will encourage cancer to explore other survival tactics – PI3K/AKT/mTOR, FGFR, integrins, and so on. This 2026 ER+ paper found that ivermectin did not significantly hit PI3K/AKT/mTOR, so it is not a complete network solution. But that isn’t unique to ivermectin; it’s how all targeted therapy works. That’s why oncology so often layers: endocrine + CDK4/6 ± PI3K/mTOR ± anti‑HER2, etc.
So a fair way to put it is: Ivermectin’s ER down‑regulation is aligned with what endocrine therapy is trying to do, not fighting it. CDK4/6 inhibitors still have their target – the CDK4/6 checkpoint – and less upstream growth signalling feeding into that checkpoint may actually support their work, not undermine it. The real complexity isn’t “ER gets too low so my drugs stop working,” it’s the broader adaptation problem: what resistance pathways has ivermectin been shown to encourage in what contexts. What does the research indicate are the pathways the cancer will lean on next, and how we plan to safely stop this.
References
Ivermectin inhibits ER, HER2, and TGF‑β pathways in ER‑positive and endocrine‑resistant breast cancer cells https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0348260
Low‑Dose CDK4/6 Inhibitors Induce the Presentation of Pathway‑Specific Tumor Antigens https://pmc.ncbi.nlm.nih.gov/articles/PMC8158036/
Inhibition of CDK4/6 Promotes CD8 T‑cell Memory Formation https://pubmed.ncbi.nlm.nih.gov/33941591/
CDK4/6 Inhibition Induces CD8⁺ T‑Cell Antitumor Immunity via MIF‑Dependent Macrophage Crosstalk https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511330
The Regulatory Role of CDK4/6 Inhibitors in Tumor Immunity and the Tumor Microenvironment https://pmc.ncbi.nlm.nih.gov/articles/PMC12176271/
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