Aminopterin, a cancer drug, effectively blocked monkeypox virus replication in laboratory cells and reduced viral infection and inflammation in animal studies, according to research published in Microbiology Spectrum. The drug was 92 times more selective at killing the virus than harming healthy cells in monkey kidney cells, and over 305 times more selective in human skin cells. While these findings are promising, aminopterin has not yet been tested in humans and remains experimental.
Scientists discovered that a drug called aminopterin, originally used to fight cancer, can effectively stop monkeypox virus from spreading in laboratory and animal studies. According to Gram Research analysis, aminopterin worked better than three other similar drugs at blocking the virus and reducing inflammation caused by infection. The drug appears to work by interfering with how the virus attaches to cells and reproduces. While these early results are encouraging, the drug still needs to be tested in humans before doctors can use it to treat monkeypox patients.
Key Statistics
A 2026 laboratory study published in Microbiology Spectrum found that aminopterin blocked monkeypox virus replication at a concentration of 36.10 nanomoles in monkey kidney cells while requiring 3,310 nanomoles to harm healthy cells, achieving a selectivity index of 91.69.
In human foreskin fibroblast cells, aminopterin demonstrated an even stronger selectivity index exceeding 305.53, stopping viral replication at 32.73 nanomoles while remaining non-toxic at concentrations above 10 micromoles, according to the 2026 Microbiology Spectrum study.
Research published in Microbiology Spectrum in 2026 showed that aminopterin outperformed three other antifolate agents (methotrexate, pemetrexed, and LSN3213218) at inhibiting monkeypox virus replication in laboratory testing.
A 2026 study found that aminopterin not only reduced monkeypox viral load in animal models but also mitigated the inflammation and tissue damage associated with high-dose viral infection.
The Quick Take
- What they studied: Whether a cancer drug called aminopterin could stop monkeypox virus from infecting cells and spreading in the body
- Who participated: Laboratory cells (both monkey kidney cells and human skin cells) and animal models; no human participants were involved in this early-stage research
- Key finding: Aminopterin blocked monkeypox virus replication at very low doses and was significantly more effective than three other similar drugs tested
- What it means for you: This is promising early research, but it’s too soon to know if aminopterin will work in humans. More testing is needed before it could become a treatment option for monkeypox patients
The Research Details
Researchers tested aminopterin against monkeypox virus using two different types of laboratory cells: monkey kidney cells and human skin cells. They measured how much drug was needed to stop the virus from replicating (called the IC₅₀) and how much drug was toxic to healthy cells (called the CC₅₀). They also compared aminopterin to three other antifolate drugs—methotrexate, pemetrexed, and LSN3213218—to see which worked best.
Next, the team tested aminopterin in living animals infected with monkeypox to see if it worked in a real body, not just in a dish. They looked at whether the drug reduced the amount of virus and decreased inflammation (swelling and irritation caused by infection).
This type of research is called ‘in vitro and in vivo’ testing—in vitro means in laboratory cells, and in vivo means in living organisms. It’s an important early step before testing drugs in humans.
Testing in both laboratory cells and animals gives scientists confidence that a drug might work in real human bodies. The fact that aminopterin worked better than other similar drugs suggests it has unique properties worth exploring further. Finding new treatments for monkeypox is important because the virus continues to spread globally and we need more options to fight it.
This study used established laboratory methods and compared aminopterin to multiple other drugs, which strengthens the findings. The researchers tested in two different cell types, which shows the results weren’t just a fluke in one system. However, this is early-stage research—animal studies don’t always predict how drugs will work in humans. The study doesn’t specify exact sample sizes for animal experiments, which makes it harder to evaluate the strength of those results.
What the Results Show
Aminopterin was remarkably effective at stopping monkeypox virus in laboratory cells. In monkey kidney cells, the drug blocked viral replication at a concentration of 36.10 nanomoles (an extremely tiny amount), while it took 3,310 nanomoles to harm healthy cells—meaning the drug was about 92 times more selective for killing virus than harming cells.
In human skin cells, aminopterin was even more impressive: it stopped the virus at just 32.73 nanomoles, and the drug didn’t harm healthy cells even at concentrations above 10,000 nanomoles—making it more than 305 times more selective. This huge gap between the dose needed to stop the virus and the dose that harms cells is exactly what doctors want in a medicine.
When tested in living animals with monkeypox infection, aminopterin not only reduced the amount of virus in the body but also decreased the inflammation and tissue damage caused by the infection. The drug appeared to work by preventing the virus from attaching to cells and by blocking the virus from making copies of itself.
A key finding was that aminopterin outperformed three other antifolate drugs (methotrexate, pemetrexed, and LSN3213218) at stopping monkeypox virus. This is important because it shows aminopterin has special properties that make it better suited for fighting this particular virus than other drugs in the same family. The ability to reduce inflammation in infected animals suggests the drug might help prevent some of the severe complications that can occur with monkeypox.
This research adds to growing evidence that antifolate drugs—medications that interfere with folate metabolism—might have antiviral properties beyond their known cancer-fighting abilities. While methotrexate is sometimes used off-label for various conditions, this study shows it doesn’t work well against monkeypox, whereas aminopterin does. This finding opens a new direction for monkeypox treatment research that hadn’t been thoroughly explored before.
This study only tested aminopterin in laboratory cells and animals, not in humans. Animal studies often don’t translate perfectly to human medicine—a drug that works in mice or monkeys might not work the same way in people, or it might have side effects that weren’t apparent in animals. The study doesn’t provide detailed information about how many animals were used or the exact experimental conditions, making it harder to fully evaluate the strength of the animal results. Additionally, we don’t yet know if aminopterin would be safe for long-term use in humans or how it would interact with other medications.
The Bottom Line
Based on this early research, aminopterin appears to be a promising candidate for further development as a monkeypox treatment. However, confidence level is moderate because this is preliminary laboratory and animal research. The next steps would be safety testing in humans and then clinical trials to see if it actually helps patients. At this stage, aminopterin should not be used to treat monkeypox outside of formal research studies.
This research is most relevant to pharmaceutical companies, researchers, and public health officials looking for new monkeypox treatments. People currently infected with monkeypox should not seek out aminopterin, as it’s not approved for this use and hasn’t been tested in humans. Healthcare providers should stay informed about this research as it progresses toward human trials.
If aminopterin moves forward, typical drug development takes 5-10 years before a new treatment becomes available to patients. First would come safety testing (1-2 years), then early human trials (2-3 years), and finally larger clinical trials (2-3 years). So realistically, if everything goes well, aminopterin might not be available as a monkeypox treatment for several years.
Frequently Asked Questions
Can I get aminopterin to treat monkeypox right now?
No. Aminopterin has only been tested in laboratory cells and animals, not in humans. It’s not approved for monkeypox treatment and is not available outside of research studies. Anyone with monkeypox should consult their doctor about currently approved treatments.
How does aminopterin stop monkeypox virus?
Aminopterin interferes with folate metabolism, a process the virus needs to replicate and attach to cells. By blocking this process, the drug prevents the virus from making copies of itself and spreading to other cells in the body.
Why is aminopterin better than methotrexate for monkeypox?
While both are antifolate drugs, aminopterin proved significantly more effective at stopping monkeypox virus in laboratory tests. Methotrexate, another antifolate drug, showed minimal antiviral activity against monkeypox in this study, suggesting aminopterin has unique properties.
When will aminopterin be available as a monkeypox treatment?
If development continues successfully, aminopterin would need to go through human safety testing and clinical trials before approval, a process typically taking 5-10 years. There’s no guarantee it will ultimately be approved for human use.
Are there any side effects from aminopterin?
This study didn’t test aminopterin in humans, so we don’t know what side effects it might cause. Aminopterin is a cancer drug, and cancer drugs typically have significant side effects. More research is needed to understand safety in humans.
Want to Apply This Research?
- Users interested in monkeypox treatment developments could track ‘antiviral drug research milestones’ by setting reminders to check for updates on aminopterin’s progress through clinical trials every 6 months
- While awaiting new treatments, users can track and maintain evidence-based monkeypox prevention behaviors: monitoring for symptoms, practicing good hygiene, and staying informed about outbreak status in their area
- Create a ‘monkeypox treatment tracker’ that monitors news and research databases for updates on aminopterin clinical trials, allowing users to stay informed about when this potential treatment might become available
This article describes early-stage laboratory and animal research. Aminopterin has not been tested in humans and is not approved for treating monkeypox. This information is for educational purposes only and should not be used to guide treatment decisions. Anyone with monkeypox should consult with a healthcare provider about appropriate, approved treatment options. Do not attempt to obtain or use aminopterin outside of authorized clinical research settings.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.