Adults with neurofibromatosis type 1 who take the MEK inhibitor mirdametinib see their plexiform neurofibromas shrink by at least 20 percent in about 2 out of 5 cases, according to clinical trial data.
People born with neurofibromatosis type 1 possess one inactive copy of the NF1 gene, which leaves a protein known as MEK overactive. This overactivity commands cells to grow and divide at an accelerated rate, frequently affecting Schwann cells that wrap around and protect nerves.
When these rapidly dividing cells accumulate further DNA copy errors, both copies of the gene stop working, leading to the formation of noncancerous tumors called plexiform neurofibromas along the nerve pathways.
Doctors evaluate whether a MEK inhibitor is working by analyzing volumetric MRI scans and assessing patient-reported symptoms. Volumetric MRI calculates a tumor’s full three-dimensional size.
In clinical trials, a tumor was considered responsive if it shrank by at least 20 percent and that reduction held steady on a second scan performed two to six months later. Because tumors take months to respond, an MRI about every six months is reasonable, though there’s no hard-and-fast rule.
Beyond imaging, clinical trials track tumor pain on a scale of 0 to 10 alongside functional questionnaires. While formal questionnaires are rarely utilized outside major academic centers, tracking quality of life involves monitoring pain, strength, appearance, vision, swallowing, and physical mobility. In a mirdametinib trial, approximately 70 percent of adults reported feeling at least a little better overall after about a year of treatment.
“Those conversations are different for every patient. Quality of life may involve pain, strength, appearance, vision, swallowing, or even the way your clothes fit,” says Brown.
In Plain English: The Clinical Takeaway
- MEK Inhibition: MEK inhibitors are oral medications that block the overactive MEK protein, slowing down the rapid growth and division of tumor cells.
- Volumetric Response: Clinical success is measured using three-dimensional MRI scans to confirm whether a tumor has shrunk by 20 percent or more.
- Symptom Relief: Physical improvements such as pain reduction often occur months before measurable tumor shrinkage appears on an MRI scan.
Drug Timelines for Tumor Shrinkage and Pain Relief
The time required to achieve tumor shrinkage varies significantly between the two evaluated drugs. In an adult trial for selumetinib, half of the responding patients observed a response within approximately four months. By contrast, the mirdametinib trial showed it took about eight months to reach that same halfway point, with patients typically requiring about 15 months to reach their smallest tumor size.
Pain relief frequently precedes visible anatomical changes. Nearly 60 percent of adults in the mirdametinib trial who experienced significant baseline pain reported meaningful relief within about four months. Response rates also differ across age groups. Both drugs worked better in trials in children, where about 50 to 70 percent responded, which may be because those children tended to have more severe or faster-growing tumors.
“Pain can start to improve as early as two months after starting treatment. People may also function better before we see the tumor shrink, possibly because they're in less pain,” says Brown.
| Medication | Adult Tumor Shrinkage (≥20%) | Time to Median Response |
|---|---|---|
| Mirdametinib | About 2 out of 5 adults | About 8 months |
| Selumetinib | 20% of adults (vs. 5% on placebo) | About 4 months |
When tumors respond, the volume reduction can be substantial. More than 60 percent of adults responding to mirdametinib saw their tumors shrink by more than half, with some reductions reaching as much as 90 percent. Among people who responded to selumetinib, tumors typically shrank by about a third. Even patients whose tumors did not reach the 20 percent threshold still experienced an average shrinkage of about 10 percent.
Long-Term Management and Treatment Duration
When MEK inhibitors successfully shrink a tumor, the results tend to last. In the mirdametinib trial, all but one of the 24 responding adults were still responding when the results were analyzed, although none of the tumors went away completely. “They only work while you're taking them, and tumors have been reported to grow back after people stop,” says Rebecca Brown.
Common practice is to stay on a MEK inhibitor for at least 18 months to two years. If patients struggle to tolerate adverse reactions, physicians might gradually taper the dosage over several months rather than halting administration abruptly, helping to prevent tumor regrowth.
Although some patients have stayed on MEK inhibitors for many years, there is not enough data yet to say how long is long enough or how long is too long.
Managing Common Side Effects During Early Treatment
Reactions linked to therapy occur very frequently; clinical studies demonstrated that virtually every participant receiving a MEK inhibitor experienced at least one adverse event, such as diarrhea, nausea, or a rash resembling acne. The initial six months present the most difficult phase of therapy, as adverse reactions peak during this timeframe while tumor reduction remains premature.
For a mild acne-like rash, that might mean a low-strength steroid lotion such as hydrocortisone. Should the skin condition intensify, medical personnel may recommend a prescription antibiotic lotion like clindamycin, an oral antibiotic such as doxycycline, or a dermatology consultation. Antidiarrheal medicines such as loperamide can ease diarrhea, but check with your care team before taking it.
Patients experiencing severe adverse effects should not independently alter their regimen. The care team may pause the medication until it improves or lower the dose. Most people can restart once things improve. Permanent discontinuation is uncommon and is reserved for significant problems. Patients must communicate with their care team when thinking about stopping medication or changing a dose.
References
- Everyday Health: Measuring Success and Clinical Trials for MEK Inhibitors in NF1
- University of Alabama at Birmingham Heersink School of Medicine Clinical Observations
- Ohio State University Wexner Medical Center Neuro-Oncology Division Data
