Imagine your oncologist calls with results from your tumor's genetic testing. "Your tumor mutational burden is 11," she says. "That's high — you may qualify for immunotherapy."
Under the rules that have been in place since 2020, she would have been right. A score of 11 cleared the bar.
As of October 16, 2026, it no longer does.
On that day, the U.S. Food and Drug Administration (FDA) granted traditional (full) approval to Keytruda (pembrolizumab) — and its under-the-skin version, Keytruda Qlex — for adults and children with unresectable or metastatic solid tumors that are tumor mutational burden–high (TMB-H), have grown after prior treatment, and have no satisfactory alternative treatment options. At the same time, the FDA raised the definition of TMB-high from at least 10 mutations per megabase (mut/Mb) to at least 13.
So this is really two stories in one. The first is good news: a major immunotherapy approval is now permanent, backed by mature data. The second is more complicated: the finish line moved, and some patients who qualified last month fall outside the label today.
This post explains both — what tumor mutational burden actually is, how it differs from the MSI-H/dMMR markers we covered in our last two posts, what the big study found, and what to ask your oncology team if your score sits anywhere near the line.
What Is Tumor Mutational Burden?
Every tumor's DNA is full of changes — mutations — that built up as the cancer grew. Tumor mutational burden (TMB) is simply a count of them: how many mutations are found in a standard-sized stretch of the tumor's DNA, measured in mutations per megabase (mut/Mb).
Think of the tumor's DNA as an instruction book for building cells.
- A tumor with a low TMB has a fairly clean copy of the book — a few typos here and there.
- A tumor with a high TMB has a book riddled with typos — page after page of garbled instructions.
Here is why that matters for treatment. Those typos change the proteins the tumor builds, and garbled proteins can look foreign to the immune system — like misspelled words a spell-checker would flag instantly. The more typos a tumor carries, the more visible it tends to be to immune cells, and the better immunotherapy drugs like Keytruda tend to work against it.
That is the whole logic of TMB as a biomarker — a measurable feature that helps predict who is likely to benefit from a treatment. A lab test on tumor tissue (or, in some cases, on blood) produces a number. Above the cutoff, the tumor is "TMB-high." Below it, it isn't.
And as this approval shows, exactly where that cutoff sits is a judgment call — one that can change as better data arrives.
A TMB score is not a grade of how "bad" a cancer is. It is a prediction tool: how visible is this tumor to the immune system, and how likely is immunotherapy to help?
TMB vs. MSI-H/dMMR: Two Different Ways to Spot a "Typo-Heavy" Tumor
If you have read our recent posts, this may sound familiar. Our post on the FDA approval of immunotherapy plus chemotherapy for stage III dMMR colon cancer and our post on Jemperli for MSI-H/dMMR rectal cancer both covered tumors whose DNA spell-check system is broken. Those tumors pile up mutations too. So how is TMB different?
- MSI-H/dMMR asks a yes-or-no question: is the tumor's spell-check machinery (mismatch repair) broken? It is one specific cause of mutation build-up.
- TMB asks a how-many question: whatever the cause, how many mutations does this tumor actually carry?
The two overlap — most MSI-H tumors also have high TMB — but they are not the same test and not the same group. A tumor can be TMB-high for other reasons (sun damage in melanoma, tobacco exposure in lung cancer, certain other repair problems) without being MSI-H at all.
That distinction is built into this approval's main study. KEYNOTE-158's Cohort M — the group enrolled specifically to test this question — was made up of TMB-high tumors that were not MSI-H. In other words, the FDA wanted to know: does TMB predict benefit on its own, even when the spell-check story isn't the explanation? The answer, at the right cutoff, turned out to be yes.
What Exactly Did the FDA Change?
Keytruda is not new to TMB-high cancers. The timeline matters:
- June 2020: The FDA granted Keytruda an accelerated approval for TMB-high solid tumors, defined then as ≥10 mut/Mb. Accelerated approval is a "yes, for now" based on early results — in exchange, the company must confirm the benefit in follow-up data.
- October 16, 2026: With the confirmatory data in, the FDA converted that to traditional (full) approval — the "yes" is now settled. But the label was narrowed at the same time: TMB-high now means ≥13 mut/Mb, measured by an FDA-authorized test.
Why narrow it? Because when the agency lined up the mature data, the benefit in patients with TMB of 10 to less than 13 did not hold up well enough to confirm. The responses were there — just fewer, and weaker, than in the ≥13 group. The FDA's own wording is plain: efficacy to support traditional approval "was not confirmed" in the 10 to <13 data-preserve-html-node="true" group.
One more practical change: the approval covers both the original IV form of Keytruda and Keytruda Qlex, a subcutaneous (under-the-skin injection) version of pembrolizumab combined with berahyaluronidase alfa-pmph, an ingredient that helps the drug spread under the skin so it can be given as an injection instead of a long IV drip.
What Did KEYNOTE-158 Find?
The approval rests on KEYNOTE-158, a large phase 2 study of pembrolizumab in previously treated advanced solid tumors. For this approval, the FDA looked at 220 patients whose tumors scored TMB ≥13 mut/Mb:
- Cohort M (145 patients): enrolled on purpose for this question — TMB-high tumors that were not MSI-H. They received pembrolizumab 400 mg IV every 6 weeks.
- Cohorts A–J (75 patients): patients with various tumor types whose TMB-high status was confirmed in a planned analysis. They received pembrolizumab 200 mg IV every 3 weeks.
Treatment continued until the cancer grew or side effects became unacceptable, for up to about two years. Here is what the study found.
Finding 1: About 1 in 3 tumors shrank
The objective response rate was 36% (95% confidence interval: 30–43%). That means in just over a third of these heavily pretreated patients, the tumor shrank measurably on scans. For people who had already been through prior treatment and had no good alternatives left, that is a meaningful number.
Finding 2: When it worked, it tended to keep working
Among the patients who responded, 58% had responses that lasted at least 12 months. Durability is immunotherapy's signature strength — when the immune system learns to hold a tumor down, the effect can persist long after a chemotherapy response would have faded.
Finding 3: The old data predicted the cutoff move
Back in the 2020 analysis that won the accelerated approval, researchers had already split the results at 13. Patients at ≥13 mut/Mb responded 37% of the time (70 patients), while patients at 10 to <13 data-preserve-html-node="true" responded only 13% of the time (32 patients). The signal was there six years ago. The 2026 action is the FDA catching the label up to what the data kept showing.
The cutoff did not move because the science was wrong in 2020. It moved because six more years of data showed exactly where the benefit is strong — and where it fades.
"My Score Was 11." What the 10–12 Group Should Know
This is the part of the story that matters most to a specific group of readers: anyone whose tumor scored 10, 11, or 12 mut/Mb.
Falling outside the new label does not mean:
- that Keytruda could never help you — the 10–<13 data-preserve-html-node="true" group did have responses (about 13% in the early analysis); the FDA's point is that the benefit wasn't strong or consistent enough to confirm for the whole group.
- that your past treatment was a mistake — if you received pembrolizumab under the old approval, you were treated under the standard of the time, with the information available then.
- that immunotherapy is off the table entirely — your tumor might qualify under a different biomarker (MSI-H/dMMR status is a separate approval path), a cancer-specific indication, or a clinical trial.
What it does mean is practical: for a new start today, an FDA-labeled TMB indication needs a score of 13 or higher. Coverage and access decisions often follow the label, so the 10–12 group is exactly who should be asking about trials and alternatives. Bring your exact number to that conversation — not just "high" or "low."
Three Myths to Avoid
Myth 1: "A higher TMB score means a worse cancer." No. TMB is not a measure of how aggressive or advanced a cancer is. It measures how many mutations the tumor carries, which mainly predicts how visible it is to the immune system. Some very treatable tumors have high TMB; some dangerous ones have low TMB.
Myth 2: "Full approval means it works for everyone with a high score." No. In KEYNOTE-158, 36% of patients at ≥13 responded — which also means about 64% did not have a measurable response. Full approval means the benefit is proven for the group, on average. It is not a promise for any one person.
Myth 3: "The FDA moved the cutoff, so the test must be unreliable." No. The test measures what it measures. What changed is where experts draw the line for "likely enough to benefit" — and lines drawn on a spectrum get refined as larger, longer studies report. A moved cutoff is the system working as intended, even when the result is disappointing for people near the line.
What Side Effects Can Keytruda Cause?
Keytruda is a PD-1 blocker: it releases a brake on the immune system so T cells can attack the tumor. That same released brake explains its side effects.
Common side effects of pembrolizumab include:
- Fatigue
- Diarrhea
- Rash and itching
- Nausea
- Muscle and joint aches
The side effects that need the fastest attention are immune-related ones, where the activated immune system inflames healthy organs:
- Colitis — inflammation of the colon: worsening diarrhea, blood or mucus in the stool, severe stomach pain
- Pneumonitis — lung inflammation: new cough, shortness of breath, chest pain
- Hepatitis — liver inflammation: yellowing skin or eyes, dark urine
- Hormone gland problems — especially thyroid: unusual tiredness, weight change, feeling cold or racing heartbeat
- Kidney or skin inflammation
These can appear during treatment or even after it ends. Immune-related diarrhea deserves special emphasis: it can be an early sign of colitis, which needs prompt medical treatment — and any ongoing diarrhea drains water and electrolytes, which brings us to supportive care.
Why Supportive Care Still Matters During Immunotherapy
This approval is about matching the right drug to the right tumor. Supportive care is about how a person gets through that treatment day to day. Learn more in our post: Cancer Treatment Is More Than Fighting the Tumor: Why Supportive Care Matters
For patients on Keytruda, the supportive-care picture looks like this:
- Diarrhea and hydration. Diarrhea is a common pembrolizumab side effect, and immune-related colitis can make it severe. Losing water, potassium, and magnesium day after day leaves you weak and strains the heart and kidneys. Report new or worsening diarrhea early — and keep up with fluids and electrolytes while your team sorts out the cause.
- Fatigue. One of the most common effects of immunotherapy. Rest planning, gentle activity, and nutrition support are part of treatment, not extras.
- The long tail of monitoring. Immunotherapy responses can last — and so can immune side effects, which can show up months after the last dose. Follow-up visits and lab checks (including thyroid) are part of the treatment, not an afterthought.
That is the supportive-care space HuMOLYTE was made for. HuMOLYTE is a medical food for use under medical supervision, designed to support hydration and electrolyte balance — including magnesium, which many standard drinks do not replace — and gut health during cancer treatment. HuMOLYTE does not treat cancer, and it does not replace Keytruda or care from your oncology team. Talk with your oncologist before starting any new product during treatment, including HuMOLYTE.
Practical steps that help during immunotherapy:
- Keep a simple symptom diary: energy, appetite, bathroom habits, skin changes.
- Report diarrhea, rash, or a new cough early — do not wait for the next infusion.
- Ask whether your thyroid and other hormone levels are being checked.
- Keep your exact biomarker results — MSI/MMR status and your TMB number — in your own records.
What Should Patients and Caregivers Watch For?
During and after Keytruda treatment, contact your care team promptly if you notice:
- Diarrhea that is new, worse than usual, or lasts more than a day — or any blood or mucus in the stool
- Severe stomach pain or tenderness
- A new or spreading rash, blistering, or severe itching
- Cough, shortness of breath, or chest pain
- Yellowing of the skin or eyes, or dark urine
- Unusual tiredness, weight change, mood change, or feeling cold all the time (possible hormone-gland effects)
- Headaches that won't go away, or vision changes
- Signs of dehydration: dry mouth, dizziness, urinating much less than usual, muscle cramps, or a racing heartbeat
Because immune-related side effects can appear even after treatment ends, keep reporting new symptoms at follow-up visits too.
Questions to Ask Your Oncology Team
- What was my tumor's exact TMB number — not just "high" or "low"?
- Was my tumor also tested for MSI and mismatch repair (MMR)? Am I MSI-H or dMMR?
- Which FDA-authorized test was used to measure my TMB?
- If my score is 13 or higher, is Keytruda an option for me — IV or the under-the-skin Qlex version?
- If my score is between 10 and 12, what changed for me with the new cutoff — and what are my alternatives?
- Are there clinical trials I should consider, especially if I fall outside the label?
- What side effects should I watch for, and which ones mean I should call right away?
- How will my thyroid and other hormone glands be monitored during treatment?
- What is our plan for fluids and electrolytes if I develop diarrhea?
- If Keytruda works, how long would I stay on it — and how will we know it is working?
The Bottom Line
On October 16, 2026, the FDA granted traditional (full) approval to Keytruda and Keytruda Qlex for unresectable or metastatic TMB-high solid tumors that progressed after prior treatment — and raised the TMB-high cutoff from ≥10 to ≥13 mut/Mb:
- In KEYNOTE-158, 220 patients at TMB ≥13 had a 36% response rate, and 58% of responders stayed in response for at least 12 months.
- The benefit at 10 to <13 data-preserve-html-node="true" did not hold up well enough to confirm — an early split of the data (37% responding at ≥13 vs. 13% at 10–<13) data-preserve-html-node="true" pointed this way back in 2020.
- The approval is tumor-agnostic: any solid tumor type can qualify if the score and the clinical situation fit.
- TMB and MSI-H/dMMR are different tests — and Cohort M proved TMB predicts benefit even in tumors that are not MSI-H.
The headline for patients is simple: know your number. A cutoff is only useful if you know which side of it you are on — and if your score is 10, 11, or 12, you now have specific, important questions to bring to your next appointment.
Frequently Asked Questions
What is tumor mutational burden (TMB)?
TMB is a count of how many mutations are found in a standard stretch of a tumor's DNA, reported in mutations per megabase (mut/Mb). More mutations usually means the tumor looks more foreign to the immune system, which can make immunotherapy more likely to work.
What did the FDA approve on October 16, 2026?
Traditional (full) approval for Keytruda (pembrolizumab) and Keytruda Qlex (the subcutaneous version) for adults and children with unresectable or metastatic TMB-high solid tumors that progressed after prior treatment, when no satisfactory alternatives remain. The approval also raised the TMB-high cutoff to at least 13 mut/Mb.
What is the difference between accelerated and traditional approval?
Accelerated approval is granted on early results, with a requirement to confirm the benefit later. Traditional (full) approval means the confirmatory evidence is in and the benefit is considered established. Keytruda's TMB approval spent 2020 to 2026 in the accelerated category.
Why did the cutoff move from 10 to 13?
Because the mature data showed the benefit was concentrated at higher scores. Response rates at 10 to <13 data-preserve-html-node="true" mut/Mb were much weaker (13% in the early analysis) than at ≥13 (37%), and the FDA concluded efficacy was not confirmed in the lower band.
Is TMB the same as MSI-H or dMMR?
No. MSI-H/dMMR asks whether one specific DNA repair system is broken. TMB counts total mutations, whatever the cause. They overlap but are different tests, and a tumor can be TMB-high without being MSI-H.
Who is eligible for Keytruda under this approval?
Adults and children with unresectable or metastatic solid tumors scoring TMB ≥13 mut/Mb on an FDA-authorized test, whose cancer progressed after prior treatment and who have no satisfactory alternative options. Any solid tumor type can qualify — the approval is tumor-agnostic.
How well did Keytruda work in KEYNOTE-158?
Among 220 patients at TMB ≥13, 36% had their tumors shrink measurably, and 58% of those responses lasted at least 12 months.
My TMB score is 11. Am I out of options?
No. It means this particular label no longer covers a new Keytruda start based on TMB alone. You may qualify through MSI-H/dMMR status, a cancer-specific indication, or a clinical trial — and some patients in the 10–12 range did respond. Ask your oncologist what applies to you.
What is Keytruda Qlex?
A subcutaneous (under-the-skin injection) version of pembrolizumab, combined with berahyaluronidase alfa-pmph to help it spread under the skin. It carries the same TMB-high indication as the IV version.
What side effects should I watch for most carefully?
Immune-related ones: worsening diarrhea or blood in the stool (possible colitis), new cough or breathlessness, yellowing of the skin or eyes, severe rash, and unusual fatigue or weight change that could signal hormone-gland effects. Report them promptly — during treatment and after it ends.
Sources
- U.S. Food and Drug Administration — "FDA grants traditional approval to pembrolizumab or pembrolizumab and berahyaluronidase alfa-pmph for unresectable or metastatic TMB-H solid tumors," FDA Approvals, October 16, 2026
- OncLive — "FDA Grants Full Approval to Pembrolizumab for Unresectable/Metastatic TMB-H Solid Tumors," OncLive, October 22, 2026
- CancerNetwork — "FDA Approves Pembrolizumab in Metastatic TMB-H Solid Tumors," Cancer Network, October 2026
- Oncology Nursing News — "FDA Grants Traditional Approval to Pembrolizumab in TMB-H Solid Tumors," Oncology Nursing News, October 2026
For more information on how HuMOLYTE can support your gut health during chemotherapy, visit our product page or consult your health care provider.
This blog was reviewed by Dr. Sourabh Kharait.
Disclaimer: This blog post is for educational purposes only and is not intended as medical advice. Always consult with your healthcare provider before making changes to your treatment plan, hydration strategies, or diet. The information provided is based on general insights and may not apply to individual circumstances.
