The drugs work. The question now is: working how?
For nearly four decades, a class of cancer drugs called HDAC inhibitors has been understood through a single, elegant mechanism — block the enzyme, restore gene expression, slow the tumor. Now, researchers at Baylor College of Medicine have quietly dismantled that certainty, finding that these drugs retain their cancer-fighting power even when stripped of their supposed primary function. It is a reminder that medicine, like all human knowledge, sometimes builds confident structures on foundations that have not yet been fully examined.
- A foundational assumption in cancer pharmacology — that HDAC inhibitors work by blocking a specific enzyme — has been directly challenged by experimental evidence, unsettling decades of drug development logic.
- Bioinformatics analysis across multiple tumor types revealed that HDAC enzyme levels do not reliably predict cancer growth or patient survival, exposing a persistent gap between textbook theory and biological reality.
- In mouse models, HDAC inhibitors including FK228 continued to fight tumors even after their enzyme-blocking ability was deliberately removed — meaning the drugs are working, but through mechanisms the field has not yet identified.
- The Baylor team is now urging the research community to abandon the single-mechanism assumption and actively search for the true molecular targets driving these drugs' anti-cancer effects.
- The stakes are high: understanding what HDAC inhibitors actually do could explain why some patients respond to treatment and others do not, and reshape how the next generation of cancer drugs is designed.
For nearly four decades, cancer researchers have operated on a clean assumption: HDAC inhibitors fight tumors by blocking enzymes that suppress beneficial gene expression. The logic was elegant — inhibit the enzyme, restore the signal, slow the cancer. A team at Baylor College of Medicine, led by Dr. Zheng Sun, has spent years testing whether that logic holds.
It does not — or at least, not entirely. Publishing in Signal Transduction and Targeted Therapy, Sun and first author Dr. Chaitra Rai present evidence that the standard model is incomplete. In some cancers, HDAC enzymes actually suppress tumor growth rather than promote it. In others, the drugs increase histone acetylation as expected, but the downstream effect on gene expression is surprisingly modest. The data kept refusing to fit the theory.
The team's most striking finding came from direct experimentation. When they engineered HDAC inhibitors — including the drug FK228 — to lose their enzyme-blocking capacity, the drugs continued to fight cancer in mouse models. The anti-tumor effect persisted without the mechanism presumed to cause it. Something else was happening.
Sun's conclusion is both clarifying and humbling: these drugs are working, but the field has been explaining them incorrectly. HDAC inhibitors appear to interfere with other proteins or pathways entirely unrelated to histone acetylation. Identifying those true molecular targets, the team argues, is now the essential next step — one that could lead to better-designed drugs, more predictable patient responses, and a more honest understanding of a treatment that has been in clinical use for years.
For nearly four decades, cancer researchers have operated from a straightforward assumption: drugs called HDAC inhibitors work by blocking a specific enzyme that drives tumor growth. The logic was clean and appealing. Inside every cell, DNA wraps around proteins called histones. Chemical modifications to these histones—particularly the addition or removal of acetyl groups—control which genes turn on and which stay silent. HDAC enzymes strip away those acetyl groups, and the thinking went that by inhibiting them, the drugs would restore beneficial gene expression patterns and slow or kill cancer cells.
But a team at Baylor College of Medicine, led by Dr. Zheng Sun, has spent the last few years pulling at this thread. What they found troubles the conventional wisdom. In a study published in Signal Transduction and Targeted Therapy, Sun and his colleagues present evidence that HDAC inhibitors may not work the way the field has long believed—or at least, not only that way.
The problem, as Sun explains it, is that reality keeps refusing to cooperate with theory. In some cancers, HDACs don't promote tumor growth at all; they actually suppress it. In other cases, HDAC inhibitors do increase histone acetylation, but the effect on gene expression is modest at best. These inconsistencies suggested something was missing from the standard model.
To investigate, Sun's team, including first author Dr. Chaitra Rai, took an unbiased approach. They analyzed data across multiple solid tumor types using bioinformatics tools, looking for correlations between HDAC levels and cancer development. What emerged was surprising: HDAC levels did not consistently predict cancer growth or patient survival across most cancers. The relationship was far messier than the textbook version suggested.
Then came the more direct test. The researchers took HDAC inhibitors—including one called FK228—and tested them in mouse models of cancer. They found that the drugs retained their anti-cancer effects even when the researchers engineered them to lose their ability to inhibit HDACs. In other words, the drugs were still working, but not through the mechanism everyone assumed. When they eliminated the enzyme-blocking capacity of a broader class of HDAC inhibitors, the drugs kept most of their tumor-fighting power anyway.
The implication is unsettling for a field that has invested heavily in a single mechanism. HDAC inhibitors appear to be doing something else—interfering with other proteins, activating different pathways, triggering effects that have nothing to do with histone acetylation. The drugs work, but the textbooks got the reason wrong.
Sun and his team are now calling for a shift in focus. Rather than assuming HDAC inhibition is the primary driver of these drugs' anti-cancer activity, researchers should hunt for the actual molecular targets. Identifying what HDAC inhibitors really do at the molecular level could lead to better drug design, more effective treatments, and a clearer understanding of why some patients respond to these drugs and others don't. The cancer drugs are working. The question now is: working how?
Notable Quotes
Some HDAC inhibitors may work through HDACs in certain contexts, but this mechanism is not as universal as previously assumed— Dr. Zheng Sun, Baylor College of Medicine
HDAC inhibitors may also interfere with other proteins, and targeting such proteins may suppress cancer— Dr. Zheng Sun
The Hearth Conversation Another angle on the story
So for forty years, everyone thought these drugs worked one way, and now you're saying they don't?
Not exactly. Some of them might work that way in certain contexts. But the evidence suggests it's not the main story, or at least not the whole story.
How did nobody catch this earlier?
Because the drugs do work against cancer. When something works, you don't always dig into why. You just use it. But when you actually look at the data—when you ask whether HDAC levels predict which patients get cancer—the connection falls apart.
What does that mean for patients taking these drugs right now?
It means the drugs are still working. But it also means we're not optimizing them. We're not targeting what actually matters. We could be designing better versions if we understood the real mechanism.
And nobody knows what that mechanism is yet?
Not yet. That's the work ahead. The drugs are interfering with other proteins, other pathways. Finding out which ones is the next step.
Does this happen often in medicine? Drugs that work for the wrong reasons?
More often than you'd think. Aspirin worked for a hundred years before anyone understood why.