Senolytics and Ageing: Have We Finally Seen Evidence That Clearing ‘Zombie Cells’ Can Help Humans?

Few areas of longevity research have generated quite as much excitement as senescent cells. They have acquired the wonderfully dramatic nickname “zombie cells”, are routinely presented as one of the villains of biological ageing, and have spawned an expanding market of supplements claiming to help the body clear them away. That has even sprawled into skincare too. 

As ever, the biology is considerably more interesting than the marketing.

Cellular senescence is a genuine and important biological process. Senescent cells accumulate in many tissues as we age and have been implicated in chronic inflammation, fibrosis, impaired tissue repair and a growing list of age-related diseases. Experiments in animals have produced some remarkable results when these cells are selectively removed, helping to establish senescence as one of the most intriguing therapeutic targets in modern ageing research.

The problem has always been translating that into humans. Improving the health of an ageing mouse is one thing. Demonstrating that targeting cellular senescence meaningfully improves human disease is another.

A small but fascinating randomised controlled trial published in Nature Metabolism has now taken an important step in that direction. Researchers treated people with metabolic dysfunction-associated steatohepatitis, or MASH, using the senolytic combination dasatinib and quercetin. After 21 weeks, almost half of those receiving treatment showed an improvement in liver fibrosis without worsening of MASH, compared with just 7% receiving placebo. Molecular analysis of liver tissue also showed reductions in gene signatures associated with senescence and fibrosis.

It is an exciting result.

It is also a long way from proving that we have discovered a drug combination that slows human ageing.

Understanding the difference between those two statements tells us a great deal about where longevity medicine currently stands.

What actually is cellular senescence?

Cells do not continue dividing indefinitely.

When a cell experiences significant stress or damage, it can enter a state known as cellular senescence. It remains metabolically active but undergoes a relatively stable arrest of the cell cycle, meaning that it no longer divides normally.

Several things can push a cell towards senescence. Persistent DNA damage is one. Telomere dysfunction is another. Oxidative stress, mitochondrial dysfunction, oncogene activation, radiation, metabolic stress and inflammatory signalling can all contribute depending upon the cell and tissue involved.

At first glance, preventing a damaged cell from dividing sounds rather sensible.

And it is.

Cellular senescence evolved for good reasons. If a cell has accumulated potentially dangerous damage, preventing it from replicating can help suppress tumour formation. Senescence also participates in embryonic development, wound healing and tissue remodelling.

This is the first important correction to the usual “zombie cell” story. Senescent cells are not simply defective cells that the body forgot to throw away.

Senescence is a biological programme with useful functions.

The problem appears to arise when senescent cells accumulate and persist.

Why do senescent cells become problematic?

A senescent cell may have stopped dividing, but it has not necessarily gone quiet.

Many senescent cells develop what is known as the senescence-associated secretory phenotype, usually abbreviated to SASP. This can involve the release of a complex mixture of inflammatory cytokines, chemokines, growth factors, proteases and other signalling molecules.

The exact SASP varies enormously according to cell type, tissue, the trigger that produced senescence and the length of time the cell has remained senescent. There is no single universal senescent-cell secretome.

Nevertheless, persistent SASP signalling can alter the surrounding tissue environment. It can promote inflammation, disturb extracellular-matrix regulation, influence immune cells and impair the behaviour of neighbouring cells. Under some circumstances, senescent cells can even encourage nearby cells to develop senescence-associated characteristics.

This creates an interesting biological paradox. A programme that may initially protect tissue can become harmful when the cells carrying that programme are not efficiently removed.

In younger tissues, immune surveillance helps identify and clear many senescent cells. With ageing, that clearance appears to become less effective while cellular damage continues to accumulate. The result can be an increasing burden of senescent cells in tissues already becoming less resilient.

That has led researchers to ask a rather obvious question: if persistent senescent cells contribute to age-related dysfunction, what happens if we selectively remove them?

This is where senolytics enter the story

Senolytics are compounds designed to preferentially kill senescent cells.

This is possible because senescent cells face an unusual problem. They exist in an environment containing considerable cellular stress and pro-apoptotic signalling, yet they resist programmed cell death. To survive, they become increasingly dependent upon particular pro-survival pathways.

Researchers realised that this dependence might represent an Achilles' heel.

Rather than attacking senescent cells simply because they have stopped dividing, a senolytic can interfere with the survival mechanisms that allow them to remain alive. If those protective pathways are disrupted, the senescent cell may undergo apoptosis while healthier neighbouring cells are comparatively less affected.

In 2015, researchers screened these senescent-cell survival networks and identified two particularly interesting compounds: dasatinib and quercetin.

Dasatinib is a tyrosine-kinase inhibitor used as a prescription cancer medicine. Quercetin is a naturally occurring flavonoid found in foods including onions, apples, berries and various vegetables.

That pairing has understandably generated enormous interest in the supplement world. Unfortunately, the sentence “quercetin is one half of an experimental senolytic drug combination” has a habit of rapidly transforming online into “quercetin clears zombie cells and slows ageing”.

Those are not the same claim.

Why combine dasatinib and quercetin?

One of the complexities of senescence is that senescent cells are not all biologically identical.

Different cell types can depend upon different survival pathways, which means a compound capable of killing one population of senescent cells may have relatively little effect on another.

Early laboratory work found that dasatinib appeared particularly effective against certain senescent human fat-cell progenitors, whereas quercetin showed activity against other senescent cell types. Combining the two therefore offered a broader approach than relying on either agent alone.

In mouse experiments, intermittent dasatinib plus quercetin reduced markers of senescent-cell burden and improved several measures of tissue function. These and other senolytic experiments generated enormous enthusiasm because they suggested that removing a relatively small population of dysfunctional cells might improve the environment of an entire tissue.

This was a subtly different proposition from many traditional treatments for chronic disease. Rather than continuously suppressing a downstream symptom, perhaps it might be possible to intermittently remove cells contributing to the dysfunctional tissue environment and then allow healthier tissue to recover.

It is a compelling idea.

But mice have been cured of an impressive number of things over the years.

Human beings remain rather less cooperative.

What had senolytics actually achieved in humans before now?

Human trials of senolytics have so far been small and exploratory.

One early open-label study administered dasatinib and quercetin to nine people with diabetic kidney disease. Participants received dasatinib at 100 mg per day and quercetin at 1,000 mg per day for three days.

When researchers examined adipose tissue and skin before and after treatment, several markers associated with cellular senescence decreased. These included cells expressing p16^INK4A^ and p21^CIP1^, along with changes in senescence-associated β-galactosidase activity and circulating components associated with the SASP.

That was an important proof-of-concept observation because it suggested that a senolytic regimen could alter markers of senescent-cell burden in human tissue.

It did not demonstrate that people lived longer, became biologically younger or experienced fewer age-related diseases.

A subsequent randomised placebo-controlled pilot trial examined intermittent dasatinib plus quercetin in 12 people with idiopathic pulmonary fibrosis. The primary purpose was to assess feasibility and tolerability rather than establish clinical efficacy. The regimen was feasible, but exploratory measures of frailty, pulmonary function and physical function did not show convincing differences between groups.

This is roughly where human senolytic research has remained: biologically intriguing, supported by compelling animal experiments, but with extremely limited evidence that clearing senescent cells meaningfully changes a human disease process.

The new liver trial is therefore particularly interesting.

The new trial targeted a disease where senescence may actually matter

Metabolic dysfunction-associated steatotic liver disease, or MASLD, has become extraordinarily common.

At its more serious end sits metabolic dysfunction-associated steatohepatitis, or MASH, where excess liver fat is accompanied by inflammation and hepatocellular injury. Over time, some people develop progressive fibrosis, cirrhosis and potentially hepatocellular carcinoma.

Fibrosis is especially important because its severity strongly predicts future liver-related complications and mortality.

Senescent cells are thought to participate in this process. Hepatocytes, hepatic stellate cells and other cell populations can develop senescence-associated characteristics within a metabolically stressed liver. Their secretory signals may then influence inflammation, extracellular-matrix deposition and the fibrotic environment.

This makes MASH an interesting disease in which to test the senolytic hypothesis.

Rather than asking whether a treatment makes somebody's “biological age” score fall by a couple of years, researchers could examine something considerably more tangible: actual liver tissue.

What did the trial do?

The phase II proof-of-principle trial was conducted at Amsterdam University Medical Center and included 31 people with biopsy-confirmed fibrotic MASH.

Participants had stage F2–F3 fibrosis and were randomly assigned to dasatinib plus quercetin or placebo. The study was double-blinded, meaning neither the participants nor the investigators assessing them knew which treatment had been assigned.

Seventeen participants received the senolytic treatment and 14 received placebo.

Treatment was deliberately intermittent. Participants in the active group received 100 mg of dasatinib and 1,000 mg of quercetin per day for three consecutive days each week for three weeks, followed by four weeks without treatment. This seven-week cycle was repeated three times, producing a total study duration of 21 weeks.

The intermittent schedule reflects the proposed biology of senolytics. If the objective is to remove an accumulated population of senescent cells, continuous exposure may not be necessary in the same way that it would be for a medicine intended to suppress a receptor or enzyme every day.

The primary endpoint was demanding and clinically meaningful: at least a one-stage improvement in liver fibrosis without worsening of MASH, assessed using paired liver biopsies.

The results were striking enough to deserve attention.

Liver fibrosis improved in almost half of the treatment group

Eight of the 17 participants receiving dasatinib plus quercetin achieved the primary endpoint, equivalent to 47% of the treatment group.

Only one of the 14 participants receiving placebo did so, equivalent to 7%.

MASH resolution was also observed in 53% of the treatment group compared with 7% receiving placebo.

Among participants who completed paired liver biopsies, measures of disease activity improved more strongly with dasatinib plus quercetin, including the NAFLD Activity Score.

For such a small trial, those are sizeable differences.

However, small trials are precisely where sizeable differences require the most restraint.

The confidence intervals around the treatment effect were wide. Four participants did not complete the study, leaving paired biopsy data for 27 people. When the investigators performed sensitivity analyses using different assumptions about the missing biopsy results, the direction of the effect remained similar, but the primary result lost conventional statistical significance under the most pessimistic scenario.

The authors therefore explicitly describe the histological finding as hypothesis-generating rather than confirmatory.

That wording matters considerably more than an exciting headline.

The molecular findings make the trial more interesting

If the study had simply shown an improvement in liver histology, an obvious question would remain: was this really a senolytic effect?

The researchers therefore went considerably deeper.

Using single-nucleus RNA sequencing of liver tissue, they examined changes in cellular populations and gene-expression patterns before and after treatment.

The dasatinib-plus-quercetin group showed reductions in gene signatures associated with cellular senescence and fibrosis, alongside changes in hepatic stellate-cell and immune-cell populations involved in the fibrotic process.

This gives the study a degree of mechanistic coherence.

There was a clinical tissue-level change — improvement in fibrosis — occurring alongside molecular changes pointing in the direction expected if senescence-related biology had been altered.

That combination is far more persuasive than simply demonstrating that somebody's circulating inflammatory marker moved after taking a supplement.

It still does not prove that selective removal of senescent cells was solely responsible for the improvement. Dasatinib has numerous biological effects and quercetin interacts with multiple signalling pathways. Neither compound is a laser-guided missile that touches senescent cells and nothing else.

But the convergence of histological and molecular findings makes the senolytic explanation biologically plausible enough to justify larger trials.

Does this mean senolytics slow ageing?

No. And this distinction is essential.

The trial involved people with a specific disease: fibrotic MASH.

It did not recruit healthy older adults and demonstrate that senolytic treatment slowed ageing. It did not measure lifespan. It did not demonstrate a reduction in cardiovascular disease, dementia, frailty or cancer. It did not establish that treated participants became biologically younger.

What it provides is preliminary evidence that targeting a biological process strongly associated with ageing may improve a specific disease in which that process appears to participate.

That is still important.

One of the central ideas in geroscience is that some of the biological mechanisms contributing to ageing may also contribute to multiple chronic diseases. Cellular senescence is one such mechanism.

If therapies directed at senescence eventually improve several apparently different age-related conditions, that would strengthen the argument that we can therapeutically target aspects of ageing biology rather than treating every disease entirely in isolation.

But we are not there yet.

One small MASH trial cannot carry the weight of the entire longevity industry on its shoulders.

Senescent cells are not simply biological rubbish

There is another reason to resist the temptation to declare war on senescent cells.

We need them.

Senescence can suppress tumour formation by preventing damaged cells from continuing to divide. Senescent cells participate in tissue repair and wound healing. They also have roles during development and in coordinating immune responses.

The therapeutic objective is therefore unlikely to be eliminating every senescent cell from the human body.

That would be rather like discovering that fire causes house damage and responding by outlawing every flame, including the one under the saucepan.

The challenge is identifying pathological senescence: cells persisting in the wrong place, at the wrong time and in sufficient numbers to contribute to tissue dysfunction.

This is made considerably harder by the fact that we do not possess one universal marker that uniquely identifies every senescent cell.

Markers such as p16^INK4A^, p21^CIP1^ and senescence-associated β-galactosidase are useful, but none is perfectly specific. Senescence is a heterogeneous cellular state rather than a single uniform cell type.

This has enormous implications for senolytic medicine. Before we can reliably remove problematic senescent cells, we need increasingly sophisticated ways of determining which cells are senescent, which are harmful and which should probably be left alone.

What about quercetin supplements?

This is where I expect things to become silly.

Quercetin is readily available as a food supplement. It is also one component of the senolytic combination used in this trial.

That does not mean taking quercetin supplements has now been shown to clear senescent cells, reverse liver fibrosis or slow ageing.

The active intervention was dasatinib plus quercetin.

Dasatinib is a potent prescription tyrosine-kinase inhibitor used in the treatment of certain cancers. It is not a wellness supplement. It can produce clinically significant adverse effects and requires appropriate medical oversight.

The original laboratory work on dasatinib and quercetin also demonstrated an important point that tends to disappear when the research reaches social media: the two compounds showed different senolytic activity against different cell types.

The combination was part of the biological strategy.

We therefore cannot remove dasatinib from the experiment, retain the ingredient available from a supplement shop and assume the result survives intact.

That would be rather convenient, but science does not generally work by keeping the ingredient we can buy online and deleting the inconvenient prescription medicine from the mechanism.

Quercetin remains an interesting flavonoid with a substantial body of biological research behind it. What this trial does not establish is that quercetin supplementation constitutes a clinically proven senolytic therapy.

The safety question becomes much bigger if we move from disease treatment to longevity

Adverse events occurred in 82% of participants receiving dasatinib plus quercetin compared with 43% receiving placebo in the MASH trial.

The investigators reported that these events were self-limiting, and the trial was not large enough to characterise uncommon adverse effects reliably.

That safety profile has to be interpreted in context.

For somebody with progressive fibrotic liver disease, accepting some treatment risk may be entirely reasonable if larger trials eventually show that senolytic therapy meaningfully prevents cirrhosis, liver failure or other serious outcomes.

The calculation becomes very different when treating an otherwise healthy 50-year-old who would simply quite like to age more slowly.

Preventive longevity therapies face an extraordinarily high safety bar because they may ultimately be given to large numbers of relatively healthy people for many years.

Even a small risk becomes important when exposure is widespread.

Intermittent senolytic treatment might theoretically help here because the drugs need not necessarily be taken continuously. But we do not yet know the optimal dose, treatment interval, duration, long-term consequences or which populations might benefit.

Nor do we know whether repeatedly clearing senescent cells over many years might interfere with beneficial functions of senescence.

These questions cannot be answered by a 21-week study involving 31 people.

Senolytics are only one way of targeting senescence

There is also more than one possible therapeutic strategy.

Senolytics aim to eliminate senescent cells.

Another approach involves senomorphics, sometimes called senostatics, which attempt to modify the harmful behaviour of senescent cells without necessarily killing them. For example, a therapy might suppress components of the SASP and thereby reduce chronic inflammatory signalling while leaving the cell itself intact.

Other strategies may enhance immune-mediated clearance of senescent cells, prevent inappropriate senescence from developing or target particular senescent-cell populations more selectively.

This may ultimately prove important because the future of senescence therapy is unlikely to involve periodically carpet-bombing every cell displaying a vaguely senescent characteristic.

Precision will matter.

The ideal therapy would target the senescent cells actively contributing to disease while preserving the protective functions of temporary, appropriately regulated senescence.

We are still some distance from that level of control.

Where does lifestyle fit into all of this?

This is where longevity discussions can become slightly upside down.

An experimental drug combination designed to clear senescent cells is understandably more exciting than advice to exercise, maintain metabolic health, avoid smoking, sleep adequately and eat a good diet.

The latter makes for a considerably less impressive biotechnology pitch deck.

Yet many of the processes that drive excessive cellular stress and senescence are influenced by the metabolic and inflammatory environment in which cells exist. Obesity, insulin resistance, oxidative stress, chronic inflammation and tissue injury are all relevant to the broader biology in which senescence develops.

Exercise in particular interacts with numerous pathways associated with ageing biology, including mitochondrial function, glucose regulation, inflammatory signalling, immune function and skeletal-muscle maintenance. Diet quality influences metabolic health and the availability of substrates and bioactive compounds involved in cellular defence and repair.

This does not mean broccoli is a senolytic or that going for a run “clears zombie cells”. We should resist dressing ordinary healthy behaviours in fashionable molecular terminology simply to make them sound more sophisticated.

The point is more straightforward.

If we eventually develop highly effective therapies targeting cellular senescence, they will probably sit on top of, rather than replace, the behaviours already known to influence healthy ageing.

A drug capable of modifying one hallmark of ageing would not make cardiovascular fitness, muscle mass, blood pressure, metabolic health or dietary quality irrelevant.

Human biology has an irritating habit of remaining multifactorial.

What this trial actually changes

The most important thing about this study is not that it has discovered an anti-ageing treatment.

It hasn't.

Its significance is that a field built largely upon cell experiments, animal models and tiny early human studies has produced a randomised, double-blind, placebo-controlled human trial in which a proposed senolytic intervention was associated with improvement in an actual disease feature measured directly in tissue.

That moves the conversation forward.

For years, the central senolytic question has been whether clearing senescent cells can improve ageing and disease in humans as dramatically as it sometimes does in experimental animals.

We still cannot answer that question.

What we can now say is that, in a small group of people with fibrotic MASH, intermittent dasatinib plus quercetin produced encouraging improvements in liver histology alongside molecular changes consistent with reduced senescence- and fibrosis-associated signalling.

The appropriate response is neither dismissal nor a rush to the supplement cupboard.

It is to run bigger trials.

Those studies need enough participants to establish whether the effect is real and reproducible, longer follow-up to determine whether improvements persist, careful monitoring of safety and ideally clinical outcomes that tell us whether altering senescence actually changes the trajectory of disease.

Beyond MASH, we need similarly rigorous trials in other conditions where cellular senescence is strongly implicated.

Only then can we begin to answer the much larger question of whether senolytics might eventually become part of medicine aimed not merely at treating individual diseases, but at modifying some of the biology that makes those diseases increasingly likely as we age.

The ‘zombie cell’ story has finally become a little more human

Cellular senescence remains one of the most fascinating areas in ageing research because the underlying idea is so compelling. A relatively small population of damaged, persistent cells may be capable of disrupting the function of much larger areas of tissue through inflammatory and fibrotic signalling. Remove the problematic cells, and perhaps some of that tissue dysfunction can improve.

In mice, versions of that story have already produced remarkable results.

In humans, the evidence has been far less impressive.

This new trial does not close that gap, but it narrows it slightly. For the first time in fibrotic MASH, we have placebo-controlled evidence linking an intermittent senolytic regimen with histological improvement and molecular changes in the tissue being treated.

That deserves genuine excitement, but excitement with its shoes firmly attached to the floor.

We have not discovered how to reverse human ageing. We have not demonstrated that quercetin supplements make people younger. We certainly have not established that otherwise healthy people should experiment with prescription cancer medicines in pursuit of longevity.

What we may have seen is something more scientifically useful: an early indication that cellular senescence is not merely an interesting marker of ageing, but a biological process that might actually be therapeutically modifiable in human disease.

If larger trials confirm that, the significance could extend far beyond the liver.

For now, senolytics have earned something they have been missing for quite a long time: a little more human evidence to accompany an enormous amount of hype.

References

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