Senolytic drugs have been the most promising near-term anti-aging therapy since the ground-breaking paper by van Deursen of Mayo Clinic published in 2011.  The body accumulates senescent cells as we age, damaged cells that send out signal molecules that in turn modify our biochemistry in a toxic, pro-inflammatory direction.  Though the number of such cells is small, the damage they do is great. Van Deursen showed that just getting rid of these cells could increase lifespan of mice by ~25%. But he did it with a trick, using genetically engineered mice in which the senescent cells had a built-in self-destruct switch.  

After that, the race was on to find chemical agents that would do the same thing without the genetically engineered self-destruct.  They must selectively kill senescent cells, while leaving all other cells unharmed. It’s a tall order, because even a little residual toxicity to normal cells can be quite damaging.  Before last week, the two best candidates were FOXO4-DRI and a combination of quercetin with dasatinib.

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I’ve written in the past (here and here) that senolytic drugs are our best prospect for a near-term lift on the road to anti-aging medicine.  

Last week, a large research group affiliated with the original May Clinic team published findings about fisetin, the latest and greatest candidate for a senolitic pill, another flavenoid, very close in structure to quercetin. 

They grew senescent and normal cells in a test tube, then tested 11 different plant-derived chemicals for power to kill the one while leaving the other unharmed.  The winner was fisetin.

(MEF stands for Mouse Embryonic Fybroblast, the cells that were cultured in the screening experiment.)

Fisetin is especially interesting because it is cheap, easily available, widely-regarded as safe, but not nearly as well studied as quercetin.

They took the winner, fisetin, and subjected it to a series of tests.  They began with in vitro (cell culture) tests and proceeded to in vivo tests with live animals, culminating with an impressive life span assay in mice.

(The runner-up was curcumin, less interesting perhaps only because it has already been extensively studied.  The curcumin molecule is unrelated to quercetin or fisetin, and is not a flavenoid. I can’t help but wonder if they had subjected curcumin to the same thorough testing that they reserved for fisetin, how would curcumin have fared?)

 

The paper’s principal findings were:

• Fisetin has lower liver toxicity (at equivalent doses for senolytic benefit) than any of the other senolytics tested so far.
• Fisetin reduces pro-inflammatory signaling in a short course given to mice and in long-term experiments where fisetin was added to the mouse chow.
• Fisetin reduces number of senescent fat cells in a short course given to mice.
• Mice fed fisetin for long periods had much more glutathione than control mice.  (Glutathione is one of very few marker molecules that seems to be wholly beneficial.)
• Most impressively, mice fed fisetin late in life lived 10-15% longer than control mice. This represents a 50% increase in the remaining lifespan after the intervention.

What we know and what we’d really like to know

We’d like to know, do humans who take large doses of fisetin live longer?  Do they have toxic side-effects? These questions require decades to answer.

Does fisetin reduce age markers in humans, especially methylation age?  This is a feasible study, since the test is mature and safety of fisetin is fairly well established for short courses.  Perhaps this experiment is being considered; I’ve written to the corresponding authors of the most recent study, in case they haven’t already thought of it.  This test would not be definitive because we know that methylation age is not perfectly correlated with biological age; but if positive it would confirm both that fisetin is accomplishing epigenetic rejuvenation and that methylation tests were correctly informing us of this; a negative result would be ambiguous.

Episodic Dosing

It makes sense that senolytics should be taken periodically, not continuously.  A high dose can be toxic to existing senescent cells, and then getting out of the way, it can allow normal cells to recover from any damage.  This sounds like good theory, but different dosing regimens have not been tested experimentally. In fact, the new paper reports positive results from both high episodic dosing and lower everyday dosing.

The Mayo group had previously tested fisetin, and found it effective in killing some kinds of human senescent cells but not others.  In previous tests, fisetin was found to be effective in senescent fat cells (pre-adipocyte, white adipose tissue), and that is where it was primarily tested in the new studies.  

Authors’ comments

They note that the episodic treatment and short half-life suggest that the benefits of fisetin come from its senolytic action, rather than other actions as an antioxidant and signal molecule.  They emphasize that clearing senescent white blood cells and making room for new, active white blood cells are activities that enhance the benefits of fisetin, since white blood cells contribute to clearing the remaining senescent cells.  

Fisetin has previously been shown to have anti-cancer activity and to inhibit inflammatory signals directly.  Here is a review of benefits of fisetin from three years ago.  Drugage lists just two previous lifespan studies with fisetin, with encouraging results from yeast and fruitflies.

The Bottom Line

If we choose to take fisetin at this stage in the science, we are early adopters, and our main concern ought to be safety.  There is little doubt that killing senescent cells will be beneficial. But what is the toxic burden of fisetin, and what dosage can we safely take without risk of damage to normal cells?  The current study covers a lot of ground but doesn’t answer this question, apparently because they are convinced that fisetin is quite safe.

Strawberries, apples, grapes, and onions all contain fisetin, but at low levels compared to a senolytic dose.  For example, the highest food concentration, 160 ppm, is found in strawberries.  A half pound of strawberries yields 36 mg of fisetin.  We’re still guessing at the therapeutic dose, based on mouse studies, and the experimental dosage in human trials is about 1,000 to 1,500 mg (based on this clinical trial), the content to 30-40 pounds of strawberries on each of two consecutive days.

In the best cases, fisetin was shown to reduce senescent cell burden by 50% and up to 75% in cell cultures.  This is a good start, and encourages us to think we can do better by combining fisetin with other agents, or perhaps with fasting.

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Also reported today,

Clearing Senescent Cells From The Brain In Mice Preserves Cognition

It sounds impressive, but I’m not impressed.  First, mouse models of Alzheimer’s have been discredited repeatedly.  Mice don’t naturally get AD, so they have to be genetically engineered to do so, and the genetically modified mice don’t share the deep causes of human AD.  Time and again, treatments have been found effective in the mouse model that fail to translate to humans. Second, the treatment used in the study to kill senescent brain cells also relied on another genetic modification, and would not be applicable to humans.  

My guess is that effective senolytic agents for humans will be available within a few years, and that they will decrease risk of all age-related disease, including Alzheimer’s.  But this study does little to advance us toward that goal.

This is the most important column I’ve ever written.  The message is quite complex–dozens of new health parameters to test for and to optimize, all of them interacting in ways that will require new training for MDs.  The message is also as simple as it can be: There is a cure for Alzheimer’s disease. You can stop reading right here, and buy two copies of Dale Bredesen’s book, one for you and one for your doctor:  The End of Alzheimer’s.

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Dr Bredesen’s spectacular success is easily lost in a flood of overly-optimistic, early hype about any number of magic cures.  This is an excuse for the New York Times, the Nobel Prize committee, and the mainstream of medical research, but it’s no excuse for me.  I’ve known Bredesen for 14 years, and I’ve written about his work in the past.  His book has been out for a year, and I should have written this column earlier.

I suspect you’re waiting for the punch line: what is Bredesen’s cure?  That’s exactly what I felt when I read about his work three years ago. But there isn’t a short answer.  That’s part of the frustration, but it’s also a reason that Bredesen’s paradigm may be a template for novel research approaches cancer, heart disease, and aging itself.

The Bredesen protocol consists of a battery of dozens of lab tests, combined with interviews, consideration of life style, home environment, social factors, dentistry, leaky gut, mineral imbalances, hormone imbalances, sleep and more.  This leads to an individual diagnosis: Which of 36 factors known to affect APP cleavage are most important in this particular case? How can they be addressed for this individual patient?

Brain cells have on their surface a protein called APP, which is a dependence receptor.  It is like a self-destruct switch whose default is in the ON position.  The protein that binds to the receptor is a neurotrophin ligand, and in the absence of the neurotrophin ligand,  the receptor signals the cell to die.

APP cleavage is the core process that led Bredesen down a path to his understanding of the etiology of AD 16 years ago.  APP is Amyloid Precursor Protein, and it is sensitive to dozens of kinds of signals, adding up the pros and the cons to make a decision, to go down one of two paths.  It can be cleaved in two, creating signal molecules that cause formation of new synapses and formation of new brain cells; or it can be cleaved in four, creating signal molecules that lead to trimming back of existing synapses, and eventually, to apoptosis, cell suicide of neurons.

In a healthy brain, these two processes are balanced so we can learn new things and we can forget what is unimportant.  But in the Alzheimer’s brain, destruction (synaptoclastic) dominates creation (synaptoblastic), and the brain withers away.

On the right, one of the fragments is beta amyloid.  Beta amyloid blocks the dependence receptor, so the receptor cannot receive the neurotrophin ligand that gives it permission to go on living.  Beta amyloid is one of the 4 pieces, when the APP molecule goes down the branch where it is split in 4.

One of the signals that determines whether APP splits in 2 or in 4 is beta amyloid itself.  This implies a positive feedback loop; beta amyloid leads to even more beta amyloid, and in the Alzhyeimer’s patient, this is a runaway process.  But positive feedback loops work in both directions–a boon to Bredesen’s clinical approach. If the balance in signaling can be tipped from the right to the left pathway in the diagram above, this can lead to self-reinforcing progress in the healing direction.  In the cases where Bredesen’s approach has led to stunning reversals of cognitive loss, this is the underlying mechanism that explains the success.

Amyloid has been identified with AD for decades, and for most of that time the mainstream hypothesis was that beta-amyloid plaques cause the disease.  (Adherents to this view have been referred to jokingly as BAPtists.) But success in dissolving the plaques has not led to restored cognitive function.  In Bredesen’s narrative, generation of large quantities of beta amyloid are a symptom of the body’s attempts to triage a dying brain.

 

To tip the balance back toward growing new synapses

Having identified the focal point that leads to AD, Bredesen went to work first in the lab, then in the clinic, to identify processes that tend to tip the balance one way or the other.  He has compiled quite a list.

Reduce APPβ-cleavage Reduce γ-cleavage Reduce caspase-6 cleavage Reduce caspase-3 cleavage (All the above are cleavage in 4) Increase α-cleavage (cleavage in 2) Prevent amyloid-beta oligomerization Increase neprilysin Increase IDE (insulin-degrading enzyme) Increase microglial clearance of Aβ Increase autophagy Increase BDNF (brain-derived neurotropliic factor) Increase NGF (nerve growth factor) Increase netrin-1 Increase ADNP (activity-dependent neuroprotective protein) Increase VIP (vasoactive intestinal peptide) Reduce homocysteine Increase PPZA (protein phosphatase 2A) activity Reduce phospho-tau Increase phagocytosis index Increase insulin sensitivity Enhance leptin sensitixity improve axoplasmic transport Enhance mitochondnal function and biogenesis Reduce oxidative damage and optimize ROS (reactive oxygen species) production Enhance cholinergic neurotransmission Increase synaptoblastic signaling Reduce synaptoclastic signaling Improve LTP (long-term potentiation)

Optimize estradiol Optimize progesterone Optimize E2:P (estradiol to progesterone) ratio Optimize free T3 Optimize free T4 Optimize TSH (thyroid-stimulating llormone) Optimize piegnenolone Optimize testosterone Optimize cortisol Optimize DHEA (deliydroepiandrosterone) Optimize insulin secretion and signaling Activate PPAR-γ (peroxisome proliferator-activated receptor gamma) Reduce inflammation Increase resolvins Enhance detoxification Improve vascularization Increase cAMP (cyclic adenosine monophosphate) Increase glutathione Provide synaptic components Optimize all metals Increase GABA (gamma-aminobutyric acid) Increase vitamin D signaling Increase SirT1 (silent information regulator T1) Reduce NF-κB (nuclear factor kappa-ligllt-chain-enhancer of activated B cells) Increase telomere length Reduce glial scarring Enhance stein-cell-mediated brain repair

This explains why no single drug can have much effect on AD; it’s because the primary decision point depends on a balance among so many pro-AD (synaptoclastic) and anti-AD (synaptoblastic) signals.  Addressing them all may be impractical in any given patient, so the Bredesen protocol is built around a detailed diagnostic process that identifies the factors that are most important in each individual case.

Three primary types of AD

Bredesen’s diagnosis begins with classifying each case of AD into one of three broad constellations of symptoms, with associated causes.

 

Type I is inflammatory. It is found more often in people with carry one or two ApoE4 alleles (a gene long associated with Alzheimer’s) and runs in families. Laboratory testing will often demonstrate an increase in C- reactive protein, in interleukin-2, tumor necrosis factor, insulin resistance and a decrease in the albumin:globulin ratio. Type II is atrophic. It also occurs more often those who carry one or two copies of Apoε4, but occurs about a decade later. Here we do not see evidence of inflammatory markers (they may be decreased), but rather deficiencies of support for our brain synapses. These include decreased hormonal levels of thyroid, adrenal, testosterone, progesterone and/or estrogen, low levels of vitamin D and elevated homocysteine. Type III is toxic.  This occurs more often in those who carry the Apoε3 allele rather than Apoε4 so it does not tend to run in families. This type tends to affect more brain areas, which may show neuroinflammation and vascular leaks on a type of MRI called FLAIR, and associated with low zinc levels, high copper, low cortisol, high Reverse T3, elevated levels of mercury or mycotoxins or infections such as Lyme disease with  its associated coinfections.   (This box quoted from Dr Neil Nathan’s book review)

There’s also a Type 1.5, associated with diabetes and sugar toxicity, a Type IV, which is vascular dementia, and a Type V which is traumatic damage to the brain.
These categories are just a start.  The patient will work closely with an expert physician to determine, first, where are the most important imbalances to address, and, second, which of the changes that cna address them are most accessible for the life style of this particular patient.

 

Success

Bredesen wrote a paper in 2014 about successes in reversing cognitive decline with his first ten patients.  As of this writing, he has treated over 3,000 patients with the protocol called RECODE (for REversal of COgnitive DEcline), and he claims success with all of them, in the sense of measurable improvement in cognitive performance.  This contrasts with the utter failure of all previous methods, which claim, at best, to slow cognitive decline.

Translation to the millions of Alzheimer’s patients will require training of local practitioners all across the country.  A few doctors have already learned parts of the Bredesen protocol, and Bredesen’s website can help you find someone to guide your program, but you will probably have to travel.  The first training for doctors is being organized now through the Institute for Functional Medicine.

 

Implications

This is a new paradigm for how to study chronic, debilitating diseases.  Type 2 diabetes comes to mind as the next obvious candidate for reversal through an individualized, comprehensive program.  Terry Wahls has pioneered a similar approach with MS.  Cancer and heart disease may be in the future.

I’ll go out on a limb and say I think Bredesen’s protocol is the most credible generalized anti-aging program we have.  (Blame me for the hyperbole, not Dr Bredesen — he has never made any such claim.) Could we adopt Bredesen’s research method to accelerate research in anti-aging medicine?  Perhaps biomarkers for aging (especially methylation age) are approaching a point where they could be used as feedback for an individualized program, but Horvath’s PhenoAge clock will probably have to be 10 times more accurate to be used for individuals.  Averaging over ~100 individuals can give this factor of 10 in a clinical trial.  Still, we don’t have the kind of mechanistic understanding of aging that Bredesen himself developed for AD before bringing his findings to the clinic; and this is probably because causes of aging are more complex and varied than AD.

Disclaimers:  I’m pre-disposed to think highly of Dale Bredesen and his ideas for 3 reasons.  He was a friend to me, and gave me a platform when I was new to the field of aging.  He believes that aging is programmed. And his multi-factorial approach parallels the research I have advocated for researching other aspects of aging.

Rhonda Patrick interviews Dale Bredesen on FoundMyFitness

I’ve been in the field of aging research from the late 1990s, just the time when Aubrey de Grey was getting his start.  Before others, Aubrey had the vision to realize that cancer, heart disease, and Alzheimer’s would never be conquered without addressing their biggest risk factor: aging.

From the beginning, I admired Aubrey’s successes in communicating with scholars and the public, and I reached out to him.  He has always been gracious and supportive of me personally, appreciating the large common ground that we share. There is, however, one foundational issue on which we disagreed from the start.

Aubrey regards aging as an accumulation of damage.  Evolution has permitted the damage to accumulate at late ages because (as Medawar theorized in 1952) there is little or no selection against it, since almost no animals live long enough in the wild to die of old age.  Aubrey’s program is called SENS, where the E stands for “engineering.”   The idea is to engineer fixes to the 7 major areas where things fall apart with age.

I regard aging as a programmed process, rooted in gene expression.  Just as we express growth genes when we are in the womb and ramp up the sex hormones when we reach puberty, so the process continues to a phase of self-destruction.  In later life, we over-express genes for inflammation and cell suicide; we under-express genes for antioxidants, autophagy (recycling), and repair of biomolecules.  I believe in an approach to anti-aging that works through the body’s signaling environment.  If we can shift the molecular signals in an old person to look like the profile of a young person, then the person will become young.  The body is perfectly capable of doing its own repair, and needs no engineering from us.

Over the years, research findings have accumulated, and both Aubrey and I have learned a thing or two.  I’m happy to say that our favored strategies are converging, even as our philosophical underpinnings continue to differ.

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A unifying idea in my research has been that aging is an evolved adaptation.  This is a statement about evolutionary biology, but I came to it before I studied evolution, by looking at the phenomenology and genetics of aging.

• The body does not appear to be doing its best to stay young.  We can see this because when the body is under stress, it has less available resources, but manages to a better job of protecting us from aging damage.  This phenomenon is called hormesis.
• There are single genes that can be disabled, greatly extending lifespan in worms. Some of these have no known detrimental side-effects (pleiotropy).  These could only have persisted in the genome if natural selection is favoring aging for its own sake.  Similar genes exist in higher organisms, though their effects on lifespan are not as dramatic as the 10-fold increase in worms’ life expectancy in worms that comes from eliminating both copies of AGE-1.
• Most genes that affect the rate of aging have been around for a long time, and do the same job.  This means they are evolutionarily conserved. For example, insulin is the most effective modulator of aging in mammals (including humans).  In higher animals, insulin is secreted by the pancreas, from whence it regulates blood sugar and fat storage. But yeast cells existed half a billion years before the first mammals, and have no pancreas, as my friend Barja has pointed out; and yet insulin was already a primary modulator of aging in yeast.

Programmed aging and optimism

There was a time when I spoke of “aging genes” and looked for drugs that could jam their targets and turn the genes off.  Meanwhile, the science of epigenetics, or gene expression, was coming of age, so to speak. We learned that genes are turned on and off, not just in different tissues, but at different times of life.  I came to think less in terms of “aging genes”, more about multipurpose genes that are deployed in appropriate combinations when we are young, keeping us strong and healthy. But as we get older, the proportions change.  Aging is not accomplished via new mechanisms of self-destruction, which evolution invented for that purpose. Rather, the proportions are re-shuffled and change gradually, with effects that are more and more detrimental over time.

• For example, the immune system is vital for protecting the body, but it becomes indiscriminate with age. In older people, the immune system fails to protect us from microbial infections, and simultaneously, immunity turns against the self.  Autoimmunity contributes to arthritis and to Type 2 Diabetes (metabolic syndrome), as well as playing a role in AD.
• For example, p53 is a gene that promotes apoptosis, or cell suicide.  We need for cells to be smart enough to destroy themselves when they are infected with a virus or if they are cancerous.  But later in life, apoptosis is on a hair trigger, and we lose muscle and nerve cells that are still healthy and functional.
• For example, inflammation is used as a primary defense against microbes, and a way to eliminate tissue around a wound so that it can be replaced; but as we get older, signals that promote inflammation are dialed up higher and higher.  Chronic inflammation contributes importantly to all the diseases of old age.

Twenty years ago, I imagined one or a few medications that would block the effects of aging genes.  I wrote that the thesis of programmed aging implied great optimism about the ease with which aging might be combatted.  I thought that merely lengthening telomeres might add many years to our lifespan.

Ten years ago, I saw that what was needed was re-balancing of signaling molecules to create a more youthful environment.  My hope was that a few transcription factors (master regulator genes) might control a large number of signal molecules and we might set the clock by controlling just a handful of master signals.

More recently, I have come to realize that shortening telomeres are only a small part of the aging program.  Worse, there is no clear line between transcription factors and hormones. Most hormones affect transcription, and most transcription factors have direct metabolic effects.  There are thousands of transcription factors in the human genome.  As a result, my robust optimism has been tempered, and I have come to think that we need to look for ways to re-balance a great number of genes to effect rejuvenation.  I still believe in a signaling approach, but I see signals as a tangled web of cause and effect, in which every cause is also an effect, and every effect has a side-effect.  Modulation of the signaling system toward a more youthful state is possible, but not easy.

Aubrey’s program, too, has changed over time

Aubrey has never believed that aging evolved as a program, but rather that aging is a manifestation of damage that is permitted to accumulate because of evolutionary neglect.  Recently, he has argued explicitly against the idea of programmed aging, not for the reasons that traditional evolutionists offer, but by an argument that is uniquely his own.  In his words, “it is impossible for a species to maintain two sets of genetic pathways whose selected actions diametrically oppose each other. Specifically, since we clearly have a great deal of anti-aging machinery…we cannot also have pro-aging machinery.”  (My response is that we have pro-aging and anti-aging machinery that are activated at different times of life.–see Aubrey’s comment below.)

Over two decades, Aubrey, too has paid attention to research results, and his thinking about what is necessary to achieve rejuvenation is changing.  I see changes in the combinations of signal molecules and call it an evolved program. Aubrey sees the same thing and calls it “dysregulation”, which is a kind of damage.  Aubrey and I agree that re-balancing of hormones and other signal molecules is going to be essential.

Aubrey now finds optimism in the existence of what he calls “cross-talk”.  If we engineer a fix for one kind of damage, the body may sometimes regain the ability to repair other, seemingly unrelated kinds of damage.  Hence, we may not have to engineer solutions to everything—some will come for free. A dramatic example is in the benefit of senolytics. Cells become senescent over time.  I see this as a programmed consequence of short telomeres; Aubrey sees it as a response to damage in the cells. But both of us were surprised and delighted to learn, a few years ago, that elimination of senescent cells in mice had 20-30% benefits for lifespan.  Even though only a tiny fraction of all cells become senescent, they are a major source of cytokines (signal molecules) that promote inflammation and can cause nearby cells to become senescent in a vicious circle; this apparently accounts for the great benefit that comes from eliminating them.  If we find appropriately selective senolytic agents that can eliminate senescent cells without collateral damage, then the signals that up-regulate inflammation will be cut way back, and a great deal of the work needed to repair inflammatory damage is obviated.

The SENS 7

The SENS web site still lists the same 7 categories of damage that Aubrey has used for many years.  But the program to address these 7 has shifted a bit from bioengineering of exogenous solutions to signaling approaches that support the body’s innate mechanisms (which we know are sufficient to keep the body in good repair through several decades of early life).  For eliminating the plaques associated with AD, SENS at one time favored the engineering of artificial antibodies that would attack them, but more recently they see promise in the discovery of Dr. Sudhir Paul that our bodies already have catalytic antibodies, each capable of destroying many antibodies and re-cycling itself for the next one.  Where once Aubrey saw the need for tissue engineering to replace worn-out body parts, he now sees promise in reprogramming somatic cells to become stem cells, so that our bodies can regenerate damaged tissues endogenously.  Aubrey’s 1999 dissertation in biochemistry was about the theory that aging was caused by the damage inflicted by free radicals generated in our mitochondria, but he has long since embraced the fact that free radicals have an important role as signal molecules, so that anti-oxidants are not helpful for anti-aging.

Aging is not the only threat to human life

One respect in which my thinking has always departed from Aubrey’s is that I see humans as part of a continuous web of life on earth, integrated into a global ecosystem.  Aubrey doesn’t worry about the Sixth Extinction that human activity has initiated because he anticipates that future humans will invent ways to support future human life as necessary.  I value nature for its own sake, and I also believe that human life depends on ecoystem support in ways for which we have seen hints, but that we have not yet begun to study. Aubrey draws a sharp line between the value of human life and the value of other life, and he is highly optimistic about the ability of our species to find new ways to sustain ourselves in a post-ecologic world.

 

The Bottom Line

In my youthful enthusiasm, I was entirely too optimistic about the prospects for near-term anti-aging fixes.  Aubrey was probably too conservative about the scope of what needed to be done to generate man-made solutions for problems the body can’t solve itself.  I have come to understand the complexity of the body’s signaling network, and the fact that it is inseparable from cellular metabolism. Aubrey has come to realize that the body has endogenous solutions that can be activated more easily than we can engineer substitutes for them.  I’ve been moving the timeline out, as he has been moving the timeline in, and there is much that we agree about.

I’m grateful to Aubrey — we all are — for the energy, the expertise, and the humor that he has brought to his chosen role, as a public advocate for bringing anti-aging strategies into the mainstream of medical research.