Despite the immense implications for the future of human health, many people are still unaware that ageing is a malleable process that can and has been successfully delayed in animals. Moreover, many drugs currently taken by humans all over the World have been shown to extend lifespan in mice. Mice, of course, are not humans, but they are close enough to us biologically that some conclusions can be drawn. If it is possible to make a mouse live longer simply by feeding them a single drug, then it should be possible, though harder, to do the same thing in a human. There is no law of nature to say that humans have to age.
How do we know whether a drug extends mouse lifespan? It’s not quite as straightforward as you may think. Currently, most of the best quality evidence about whether a drug extends mouse lifespan comes from something called the Interventions Testing Program (ITP). It’s worth taking some time to talk about the ITP and exactly why it’s held in high regard, but if you would rather simply read about results, you can skip to that section.
The ITP is part of the US National Institute on Ageing (NIA), and is an initiative to test compounds that may extend lifespan in mice. The ITP is a peer reviewed program to which any organisation can recommend a compound for testing. The ITP will then review evidence for that compound and decide whether or not it is worth including.
What makes the ITP different from most other studies? There are a few factors:
Genetically diverse mice: Most studies use a strain of highly inbred mice called C57BL/6 mice (black 6 or B6). Despite their widespread use in scientific research, these mice pose significant problems when it comes to testing lifespan-extending drugs. Because they are so inbred, most B6 mice are almost genetically identical, and develop severe health problems. This means that findings in B6 mice often do not translate to normal mice, let alone humans. Testing a lifespan extending drug on B6 mice is a bit like testing an Azlheimer’s drug on identical triplets with an ultra rare form of hereditary Ahlzeimer’s disease that is unique to them.
In contrast, the ITP uses a different mouse model called UM-HET3. These mice are the result of cross breeding four strains of inbred mice, one of which is B6. This means that any population of UM-HET3 mice will contain the same pool of genes (which is crucial for reproducibility), but any two mice will be genetically different as there are thousands of different combinations of chromosomes that a given mice could inherit. While they are still not as genetically diverse as mice breeding in the wild, they are far more representative of a normal population than B6 mice.
Reproducibility: Even when scientists attempt to replicate the conditions of a previous experiment perfectly, different labs can sometimes produce different results due to random variables that were outside their control. When the ITP tests a drug, they run the experiments in three different labs in different parts of the United States simultaneously. This immediately tells us how reproducible any lifespan extending effects are. If two labs find significant lifespan extension but the third does not, this suggests that some variable unique to that lab may have interfered with that experiment. Had the drug been tested only in that site, this might have led to the false conclusion that the drug had no effect.
Male and female mice: The ITP measures lifespan extension separately in both male and female mice. This is important because lifespan extending drugs have different effects in males and females and it is important to measure both so that benefits that only apply to one sex are not overlooked. Studying how lifespan extending drugs affect males and females differently may also tell us something about how the biology of ageing differs between the sexes.
Sample sizes: Having a sufficiently large sample size is vital for any study. The ITP generally uses between 150-300 mice per sex per drug throughout the three test sites. This is a respectable sample size that yields decent statistical power.
Dosing strategies: The ITP often tests the same compound at different doses, and will begin administering it to the mice at different points during their lifespan. This gives us important information about what dose is necessary to produce an effect and whether that effect increases with a larger dose. It also tells us whether the drug must be given while the mice are still young, or whether it remains effective in old mice. This last point is important because if we develop a lifespan-extending drug for humans, we want that drug to work for everyone. A drug that only works when started in youth would still be ground-breaking, but not ideal.
This table summarises the results for all compounds tested in the ITP so far. To understand this table, we need to talk about how the ITP actually measures lifespan extension. Two metrics are used: the median lifespan extension, and the p90 lifespan extension.
The median lifespan is simply the age by which 50% of the mice are dead. An increase in median lifespan means that more mice are surviving into old age, but does not necessarily mean that the oldest mice are living longer. This is measured by P90 lifespan extension, which is the age by which 90% of the mice have died. Why not 95% or 99%? The higher the bar is set, the fewer mice will survive to that age and the more susceptible the experiment will be to a few mice randomly living longer than normal. The ITP selects 90% in advance so that there is no potential to cherry pick the value that makes the experiment look more successful.
What is it?
Rapamycin is an antibiotic produced by bacteria discovered on Easter Island, AKA Rapa Nui. Rapamycin was initially found to suppress the immune system in humans, but the effects of rapamycin turned out to be a lot more complicated. Rapamycin suppresses an important signalling molecule called mTOR, which is present in all of our cells and boosts the rate at which cells grow, divide and synthesise new proteins, while suppressing the recycling of damaged components. More mTOR activity is considered to be a bad thing with respect to the ageing process. To age more slowly, we generally want our cells to focus on repairing and maintaining themselves, as opposed to dividing quickly.
Rapamycin has been one of the most consistent ‘winners’ of ITP trials. Both median and P90 lifespan have been extended by over 20% at the highest doses of rapamycin. However, rapamycin has shown significant benefits using several different dosing strategies. In particular, rapamycin significantly extends median and P90 lifespan even when administered when mice are 20 months old, which is roughly equivalent to 65 human years. This is encouraging because it suggests that rapamycin still works even after many of the biological changes associated with the ageing process have already taken place.
What is it?
Acarbose is a drug used to treat type II diabetes mellitus. It works by inhibiting enzymes in the gut that break down carbohydrates, and this delays the absorption of glucose into the blood. This is relevant to the ageing process as high blood sugar may promote ageing through a number of mechanisms. Acarbose may also have some direct anti-inflammatory effects.
Acarbose has been found to extend median lifespan at lower doses and P90 lifespan at higher doses. It works even when given in ‘middle age’, though not as well as it does when given to young mice. It also consistently benefits males more than females. It’s not known for certain why this is the case, but it may be that males are more negatively affected by high glucose levels.
What is it?
17α-estradiol is a form of oestrogen that is around 100 times less potent than its cousin, 17β-estradiol. In mice, 17α-estradiol is ‘non-feminizing’, meaning that it doesn’t induce female sexual characteristics in male mice. Since female mice live intrinsically longer on average than male mice, it was proposed that 17α-estradiol might make male mice live longer.
When given at sufficiently large doses, 17α-estradiol extends both median and P90 lifespan in male mice, regardless of whether the mice got it when they were young or in late middle-age. It did not have any significant effect on female mice. Yet surprisingly, 17α-estradiol didn’t just allow the males to ‘catch up’ – males taking 17α-estradiol actually lived significantly longer than females taking the drug. This suggests that 17α-estradiol is having some unique effect in males. 17α-estradiol is quite understudied and work is currently underway to figure out why these sex differences exist and whether they can be overcome.
What is it?
Like Acarbose, canagliflozin is a drug used to treat type II diabetes mellitus. It works by blocking the reabsorption of glucose by the kidneys. This means that more glucose is excreted into the urine, reducing the amount of glucose in the blood.
Just like acarbose, canagliflozin increased both median and P90 lifespan in male mice only. Given that acarbose and canagliflozin both work by lowering blood glucose, this again hints that males might be more sensitive to the negative effects of high glucose levels.
What is it?
Glycine is an amino acid – a protein building block. It’s the smallest amino acid and it is nonessential, meaning our bodies can synthesise it. Glycine is of interest when it comes to delaying the ageing process because it appears to be a necessary building block for certain pro-longevity proteins, and may counteract the negative effects of methionine, another amino acid.
Glycine did significantly extend median lifespan in both sexes and extended P90 lifespan in males. However, in each case lifespan was only extended by about 5%, making it one of the least impactful yet successful compounds to be tested. It remains noteworthy for being the only compound so far that has extended lifespan equally in both males and females.
What is it?
Captopril is a drug used to treat hypertension. It works by blocking an enzyme called angiotensin converting enzyme (ACE), which is involved in maintaining blood pressure. Blood pressure increases with age and is intertwined with many processes related to ageing.
Captopril significantly increased median and P90 lifespan in both sexes, though the effects on male mice were ultimately deemed indeterminate due to an experimental problem. Mice do not usually die from problems related to hypertension, which makes these findings interesting. The ITP is planning to investigate captopril again at higher doses.
The ITP has also investigated whether combining rapamycin with other drugs can enhance lifespan extension. Rapamycin has been tested in combination with both acarbose and metformin, which is another drug for type II diabetes. One of the rationales behind these choices is that rapamycin has the side effect of reducing sensitivity to the blood sugar-lowering hormone insulin, so combining it with blood sugar-lowering drugs might help counteract this downside.
The combination of rapamycin and metformin significantly extended lifespan in all sexes. Based on comparisons with previous studies where rapamycin was given alone, metformin + rapamycin seemed to have added benefit. Similar findings were produced for the acarbose/rapamycin combination, which extended median and P90 lifespan significantly, but was only significantly better than rapamycin alone in the males.
Multiple other compounds have been found to slightly extend mouse lifespan, but only median lifespan and only in males, so they’ll be briefly covered here. Aspirin, NDGA (a plant antioxidant), protandim (a commercial dietary supplement containing various ingredients), astaxanthin (another plant antioxidant) and meclizine (an anti-nausea medication) have been found to increase median lifespan in male mice by between 7% and 12%.
Unfortunately, not all compounds tested by the ITP are found equally worthy, and some relatively promising candidates have shown no benefit at all.
Foremost among these disappointments is metformin. Metformin is a drug for type II diabetes mellitus that has been a focus of longevity research for some time. It lowers blood sugar though various mechanisms, including reducing the production of glucose in the liver. Metformin gained the attention of researchers when diabetic patients taking it seemed to live longer than nondiabetics in a clinical trial, though this conclusion has been called into question due to the way said trial was conducted. Even so, numerous studies appear to show that metformin extends lifespan in mice. Unfortunately, the ITP found no significant increase in lifespan when mice were given metformin. This may be related to dosing strategies, however.
Other notable disappointments include resveratrol, an antioxidant mostly found in red grapes, and nicotinamide riboside (NR). NR is a precursor to NAD, a molecule thought to play a key role in ageing. Plenty of research shows that depletion of NAD is associated with ageing and that restoring NAD levels can mitigate ageing in various ways, yet NR had no significant effect on lifespan. Again, this could have been related to dosing.
A more recent disappointment is the failure of fisetin. Fisetin is a type of drug called a senolytic. Senolytic drugs help the body remove senescent cells, which are cells that have lost the ability to divide but refuse to die. Senescent cells are thought to contribute to ageing, but in the ITP study fisetin failed to significantly reduce the number of senescent cells in the mice.
The ITP studies show that simply by putting something in a mouse’s food, we can make that mouse live significantly longer than they would otherwise be capable of. This alone is a revelation that should make us think differently about what human health could look like. But what about the present? Is there anything we have learnt from the ITP that we can act on to improve our own chances at living longer? The answer, arguably, is yes!
Perhaps the most important revelation is that most drugs were effective past ‘middle age’, suggesting that ageing remains malleable and that anti-ageing interventions may remain effective even in later life. Another is the apparent significance of glucose (sugar), especially in males. Several of the most successful drugs in the ITP have been blood sugar-lowering drugs. This suggests that high blood sugar is very bad and that we should be trying to limit it as much as possible. Finally, the success of rapamycin suggests that its target, mTOR, is important. mTOR controls the cell’s behaviour in response to the availability of nutrients, and more mTOR activity seems to be bad. The best way to reduce mTOR activity without drugs is through dietary restriction strategies such as fasting.
Exciting as they are, we should remember that ITP studies are only the beginning of a long journey. Proving that a drug extends human lifespan is much harder and will require significant efforts, but will be extremely worthwhile in the long term.
A list of tested interventions, including links to the relevant publications, can be found on the NIH website.
ITP Supported Interventions https://www.nia.nih.gov/research/dab/interventions-testing-program-itp/supported-interventions
Title image by Dawid Zawiła, Upslash