What are the different research avenues? Are there special considerations when trying to develop treatments for aging? What are the pitfalls? It also includes an overview of the field as it stands today. Different drugs or substances that are especially promising or interesting are discussed. Can we meaningfully impact healthy lifespan through pharmacological means?

The second part is an ambitious proposal for testing a mixture of promising compounds for their effect on the health and lifespan of mice. This includes seven compounds tested individually side by side for their effects on longevity and a variety of health-span measures. We will also try to determine the optimal dose and see if the compounds together have an additive or even possibly synergistic effects on lifespan.

We hope you enjoy it, and if you do, there is much more coming in the Longevity Cookbook.

Pharmacological Extension of Lifespan

As we age, our risk of disease and death increase dramatically. This, of course, is something that we want to postpone, reduce or even eliminate. There are different ways of approaching aging. In this chapter, we will discuss what we know today about pharmacological interventions.

Can we impact lifespan with pharmaceuticals? The average lifespan of humans has increased unevenly throughout history, but since the beginning of the industrial revolution it has taken rapid and steady strides upward. Much of this increase has been due to better sanitation, nutrition and living standards. Improvements in medicine, such as vaccines and antibiotics, have been very successful in combating infectious disease. This used to be the main cause of death for humanity but we have been so successful in combating them (world wide) that the main causes of death are now chronic age-related disease and cancer. These are what we hope to tackle now. The risk of death increases exponentially from about age 35. But it is not only the risk of death and age related diseases we hope to combat, but also the general frailty that accompanies old age. This is not recognized as a disease but is still a very undesirable part of aging. The closest thing to frailty that is a recognized disease might be osteoporosis. This will be discussed further when we look at drugs that may affect bone density.

We will not cover all of the compounds that have been studied to extend lifespan, but those that I find most promising or most interesting. We will also discuss compounds that possibly work through different mechanisms. This could mean that we are neglecting some potentially important compounds, so we will try to explain what we think constitutes good evidence for a compound being promising. Extending lifespan in a model organism is certainly an important factor. If the compound extends lifespan in multiple model organisms, that strengthens the case, making it less likely that effect is idiosyncratic to that specific organism or strain. The type of organism is also important where organisms closely related to humans or data from humans being given larger weight. The magnitude of the extension is also important. Large effects make it more likely that aging is really affected and not some marginal effect on metabolism. Another consideration is the effect on median and maximum lifespan. Some would argue that aging is not truly affected unless there is an effect on median as well as maximum lifespan. Another possible consideration is whether the compound can be added late in life or needs to be started early, since we are not going to start treating babies but more likely people who have already reached middle age.

In this chapter we will be exploring how we discover potential lifespan and healthspan extending compounds. The compounds we discuss as promising candidates should not be taken as recommendations. The gold standard, large randomized placebo controlled clinical trials for aging have not been done on these compounds but we hope this type of research will get done. It should be noted that although compounds are available as supplements, the supplement industry is largely unregulated and while there are ethical suppliers out there, there are also many that will sell you pills with reduced or no active ingredient, or even worse, unlisted ingredients with pharmacological properties, or even real drugs. The spot checks that have been done do not look promising [1, 2].

Considerations for identifying lifespan-extending compounds

Model organisms

There are many lessons that can be learned from studies in humans such as treating patients with certain drugs for one disease and noticing this confers protection from other diseases, or reduces all cause mortality. This is certainly a very promising avenue of research that we think will yield results in the near term; however, in order to not be limited by only testing existing drugs we need other models we can work with. What about using cells or tissues? Ideally we would know enough about the biology and the aging process that we could rationally design therapies that could be tested on cells or tissues. 

Unfortunately, these types of models are not the same as a living organism where different parts of the body influence each other and work together. It is also very hard to assess effects on lifespan or healthspan on cells. Just because a cell lives long or is immortal does not mean that will be beneficial for the organism. Cancer cells are immortal in the sense that they can keep dividing forever, but that is not very helpful for their host. There is no one ideal model organism. Each one comes with different strengths that help us learn what treatments will work to combat the aging process.

One criticism of how model organisms are used is that they lack genetic diversity. 

Generally, animals are inbred strains that are almost like identical twins. This increases the chance that the intervention you are testing only works because of some quirk of the specific strain you are working with. There are ways to try to get around this, but they are rarely used. We will discuss this more below.

Yeast, nematodes, fruit flies, mice, and rats are the most commonly used model organisms for aging studies, especially when testing compounds. Other species can be informative when comparing the aging process between different species. Generally, larger animals live longer, but there are exceptions that are interesting. For example, naked mole rats live about 30 years and they weigh about as much as a mouse, which can live 3 years. They also show very few signs of aging as they get older [3]. Then there are extremely long-lived animals such as clams that can be over 400 years old. Some animals might not age at all, such as Hydra, which some argue show no age-based increase in mortality[4].

All model organisms have their strengths and weaknesses. Mice are of course closer to humans than yeast or nematodes, but they are expensive and lifespan experiments take a very long time. In yeast and nematodes large screens (more on this in the section on screens) can be conducted, optimal doses can be tested in a matter of weeks, and more can be learned about the biology by rapidly employing genetic tools and using lifespan as an outcome. This is very slow in mice. One way to use the relative strengths of the organism is to triage compounds in less expensive and less labor-intensive organisms before moving them up the ladder to organisms more related to human physiology. One way at attempting this is the Caenorhabditis intervention testing program (CITP). 

Caenorhabditis is a genus of nematode or roundworm. The most studied species is Caenorhabditis elegans (C. elegans). The CITP is divided to three testing centers across the United States where each center does the same experiment as the other sites to ensure reproducibility. They also introduce genetic diversity by having 22 different strains over three different species of nematode. If the compound can extend lifespan robustly across such genetic diversity, it is more likely to target conserved mechanisms, and therefore be more likely to work in mammals or even humans. Clearly there are mechanisms in human aging that cannot be addressed by looking at yeast or nematodes, but there are many commonalities also.

Pharmaceutical companies have had a hard time developing treatments for the diseases of aging. Perhaps it is because the models that they use are lacking the underlying causes. Young animals are generally used to model the diseases of aging. These models can be created by introducing mutated genes or by chemically disrupting cells or tissues. This causes the animals to develop symptoms similar to human diseases early. These animals did not get their disease for the same reasons as humans do. It seems being old is the largest risk factor for getting age related diseases. Perhaps more studies should be done on old animals.

The intervention testing program (ITP) is a consortium under the National Institute on Aging (NIA) that use mice. It treats mainly old animals, and measures their lifespan. This is done across three centers and great care is taken to standardize procedures. To introduce genetic diversity, they employ a four way cross, so that genes from four different strains end up in the mice that are receiving the treatment. Genetic diversity is important if you want to know that the treatment is more generally applicable. The ITP has had some successes, with rapamycin being the most prominent. If you have a good candidate with some promising data to back it up, they will consider your compound for testing [5].

Target-based screens

A screen is basically when you start with a large number of compounds and try to find one that works. When preforming target-based screens, the target needs to be known beforehand. The screen can then proceed in two basic ways: either through a biochemical assay which measures how strongly the compound binds to the target, or through a cell-based assay in which the protein is overexpressed in a mammalian cell line and some kind of output of its activity can be measured. The screen then proceeds by taking a large compound library, and testing all the compounds against the target. Compound libraries can come in many different forms. They can be natural products, derived from plants and animals. They can be enriched for compounds that look “drug like”. They can be variations of known compounds. The common denominator is that it is a lot of compounds, thousands, or even millions. Once a hit compound is found, it is then tested to see if it has the desired effect in a live organism.

Unbiased screens

Unbiased phenotypic screens don’t concern themselves with a particular target. Instead what you screen for is the effect on some visible phenotype. Such a phenotype could be many things, but preferably it should be easy to score. For example, if you have marked aggregating proteins with fluorescent labels you could visibly score for protein aggregation. The most straightforward measurement in a screen for lifespan extending compounds, so called “geroprotectors “, is lifespan. For this, very few organisms are practical, such as yeast or the nematode C.elegans. This is due to the fact that they can live out their life in a 96 well plate format. Different compounds are put in each well, and after a period of time the wells are then checked to see if there are any survivors. If the well with the compound has more survivors than the control wells, this would be considered a hit.

After the hits are independently validated, the process to find the target begins. While it is not strictly necessary to know the target for the drug to work, it is highly valued in industry and in academia. There are still many drugs for which the targets are unknown, but methods are being developed that make target identification easier. These include new ways to fish out the target chemically, and in silico (computational) ways of finding binding partners[6].

Using phenotypic screens like this targeted on aging, we can find promising compounds and also learn more about the aging process. If we can find the target, we can uncover new ways to combat the aging process that we might not have thought of using a target-based approach.

When a disease is well characterized, with known targets, targeted screens may be preferred. Since the 80’s, targeted screens have generally been favored by pharmaceutical companies. Some credit this preference with the failure to produce new first-in-class drugs[7, 8]. With aging, it is a mixed bag: there are known targets we would like to modulate, but also a lot of unknown biology, so there is certainly room for both targeted and phenotypic screens.

Mining existing compounds

Drugs that are already approved for human use have some huge advantages. Since they have been used by thousands of people, they are better understood. In order to be used in humans, they have passed important toxicity tests and they have some biologically important effect. Many times a drug is developed for one indication and as we learn more about it and realize that it is an even better treatment for something else. This type of research is certainly relevant for geroprotectors, and there are quite likely compounds already on the market that could be used to prolong the lifespan of healthy individuals.

One way to do this is through epidemiological studies that follow large groups of people, taking certain drugs and looking at their mortality or risk of certain diseases. This can certainly be very informative, and several drugs have shown promise through these types of studies.

One criticism of this approach is that we should already be seeing people living for a very long time if this were true. The fact is that life expectancy has risen by 6.2 years worldwide from 1990 to 2013 and it is mostly good healthy years being added. 5.4 healthy years are being added and 0.8 years with disability [9]. We would certainly like this to rise much faster. 2.7 years per decade falls short of 1 year per year, which would be nice. Still, progress is being made, and the fact that it is mostly healthy years being added is very hopeful to us.

So can only modest gains be had from adapting drugs that are already on the market? Not necessarily. Different dosing regimes, or new formulations of the drugs, could ameliorate the side effects and increase the potency. Combinations of these drugs might prove effective. Likely though, we do not have drugs on the market that combat every aspect of the aging process. New research on new classes of geroprotectors is certainly needed.

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