Difficulties of finding the pill of immortality (8)

Problems of screening anti-aging drugs

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Several drugs have shown great promise in laboratory experiments to increase healthy life expectancy and life expectancy of various species, including mice, which indicates the reality of effective pharmacological anti-aging interventions. However, conducting an unbiased screening of new potential geroprotectors on mammals is an almost impossible task. Considering that some cellular signaling pathways affecting longevity have been preserved during evolution, invertebrate models may be quite useful for conducting such screenings. At the same time, some of the known molecular factors that have a pronounced effect on the life expectancy of mammals (for example, growth factor) differ quite significantly in invertebrates and mammals. Therefore, screening of small molecules based solely on the use of invertebrate models will not reveal all compounds that have a pronounced effect on mammalian aging. Moreover, many key physiological parameters of humans and other mammals are poorly modeled on invertebrates that do not have such specific tissues as heart and kidney tissues, and complex endocrine, nervous and circulatory systems that are critical targets in the fight against aging and age-related pathologies. Most invertebrate models of aging have limited regenerative abilities and do not fully reproduce such processes as the renewal of the stem cell pool, which are necessary for the mechanisms of tissue damage restoration, ensuring the maintenance of tissue homeostasis in mammals, which is necessary to maintain the functioning of organs for many years and decades.

The development of new short-lived vertebrate models of aging can greatly facilitate such screening. In this context, an attractive model system is the short-lived vertebrate fish Nothobranchius furzeri. Recently Harel et al. Using the synthesized de novo genome and CRISPR/Cas9 technology, a genotype-to-phenotype platform model was described for N.furzeri, which opens up the possibility of integrative screening for gene mutations and drugs that increase the lifespan of this organism. One of the main limitations of the possibilities of using N.furzeri is the need for separate maintenance, which significantly increases financial costs. Moreover, it is possible that some factors modulating the aging of fish and other cold-blooded vertebrates may not coincide with factors effective against mammals. Despite the fact that mice can reproduce well many aspects of aging and age-related human diseases, their use for primary screening/testing of a large number of potential anti-aging compounds is impractical due to high financial costs. The use of progeroid models, such as Ercc1 hypomorphs or Lmna mutants, which are characterized by accelerated development of pathologies and short life expectancy, will allow testing much more compounds than using wild-type animals. However, the question of whether these animals really suffer from aging as such is currently hotly debated. It is also possible that a thorough assessment of adequate markers of aging, for example, an increase in p16 expression or changes in DNA methylation, will allow for a primary assessment of the anti-aging effects of a large number of compounds in mice without conducting studies whose duration corresponds to the lifespan of mice, using a variety of cohorts receiving various potential anti-aging compounds. In this regard, the Horvath group has developed an approach that allows estimating the age of most types of tissues and cells based on age-related changes in DNA methylation levels in CpG zones, but such methods have not yet been applied to mice.

The search for anti-aging compounds at the present stage is carried out using two main approaches. One of them is phenotypic, that is, it is a screening of compounds on cellular or animal models, carried out to identify drugs that have the desired biological effects, that is, an increase in life expectancy. Despite the fact that this approach has demonstrated its exceptional value in many areas of biochemical research, the identification of life-modulating drugs is much longer, more complex and costly than working with many other phenotypes. The insufficiently used model for screening anti-aging molecules is baker's yeast S.cerevisiae. Two different forms of aging have been described in these organisms: replicative and chronological (population). In principle, any of them can be used for screening anti-aging drugs, although chronological aging is much more suitable for high-performance analysis. An additional approach involves targeted screening of modulators of mechanisms that with a high degree of probability modulate the rate of aging. However, such approaches, by definition, only with a small degree of probability will allow identifying new cellular factors and signaling pathways involved in longevity.

In overcoming these difficulties, a powerful mechanism may be the use of a holistic approach for the search for anti-aging drugs, which implies simultaneously using experiments on invertebrates, mammalian cells and mice. In this context, the authors of this article, employees of the Paul F. Glenn Center for Aging Research at the University of Michigan, conduct screening of compounds for their ability to increase the duration of healthy life and the lifespan of fruit flies and nematodes C.elegans, as well as to increase the resistance of mammalian fibroblasts to stress, correlated with the longevity of mammals. Compounds effective in all three tests are candidates for deeper mechanistic evaluation and further testing in mice (Figure 3).

Figure 3. Drugs that have shown an increase in life expectancy and preservation of health in drosophila and nematodes and increased resistance to stress of mammalian fibroblasts are potential candidates for further in-depth evaluation and testing in mice.

Another similar problem in the study of aging today is the lack of humanoid (primate) model systems with an acceptably short lifespan for preclinical testing of potential anti-aging drugs. Rhesus macaques, whose life expectancy is 30-40 years, are most often used as such a model. In terms of size, accessibility and other biological parameters, common monkeys have a number of advantages over Rhesus monkeys. Due to the smaller body size, their maintenance is cheaper. Moreover, the pregnancy of monkeys lasts about 147 days and leads to the appearance of 2-3 cubs. Some features of monkeys, including the predisposition profile to diseases, are more similar to human characteristics than in macaques. In Europe, monkeys are used as a model for testing the safety and toxicological parameters of medicines. Thus, in a recent study by Tardif et al. the dosing procedure, pharmacokinetics and changes in signaling pathways for rapamycin were described on monkeys. However, their life expectancy is about 17 years, which is less than the life expectancy of rhesus macaques, but still little applicable to the study of pharmacological interventions designed to increase life expectancy. The creation of new models of mammalian aging would greatly facilitate the study of the biological processes underlying aging and accelerate the transfer of pharmacological interventions from the laboratory to the clinic.

One of the models considered in this regard are dogs that share their environment with a person. Moreover, the aging and diseases of dogs are well studied, their body size and life expectancy vary very widely, due to the huge genetic diversity. Dogs can be a relatively inexpensive model system, especially given the desire of some owners to test life-extending drugs on their pets that have previously been validated in invertebrate models and rodents. Matthew Kaeberlein and Daniel Promislow from the University of Washington in Seattle have started a pilot study on 30 dogs dedicated to testing the effectiveness of rapamycin in improving overall health and increasing the life expectancy of large dogs, usually living up to 8-10 years.

Conducting clinical trials of potential anti-aging drugs is associated with enormous difficulties. It is very unlikely that pharmaceutical companies will agree to conduct clinical trials lasting several decades. Evaluation of short-term surrogate phenotypes, such as molecular markers or age-related disorders, such as a weakened response to vaccination, can be used for the initial clinical evaluation of potential anti-aging compounds, carried out in a more acceptable time frame.

Conclusion

Since ancient times, people have dreamed of interventions that slow down the aging process and increase life expectancy. However, it is only very recently that biological studies of aging have reached the level at which such interventions already look quite real. The result of a large number of studies on invertebrate models and rodents has become an ever-growing list of molecules that can potentially increase the lifespan of mammals and contribute to their preservation of good health in old age. Given the fact that there is a close relationship between aging and morbidity, such drugs, if they overcome all the problems associated with their testing and development, can significantly improve human health.

For references to literary sources, see the original article.

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14.09.2016