LEV Foundation on Senolytics as One Part of a Combination Rejuvenation Therapy – Fight Aging!

The primary focus of the Longevity Escape Velocity (LEV) Foundation is to demonstrate that therapies based on the repair of forms of underlying molecular damage that cause aging can be combined to produce greater rejuvenation. Research of recent years has demonstrated quite comprehensively that the alternative strategy for treating aging, to manipulate metabolism into a state in which aging occurs modestly more slowly, has so far produced therapies that largely cannot be combined. The combination of any two or more metabolic alterations, induced by supplements or other small molecules, that individually modestly slow aging in animal models will likely result in no effect or even a modest acceleration of aging. The advocacy community might do well to use this as a teaching moment, to refocus efforts on the better path of damage repair.

In this article the LEV Foundation staff discuss the use of senolytics to clear senescent cells in their combination studies in mice, and the relevance of this approach to the bigger picture of combined therapeutics to produce rejuvenation. Since aging is a condition caused by a number of interacting but quite different forms of molecular damage and disarray, it will require a panoply of different therapies to repair aged tissue. Reversing mitochondrial dysfunction, repairing stem cell populations, removing senescent cells, clearing intracellular and extracellular waste products, and so forth. Once widespread in the clinic, senolytic drugs to clear senescent cells will be the first rejuvenation therapy worthy of the name. It will be useful to have some degree of evidence in aged animal models to demonstrate that this treatment can combine well with other approaches.

The Case For: Senolytics

As of 2023, the flagship research program at LEVF is our Robust Mouse Rejuvenation (RMR) studies, the first of which (RMR-1) was initiated in early 2023. In this program, we seek to investigate the potential lifespan-enhancing effects of combining multiple interventions in middle-aged mice which have been previously shown to extend the lifespans of lab mice. For RMR-1, we decided to test a senolytic intervention as one of the four interventions administered in combination to our study mice at Ichor Life Sciences. Those four interventions are: (a) Senescent cell ablation via galactose-conjugated Navitoclax (“Nav-Gal”); (b) Rapamycin in food at 42 ppm; (c) Enhanced telomerase expression via repeated TERT gene therapy (via nasally administered AAV–mTERT); (d) Hematopoietic stem cell transplantation. In this essay, we hope to answer readers’ questions about the role of cellular senescence in aging and the role of senolytic therapies in rejuvenation. We will further speculate on how reducing the burden of senescent cells might synergize with other rejuvenation therapies.

The word senolytics is a blend of two words – senescence and lysis. Lysis means “a process of disintegration or dissolution”. So, the use of senolytics is an effort to selectively and deliberately induce disintegration or elimination of senescent cells. One could reasonably want to be cautious about purposefully inducing the death of cells in the body, but there have been multiple reports of health benefits associated with administration of senolytics, particularly when done in animals with elevated senescent cell burden such as animals that are older or have been treated with chemotherapy or radiation – both of which are known to elevate the prevalence of senescent cells. There appear to be multiple mechanisms by which senolysis can be accomplished. The most common mechanism is to inhibit proteins associated with cell survival during stressful situations. The survival of senescent cells is dependent on proteins and processes that are different than those used for survival by non-senescent cells. We can exploit these differences to specifically target senescent cells while leaving non-senescent cells relatively unaffected.

We found Navitoclax, also called ABT-263, particularly interesting for our first RMR study for several reasons. First, Navitoclax appears to be effective at inhibiting Bcl-2, Bcl-w, and Bcl-XL. Because different cell types can overexpress different survival proteins when they become senescent, the ability of Navitoclax to inhibit all three of these proteins means that it might be relatively more effective at reducing the elevated numbers of senescent cells in many different tissues in the body. Second, Navitoclax also seems to be increasingly well studied. There have been many scientific reports about its effects on both normal and senescent cells, and this gives us confidence about its possible effects in older animals. However, Navitoclax has a drawback: it has been shown to be toxic to normal, healthy platelets and other immune cells. Fortunately, some researchers have designed a method to substantially reduce the toxicity of Navitoclax to non-senescent cells. Attaching galactose to Navitoclax reduced the toxicity of Navitoclax for normal cells but retained its toxicity for senescent cells, as senescent cells contain a lot of beta-galactosidase, an enzyme that cleaves galactose molecules from the other molecules they are attached to.

There is evidence that a persistent, elevated prevalence of senescent cells inhibits wound healing, immune function, tissue maintenance, and possibly stem cell function, and that these effects might limit the lifespans of aged mice (and we suspect, humans). We imagine that the elimination of senescent cells will enable the more anabolic interventions – such as TERT gene therapy and hematopoietic stem cell (HSC) transplantation – to work more effectively. Particularly in the case of HSC transplantation, we suspect that ridding the body of excessive senescent cells and senescence-associated secretory phenotype (SASP) signaling might enable those transplanted HSC to function better than they otherwise would. For example, consider the case of the bone marrow, which houses both mesenchymal stem cells (MSC) and HSCs. There is some evidence that mesenchymal stem cells become senescent during aging and secrete proteins which alter the bone marrow microenvironment, which in turn impairs HSC function. We imagine that a senolytic intervention which reduces the prevalence of senescent MSCs in the bone marrow could enhance HSC function, which includes the generation of red blood cells and immune cells.

In addition, there is some evidence to suggest that rapamycin and senescent cell ablation might be synergistic. This comes from evidence that rapamycin inhibits a protein complex called mTOR and upregulates autophagy – a process by which tissues and cells recycle their molecular materials. A study found that senescent cells seem to upregulate autophagy, but then also upregulate mTOR to survive the upregulated autophagy. It may be that inhibiting mTOR and enhancing autophagy (both accomplished by rapamycin) might facilitate greater senolysis by making senescent cells more susceptible to cell death – they might experience elevated autophagy and will fail to survive it when pro-survival mTOR is inhibited by rapamycin. So, we’ll be looking for this interaction between rapamycin and senolytics and should be evident by a greater reduction in senescent cell prevalence in the mice administered both a senolytic and rapamycin, relative to mice administered a senolytic alone.