R&R#5: Novel and Lesser Known Senolytic Strategies

Scientists and entrepreneurs are trying many different strategies to kill senescent cells. What is the golden senolytic(s)? Here are some potentially powerful strategies from academia.

Most people who are interested in the field of aging know of senescent cells, the dysfunctional “zombie” cells that hang around too long after cellular damage (aging or otherwise). They are known to have a beneficial role in would healing but have been implicated in the majority of age-related diseases and are generally seen as detrimental with age. They are established hallmarks of aging and a SENS hallmark.

Senescent cell types and tissue level disease, Borghesan et al 2020

They damage tissue through their secretions, known as the Senescence Associated Secretory Phenotype (SASP). These include:

·        Inflammatory cytokines and chemokines, directly contributing to inflammation

·        Pro angiogenesis (blood vessel growth) factors, notably VEGF

·        Pro-fibrotic factors, most notably TGF-beta. Fibrosis, while necessary in wound healing, is a common aspect of aging diseases

·        Matrix metalloproteinases which break down the integrity of the ECM and cellular barriers

SASP secreted molecules, Zorina et al 2022

Most efforts to develop anti-senescence therapies for aging diseases have focused on senescent cell removal, a class of compounds known as senolytics. Genetically induced senescent cell clearance in mice has been shown to extend lifespan as well as to improve aging phenotypes in several tissues, including kidneys, heart, fat cells, skeletal muscle, bone marrow, lens (cataract), spine (lordokyphosis), and brain. Thus, the promise of eliminating senescent cells is extremely high, but it has proven much harder to develop targeted senolytic therapeutics. The struggle is related to sensitivity vs specificity – how do we make a treatment that destroys the majority of existing senescent cells that also kills little or no healthy non-senescent cells? Senolytic small-molecule compounds like dasitinib/quercetin have been known for a while, but are far from the ideal senolytic compound which would have near perfect selectivity and sensitivity.

An in-depth review of companies focusing on developing senolytics  and their assets is available on the Longevity Marketcap Newsletter. However, today I’d like to briefly highlight a few interesting technologies published over the last few years but not yet being developed commercially (to my knowledge) which I find very exciting.

The use of immune cell therapies to target senescent cells is very interesting as senescent cells are naturally removed by a functional immune system. The most exciting development in this regard in one study that showed that senolytic CAR-T cells are able to effectively remove senescent cells both in vivo and in vitro. CAR-T cells, or Chimeric Antigen Receptor T-cells, are specialized immune cells genetically engineered to target a specific cell surface antigen, in this case a protein called uPAR which is primarily found on senescent cells. The treatment was examined in mouse models of both diet induced and chemical induced liver fibrosis, effectively penetrating tissue and showing both a profound reduction in senescent cells as well as an improvement in disease specific endpoints such as liver enzyme levels. They also improved survival outcomes of mice with lung adenocarcinomas treated concurrently with more traditional lung cancer therapies (MEK and CDK4/6 inhibitors). They showed strong selectivity results as well. I think this idea is exciting as CAR-T cells (and naturally occurring T-cells for that matter) are designed to do exactly what is needed in the case of senolytic development – to selectively and exhaustively kill specific types of cells. The work was also done by scientists at MSKCC, where my PhD lab was located.

Natural Killers (NK) cells and macrophages are naturally involved in clearing out senescent cells, among other maintenance tasks. They are less targeted by nature than T-cells as they lack T-cell receptors, which enable bespoke antigen targeting, though they have other methods to selectively target damaged cells rather than healthy cells. One study showed a reduction in senescent cell levels in 18-month old mice with the addition of adoptive NK cells alongside acein, which stimulated dopamine release and functioned synergistically. Though the reduction of senescent cell markers was not absolute, significant improvement was also observed in inflammation biomarkers as well as liver disease endpoints.

Another strategy may be to block existing signaling by senescent cells which prevents clearance via NK cells and T-cells. The interaction between HLA-E and NKG2A was shown to mediate this immune evading ability of senescent cells, and inhibiting this interaction increased immune response. Inhibition of this or similar pathways is a promising strategy to me for two reasons. The first is that it is similar as a general idea to cancer immunotherapies like PD-1/PD-L1 inhibitors which also prevent immune evadence (some of the most successful cancer drugs to emerge in the past decade such as pembrolizumab and nivolumab fall into this category). The other is that it can in theory function synergistically with some of the cell therapies listed previously to enhance cellular toxicity.

Another interesting situation is when we have cell types in which selectivity matters less given that populations can quickly replenish. An interesting example of this case is microglia, the brains immune cells which (uniquely among brain cells) are derived and replenished from Hematopoietic Stem Cells (HSCs). Thus microglia are expendable, and even after 99% of microglia are removed from a mouse’s brain it can survive as the cells are replaced within a week. Senescent microglia are a type of Damage Associated Microglia (DAM) which have lately been of huge interest to the neurodegeneration community for their causative role in Alzheimer’s and potentially other disorders. CSF1R inhibitors selectively kill monocyte lineage cells including microglia, and treatment with them eliminates senescent microglia and improves disease phenotypes (1,2).

Thus, while they are not senolytics in a classic sense, they have the potential to eliminate some of the worse senescent cells in the body. They may have off-target effects elsewhere in the body though due to broad impact on other cells in the lineage such as macrophages, and are associated with some side effects overall. However, positive impacts have also been shown elsewhere including a reduction in plaques observed in atherosclerosis models (senescent macrophages are major contributors to atherosclerosis). I personally think CSF1R inhibitors are some of the most exciting areas of research in Alzheimer’s, and if specificity is a concern despite microglial capacity for repopulation, they could also potentially be directed to senescent cells via localized delivery methods targeting senescent surface markers.

This brings me to my last recent point of interest relating to senescent cells, which is that of targeted delivery strategies. One really interesting strategy is that of CD9 antibody coated nanoparticles for drug delivery (CD9 is another marker for senescent cells). The authors used these nanoparticles to deliver senolytic small-molecule drugs specifically to senescent-lesion areas of atherosclerotic plaques in mice, alleviating plaque progression. Ostensibly a similar strategy could also be applied with CSF1R inhibitors. In both case you are using two targeting methods together – a senescent cell targeting carrier combined with either a senescent cell killing drug or a microglia/macrophage killing drug – to enhance overall specificity above that of either component alone. Another previously developed strategy is to apply galacto-oligosaccharide coated drugs given that senescent cells highly express the enzyme lysosomal β‐galactosidase which breaks these carbohydrates down. A review on several strategies for specific delivery to senescent cells via nanocarriers can be found here.

Besides sensitivity and specificity, cost is likely to be a concern and potentially an initial limiting factor for expensive-to-produce treatments such as CAR-T cells. However, if the type of profound rejuvenation available in animals is reproducible in humans, the cost will likely be justified based on the number of Quality Adjusted Life Years (QALYs) gained. This is especially true if the onset of further senescent cell development is pushed off at least several years, as single-time or periodic rejuvenation treatments are more cost-effective by nature than continuous treatments.

Another relevant factor in terms of the cost-effectiveness will be how many tissues can be targeted at once: a therapy which eliminates senescent cells in all tissues is much more effective than one that does so only in the liver or brain, for example, because the latter situation would require multiple therapies to extend benefit to other tissues as well. However, FDA approval is disease indication specific, and thus the most likely scenario is that the first breakthroughs will relate to specific diseases and subsequently be extended into other indications as pharmaceutical companies seek to extend exclusivity lifespan for their assets and find additional sources of revenue.

There are so many senolytic strategies being tested, it is comforting that few doors are being left unturned on this very important question relating to rejuvenation and repair. These are the ones that I find most interesting, but do you have thoughts on other strategies or recent breakthroughs? If so, please leave in the comments!