Against Senolytics

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There is no consensus in science that is so strong as to have no heretics. So here we have an interview with a naysayer on the matter of senolytic treatments, who argues that the loss of senescent cells in aged tissues will cause more harm to long-term health than the damage they will do by remaining. To be clear, I think this to be a ridiculous argument given the present evidence. To make it one has to declare the existing results showing extension of healthy life span in mice to be something other than credible data, which just isn’t the case. Further, it seems shaky on theoretical grounds to suggest that removal of something like 1% of cells will put onerous stress on the remaining 99%, particularly given that the 1% were contributing to declining stem cell activity via inflammatory signaling. All told, it is hard to take seriously the idea that loss of senescent cells can possibly produce greater degrees of dysfunction in tissue than is caused by the inflammatory signaling of senescent cells.

Your new review on senolytics suggests that senolytics may cause more harm than good. Can you summarize your objections and concerns?

Here is the argument: 1) theoretically, senolytics should make things worse and 2) the available data support this theoretical concern. To use an analogy, imagine that you have a factory in which 10 of the 100 factory workers are feeling overworked and tired. Furthermore, their complaints are disrupting the other workers. You have two possible interventions. You can: (a) Fire the 10 workers, thereby removing the complainers. The result is that the remaining 90 workers are now overworked, and they, too, begin to complain. You end up with 30 workers who are now complaining and disrupting your factory. This is the senolytic approach. (b) Improve the health and conditions of the 10 workers who are overworked and complaining. You now have 100 workers who are doing an excellent job. This is the telomerase therapy approach.

In the first case, your factory has a problem and you make it worse. In the second case, your factory has a problem and you solve the problem. This figure from my new paper illustrates the same point in terms of nine cells subjected to senolytics, with the result being temporary short-term improvement followed by decline and a worse situation than we started with.

This does not take into account the idea of replacing that pool of “workers” by bringing in fresh stem cells.

You have to keep a few points in mind. 1) Will the stem cells populate as desired? 2) If you do get a stem cell population, that requires cell division, which shortens telomeres, which accelerates cell senescence, and once again you have accelerated pathology. 3) Why would you bother recruiting stem cells when you can much more easily reset cell senescence in the resident cells of the tissue? 4) The long-term data (what there is of it) supports the failure of senolytics. Again: remember where those “new cells” come from: you are accelerating senescence in the stem cell pool. The only way to “replace them with healthy working cells” is to simply and effectively reset gene expression, taking senescing cells and turning them into functionally young cells.

It seems that we can only speculate on these issues, as these long-term follow-ups have not yet been done. However, senolytics have been shown to increase median lifespan and healthspan in murine models.

I don’t see any credible data that supports the contention that “senolytics have been shown to increase median lifespan and healthspan in murine models”.


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