Another Example of a Galactose-Conjugated Senolytic Prodrug

Killing cells is easy. Killing only the cells that you want to kill, while leaving all other cells untouched, is very much more challenging. The ability to do this is fundamental to much of the future of medicine, however. The aging body contains many cell populations that cause significant harm and should be removed, including misconfigured T cells, age-associated B cells, precancerous cells, and of course senescent cells of many different types. Great benefits to health and longevity might be obtained via efficient means of targeting that enable therapies to only destroy unwanted, harmful cells.

This point is well illustrated by present efforts to selectively destroy senescent cells. Today's open access paper is one of a number of recent publications that focus on using galactose conjugation to produce prodrugs that are highly selective to senescent cells. Senescent cells produce a lot of β-galactosidase, a protein that acts to strip galactose from other molecules. It is thus possible to combine any one of a range of toxic cell-killing compounds with galactose to produce molecules that are entirely innocuous until they encounter β-galactosidase, making the therapy very specific to senescent cells.

Researchers have tried this approach with the overly toxic senolytic drug navitoclax, with some success, but one really doesn't have to be clever about the drug used. In principle any of the cytotoxic compounds employed widely in the cancer research community will work. Thus other groups have used duocarmycins, while the researchers noted here instead chose gemcitabine, and a long list of alternative options exist beyond these.

Elimination of senescent cells by β-galactosidase-targeted prodrug attenuates inflammation and restores physical function in aged mice

Previous studies have shown that compounds termed 'senolytics' could kill senescent cells. Reported senolytics target anti-apoptotic pathways, which are up-regulated to inhibit apoptosis in senescent cells. These senolytics have been reported to eliminate certain types of senescent cells and have shown the potential to improve physiological function in several tissues. However, senolytic drugs have significant limitations in killing senescent cells in terms of specificity and broad-spectrum activity because of the dynamic and highly heterogeneous nature of the senescence program, which leads to the varying sensitivity of different types of senescent cells to current senolytic drugs. To overcome these challenges, it is highly demanded to develop a new strategy that permits selectively deleting senescent cells in a wide spectrum of cell types or tissues for anti-aging interventions.

To specifically target senescent cells, we focused on one primary characteristic of senescent cells - the increased activity of lysosomal β-galactosidase, exploited as senescence-associated β-galactosidase (SA-β-gal). Notably, SA-β-gal in diverse types of senescent cells is one widely used marker for identifying senescence in vitro and in vivo, which is linked to the increased content of lysosomes. Therefore, we hypothesized that lysosomal β-gal could be utilized for the design of a galactose-modified prodrug to target senescent cells in a broader spectrum. This prodrug could be processed into a cytotoxic compound by β-gal and subsequently delete senescent cells in a specific manner, a strategy that could overcome the limitations of current senolytic drugs.

Here, we designed a new prodrug, SSK1, that was specifically cleaved by lysosomal β-gal into cytotoxic gemcitabine and induced apoptosis in senescent cells. This prodrug eliminated both mouse and human senescent cells independent of the senescence inducers and cell types. In aged mice, our compound reduced SA-β-gal-positive senescent cells in different tissues, decreased senescence- and age-associated gene signatures, attenuated low-grade chronic inflammation, and improved physical function.

While SA-β-gal is widely used as a marker of cellular senescence, its elevated activity can be found in some other cells such as activated macrophages. These SA-β-gal-positive macrophages can be harmful and have been found to accumulate in injured and aged tissues contributing to chronic inflammation. Importantly, we have shown that SSK1 decreases the number of SA-β-gal-positive macrophages in injured lungs and aged livers, which is consistent with our observation of reduced secretion of chronic inflammation-related cytokines. Therefore, eliminating macrophage accumulation by SSK1 might reduce chronic inflammation and benefit aged organisms.

Comments

There article doesn't elaborate how much improvement was witnessed in the mice

Posted by: Cuberat at May 6th, 2020 9:08 PM

@Cuberat

It says: "Since SSK1 reduced chronic inflammation locally and systematically, we further tested the functional improvement in SSK1-treated aged mice. To this end, we evaluated their physical performance by a series of functional assays which were all declined in aged mice (Fig. 6a-e). Compared with vehicle treatment, SSK1 treatment for 8 weeks significantly increased the maximal rotarod time (Fig. 6a), treadmill distance (Fig. 6b), grip strength (Fig. 6c), and rearing exploration time (Fig. 6d) as well as decreased the balance beam crossing time (Fig. 6e) in aged female mice, without a significant effect on body weight (Supplementary information, Fig. S8a). [... ] The same results of SSK1 in rotarod, treadmill, and grip strength performance were also observed for aged male mice (Supplementary information, Fig. S8b-d). These results of mouse functional assays demonstrated an improvement in motor function, balance ability, exercise endurance, skeletal muscle capacity and spontaneous exploration. Notably, after SSK1 treatment, the performance of rotarod and beam balance was improved in aged mice compared with the pretreatment state (Supplementary information, Fig. S8e, f)."

Later: "Additionally, we further compared SSK1 with other three reported senolytic drugs (ABT263, dasatinib plus quercetin, and fisetin) in their ability of improving physiological functions in aged mice.11,12,15 SSK1 improved the physical function in all four tested assays, but the effect of the other senolytic drugs was limited (Supplementary information, Fig. S8g-j). Fisetin increased the performance in two functional assays (grip strength and rearing frequency); ABT263 improved motor coordination and balance of mice in the rotarod test, while showed no obvious effect in other three assays; D + Q only led to a tendency of improvement in one functional assay (treadmill) (Supplementary information, Fig. S8g-j). Collectively, SSK1 could alleviate the dysfunction in all tested functional assays suggesting that SSK1 was more efficacious to improve physical function in aged mice."

The improvements are quantified in the figures.

Posted by: gheme at May 6th, 2020 10:31 PM

Good news I guess - more shots on goal equals more chance of success and better overall removal of senescent cells.

I'm curious if Oisin's and others senescent cell removal method can be compared to this using presently available data? Did Oisin ever release any metrics similar to the ones in this study?

I'm looking forward to seeing if any of these (supposedly more effective) PROTAC small molecule senolytics have a greater lifespan extending effect than the currently tested candidates. Although Oisin's targeted suicide gene therapy didn't extend mouse lifespan (except against the control group) if I recall correctly, so maybe this is unlikely?

Posted by: jimofoz at May 7th, 2020 2:28 AM

@gheme
Thanks. Now I will have assess the individual figures. As @jimofoz said we have more shots. It seems we are getting to generation 2 Senolytics already (wire Oisin is quite different it cold be lumped here too) .

What are the chances of having of having this compound in human b trials in the next couple of years?

Posted by: Cuberat at May 7th, 2020 4:53 AM

@jimofoz
"didn't extend mouse lifespan (except against the control group)" - in these scientific papers they do not use everyday's english terminology. By using therm "extend mouse lifespan" they mean "beat the record of the longest life span ever recorded for a species". Since we use natural or mostly natural substances or known drugs (although artificially combined into new substance like in this study) so far it's may be hard to beat such a record. And, additionally, as sparse so far studies of 'super-centenarians cause of death' reveal, while senescent cells shorten life span, removing them do not beat the longest life ever recorded which should be read as there are other limiting factors (like AGEs, lipofuscin, telomeres, genetic program, random DNA drift, DNA methylation, etc. etc.) that limit life span of a given species, and senolytics do not address this (yet unknown) limiting factor/factors.
But extending the life span you expect by 25%-50%-300% (ie. from 90 into 120, from 80 into 120, or from 40 into 120 years) it's not THAT bad :D

Posted by: SilverSeeker at May 7th, 2020 6:33 AM

I have checked the mouse performance diagrams after treated with SSK. The treated mice have better performance than the old ones and getting close but still worse than middle-aged controls. Since we don't tweak the methabolism here but rather fight chronic inflammation I think this results will translate nicely to humans. So making 70s the new 60s or even 50s becomes realistic, at least as quality of life. I would worry much less about extending the max life span since the average life expectancy in EOCED countries is less than 90, and max life span is 120, so there is quite a room there . Also, it is more important to add better health to the median population than add a few years to one-in-a-million outlier. Also, from political point of view improving health and making people feel younger without actually increasing the lifespan is much safer approach. None of the pesky arguments even have to be brought. You just become healthier and feel better when you are old. This drugs might have nice path to commercialization too with a broad target audience , well-known side effects for the non-conjugated versions, known production and logistics.

Posted by: cuberat at May 7th, 2020 8:16 AM

@Trevor Bingley
Good question. The sooner the better. I am not getting any younger.
I guess there might be some safety test for other anyway models but since the underlying molecule is already approved going to human trials might be faster. You need some clinical indication as a target for the treatment. I ask not sure the cancer cells exhibit elevated galactosidase but if they do it is a nice target to improve the existing structure and reduce of target effects

Posted by: Cuberat at May 10th, 2020 5:55 PM

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