Of the most deadly diseases in the developed world, Alzheimer’s disease (AD) and related neurodegenerative diseases stand out for the deplorable lack of progress in their treatment over the last several decades. The general failure of anti-amyloid medications to improve patient outcomes and slow disease progression has led researchers to explore other potential therapeutic avenues. Among the most recent examples is the use of senolytics – drugs which eliminate damaged, senescent cells – in order to halt the spread of neurodegenerative disease and improve cognitive trajectories. With the recent publication of the first clinical trial of senolytics for AD treatment, popular interest in these drugs has swelled, but is the excitement justified?
What are senolytics?
Cellular senescence, one of the hallmarks of cellular aging, is the irreversible cessation of cell division and is driven by a complex set of stimuli related to cellular stress, damage, and dysfunction. The arrest of cell division prevents damage from passing to the next generation of cells, but the damaged, senescent cells themselves are able to persist through the activation of anti-apoptotic (i.e., pro-survival) pathways. These cells undergo changes in gene expression and metabolism and can exhibit a so-called “senescence-associated secretory phenotype” (SASP) characterized by increased secretion of pro-inflammatory molecules and factors which can damage nearby cells, potentially leading to the propagation of senescence.
The accumulation of these damaged cells – and the increase in inflammation that accompanies them – has been implicated in the development and progression of myriad age-related conditions and diseases (including AD), and thus, pharmacological strategies for eliminating senescent cells have received growing interest in recent years. Of these “senolytic” drugs, currently the best-characterized are dasatinib (D) – an FDA-approved medication for treating certain types of leukemia – and quercetin (Q), a flavonoid compound naturally found in many fruits and vegetables. These drugs have been shown to be effective in clearing senescent cells in animal models while leaving non-senescent cells intact, and the two are typically used in combination in order to target a broader range of senescent cell types than either drug alone. Moreover, preclinical work has demonstrated that D+Q can be administered intermittently without a loss of efficacy, as the process of accumulating senescent cells requires weeks to months.
A Pilot Study of Senolytics for AD Treatment
In mouse models of AD, D+Q has been reported to improve cognitive deficits, lessen neuroinflammation, and reduce amyloid plaque load, and human studies of D+Q for other indications have demonstrated favorable safety profiles. In light of these exciting findings, investigators Gonzales et al. conducted a small, phase 1 pilot study in humans on the use of oral D+Q senolytic therapy for the treatment of mild cognitive impairment (MCI) or early-stage AD. The primary goal of the study was to evaluate the ability of D+Q to enter the central nervous system (CNS), as assessed by measurement of each compound in cerebrospinal fluid (CSF), though additional endpoints included metrics of cognition, AD biomarkers, inflammation, and safety. (Phase I studies are primarily about safety and about confirming methodology – in this case, the manner by which the investigators determine if the drugs reach the CNS. They are typically not powered to determine a meaningful clinical outcome.)
For this single-site, proof-of-concept study, Gonzales et al. enrolled five (that’s right, five) participants ages ≥65 with a diagnosis of amnestic MCI or early AD. Participants completed a first baseline visit consisting of a fasting blood draw and lumbar puncture (baseline 1), followed by a second baseline visit consisting of evaluations of cognition and functional status using a variety of established, standardized assessments, as well as an optional brain MRI (baseline 2). D+Q (100 mg D, 1,000 mg Q) was then given once every other week for a total of six doses, with the first dose administered 3-10 days after baseline 2. The drugs were given open-label – meaning that participants were informed of their treatment – and ~80-150 minutes after the final dose, baseline 1 tests were repeated, followed by a repeat of baseline 2 tests 3-10 days later.
What did the study find?
At baseline, no D was measured in plasma or CSF in any participants, whereas Q was present at trace levels in plasma in two of the five participants – likely due to the natural presence of Q in many plant-based foods and beverages. As expected, treatment resulted in higher levels of D and Q levels in plasma, with both compounds detected in all five participants in concentrations ranging from 12.7 to 73.5 ng/ml (D) and 3.29 to 26.30 ng/ml (Q). Post-treatment concentrations in the CSF, however, indicated little penetration of the drugs into the CNS. D was detected in the CSF of only four of the five participants and at levels ranging from 0.281 to 0.536 ng/ml – only slightly above the limit of quantitation (LOQ = 0.2 ng/ml). Meanwhile, no Q was detected in CSF across any participants, though Q metabolites were observed in a single participant at a level just above the LOQ. D metabolites were undetectable in CSF across all samples.
Given the evidently limited entry of D+Q into the CNS, it should come as no surprise that metrics of cognition, brain/hippocampal volume, and AD biomarkers were largely unchanged by the course of treatment. Of these readouts, only one – Hopkins Verbal Learning Test-Revised (HVLT-R) Immediate Recall – was found to be significantly different between baseline and post-treatment, and the direction of the change was negative, meaning that participants declined in this cognitive parameter over the study duration. Likewise, though differences in AD biomarkers did not achieve significance, the authors noted a trend (t(4) = 2.34, P=0.0795, 95% CI: −170.6-14.6) toward higher amyloid (specifically, Aβ42) post-treatment.
A Failed “Proof-of-Concept”?
The primary aim of this study was to show that D+Q reached the CNS, yet the results show little to no D or Q in CSF. While it’s technically possible that this observation might be due to rapid degradation of these compounds, the absence of even their metabolites across nearly all samples argues against this explanation, as does the general absence of any cognitive, functional, or other neurological changes.
Importantly, even if cognitive effects had been observed, we wouldn’t be able to discern “real” treatment effects without a control group, as within-group changes (i.e., from baseline to post-treatment) could be caused by any number of other factors, including the presence of the disease of interest itself. Indeed, the statistically significant negative finding in HVLT-R Immediate Recall was probably unrelated to the D+Q treatment and instead caused by the progression of preexisting MCI/AD. Although this would mean that D+Q does not worsen cognitive trajectories, it also unfortunately suggests that this therapy is ineffective in halting the progression of neurodegenerative disease – though again, in the absence of a placebo control group, we cannot completely rule out the possibility that cognitive decline with D+Q occurred more slowly (or for that matter, more quickly) than it would have without the treatment.
Yet insofar as this trial was meant to be a “proof-of-concept” for using senolytics for AD treatment, these results are strongly discouraging. The “concept” in question relies heavily on the penetration of D+Q into the CNS, and we now have reason to believe this effectively does not occur. But the apparent lack of efficacy as an AD treatment does not imply that senolytics have no benefits for human health more generally.
Other Benefits of Senolytics Are Still Possible
In addition to neurological metrics, the researchers also collected data on circulating markers of inflammation. They report that plasma levels of numerous inflammatory markers (specifically, IL-17E, IL-21, IL-23, IL-17A/F, IL-17D, IL-10, VEGF, IL-31, MCP-2, MIP-1β, and MIP-1α) were significantly lower in post-treatment tests than at baseline, reflecting a general decrease in systemic inflammation.
It should be noted that these changes were only statistically significant when using a significance threshold that had not been corrected for multiple comparisons, as would have been appropriate in this circumstance. Yet given the high consistency in the directionality of the effect (i.e., toward reduced inflammation), it’s possible that these results do indeed represent a true effect that might reach more robust significance standards if studied in a larger cohort. After all, this pilot included only five subjects. Such an effect would make biological sense in light of the known pro-inflammatory nature of the SASP, and it could conceivably have several downstream benefits in reducing the risk of chronic diseases for which inflammation plays an important role, such as atherosclerosis and type 2 diabetes. However, further research would be necessary to determine how the magnitude of such effects might compare to existing anti-inflammatory therapies.
Given the lack of progress in treating AD despite decades’ worth of research, any novel approach to therapy is sure to inspire a degree of excitement, and I commend Gonzales et al. for their strides toward exploring and validating novel therapeutic options. But unfortunately, venturing into new territory comes with a risk of failure, and in our desperation to find effective AD treatments, we must nevertheless remain rational in our evaluation of data. This study indicates that senolytic therapy with D+Q may not be the most promising avenue for future AD research if we believe that CNS penetration is necessary, but the door remains open for other senolytics with greater ability to enter the CNS. Likewise, these data offer a degree of hope for the use of D+Q for peripheral targets, pending controlled investigations in larger cohorts. So although the results of this trial may darken the outlook for D+Q as an AD treatment, the endeavor was hardly in vain, as it informs strategic decisions for future, more fruitful research.
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