Senolytic Therapy: Mechanisms, Targets, and Emerging Perspectives

Cells that enter a state of senescence no longer divide but remain metabolically active, releasing inflammatory molecules that contribute to aging and disease. While this process prevents cancer by halting the proliferation of damaged cells, its accumulation over time leads to tissue dysfunction and age-related conditions. Scientists are developing senolytic therapies to selectively eliminate these harmful cells, aiming to improve healthspan and potentially extend lifespan.

Research into senolytics has identified key mechanisms of cellular senescence, molecular targets for intervention, and various classes of compounds capable of clearing senescent cells. Understanding these aspects could lead to new therapeutic strategies for aging-related diseases.

Mechanisms Of Senescence

Cellular senescence occurs when stressors trigger irreversible growth arrest. Leonard Hayflick first described this phenomenon in the 1960s, observing that human fibroblasts divide a finite number of times before ceasing proliferation. Research has since identified multiple pathways driving senescence, including DNA damage, telomere attrition, and oncogene activation. Tumor suppressor networks, primarily the p53/p21 and p16INK4a/Rb pathways, enforce cell cycle arrest to prevent the spread of damaged or potentially cancerous cells.

Telomere shortening is a primary driver of senescence, occurring as cells divide. Telomeres, protective chromosome caps, erode with each replication cycle due to the end-replication problem. Critically short telomeres trigger the DNA damage response (DDR), stabilizing p53, which induces p21 expression to halt cell cycle progression. Persistent DNA damage from oxidative stress, ionizing radiation, or genotoxic agents also activates DDR, reinforcing the senescent state even in cells with intact telomeres.

Oncogene-induced senescence (OIS) represents another major pathway. Oncogenes like RAS, RAF, or MYC, when hyperactivated, cause excessive mitogenic signaling, leading to replication stress and DNA damage. This paradoxically triggers senescence as a safeguard against uncontrolled proliferation. Unlike replicative senescence, which occurs gradually, OIS can be triggered rapidly in response to oncogenic mutations, acting as an early tumorigenesis barrier.

Senescent cells undergo significant gene expression and morphological changes. They often appear enlarged and flattened, exhibit increased lysosomal activity, and display altered chromatin organization, including senescence-associated heterochromatin foci (SAHF). These chromatin changes silence proliferation-associated genes, ensuring permanent growth arrest. Additionally, senescent cells develop a distinct secretory profile known as the senescence-associated secretory phenotype (SASP), which includes pro-inflammatory cytokines, chemokines, growth factors, and matrix metalloproteinases. While SASP can reinforce senescence through autocrine and paracrine signaling, its accumulation contributes to chronic inflammation and tissue dysfunction.

Targets For Senolytic Compounds

Senescent cells resist programmed cell death by upregulating survival signals. Unlike apoptotic cells, they persist by activating pathways that protect them from intrinsic and extrinsic stressors. Senolytic compounds disrupt these survival mechanisms, sensitizing senescent cells to apoptosis.

One well-characterized survival network in senescent cells is the BCL-2 family of anti-apoptotic proteins, including BCL-2, BCL-xL, and BCL-w. These proteins prevent apoptosis by sequestering pro-apoptotic factors like BAX and BAK. Senescent cells rely heavily on BCL-2 family proteins, making them vulnerable to BCL-2 inhibitors like navitoclax (ABT-263), which disrupts these interactions, restoring apoptotic signaling and triggering cell death. However, navitoclax also affects non-senescent cells, particularly hematopoietic stem cells and platelets, leading to dose-limiting toxicities such as thrombocytopenia.

Beyond apoptotic resistance, senescent cells depend on dysregulated PI3K/AKT and mTOR signaling to maintain metabolic activity and resist cellular stress. The PI3K/AKT pathway promotes survival by inhibiting pro-apoptotic proteins while enhancing glucose uptake and anabolic processes. Similarly, mTOR signaling supports protein synthesis and mitochondrial function. Inhibiting these pathways with compounds like dasatinib has shown senolytic potential, particularly in senescent fibroblasts and endothelial cells.

Another key target is HSP90, a molecular chaperone stabilizing proteins involved in survival and stress response. HSP90 inhibitors such as 17-AAG selectively induce apoptosis in senescent cells by destabilizing survival proteins like AKT and CDK4. Senescent cells experience heightened proteotoxic stress due to accumulated misfolded proteins, relying on chaperones for proteostasis. Disrupting this balance forces a proteotoxic crisis, leading to senescent cell death.

Classes Of Senolytic Agents

Senolytic compounds eliminate senescent cells by targeting their survival mechanisms while sparing normal cells. These agents fall into distinct categories based on their molecular targets and mechanisms of action.

BCL-2 Inhibitors

Senescent cells depend on BCL-2 family proteins to evade apoptosis. Navitoclax (ABT-263) disrupts interactions between BCL-2, BCL-xL, and BCL-w with pro-apoptotic proteins like BAX and BAK, triggering mitochondrial outer membrane permeabilization and caspase activation. Preclinical studies show navitoclax clears senescent cells in aged mice, improving tissue function and reducing inflammation. However, its clinical use is limited by off-target effects, particularly thrombocytopenia due to BCL-xL inhibition in platelets. Efforts to develop more selective BCL-2 inhibitors, such as venetoclax, aim to mitigate these adverse effects while preserving senolytic efficacy. Combination strategies using lower doses or alternative senolytics are also being explored to enhance outcomes while minimizing toxicity.

Senescence-Associated Secretory Phenotype Blockers

The SASP includes pro-inflammatory cytokines, chemokines, growth factors, and matrix metalloproteinases that reinforce senescence and contribute to chronic inflammation. Targeting SASP components can promote senescent cell clearance by disrupting their pro-survival signaling. JAK inhibitors, such as ruxolitinib, suppress SASP-related cytokine production by blocking JAK/STAT signaling, reducing inflammation and improving tissue homeostasis. Inhibiting NF-κB, a transcription factor regulating SASP gene expression, also shows promise. Compounds like metformin and resveratrol attenuate NF-κB activity, dampening SASP-driven inflammation. While SASP blockers do not directly induce apoptosis, they create a less permissive environment for senescent cell survival, potentially enhancing the effects of other senolytic agents.

Natural Extract-Derived Agents

Several naturally occurring compounds exhibit senolytic properties by targeting senescent cell survival pathways. Quercetin, a flavonoid found in fruits and vegetables, inhibits PI3K/AKT signaling, impairing the metabolic adaptations that sustain senescent cells. Fisetin, another flavonoid, reduces senescent cell burden and extends healthspan in preclinical models. These compounds offer a promising alternative to synthetic senolytics due to their lower toxicity and potential for long-term use. Curcumin, derived from turmeric, modulates oxidative stress and SASP-related inflammation, though its bioavailability remains a challenge. Research is ongoing to optimize the delivery and efficacy of these natural compounds through structural modifications or combination therapies.

Neural Cells And Senolytic Strategies

Senescent cells in neural tissue present unique challenges due to the brain’s complexity and limited regenerative capacity. Unlike other organs, where cellular turnover compensates for lost cells, neurons do not divide and are not traditionally susceptible to senescence. However, glial cells, particularly astrocytes and microglia, can undergo senescence, contributing to neuroinflammation, synaptic dysfunction, and cognitive decline. The accumulation of senescent glial cells has been implicated in neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

Targeting senescent glial cells with senolytic therapies requires caution, as excessive cell removal could disrupt neuroprotective functions. Studies show that agents like dasatinib and fisetin selectively eliminate senescent astrocytes and microglia in preclinical models, reducing neuroinflammation and improving cognitive function. Optimizing delivery methods, such as using blood-brain barrier-permeable compounds or localized administration, may enhance precision in the central nervous system.

Observations From Laboratory Models

Experimental models have provided key insights into the effects of senescent cell clearance on aging and disease. Mouse models demonstrate that eliminating senescent cells improves tissue function, delays age-related pathologies, and extends lifespan. In transgenic mice engineered to selectively eliminate p16^INK4a-positive senescent cells, periodic clearance increased lifespan, reduced age-related diseases, and improved physical function.

Pharmacological approaches further validate these findings. Aged mice treated with dasatinib and quercetin show enhanced cardiovascular function, reduced osteoarthritis severity, and improved cognition. Similar benefits are observed in neurodegenerative disease models, where senolytics reduce neuroinflammation and preserve neuronal integrity. However, translating these findings to humans remains challenging due to differences in metabolism, immune response, and tissue architecture. Continued research in diverse preclinical models, including non-human primates, is necessary to assess the long-term feasibility of senolytic interventions.