Researchers at Imperial College London, in collaboration with AstraZeneca, have demonstrated that a next-generation proteolysis-targeting chimera can hunt down and eliminate the zombie-like senescent cells that accumulate in the diseased airways of COPD patients, and, crucially, leave healthy cells entirely unharmed.
The Burden of Ageing Airways
Chronic obstructive pulmonary disease (COPD) sits at the intersection of environmental exposure and progressive cellular dysfunction. Affecting more than 200 million individuals worldwide and responsible for millions of deaths annually, the disease has long been associated with chronic inhalation of cigarette smoke, biomass fuels, and airborne pollutants. These exposures initiate a cascade of inflammation, tissue remodeling, and irreversible airflow limitation. Yet, beyond these established drivers, a deeper biological layer has emerged,one defined by the accumulation of senescent cells within the lung.
Cellular senescence represents a state in which cells permanently exit the cell cycle while remaining metabolically active. These cells do not simply persist passively; they actively secrete a complex mixture of cytokines, proteases, and signaling molecules collectively termed the senescence-associated secretory phenotype (SASP). In COPD, senescent cells accumulate disproportionately within the small airway epithelium and stromal compartments. Epithelial cells lining airways less than 2 mm in diameter, including basal, club, and ciliated cells, exhibit elevated levels of canonical senescence markers such as p21CIP1 and p16INK4a. Similarly, lung fibroblasts display reduced proliferative capacity, increased SA-β-galactosidase activity, and a pro-inflammatory secretory profile.
These cells function less as inert remnants and more as persistent drivers of pathology. Their continued presence sustains inflammation, disrupts tissue repair, and reinforces structural decline. In this sense, senescent cells behave like chronic sources of damage,subtle, persistent, and self-reinforcing. For a disease with limited therapeutic options beyond symptomatic management, this biology presents a compelling opportunity: if senescent cells actively drive disease progression, their selective removal may alter its trajectory .
The Science of Senolytics: Killing the Unkillable
The concept of senolytics agents that selectively eliminate senescent cells has evolved from foundational studies demonstrating that clearance of senescent populations can extend health span and improve tissue function. Central to this strategy is the observation that senescent cells evade apoptosis through upregulation of anti-apoptotic proteins, particularly members of the BCL-2 (B-cell lymphoma 2) family. Among these, BCLXL B-Cell Lymphoma-extra-large) plays a dominant role in COPD-associated senescent cells, enabling their survival despite extensive cellular damage.
Pharmacological targeting of BCLXL has therefore emerged as a logical therapeutic strategy. Navitoclax, a BH3 mimetic that inhibits BCL-2 family proteins, demonstrated early senolytic potential but was limited clinically by thrombocytopenia, a consequence of platelet dependence on BCLXL for survival. This toxicity significantly restricts its therapeutic window and highlights the challenge of targeting broadly expressed survival proteins.
The limitation of conventional inhibitors has driven the development of alternative strategies capable of achieving both potency and selectivity. Among these, PROTAC technology introduces a fundamentally different mechanism, one that does not merely inhibit protein function but eliminates the protein entirely.
PROTACs: Molecular Matchmakers That Redirect the Cell’s Own Trash Disposal
Proteolysis-targeting chimeras (PROTACs) operate by co-opting the cell’s endogenous protein degradation machinery. Structurally, these molecules consist of two functional domains connected by a linker: one binds the target protein, while the other recruits an E3 ubiquitin ligase. This induced proximity results in ubiquitination of the target protein, marking it for degradation by the proteasome.
In the context of COPD, the BCLxL-targeting PROTAC (PZ15227) recruits the E3 ligase Cereblon (CRBN), which is minimally expressed in platelets. This design is critical, as it allows degradation of BCLXL in lung cells while sparing platelet survival pathways, thereby avoiding the dose-limiting toxicity observed with navitoclax. Importantly, PROTACs function catalytically, each molecule can degrade multiple copies of the target protein, allowing for high potency at low concentrations.
Pharmacological characterization of BCLXL -PROTAC reveals exceptional efficiency, achieving near-complete degradation of BCLXL at sub-nanomolar concentrations while exhibiting strong selectivity over related proteins such as BCL-2. Mechanistic validation confirms that degradation is dependent on proteasomal activity and E3 ligase engagement, establishing that the compound operates through its intended pathway .
Eliminating Senescent Cells from COPD Lung Tissue
Clearing the Undead in Small Airway Epithelial Cells
Application of BCLXL-PROTAC to primary small airway epithelial cells derived from COPD patients results in a pronounced reduction in senescence burden. Treatment leads to significant downregulation of BCLXL, accompanied by decreases in p21CIP1 and p16INK4a expression. Functionally, the proportion of SA-β-galactosidase-positive cells is markedly reduced, indicating effective clearance of senescent populations.
This elimination is mediated through apoptosis, as evidenced by robust activation of caspase 3/7 following treatment. The temporal dynamics of caspase activation suggest a coordinated apoptotic response, with peak activity preceding the removal of senescent cells. At the level of secretory function, reductions in specific SASP components, such as PAI-1, further indicate a shift away from the pro-inflammatory phenotype associated with senescence.
The Critical Test: Does It Spare Healthy Cells?
Selectivity remains the defining criterion for any senolytic strategy. Notably, BCLXL -PROTAC degrades BCLXL efficiently in both senescent and non-senescent cells; however, apoptosis is induced exclusively in senescent populations. Healthy epithelial cells show no activation of caspase pathways, no changes in senescence markers, and no disruption of viability.
This differential response reflects a key biological distinction: senescent cells are uniquely dependent on BCLXL for survival, whereas healthy cells retain redundant survival mechanisms. The selective vulnerability of senescent cells therefore enables targeted elimination without collateral damage.
Activity in COPD Small Airway Fibroblasts
Beyond epithelial cells, BCLXL-PROTAC demonstrates senolytic activity in small airway fibroblasts, a cell population implicated in airway remodeling and fibrosis. Treatment reduces senescence markers and decreases SA-β-galactosidase activity, while also lowering secretion of MMP-9, a matrix-degrading enzyme associated with tissue remodeling. These findings suggest that senolytic targeting may influence both inflammatory and fibrotic components of COPD pathology.
Rejuvenation: When Zombies Die, Life Returns
A central premise of senolytic therapy is that removal of senescent cells not only eliminates harmful signaling but also restores the capacity for tissue renewal. Following BCLXL-PROTAC treatment, epithelial and fibroblast populations exhibit increased proliferative activity, as indicated by elevated Ki-67 expression and enhanced growth dynamics.
Morphologically, treated cultures shift from a senescent phenotype,characterized by enlarged, flattened cells with high senescence marker expression,to a more regenerative state composed of smaller, actively dividing cells. These observations support the concept that senescent cells occupy both physical and signaling niches that constrain regeneration, and that their removal enables functional recovery.
Validation in Living Lung Tissue: Precision-Cut Lung Slices
To extend these findings into a more physiologically relevant system, the study employs precision-cut lung slices (PCLS), which preserve the three-dimensional architecture and cellular diversity of lung tissue. In this ex vivo model, BCLXL-PROTAC treatment significantly reduces p21CIP1 expression within the small airway epithelium, the primary site of senescent cell accumulation in COPD.
Importantly, this reduction is spatially restricted, reinforcing the concept that senescence is localized rather than uniformly distributed across lung tissue. The ability to modulate senescence within intact tissue structures provides a critical translational bridge between cell culture and in vivo application .
Conclusion: A New Therapeutic Paradigm for COPD?
This study establishes a coherent framework in which targeted degradation of BCLXL selectively eliminates senescent cells, reduces inflammatory signaling, and promotes regenerative capacity within COPD lung tissue. By combining mechanistic specificity with functional selectivity, BCLXL-PROTAC represents a refined senolytic strategy with clear translational potential.
For a disease defined by progressive decline and limited treatment options, this approach shifts the focus from symptom management to modification of underlying pathology. The path forward will require careful evaluation of safety, efficacy, and disease heterogeneity, but the underlying principle is clear: removing the cellular drivers of dysfunction may unlock the capacity for tissue recovery. In that sense, senolytic PROTACs introduce not just a new therapy, but a fundamentally different way of thinking about chronic lung disease
Reference
Devulder JV, Fenwick PS, Kolosionek E, Al-Sahaf M, Viola P, Lemaire R, Razdan N, Kudo H, Sinadinos A, Odqvist L, Donnelly LE, Barnes PJ. Clearance of Senescent Cells by BCLXL-PROTAC: A Novel Approach to Treat COPD? Aging Cell. 2026 Apr;25(4):e70487. doi: 10.1111/acel.70487.
Image credit: Portions of the figure in this article were generated using ChatGPT (OpenAI) or Google Gemini.
Disclaimer: This blog post is intended solely for educational and scientific informational purposes. Any mention of therapeutic drug names, including FDA-approved medications, is for the purpose of accurate reporting and discussion of biomedical research and does not constitute medical advice, endorsement, or promotion. Readers should not interpret the content as a recommendation for any specific treatment. Always consult a qualified healthcare professional for medical advice or treatment decisions.
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