Regenerative & personalized
Geroscience and senolytics
Senescence as a secretory phenotype, anti-apoptotic dependence as the target, and intermittent dosing as a direct consequence of killing rather than modulating.
Cellular senescence is neither wear nor death. It is a durable cell-cycle arrest that a cell imposes on itself in response to damage: telomere shortening, DNA breaks, oxidative stress or oncogene activation. The programme runs through the cyclin-dependent kinase inhibitors p16^INK4a and p21, and it is protective — a cell with a damaged genome is guaranteed not to become a tumour. The problem is that it does not then leave.
The harm is the secretome, not the presence
An arrested cell stays metabolically active and switches to the senescence-associated secretory phenotype, SASP: interleukins IL-6 and IL-1β, chemokines, matrix metalloproteinases, growth factors. Its regulatory core is NF-κB together with the cytosolic cGAS–STING pathway, which senses fragments of the cell’s own DNA that have escaped the nucleus. SASP is why a relatively small number of senescent cells produces a disproportionate effect: the signal is paracrine, it converts healthy neighbours into the same state, and it sustains the chronic sterile inflammation underlying most age-related disease. Transplanting a modest number of senescent cells into young mice reproduces dysfunction phenotypes — the experiment that makes the relationship causal rather than correlational.
Why a senolytic kills rather than treats
A senescent cell resists its own apoptosis by upregulating anti-apoptotic BCL-2 family proteins, BCL-xL above all, and by leaning on survival signalling nodes. These senescent-cell anti-apoptotic pathways are the target. Dasatinib with quercetin acts on kinase and survival nodes; navitoclax is a direct BCL-2/BCL-xL inhibitor. The defining feature of the class is that the drug does not normalise the cell — it pushes it across a death threshold.
The dosing regimen follows from that pharmacology. Because the effect is a one-off clearance of a population rather than suppression of a process, the drug can be given in short courses with long intervals until the population rebuilds; continuous exposure is unnecessary and harmful. The same logic bounds toxicity: navitoclax causes dose-limiting thrombocytopenia precisely because platelets depend on BCL-xL for survival. Class selectivity is not absolute and is strongly tissue-dependent — senescent fibroblasts, endothelial cells and preadipocytes rely on different combinations of survival pathways, so there is no universal senolytic.
What is unsettled
The field’s weak point is measurement. Senescence is defined by a set of features rather than one marker: p16^INK4a, β-galactosidase activity at pH 6, absence of proliferation, SASP. None is specific on its own, and there is no reliable clinically usable biomarker of senescent-cell burden in a living human — which makes it hard for a trial to show that the drug did the thing it was given to do. Clinical evidence remains limited to small open-label and early randomised studies in idiopathic pulmonary fibrosis, diabetic kidney disease and osteoarthritis; no senolytic is approved for an age-related indication. And there is a substantive counter-argument: senescent cells contribute to wound healing, limiting fibrosis and tumour suppression, so clearing them indiscriminately is not automatically neutral.