Senescent cells stop dividing but don't die off the way healthy cells eventually do. Instead they accumulate in tissue over time and secrete a mix of inflammatory signals, collectively called the senescence-associated secretory phenotype, that damage nearby healthy cells and are linked to a wide range of age-related conditions. Senolytics are compounds designed to selectively trigger the death of these senescent cells while leaving healthy cells alone.
The research foundation here, built substantially on work by Judith Campisi and James Kirkland, is older and more clinically tested than reprogramming: early human trials of senolytic combinations (most notably dasatinib plus quercetin) have shown an acceptable safety profile and measurable effects on some inflammatory biomarkers. What hasn't yet been established at scale is durable clinical benefit on hard outcomes like disease incidence or lifespan in humans, which is why the evidence level here is best described as mixed and still developing rather than settled.
Unity Biotechnology is the biotech most identified with pushing senolytics into human trials specifically. For the fuller mechanism, evidence review, and current research status, see the companion article on senolytics.