Senescent cells are under constant pro-apoptotic pressure from accumulated DNA damage and oncogenic stress. To survive this pressure, they massively upregulate BCL-2 family anti-apoptotic proteins (BCL-2, BCL-XL, BCL-W). Senolytics inhibit these survival proteins, tipping the balance toward apoptosis specifically in cells that depend on them — creating selectivity for senescent over healthy cells.

Why don't senolytics kill healthy cells?

Healthy cells are not under high pro-apoptotic pressure (they don't have persistent DNA damage signals driving BAX/BAK activation). BCL-2 inhibition in healthy cells is insufficient to trigger apoptosis. Only cells under intense pro-apoptotic pressure (senescent cells) are vulnerable to BCL-2 family inhibition.

Do cancer cells also express high BCL-2?

Yes — this is why senolytic-like drugs (navitoclax, venetoclax) are used in cancer treatment. BCL-2 family overexpression helps cancer cells resist apoptosis. Senolytic doses in healthy aging individuals are lower than cancer treatment doses, targeting the senescent cell population rather than cancer cells.