IF aged male C57BL/6 mice (22–24 months) with bleomycin-induced pulmonary fibrosis receive a sequential two-hit SA-β-gal-targeted senolytic regimen — SSK1 administered first (intraperitoneal, based on established dosing in the bleomycin model) followed 72 hours later by Nav-Gal (intraperitoneal, based on GLB1-dependent activation pharmacology) — rather than either agent as monotherapy or DT2216 (BCL-xL PROTAC) alone,

THEN pulmonary senescent cell burden will be reduced by ≥60% (vs. ≤40% for either monotherapy), lung fibrosis scores (Ashcroft scale, hydroxyproline content) will decrease significantly beyond monotherapy levels, and lung compliance will improve, without additive thrombocytopenia or off-target tissue toxicity compared to either monotherapy,

BECAUSE the two agents exploit orthogonal but sequentially synergistic apoptotic mechanisms within the same SA-β-gal-positive senescent cell population in the lung, a concept imported from oncologic "BH3 priming" that has never been applied to senolytics:

1. Senescent pulmonary cells accumulate high SA-β-gal (GLB1) activity, enabling both SSK1 and Nav-Gal to activate preferentially within the lysosomal compartment of senescent — but not normal — lung cells. (SSK1 clears senescent cells in bleomycin lung injury with RNA-seq-confirmed downregulation of senescence gene sets and fibrosis-associated genes including Col1a1)[https://doi.org/10.1038/s41422-020-0314-9]

2. SSK1 releases gemcitabine inside SA-β-gal+ cells, causing replication-independent DNA damage and stalled ribonucleotide reductase activity. This engages the intrinsic DNA damage response (DDR), which in post-mitotic or arrested senescent cells upregulates BH3-only proteins — particularly PUMA and NOXA — that bind and occupy anti-apoptotic BCL-2 family members (BCL-xL, BCL-2, BCL-W), a state termed high mitochondrial apoptotic priming. (SSK1 reduces senescent cell burden in vivo in aged mice and bleomycin lung injury; physical function improvements confirmed)[https://doi.org/10.1038/s41422-020-0314-9] [SPECULATIVE: direct BH3 priming by SSK1-released gemcitabine in senescent lung cells has not been directly measured — inferred from cancer cell biology of nucleoside analog DDR signaling]

3. 72 hours post-SSK1, the surviving primed-but-not-yet-apoptotic SA-β-gal+ cells are now mitochondrially poised at a reduced apoptotic threshold. Nav-Gal is then administered and undergoes GLB1-dependent cleavage in the same lysosomal compartment of senescent cells, releasing navitoclax (BCL-2/BCL-xL inhibitor). (Nav-Gal is a GLB1-dependent senolytic prodrug with broad activity; pharmacologically inactive in non-senescent cells due to galactose caging)[https://doi.org/10.1111/acel.13142]

4. Navitoclax released by Nav-Gal displaces the BH3-only proteins (PUMA, NOXA) that SSK1-driven DDR loaded onto BCL-xL/BCL-2, precipitating cytochrome c release and caspase-dependent apoptosis in cells that would have survived navitoclax m...

SENS category: LysoSENS

Key references: • doi.org/10.1038/s41422-020-0314-9] • doi.org/10.1111/acel.13142] • doi.org/10.1111/acel.13133] • doi.org/10.1016/j.ebiom.2017.04.013] • doi.org/10.1038/s41422-020-0314-9];