IF a bispecific antibody-PROTAC conjugate (herein designated uPAR-AbTAC-BCL-xL) — comprising (1) an anti-uPAR scFv or nanobody targeting module, (2) a cleavable lysosomal-labile linker, and (3) a CRBN-recruiting BCL-xL PROTAC warhead derived from the ABT-263 scaffold — is administered systemically (intravenous, 10–30 mg/kg, bi-weekly) to aged (22–24 month) male and female C57BL/6J mice with confirmed elevated senescent cell burden,

THEN a ≥50% reduction in p16^INK4a-positive senescent cell burden across liver, lung, adipose, and kidney tissue will be observed within 8 weeks, with preservation of platelet counts within ≤15% of vehicle-control baseline and no significant thrombocytopenia (platelet counts >800 × 10³/µL), alongside improvements in physical function (grip strength, rotarod), plasma SASP cytokine reduction (IL-6, MMP-3, GDF-15 ≥30% reduction), and tissue-level BCL-xL protein depletion restricted to uPAR⁺ cells as measured by co-immunofluorescence (p16/uPAR/BCL-xL triple staining),

BECAUSE the following mechanistic chain is operative:

1. Senescent cells, particularly p16^INK4a-high cells, selectively upregulate urokinase plasminogen activator receptor (uPAR) on their plasma membrane as part of the senescence-associated surface proteome remodeling, generating a cell-type-restricted targetable epitope absent from platelets, muscle stem cells, and most quiescent non-senescent tissues — making uPAR a senescence-specific delivery handle [SPECULATIVE for complete specificity; supported by surface proteomics literature cited in research context describing uPAR/DPP4/B2M/CD36 as senescent surface markers].

2. BCL-xL is the dominant anti-apoptotic protein sustaining senescent cell survival across diverse tissue types, and converting the BCL-xL inhibitor ABT-263 (navitoclax) into a CRBN-recruiting PROTAC retains or improves senolytic efficacy in multiple senescent cell types while substantially reducing platelet toxicity relative to the parental inhibitor, establishing the CRBN-BCL-xL PROTAC warhead as a validated senolytic degrader component (Using proteolysis-targeting chimera technology to reduce navitoclax platelet toxicity)[https://doi.org/10.1038/s41467-020-15838-0].

3. The antibody-PROTAC conjugate (AbTAC) architecture — analogous to antibody-drug conjugate (ADC) design — restricts intracellular PROTAC release to cells that internalize the targeting antibody via receptor-mediated endocytosis; upon uPAR-mediated internalization in senescent cells, the lysosomal-labile linker (e.g., cathepsin B-cleavable valine-citrulline dipeptide, borrowed from clinical ADC chemistry) releases the active CRBN-BCL-xL PROTAC intracellularly [SPECULATIVE: linker cleavage efficiency in senescent cells vs. cancer cells requires validation; cleavage kinetics in senescent lysosomal compartments, which show lysosomal stress, are not yet characterized].

4. Liberated CRBN-BCL-xL PROTAC forms a ternary complex (BCL-xL:PROTAC:CRBN/DDB1/CUL4A), ubiquitinates B...

SENS category: RepleniSENS

Key references: • doi.org/10.1038/s41467-020-15838-0]. • doi.org/10.1038/s41467-020-15838-0] • doi.org/10.1038/s41467-020-15838-0