If given a way to more effectively target senolytic drugs towards senescent cells, reducing off-target effects, then navitoclax is a good drug to test with. Navitoclax is arguably the worst of the first set of drugs found to be meaningfully senolytic; it certainly compares unfavorably with dasatinib. While navitoclax can kill a usefully large fraction of senescent cells in aged tissues, the dose required for that outcome will also kill a lot of normal cells along the way. More unpleasant drugs have since been discovered to be usefully senolytic, such as cardiac glycosides, but there is much less data on their senolytic use at this stage. In comparison, a good deal of data exists for the ability of navitoclax to destroy senescent cells. Its unpleasant side effects are quantified as a result of its development as a potential chemotherapeutic.
Finding ways to target highly toxic drugs to specific cell populations is a well established line of work in the cancer research community. Even if it were not the fact that many senolytic drugs were initially characterized for use as cancer therapeutics, it would not be surprising to find analogous cell targeting research taking place in the field of senolytics. This is a well understood approach to making toxic drugs more useful.
You may recall that researchers have been exploring the possibility of conjugating toxic drugs with galactose, producing a far less harmful prodrug molecule. Senescent cells are characterized by the high expression of senescence-associated β-galactosidase. This enzyme acts to remove the galactose from the prodrug, restoring the original toxic molecules that then kill the cell. In today's open access paper, an variant on this approach is demonstrated to improve the characteristics of navitoclax as a senolytic drug.
Recent research has identified targetable vulnerabilities of senescent cells that can be exploited by a novel group of drugs called senolytics. These compounds preferentially kill senescent cells by different mechanisms. Senolytics include the BCL-2 family inhibitors Navitoclax (ABT-263) and ABT-737; the flavonoid fisetin; combinations of tyrosine kinase inhibitors and flavonoids (e.g. dasatinib and quercetin); FOXO4-p53 interfering peptides; HSP90 chaperone inhibitors; and other compounds such as piperlongumine and cardiac glycosides. Senolytics have emerged as promising agents for treatment of pulmonary fibrosis, atherosclerosis, osteoarthritis, type 1 diabetes mellitus, type 2 diabetes mellitus, and neurocognitive decline. They can also rejuvenate aged hematopoietic stem cells and muscle stem cells and extend the lifespan of naturally aged mice.
Despite successful preclinical proofs-of-concept for senolytics, their potential translatability is hampered by their associated toxicities, necessitating the development of more specific, and less toxic, second-generation senolytics. Navitoclax has been validated in a variety of preclinical models showing high potency in killing senescent cells - however, it also has significant on-target haematological toxicity, including thrombocytopenia.
One consistent feature of senescent cells is their enrichment in lysosomes and lysosomal proteins, including senescence-associated β-galactosidase (SA-β-gal) which is widely used as a marker of senescence. We previously showed that the encapsulation of nanoparticles with galacto-oligosaccharides (GalNPs) is an efficient method to preferentially deliver cytotoxic drugs and tracers to the lysosomes of senescent cells where SA-β-gal activity digests the galacto-oligosaccharides, thereby releasing the cargo. We have also shown that a fluorescent probe covalently linked to multi-acetylated galactose is preferentially digested by senescent cells, releasing the free fluorophore.
Here, we have modified Navitoclax with an acetylated galactose to exploit the enriched SA-β-gal activity of senescent cells. Using a variety of model systems, we show that galacto-conjugation of Navitoclax, which we name Nav-Gal, results in a prodrug with selective, pro-apoptotic senolytic activity released in senescent cells that is dependent on GLB1 activity. Galacto-conjugation of Navitoclax reduces thrombocytopenia in treated mice at therapeutically effective doses, as well as apoptosis of platelets in human blood samples treated ex vivo. Overall, we propose galacto-conjugation of cytotoxic drugs as a versatile methodology for developing second-generation prodrugs with high senolytic activity and reduced toxicity. We provide evidence of the efficacy of combining senescence-inducing chemotherapies with senotherapies in cancer, with potential for clinical application.