As ever more researchers turn their attention to cellular senescence as a cause of aging and age-related disease, more potential approaches to selectively targeting these unwanted cells are emerging. In the paper I'll point out here, the cell surface receptor CD9 is used to target nanoparticles carrying a therapeutic payload into senescent cells. The researchers chose to use rapamycin as the drug payload, as for one it doesn't matter too greatly if it gets into other cells, and secondly there is a fairly active line of research involving mTOR and its influence over the behavior of senescent cells. Rapamycin, as you'll recall, inhibits mTOR, but has some unpleasant side-effects that make it a poor option for a therapeutic. Targeting via nanoparticles in this way greatly lowers the provided dose; it is a way to deliver potentially harmful drugs in order to obtain a narrow set of benefits while minimizing the unwanted side-effects.
For my money, the best use of targeting mechanisms in the case of senescent cells is to deliver cell-killing mechanisms rather than the sort of cell-adjusting mechanisms used here, but when killing cells the targeting method has to have a very high degree of discrimination. To my knowledge, no-one has made it all that far down that road yet. The present approaches to destroying senescent cells, those under active development and heading towards the clinic, don't even try to deliver their therapeutic agents selectively to senescent cells. They are applied to all cells and target senescence in the sense of preferentially activating inside senescent cells. Some are more effective in that discrimination than others, but the basic concept certainly works. So it is interesting to see a group working on the more traditional method of steering delivery via cell surface markers, in order to place the therapeutic into the target cell population only, or at least to the greatest degree possible. A few years back, I had predicted that this would be the sort of technology first used to destroy senescent cells, and was completely incorrect on that front.
Cellular senescence refers to a state of irreversible growth arrest and altered function of normal somatic cells after a finite number of divisions. Senescent cells are characterized by a flattened shape, senescence-associated β-galactosidase (SA-β-gal) activity, and hypersecretion of cytokines, chemokines, and proteases, the senescence-associated secretory phenotype (SASP). Senescence partly depends on mechanistic target of rapamycin (mTOR) signaling that mainly regulates tumor suppressor pathways p53/p21 and Rb/p16, and leads to disease development/progression through tissue function impairment. In addition, progressive inability of the immune system to destroy senescent cells during aging results in the accumulation of "death-resistant" cells that accelerate aging and disease development by altering neighboring cell behavior, lowering the pool of mitotic-competent cells, degrading the cellular matrix, and stimulating cancer. Diverse age-related diseases result from cellular senescence progression. Therefore, strategies for the prevention, treatment, or removal of senescent cells are of prime interest for clinical applications.
A recently reported proof-of-concept demonstrated the use of capped mesoporous silica nanoparticles for targeted cargo delivery inside senescent cells mediated by β-galactosidase activity. However, it fails to justify cell-specific uptake of these nanosystems to senescent cells following intravenous or subcutaneous delivery. A mechanism driven approach for specific interaction and uptake of nanoparticles by senescent cells has thus become a challenging necessity. Hence, we proposed a proof-of-concept regarding delivery of rapamycin (Rapa) loaded calcium carbonate (CaCO3) nanoparticles with CD9 receptor mediated targeting, in addition to utilization of β-galactosidase activity, in senescent cells.
Rapamycin (Rapa), an mTOR inhibitor, was found to prevent replicative senescence in rat embryonic fibroblasts by affecting the p53/p21 pathway. In addition, several studies have indicated the beneficial effects of Rapa for life span extension in aging models. More importantly, CD9 - a glycoprotein receptor of the tetraspanin family that regulates cellular activity, development, growth, and motility - is overexpressed in senescent cells and thus, can potentially be used in targeted drug delivery. Although contradictory reports on CD9 receptors in different cancer cells suggest either enhancement or inhibition of growth and motility functions, implying cell type-specific activity, senescent cells are closely related to cancer development. Our study is the first report for the utilization of CD9 receptors in targeting drug-loaded nanoparticles to senescent cells and can be a stepping stone for further research in the field of targeted therapy to senescent cells.
In our study, CD9 monoclonal antibody-conjugated lactose-wrapped calcium carbonate nanoparticles loaded with rapamycin (CD9-Lac/CaCO3/Rapa) were prepared for targeted rapamycin delivery to senescent cells. The nanoparticles exhibited an appropriate particle size (~130 nm) with high drug-loading capacity (~20%). In vitro drug release was enhanced in the presence of β-galactosidase suggesting potential cargo drug delivery to the senescent cells. Furthermore, CD9-Lac/CaCO3/Rapa exhibited high uptake and anti-senescence effects (reduced β-galactosidase and p53/p21/CD9/cyclin D1 expression, reduced population doubling time, enhanced cell proliferation and migration, and prevention of cell cycle arrest) in old human dermal fibroblasts. Importantly, CD9-Lac/CaCO3/Rapa significantly improved the proliferation capability of old cells along with significant reductions in senescence-associated secretory phenotypes (IL-6 and IL-1β). Altogether, our findings suggest the potential applicability of CD9-Lac/CaCO3/Rapa in targeted treatment of senescence.