Our major goal is to demonstrate the efficacy of local delivery of anti-senescence drugs when used for immunomodulation at the local cellular level for bone healing. In the immune system, macrophages play a major role in switching inflammation on or off during healing, and they balance activities of bone-forming and bone-resorbing cells. We recently reported that a thin layer of biomimetic calcium phosphate could be used in a novel way as a transient barrier layer (TBL) to sequentially deliver two drugs in a step-wise fashion to switch macrophage phenotype from pro-inflammatory to pro-regenerative. We discovered that macrophages are able to make holes through the TBL and thereby control delivery timing to a drug below the TBL without release of drugs into the media. We now propose to apply this TBL technology to improve macrophage transitions impaired by age as a means to improve bone healing in the elderly. In aging, as well as in metabolic disorders, the immune response is affected by senescent cells that no longer replicate, but have a senescence- associated secretory phenotype (SASP) that produces high levels of proinflammatory molecules. Certain chemotherapy drugs, known as senolytics, however, kill senescent cells and other drugs, called anti-SASP drugs, block their pro-inflammatory cell signaling. Both approaches have been shown to enhance bone density in older mice when given systemically over months, yet both types of anti-senescence drugs have unwanted side effects, and neither approach has been investigated in bone healing contexts. We now propose using the TBL on a bone graft substitute to control local delivery timing of either a senolytic or an anti-SASP drug in a bone microenvironment to maximize calvarial bone repair in old mice. Our preliminary studies support our hypothesis that delivery timing is critical to improve macrophage switching, osteoblast differentiation, and scaffold resorption, and restrict osteoclast maturation all required for optimal bone repair.
Aim 1 is a mechanistic in vitro study to understand how appropriately-timed delivery of senolytic ABT-263 or anti-SASP Rux restores osteogenic communication between osteoblasts, macrophages and osteoclasts from old mice compared to cells from old humans relative to young controls. We will broadly capture effects of these drugs on in vitro cell cross-talk by RNAseq. These studies will establish commonalities between human and mouse cell culture responses to the drugs and establish our model for eventual translation to human subjects. The in vivo studies of Aim 2 (with ABT-263) and Aim 3 (with Rux) will determine the appropriate dose and timing of anti- senescence drugs in old mice to suppress senescent cell activity and increase osteoprogenitor activity/decrease osteoclast activity guided by timely macrophage transitions. Our results will reveal the fundamentals of timed, local delivery of drugs to modulate macrophage transitions in injured old mice with heightened inflammation, as well as accelerating the safe and effective implementation of anti-senescence drugs. This will have dramatic implications for healing in the elderly as well as for obese and diabetic patients.
Immune suppression and impaired wound healing are drastic side effects of non-specific oral doses of anti- senescent drugs recently discovered to turn back the clock on aging and prevent many of the diseases of aging. Substantial evidence indicates controlled delivery timing by a biomaterial layer on a bone graft substitute will enable safe and effective dosing of anti-senescence drugs for bone healing in injured elderly patients. This grant will elucidate the effects of suppressing senescent cells on the interactions between the cells involved in bone repair in an aging bone model.
