Targeting strategies for bone diseases: signaling pathways and clinical studies

Abstract

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Fig. 1

An overview of intracellular regulation of osteoclastogenesis and resorption activities. The early differentiation of myeloid progenitors to pOCs is mediated by M-CSF stimuli via PI3K/Akt and GRB2/ERK signaling. The binding of RANKL to RANK further promotes the differentiation of pOCs and activates intracellular TRAFs/NF-κB and TRAFs/MAPK signaling to increase transcription factors such as MYC, FOS, and NFATc1, upregulating the expression of osteoclast activation-related proteins and acid secretion. Phosphorylation of Plcγ2 by ITAM stimuli is also required for RANKL-induced NFATc1 activation. LGR4 activation can suppress RANKL-induced osteoclastogenesis via the GSK3β/MAPK pathway. Sema3A can inhibit ITAM-induced Plcγ2 activation and M-CSF-induced osteoclast differentiation through the RhoA signaling pathway

Fig. 2

Angiogenesis, homing, and the immune microenvironment in the bone marrow. a Th17 cells promote osteoclast differentiation by secreting IL-17A to upregulate RANKL, TNF-α, and IL-6, while Treg cells suppress osteogenesis by secreting inhibitory cytokines such as TGF-β, IL-4, and IL-10 and enhance WNT-10b expression via interaction with CD8⁺ T cells. CTLA-4 expressed by Treg cells can degrade tryptophan and promote pOC apoptosis by binding to CD80/86 on the surface of pOCs. SLIT3, PGDF-BB, and VEGF secreted by osteoblasts, osteoclasts, pOCs, and chondrocytes can promote type H angiogenesis. Endothelial Notch/Dll4 signaling can increase Noggin secretion from type H endothelial cells (ECs), which promotes osteogenesis and chondrocyte hypertrophy maturation. RANKL and MMP9 derived from type H ECs can facilitate osteoclast chemotaxis and osteoclastogenesis. b CXCR4/7, integrin α4β1 (VLA-4), and S1PR can respond to CXCL12, VCAM-1, and S1P to mediate the homing of BMSCs and pOCs. CD47 on hematopoietic stem cells (HSCs) serves as a ‘marker of self’ that binds to CD172α (S1RPα) on phagocytes to reduce depletion from mononuclear phagocyte system during homing

Fig. 3

Therapeutic targets for improving bone homeostasis. Intercellular activities are mediated by specific protein–protein interactions (PPIs). By targeting key gene or protein expression, PPIs during bone remodeling can be regulated to improve bone formation and alleviate bone resorption

Fig. 4

Loop2 and loop3 of sclerostin are key binding targets for sclerostin antibodies (Scl-abs). Both of them can mediate the bone formation suppression effect of sclerostin, while loop2 possesses a cardiovascular protective effect by decreasing inflammatory cytokines and chemokines such as IL-6, MCP-1, TNF-α, interferon-γ, et al., in VSMCs and macrophages. Scl-abs inhibit the functions of both loop2 and loop3, thus promoting bone formation but increasing cardiovascular risk. In contrast, the loop3-aptamer inhibits sclerostin’s bone suppression effect while preserving the cardioprotective effect of loop2

Fig. 5

Cellular crosstalk among osteoblasts, osteoclasts, and osteocytes during the remodeling process. Osteoblast- and osteocyte-derived OPG can suppress the fusion of osteomorphs into osteoclasts. Sema4D promotes osteoclastic resorption by binding CD72 on pOCs, while Sema3A, produced by osteocytes and osteoblasts, inhibits osteoclastogenesis. WNT5a expressed by osteoblasts can stimulate the differentiation of pOCs in the noncanonical pathway by binding to ROR2 and reversing the inhibitory effects of WNT16 on RANKL-induced osteoclastogenesis. EphrinB2 secreted by osteoclasts can bind to EphB4 on osteoblasts and promote osteogenic differentiation by inhibiting the small GTPase RhoA, whereas reverse Eph signaling on pOCs can inhibit osteoclastogenesis by downregulating c-FOS and NFATc1 expression. Apoptotic osteoclasts secrete miR-214-3p, which suppresses osteoblast-specific transcription factors such as Osterix and ATF4 and promotes osteoclastogenesis by decreasing PTEN through the PI3K/Akt pathway. Conversely, RANK secreted by apoptotic osteoclasts can activate Runt-related transcription factor 2 and the intracellular PI3K-Akt-mTORC1 pathway. Sclerostin secreted by osteocytes can inhibit osteogenesis by binding to the LRP5/6 coreceptor to promote GSK3β complex-mediated inhibition of anabolic β-catenin signaling and inhibiting BMP-2/SMAD1/5-induced osteogenesis. Osteocyte apoptosis is accompanied by the secretion of RANKL, which promotes the resorption process. Sema4D inhibits IGF-1-mediated osteoblastic formation by binding the Plexin-B1 receptor expressed on osteoblasts. Sema3A acts on Plexin-A and neuropilin 1 (Nrp1) on pOBs to promote osteogenesis through the Rac signaling pathway

Fig. 6

Bone-targeted drug delivery ligands. a Bone matrix-targeted ligands. TC and DSS target the bone formation site, while Asp and Glu target the resorption site. BPs and PA are less influenced by surface property. CBD and WYRGRL target type I and II collagen, respectively. b Bone marrow- and bone cell-targeted ligands

Fig. 7

a Structures of hydroxyapatite-targeted ligands. b R1 and R2 group of different bisphosphonates c Structure of small molecule agents mentioned in the text