The clinical treatment of OP mainly depends on anti-resorption drugs. However, a basic common disadvantage of these drugs is that they cannot restore the lost bone mass but prevent bone absorption [47]. Compared with anti-bone resorption drugs, bone anabolic agents such as parathyroid hormone (PTH) and Evenity can induce bone formation. However, both of them have distinct disadvantages: PTH requires long-term continuous use (at least 2 years), because sudden termination of use will lead to a rapid decline in bone mineral density; Evenity may increase the risk of myocardial infarction, stroke, and cardiovascular death [48,49]. Therefore, it is urgent to develop new innovative treatment strategies based on promoting bone regeneration in OP patients. At present, most of the approved drugs for clinical treatment have poor selectivity to pathological tissues, which reduces their effectiveness and safety. The tissue-specific targeting can improve the bioavailability and toxicity of therapeutic drugs. Adapters are single-stranded DNA/RNA oligonucleotides that can bind to target molecules with high affinity and specificity in a three-dimensional structure [50]. Previous studies have confirmed that aptamers can target and identify pathological tissues, and specifically attack pathological tissues with covalently or physically aligned therapeutic compounds [51]. Aptamer functionalized drug loaded Exos have been reported to efficiently deliver molecular drugs/fluorophores to tumor cells, providing a promising delivery platform for cancer treatment [52]. According to these characteristics of aptamers, some scholars have constructed an aptamer functionalized BMSC-Exos (BMSC-Exo-Apt) prepared by conjugating BMSC specific aptamers with BMSC-Exos through group modification [53]. The 5′-UTR of the aptamer can be modified by an aldehyde group to form a stable Schiff base with the amino reaction of Exos membrane protein. This specific aptamer can significantly promote the internalization of BMSC-Exos into BMSCs in vitro. Furthermore, Luo et al. [54] applied BMSC-Exo-Apt complex to OP mice after OVX to verify whether Exos can preferentially accumulate in bone tissue to promote bone regeneration. OP mice were injected with BMSC-Exos-Apt intravenously once a week for two months. The results showed that compared with the unmodified BMSC-Exos group, BMSC-Exos-Apt treatment could effectively increase the bone mass of OVX mice by promoting osteogenesis [54], indicating that BMSC targeting aptamer functionalized BMSC-Exos could target bone tissue to avoid rapid metabolism and clearance, thus promoting BMSC-Exos induced bone regeneration. This study proposed an aptamer-based bone-specific targeting method for Exos delivery. The effect of the BMSC-Exo-Apt complex represents a new promising treatment strategy for OP. In addition, surface modification of Exos by introducing specific receptors or ligands is another potential strategy to regulate the distribution characteristics of Exos in vivo. Stromal cell derived factor 1 (SDF1) is a ligand of C-X-C motor chemokine receptor 4 (CXCR4) expressed mainly by BMSCs [55]. The high level of SDF1 in bone marrow can recruit CXCR4+ peripheral HSC for homing, while the up-regulation of CXCR4 expression in hematopoietic stem cells can enhance the homing of hematopoietic stem cells [56]. Therefore, SDF1/CXCR4 axis plays a key role in the transport of hematopoietic stem cells and the homing of BMSCs. According to this characteristic, Hu et al. [56] constructed highly expressed CXCR4 on the Exos surface derived from genetically engineered NIH-3T3 cells (mouse embryonic fibroblast cell line), which has the characteristic of specifically targeting bone. Increased miR-188 was found in BMSCs from elderly patients. Knockout of miR-188 inhibits adipogenic differentiation of elderly BMSCs, suggesting that targeting miR-188 can be used as a therapeutic means to promote bone formation. Hu et al. [57] fused the constructed Exos with liposomes carrying antagomiR-188 to obtain hybrid nanoparticles (NPs) with bone targeting and anti-miR-188 capabilities. The team further injected 18-month-old male mice with hybrid NPs, antagomiR-188, or antagomiR-188-loaded hybrid NPs intravenously every week for 8 weeks. The results showed that, compared with the other two groups, antagomiR-188-loaded hybrid NPs significantly reversed the age-related bone trabecular loss and reduced cortical bone porosity by inhibiting fat production and promoting osteogenesis of BMSCs in aged mice, showing significantly higher bone mass retention. It is suggested that this study provides a bone targeted RNA interference delivery strategy based on Exos modification, which is a promising anabolic therapy for age-related bone loss. In addition, alendronate (Ale) is the most widely used first-line anti-OP drug containing a P-C-P group of the pyrophate analog, which inhibits bone absorption by combining with hydroxyapatite on the bone surface with high affinity and strength [58]. However, the drug may cause side effects such as esophagitis and gastrointestinal discomfort [59]. Nanocarriers as drug delivery systems can prolong the drug circulation time of Ale, thereby reducing the dosage and side effects of drugs. Although Ale is a kind of bone targeting drug molecules, it is difficult to couple it to the drug carrier surface under normal conditions. As an efficient and convenient chemoselective conjunction method under mild conditions, “click chemistry” can solve this problem [60]. As we all know, the discoverer of “click chemistry” won the Nobel Prize in 2022. “Click Chemistry” has mild reaction, simple operation, easy purification, and no harmful by-products. It is widely used in the synthesis of functional polymers and cell markers [61]. Wang et al. [62] constructed an Ale-MSC-EVs complex by coupling an azide (N3) group modified Ale molecule and an alkynyl (DBCO) group modified MSC-EVs through “click chemistry”. Subsequently, the team evaluated the bone regeneration capacity of the Ale-MSC-EVs complex. It was found that Ale-MSC-EVs was well-tolerated without side effects. Ale-MSC-EVs can not only promote the proliferation and differentiation of BMSCs in vitro, but also effectively prevent OVX induced bone loss in OP rats, suggesting that Ale-MSC-EVs with specific bone targeting ability is a potential therapeutic strategy for OP. It also shows that the covalent binding method of “click chemistry” provides a new insight into for targeting ligand modification of EVs surface film, which can be used to improve the specific bone targeting ability of EVs and has broad application prospects.

In summary, the specific bone targeting ability of MSC-EVs modified by genetic engineering techniques with aptamers, ligands, or drugs is significantly enhanced compared to natural MSC-EVs. Although the exploration of bone-targeted MSC-EVs is still in its initial stages, the great potential of this direction of research in improving osteogenic differentiation points to new directions for further research in the clinical treatment of OP. In addition, despite the ability of Ale to confer bone targeting to EVs, it remains to be further investigated whether Ale-EVs have an inhibitory effect on osteoclasts. Moreover, since Ale-EVs act as a bone target delivery system, future studies may also try to load other drugs such as nucleic acid drugs, small molecule drugs, etc. into the targeted drug delivery system for combined treatment of OP.