Bone-targeting engineered exosomes delivering teriparatide mRNA for the treatment of osteoporosis

Teriparatide (TPTD) is an established osteoanabolic agent for the treatment of osteoporosis; however, its short circulating half-life necessitates repeated administration, while limited skeletal selectivity may reduce delivery efficiency. To address these limitations, we developed an Asp6-functionalized, bone-targeting exosome system carrying TPTD mRNA (Asp6-Exo@TPTD). An Exo-Motif sequence was incorporated into the TPTD mRNA construct to facilitate hnRNPA2B1-associated enrichment of the transcript in exosomes, while surface modification with the hydroxyapatite-binding peptide Asp6 was used to enhance skeletal affinity. BMSC-derived exosomes may additionally provide intrinsic regenerative signals that complement the osteoanabolic activity of TPTD.

Mouse bone marrow mesenchymal stromal cells (BMSCs) were incubated with different exosome formulations to evaluate intracellular TPTD mRNA transfer, TPTD protein production, CRE-dependent reporter activity, and osteogenic differentiation. Free TPTD was included as a pharmacological reference in the functional, osteogenic, and animal efficacy experiments. The bone-binding properties of the engineered exosomes were assessed using hydroxyapatite-containing bone samples and ex vivo bone tissues. In vivo biodistribution was evaluated by fluorescence imaging after intravenous administration. Therapeutic efficacy was investigated in an ovariectomy-induced osteoporosis mouse model by assessing trabecular microarchitecture, histological changes, serum bone-formation markers, and dynamic bone formation. Preliminary tolerability was evaluated by major-organ histology and serum biochemical analyses, including serum calcium and hepatic and renal function indices.

Asp6-Exo@TPTD increased intracellular TPTD mRNA levels and TPTD secretion in recipient BMSCs and enhanced CRE-dependent luciferase activity, osteogenic gene expression, alkaline phosphatase staining, and extracellular matrix mineralization. Asp6-Exo@TPTD exhibited greater binding to bone tissues than both unloaded Exo and non-targeted Exo@TPTD in vitro and ex vivo. Following systemic administration, bone-associated fluorescence remained detectable for at least 72 h, with greater femoral and tibial fluorescence and lower relative hepatic fluorescence than those observed with unmodified exosomes. In ovariectomized mice, weekly administration of Asp6-Exo@TPTD improved trabecular microarchitecture, increased serum procollagen type I N-terminal propeptide levels, and enhanced the mineral apposition rate. No overt histopathological abnormalities or significant alterations in serum calcium or routine hepatic and renal biochemical indices were detected.

Asp6-functionalized exosomes carrying TPTD mRNA promoted osteogenic activity, enhanced bone-associated distribution, and attenuated ovariectomy-induced bone loss. These findings support Asp6-Exo@TPTD as a promising exosome-based strategy for bone-targeted TPTD mRNA delivery and osteoporosis treatment.