Acid-Responsive Nanocarriers for Site-Specific Osteoclast Inhibition and Osteoporosis Therapy: A Review.

The fundamental cause of osteoporosis lies in the imbalance of bone remodeling triggered by the overactivation of osteoclasts. Although existing anti-resorptive medications demonstrate definitive therapeutic efficacy, their lack of lesion specificity often leads to off-target systemic exposure. This, in turn, frequently results in clinical side effects-such as excessive suppression of bone turnover and osteonecrosis of the jaw-which severely compromise the safety and patient compliance of long-term treatment. Consequently, there is an urgent clinical demand for precision delivery strategies with lesion-specific targeting. During the process of bone resorption, osteoclasts actively secrete protons into the sealed zone via proton pumps, establishing a localized, extreme acidic microenvironment. This biological phenomenon provides a natural physicochemical "switch" for achieving site-specific drug delivery. Based on these considerations, this paper introduces the "differential effective site exposure" strategy. This approach aims to leverage the acidic gradient to drive the spatial sequestration and active responsiveness of nanocarriers, thereby maximizing the effective drug exposure gain at bone resorption sites relative to non-target tissues. We systematically review the design principles and drug release kinetic profiles of three categories of acid-responsive nanocarriers based on chemical bond cleavage, charge reversal, and inorganic matrix degradation. Furthermore, this study highlights how biomimetic materials, represented by amorphous calcium carbonate, restore the balance of bone remodeling through a synergistic mechanism of neutralizing the pathological acidic environment and releasing osteogenic active ions. Finally, the paper evaluates the challenges posed by disease heterogeneity and discusses the translational bottlenecks in industrial-scale production and long-term biosafety. This work is intended to provide a theoretical framework and design rationale for the development of highly selective and safe next-generation precision anti-osteoporotic therapeutics.