Clodronate liposomes are lipid-based nanoparticles that encapsulate clodronate, a bisphosphonate compound traditionally used to treat bone-related disorders. These liposomes are composed of phospholipid bilayers that serve as a delivery vehicle for clodronate, allowing for targeted and efficient cellular uptake. The unique structure of clodronate liposomes facilitates the selective elimination of phagocytic cells, primarily macrophages, by exploiting the natural propensity of these cells to engulf lipid-based particles. The typical structure of clodronate liposomes includes phospholipids, cholesterol, and clodronate. The phospholipids form a bilayer, while cholesterol stabilizes the liposome structure, providing fluidity and integrity. This configuration ensures the effective encapsulation of clodronate, which can then be released intracellularly upon liposome uptake by target cells, primarily macrophages.

Clodronate is a hydrophilic molecule that can be encapsulated within phospholipid bilayers. Free clodronate does not easily cross cell membranes, and is rapidly cleared (i.e. within minutes) from circulation by the renal system. However, when entrapped in a liposome, the clodronate liposome is ingested by macrophages and cannot escape it . The phospholipid bilayers are digested by lysosomal phospholipases, whereas clodronate is not digested and remains in the macrophage. The more phospholipid bilayers and liposomes are ingested by the macrophage, the more clodronate will accumulate within the macrophage. Exceeding a certain intracellular concentration, clodronate will eliminate the macrophage by initiating its programmed cell death, i.e. apoptosis.

Figure : Mechanism of macrophage depletion using liposomal clodronate.

Liposomal clodronate is taken up by the macrophage through endocytosis. Then, the bilayers of the liposome are disrupted by phospholipases from lysosomes. This results in the release of clodronate, causing cell death(1).

Once clodronate is encapsulated in liposomes, it can specifically target phagocytes. Liposomes can be quickly recognized and swallowed by macrophages, allowing an effective clodronate concentration threshold to trigger target cell apoptosis. Clodronate encapsulated in liposomes is recognized as a foreign body by phagocytes and is taken into internal vesicles called phagosomes. After phagocytosis, the phagosome fuses with the lysosome containing phospholipase, which destroys the phospholipid bilayer of the liposome, thereby allowing clodronate to be released in the cell. Once clodronate is released in the cytoplasm, aminoacyl-tRNA synthetase will mistakenly identify it as pyrophosphate and will use clodronate to generate the non-hydrolyzable ATP analogue (AppCCl2p). Then, this ATP analog translocates to the mitochondria, where it irreversibly binds to the ATP/ADP translocase, causing blockage of the respiratory chain, leading to changes in mitochondrial function and integrity. These altered mitochondria release molecular signals that will initiate cell apoptosis. Unencapsulated clodronate cannot pass through the cell membrane to trigger cell death. Since the control liposome does not contain clodronate, the phagocytes will not be killed.

Reference

1. Al Nuaimi 12 PhD - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Mechanism-of-macrophage-depletion-using-liposomal-clodronate-Liposomal-clodronate-is_fig16_369949448 [accessed 25 Aug 2025]