“In Vivo” Depletion of Macrophages by Liposome-Mediated “Suicide”

Macrophages are multifunctional cell and play a key role in natural and acquired host defense reactions, in homeostasis, and in the regulation of numerous biological processes. The main tools used are phagocytosis followed by intracellular digestion and production and release of soluble mediators, such as cytokines, chemokines, and nitric oxide (NO). Macrophages can be found as resident cells in all organs of the body, and they can be recruited to sites of inflammation. The depletion of macrophages followed by functional studies in such macrophage-depleted animals forms a generally accepted approach to establish their role in any particular biomedical phenomenon. The methods for depletion of macrophages are based on the liposome-mediated intracellular delivery of the bisphosphonate clodronate. Liposomes are used as a Trojan horse to get the small clodronate molecules into the macrophage. Once ingested by macrophages, the phospholipid bilayers of liposomes are disrupted under the influence of lysosomal phospholipases. The strongly hydrophilic clodronate molecules intracellularly released do not escape from the cell. As a result, the intracellular clodronate concentration increases as more liposomes are ingested and digested. At a certain clodronate concentration, irreversible damage causes the macrophage to be killed by apoptosis. Free clodronate molecules show an extremely short half-life in circulation and body fluids. They are removed by the renal system. Clodronate in its free form is used widely as a drug for the treatment of malignant hypercalcemia and painful bone metastasis caused by hormone-refractory prostate cancer, emphasizing its nontoxic nature.