Administration protocols should be chosen carefully and depend on several factors, for instance: the type of macrophage you intend to deplete (e.g. Kupffer cells, alveolar macrophages, microglia, osteoclasts, red pulp macrophages), the time in which you intend to maintain depletion (e.g. short or long term), the (animal) model, and other experimental factors. In vitro application of clodronate liposomes is possible, albeit that they are specifically suitable to study macrophages in vivo. Below you can see schematic representations of the tissues and macrophages that can be reached through different administration routes. Please note that not all tissues and routes are represented here.
Abbreviations: AL = lung alveoli, BM = bone marrow, BR = brain, BV = blood vessels / circulation, DA = draining area of lymph node, EY = eye, GU = gut / intestines, IV = intravenous delivery, KI = kidney, LI = liver, LN = lymph node, LU = lung, LV = lymph vessels, PE = peritoneal cavity, SP = spleen, SY = synovial cavity in joint, TE = testis, TR = trachea.
In general, it is recommend to inject 100 μL of suspension / 10 grams of animal weight for intravenous injection. Raising the dosage considerably may lead to blockage of capillaries. Intravenous administration of clodronate liposomes will lead to maximum depletion of liver and spleen macrophages in ca. 24 hours. Dependent on the subset of macrophages they will remain depleted for ca. 5 days. After that time, new macrophages will replace the depleted ones: macrophage precursors, monocytes, that are formed in bone marrow and released in circulation will arrive at their destination and further differentiate into mature macrophages. Monocytes, macrophage precursors, can also be depleted. To prevent macrophage repopulation, multiple injections can be administered every 2-3 days to target monocytes. See for instance: Sunderk?tter, C., Nikolic, T., Dillon, M. J., Van Rooijen, N., Stehling, M., Drevets, D. A., & Leenen, P. J. (2004). Subpopulations of mouse blood monocytes differ in maturation stage and inflammatory response. The Journal of Immunology, 172(7), 4410-4417.
For intraperitoneal administration, the recommended injection dosis (i.e. 100 μL of suspension / 10 grams of animal weight) can be increased considerably. This route also depletes peritoneal macrophages, but will take longer to deplete macrophages in liver and spleen (ca. 3 days). Depletion is slower and more gradual, since the liposomes have to be carried from the peritoneal cavity to circulation by lymph flow via the thoracic duct, which is a passive form of transport.
Local administration is often required to target macrophages that are difficult to reach, such as macrophages in testis or the phagocytic synovial lining cells.
For depletion of alveolar macrophages, clodronate liposomes can be administered intranasally as well as intratracheally. The difference is that intranasal liposomes may be spoiled in the oesophagus if not administered properly, whereas intratracheal instillation will deliver all liposomes in the lung. Please note that these routes only target alveolar macrophages, and not interstitial macrophages. These can be targeted through depletion of blood monocytes in circulation. See for instance: Huang, L., Nazarova, E. V., Tan, S., Liu, Y., & Russell, D. G. (2018). Growth of Mycobacterium tuberculosis in vivo segregates with host macrophage metabolism and ontogeny. Journal of Experimental Medicine, 215(4), 1135-1152.
For subcutaneous injection the maximum volume to be injected depends on the storage capacity of the injection site.