Physicochemical and biological aspects of macrophage-mediated drug targeting in anti-microbial therapy

Macrophages are important drug targets as they mediate a wide variety of infectious diseases. Visceral leishmaniasis (VL), schistosomiasis, brucellosis, and salmonellosis are some of the well-known infectious diseases in which macrophages play a prominent pathophysiological role. For instance, VL parasites exclusively house in the macrophages of liver and spleen. They are resistant to lysosomal degradation by unknown mechanisms, they survive and thrive safely within macrophages, they multiply, and they ultimately affect visceral organs, leading to severe pathological and sometimes even fatal conditions. The majority of routinely used drugs administered in free form distribute all over the body via systemic circulation, leading to relatively low therapeutic activity and a certain degree of toxicity. Unlike for nonmicrobial diseases, targeting parasites procuring resistance and ineffective therapeutic outcome can be obviously speculated in case of infectious disease. The preferential uptake by macrophages, intended to improve the balance between efficacy and toxicity, can be achieved by the use of nanomedicines, i.e. submicron-sized macromolecular or particulate drug delivery systems. This insight has stimulated researchers to use nanomedicines--which tend to be recognized by macrophages as 'foreign' and consequently are taken up by the intended target cells much more effectively than their free drug counterparts--to improve the treatment of infectious diseases. The literature reports extensively on such approaches; however, there are several constraints that limit the application of nanomedicine in macrophage-mediated drug targeting. Here, we briefly describe the strategies that are used to achieve effective drug targeting to macrophages, using VL as a model disease, and we also put forth an understanding of the most important limiting factors. Various physicochemical and biological factors used by researchers as reported in the literature are addressed, and the most important mechanisms and modes by which macrophage-specific drug targeting can be achieved are summarized. Based on the evidence obtained to date, it can be concluded that targeting macrophages is a valuable and validated strategy for improving the treatment of infectious diseases.