Inhalable particles containing rapamycin for induction of autophagy in macrophages infected with Mycobacterium tuberculosis.

We investigated whether particles suitable for delivery to alveolar macrophages may provide a means of targeting rapamycin, an inducer of autophagy, to alveolar macrophages as a host-directed antituberculosis agent. Inhalable particles were prepared by spray-drying and characterized using laser scattering and electron microscopy. Their aerodynamic diameter was calculated from bulk and tapped densities. In vitro drug release was studied in PBS containing 1% SDS. In vitro uptake of particles by THP-1 derived macrophages was studied by flow cytometry. Cytotoxicity of the particles toward macrophages and their efficacy against intracellular Mycobacterium tuberculosis were studied using a methyltetrazolium assay and counting bacterial colonies obtained when cell lysates were plated on agar. The encapsulation efficiency was 88.8 ± 1.13% and drug content 22 ± 4% w/w. The particles had a median diameter of 2.88 ± 0.8 μm and appeared as collapsed spheres. Their calculated aerodynamic diameter was about 1 μm. In vitro drug release from the particles was first-order and extended beyond 10 days. Flow cytometry indicated that the particles were taken up by macrophages within 3 h. Macrophages exposed to the particles or rapamycin in solution at a concentration of 100 μg/mL over a 24 h period maintained 79.37 ± 0.72% and 58.33 ± 1.39% viability, respectively. Efficacy studies concluded that particles were more effective in clearing intracellular mycobacteria than rapamycin in solution. It was concluded that the preparation was suitable for formulating as a dry powder inhalation to test efficacy of inhaled, macrophage-targeted rapamycin against TB.