Evaluation of the intracellular anti-mycobacterial activity of a clofazimine nanoparticle formulation in macrophages

Abstract

Tuberculosis is a disease caused by the Gram-positive acid fast bacillus, Mycobacterium tuberculosis (M. tuberculosis). This intracellular pathogen has infected one-third of the world’s population and accounted for over two million deaths per year. The long-term persistence of M. tuberculosis, TB-human immunodeficiency virus coinfections, and the emergence of multidrug resistant (MDR) and extensively resistant (XDR) M. tuberculosis strains are the main factors contributing to the high global burden of this disease. Management of TB remains a great challenge despite the use of BCG vaccination and the availability of anti-tuberculous therapies, many of which are usually administered for longer periods. Hence ongoing research is aimed at developing new, efficacious, orally available drugs with the ability of shortening and simplifying the long and complex treatments of both drug-sensitive and drug-resistant tuberculosis. The success of such developments will be of great value if emphasis is based on re-formulation of the older drugs for the treatment of TB, such as clofazimine. Previous studies on clofazimine demonstrated that it was highly effective against M. tuberculosis in vitro, as well as in murine models of experimental, disseminated TB, but was of limited, if any, value in the treatment of TB in humans. This is probably due to the pharmacokinetic properties of this riminophenazine antimicrobial agent, which appears to have poor penetration into the airways. A renewed interest in clofazimine as an anti-TB agent was elicited by the emergence of MDR and extensively drug-resistant (XDR) TB, together with advances in technology for the delivery of lipophilic drugs to target organs. The primary objective of the laboratory research described in this dissertation was to compare the activities of a conventional preparation and a novel nanoparticle formulation of clofazimine against intracellular M. tuberculosis concealed in human monocytes/ macrophages. In the present study, a procedure was established for isolating monocytes from human peripheral blood and subsequently maturing these cells into homogenous populations of monocyte-derived macrophages. Exposure of the monocytes to the cytokine growth factors, IL-3 and GM-CSF, over a 7 day incubation resulted in a significant increase in the level of expression of CD14+/ CD16+ markers showing accelerated differentiation. Importantly, this cell population was highly homogeneous and accounted for >95% of the total cell population. These findings confirmed that the modified procedure results in the acquisition of synchronised human monocyte-derived macrophages, in adequate numbers. The monocyte-derived macrophages were then infected with H37Rv M. tuberculosis laboratory strain at a 1:10 macrophage: bacteria multiplicity of infection. Concentrations ranging from 0.15 - 2.5 ìg/ml for both the native and spray-dried clofazimine preparations together with corresponding drug-free controls were used to treat the infected cells for 48 hours. The intracellular bioactivities of the two preparations were determined by quantifying the number of surviving bacteria inside macrophages following treatment. Findings demonstrated comparable efficacy, with dose-dependent response inhibition of colony formation when M. tuberculosis was exposed to either of the clofazimine preparations, with significant inhibition of growth at 1.25 ì/ml, with no detectable colonies at 2ìg/ml. No statistically significant differences were found between the treatments with both preparations. These observations demonstrate that the intracellular bioactivities of both clofazimine preparations are equivalent with respect to efficacy against M. tuberculosis. This study may therefore provide justification for future studies on the therapeutic efficacy of the spray-dried formulation of clofazimine in murine models of experimental chemotherapy. Copyright