Anti-TB inhalation therapy: Design of mannose-based functionalised Solid Lipid Microparticles for an active targeting to alveolar macrophages

Human tuberculosis (TB) is mainly a disease of the lung characterised by a long chronic stage of infection and progressive pathology that compromise the respiratory system. This is a curable infectious bacterial disease caused by the Mycobacterium tuberculosis (Mtb). TB therapies have exploited conventional routes of administration, such as oral and intramuscular1. The pulmonary route appears the most reasonable and effective way to target the alveolar macrophages (AM) and eradicate surviving Mtb at the primary infected site of TB, especially considering that 75-80% of cases remain localised in the lungs. The anti-TB therapy by inhalation offers benefits compared with the current treatment in terms of patient’s compliance improvement, reduction in dose amount and frequency, treatment duration and TB diffusion in other organs, thus minimising the risk of drug-resistant mutants, toxicity and side effects. For a direct intramacrophagic antitubercular therapy using Dry Powder Inhaler (DPI) devices, Solid Lipid Microparticles (SLM), produced using the melt emulsifying technique followed by freeze-drying, were developed to load rifampicin, a first-line antitubercular drug. In the present project, SLM were modified to improve drug loading level and release as well as AM targeting. Several biocompatible lipid components such as fatty acids and their derivatives, diglycerides and triglycerides, were processed using mixtures of biocompatible stabilisers (sodium taurocholate and methyl mannopyranoside) in order to obtain SLM with maximum efficiency in terms of drug loading and release in simulated lung fluid. Lipids in the liquid physical state embedded into SLM provided Microstructured Lipid Carriers (MLC) that are known to exhibit superior advantages over SLM such as enhanced drug loading capacity and prevention of drug expulsion intended to maximise the drug concentration at the primary site of TB infection. The obtained microcarriers were examined for their intrinsic properties such as size and size distribution, morphology and shape, surface charge, bulk and tap density, aerodynamic diameter, physical state of the components, wettability, drug loading and release. Macrophages, as is common knowledge, possess mannose-specific membrane receptors (MR) that can be recognised by carriers bearing mannose residues, facilitating their internalisation 2, 3.
Therefore, the functionalisation of SLM surface by mannose derivatives used as the co- stabiliser in the SLM formulation was used to achieve an active targeting. The actual presence of mannose on SLM surface was investigated by means of X-ray Photoelectron Spectroscopy for Chemical Analysis (XPS) and Energy Dispersive X-ray Analysis (EDX).