Resistant tuberculosis: The latest advancements of second-line antibiotic inhalation products

Drug-resistant tuberculosis (TB) can be considered the man-made result of interrupted, erratic or inadequate TB therapy. As reported in WHO data, resistant Mycobacterium tuberculosis (Mtb) strains continue to constitute a public health crisis. Mtb is naturally able to survive host defence mechanisms and to resist most antibiotics currently available. Prolonged treatment regimens using the available first-line drugs give rise to poor patient compliance and a rapid evolution of strains resistant to rifampicin only or to both rifampicin and isonia-zid (multi drug-resistant, MDR-TB). The accumulation of mutations may give rise to extensively drug-resistant strains (XDR-TB), i.e. strains with resistance also to fluoroquinolones and to the injectable aminoglycoside, which represent the second-line drugs. Direct lung delivery of anti-tubercular drugs, as an adjunct to conventional routes, provides high concentrations within the lungs, which are the intended target site of drug delivery, representing an interesting strategy to prevent or reduce the development of drug-resistant strains. The purpose of this paper is to describe and critically analyse the most recent and advanced results in the formulation development of WHO second-line drug inhalation products, with particular focus on dry powder formulation. Although some of these formulations have been developed for other lung infectious diseases (Pseudomonas aeru-ginosa, nontuberculous mycobacteria), they could be valuable to treat MDR-TB and XDR-TB.