Prospects of Inhalable Formulations of Conventionally Administered Repurposed Drugs for Adjunctive Treatment of Drug-Resistant Tuberculosis: Supporting Evidence from Clinical Trials and Cohort Studies

Background: Drug resistant tuberculosis is a major public health concern, since the causative agent Mycobacterium tuberculosis is resistant to the most effective drugs against tuberculosis treatment ie., rifampicin and isoniazid. Globally, it accounts 4.6 percent of the patients with tuberculosis, but in some low socioeconomic areas this proportion exceeds to 25 percent. The treatment of drug resistant tuberculosis is prolonged (9-12 months) and often have less favorable outcome with novel as well as recently repurposed drugs administered by conventional routes. Materials and Methods: Clinically, these repurposed drugs have shown several major concerns including low penetration of the drugs to the pulmonary region, emergence of resistant forms, first pass effects, drug-drug interactions, food effects, and serious side effects upon administration by conventional route of administration. Although, several antimicrobial agents have been either approved or are under investigation at different stages of clinical trials and in pre-clinical studies via inhalation route for the treatment of respiratory infections, inhalable formulation for the treatment of drug resistant tuberculosis is most untouched aspect of drug delivery to validate clinically. Only a single dry powder inhalation formulation of capreomycin is able to reach the milestone, ie., phase I for the treatment of drug resistant tuberculosis. Results: Administering inhalable formulations of repurposed drugs as adjuvant in the treatment of drug resistant tuberculosis could mitigate several concerns by targeting drugs directly in the vicinity of bacilli. Conclusion: This review focuses on the limitations and major concerns observed during clinical trials of repurposed drugs (host directed or bactericidal drugs) administered conventionally for the treatment of drug resistant tuberculosis. The outcomes and the concerns of these clinical trials rationalized the need of repurposing formulation which could be administered by inhalation route as adjunctive treatment of drug resistant tuberculosis.