SILA-421 activity in vitro against rifampicin-susceptible and rifampicin-resistant Mycobacterium tuberculosis, and in vivo in a murine tuberculosis model

• SILA-421 has bactericidal activity towards Mycobacterium tuberculosis strains of the Beijing genotype.
• SILA-421 enhanced the mycobacterial killing capacity of isoniazid and rifampicin.
• SILA-421 prevented the emergence of isoniazid resistance.

Due to the emergence of multidrug-resistant and extensively drug-resistant tuberculosis (TB), there is an urgent need for new TB drugs or for compounds that improve the efficacy of currently used drugs. In this study, time–kill kinetics of SILA-421 as a single drug and in combination with isoniazid (INH), rifampicin (RIF), moxifloxacin (MXF) or amikacin (AMK) against Mycobacterium tuberculosis were assessed. Therapeutic efficacy in vivo in a mouse TB model was also studied. Further in vitro analysis was performed with a RIF-susceptible and RIF-resistant strains of M. tuberculosis. When used as a single drug, SILA-421 in vitro showed concentration-dependent and time-dependent bactericidal activity. SILA-421 also enhanced the activity of INH and RIF, resulting in synergy in the case of INH. Emergence of INH resistance following exposure to INH can be prevented by the addition SILA-421. SILA-421 had no additional value in combination with MXF or AMK. Furthermore, SILA-421 enhanced the activity of RIF towards a RIF-resistant strain and resulted in complete elimination of RIF-resistant mycobacteria. Unfortunately, in mice with TB induced by a Beijing genotype strain, addition of SILA-421 to an isoniazid–rifampicin–pyrazinamide regimen for 13 weeks did not result in enhanced therapeutic efficacy.