Preclinical in vivo assessment of replacing linezolid for spectinamide-1599 in the Nix-TB regimen

Abstract Tuberculosis (TB) patients diagnosed as drug-resistant TB have limited treatment options. Nix-TB trial is testing three-oral-drugs, namely Bedaquiline (B), Pretomanid (Pa) and Linezolid (L) (BPaL regimen) against drug-resistant TB. The BPaL regimen shows excellent favorable outcomes, but toxic effects are observed due to the long-term administration of Linezolid. Spectinamide-1599 (1599) is a potent protein inhibitor of Mycobacterium tuberculosis (Mtb) without known adverse effects and cross-resistance with other TB drugs. We hypothesized that inhaled 1599 could replace L in the BPaL regimen and provide similar or higher efficacy as BPaS regimen. Therefore, we tested BPaL and BPaS regimens in Balb/c (TB resistant) and C3HeB/FeJ (TB susceptible) mice infected with low-dose Mtb, and evaluated bacterial burden using colony-forming units (CFUs). Compared to untreated mice, CFUs in lungs of BPaL and BPaS treated Balb/c mice were decreased by >1 and 5 logs after 2- and 4-weeks treatment, respectively. In C3HeB/FeJ mice, 4-weeks of BPaL and BPaS treatment decreased CFUs by >3 logs compared to untreated control. However, no statistically significant differences were observed between treatment groups in both the mice strains. Cellular profiling using flow cytometry showed increased monocytes (CD11bhi CD14hi CCR2hi) and neutrophils (CD11b+ Ly6G+) in the bone marrow of BPaS mice compared to BPaL. These results along with cytokine profiling of the two treatment groups showed enhanced inflammatory response in BPaS mice than BPaL. We concluded that inhaled 1599 is a potential replacement for Linezolid if combined as BPaS regimen, and further studies are warranted. This project was funded by National Institute of Health under the funding number NIH-NIAID-Allergy and Infect Diseases AI120670.