MUT056399 (Fab-001) is a highly potent inhibitor of the FabI enzyme in both S. aureus and E. coli, with IC50 values of 12 nM and 58 nM, respectively.

MUT056399 is a highly potent new inhibitor of the FabI enzyme of both Staphylococcus aureus and Escherichia coli. In vitro, MUT056399 was very active against S. aureus strains, including methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), linezolid-resistant, and multidrug-resistant strains, with MIC(90)s between 0.03 and 0.12 μg/ml. MUT056399 was also active against coagulase-negative staphylococci, with MIC(90)s between 0.12 and 4 μg/ml. The antibacterial spectrum is consistent with specific FabI inhibition with no activity against bacteria using FabK but activity against FabI-containing Gram-negative bacilli. In vitro, resistant clones of S. aureus were obtained at a low frequency. All of the resistant clones analyzed were found to contain mutations in the fabI gene[1].

In vivo, MUT056399, administered subcutaneously, protected mice from a lethal systemic infection induced by MSSA, MRSA, and vancomycin-intermediate S. aureus strains (50% effective doses ranging from 19.3 mg/kg/day to 49.6 mg/kg/day).In the nonneutropenic murine thigh infection model, the same treatment with MUT056399 reduced the bacterial multiplication of MSSA and MRSA in the thighs of immunocompetent mice[1]