Select -Lactam Combinations Exhibit Synergy against Mycobacterium abscessus In Vitro

Mycobacterium abscessus is a nontuberculous mycobacterium that causes invasive pulmonary infections in patients with structural lung disease. M. abscessus is intrinsically resistant to several classes of antibiotics, and an increasing number of strains isolated from patients exhibit resistance to most antibiotics considered for treatment of infections by this mycobacterium. Therefore, there is an unmet need for new regimens with improved efficacy to treat this disease. Synthesis of the essential cell wall peptidoglycan in M. abscessus is achieved via two enzyme classes, L,D- and D,D-transpeptidases, with each class preferentially inhibited by different subclasses of -lactam antibiotics. We hypothesized that a combination of two -lactams that comprehensively inhibit the two enzyme classes will exhibit synergy in killing M. abscessus. Paired combinations of antibiotics tested for in vitro synergy against M. abscessus included dual -lactams, a -lactam and a -lactamase inhibitor, and a -lactam and a rifamycin. Of the initial 206 combinations screened, 24 pairs exhibited synergy. A total of 13/24 pairs were combinations of two -lactams, and 12/24 pairs brought the MICs of both drugs to within the therapeutic range. Additionally, synergistic drug pairs significantly reduced the frequency of selection of spontaneous resistant mutants. These novel combinations of currently available antibiotics may offer viable immediate treatment options against highly-resistant M. abscessus infections.