In Vitro and Intracellular Activity of Imipenem Combined with Rifabutin and Avibactam against Mycobacterium abscessus

Repurposing drugs may be useful as an add-on in the treatment of Mycobacterium abscessus pulmonary infections, which are particularly difficult to cure. M. abscessus naturally produces a beta-lactamase, Bla(MAb), which is inhibited by avibactam. The recommended regimens include imipenem, which is hydrolyzed by Bla(MAb) and used without any beta-lactamase inhibitor. Here, we determine whether the addition of rifabutin improves the activity of imipenem alone or in combination with avibactam against M. abscessus CIP104536. Rifabutin at 16 mu g/ml was only bacteriostatic (MIC of 4 mu g/ml) and was moderately synergistic in combination with imipenem (fractional inhibitory concentration [FIC] index of 0.38). Addition of rifabutin (16 mu g/ml) moderately increased killing by a low (8 mu g/ml) but not by a high (32 mu g/ml) concentration of imipenem. Addition of avibactam (4 mu g/ml) did not further increase killing by the former combination. In infected macrophages, rifabutin (16 mu g/ml) increased the activity of imipenem at 8 and 32 mu g/ml, achieving 3- and 100-fold reductions in the numbers of intracellular bacteria, respectively. Avibactam (16 mu g/ml) improved killing by imipenem at 8 mu g/ml. A 5-fold killing was obtained for a triple combination comprising avibactam (16 mu g/ml) and therapeutically achievable doses of imipenem (8 mu g/ml) and rifabutin (1 mu g/ml). These results indicate that the imipenem-rifabutin combination should be further considered for the treatment of M. abscessus pulmonary infections in cystic fibrosis patients and that addition of a beta-lactamase inhibitor might improve its efficacy. Mechanistically, the impact of Bla(MAb) inhibition by avibactam on antibiotic activity was assessed by comparing CIP104536 and a beta-lactamase-deficient derivative.