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 β-lactamase, BlaMAb, which is inhibited by avibactam. The recommended regimens include imipenem, which is hydrolyzed by BlaMAb and used without any β-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 μg/ml was only bacteriostatic (MIC of 4 μg/ml) and was moderately synergistic in combination with imipenem (fractional inhibitory concentration [FIC] index of 0.38). Addition of rifabutin (16 μg/ml) moderately increased killing by a low (8 μg/ml) but not by a high (32 μg/ml) concentration of imipenem. Addition of avibactam (4 μg/ml) did not further increase killing by the former combination. In infected macrophages, rifabutin (16 μg/ml) increased the activity of imipenem at 8 and 32 μg/ml, achieving 3- and 100-fold reductions in the numbers of intracellular bacteria, respectively. Avibactam (16 μg/ml) improved killing by imipenem at 8 μg/ml. A 5-fold killing was obtained for a triple combination comprising avibactam (16 μg/ml) and therapeutically achievable doses of imipenem (8 μg/ml) and rifabutin (1 μg/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 β-lactamase inhibitor might improve its efficacy. Mechanistically, the impact of BlaMAb inhibition by avibactam on antibiotic activity was assessed by comparing CIP104536 and a β-lactamase-deficient derivative.