Mycobacterium abscessus strain variability in preclinical drug development: does it really matter?

New treatment options for Mycobacterium abscessus infections are urgently needed. Since a correlation between MICs and clinical outcomes is not clearly established, potency of novel drugs needs to be evaluated using additional in vitro drug activity assays. Preclinical drug activity assays generally use the M. abscessus type strain ATCC 19977. However, M. abscessus complex entails a genetically and morphologically diverse group, and it is questionable whether drug activity observed against ATCC 19977 is representative of drug activity against clinical M. abscessus isolates.

To assess whether the relationship between MIC and the quantitative antimycobacterial activity of amikacin, imipenem and clofazimine differs between the ATCC 19977 strain and clinical M. abscessus isolates.

Experiments were performed with M. abscessus ATCC 19977 and a subset of six clinical isolates covering the three M. abscessus subspecies and the smooth and rough morphotypes. Cultures were exposed to the drugs at 4-fold increasing, MIC-standardized concentrations, and the mycobacterial load was assessed over time.

Concentration- and time-dependent activity of amikacin, imipenem and clofazimine against the six clinical isolates was similar. Only slight variations in drug activity were observed between ATCC 19977 and clinical isolates.

Time- and concentration-dependent drug activity against the ATCC 19977 strain seems indicative for in vitro drug behaviour against M. abscessus complex clinical isolates. Including one clinical smooth morphotype isolate alongside ATCC 19977 seems appropriate for reliable interpretation of this particular in vitro drug activity assay as part of the M. abscessus preclinical drug development pipeline.