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In vitro susceptibility testing of GSK656 against Mycobacterium tuberculosis complex isolates to establish the epidemiological cut-off values and MIC distribution

A Ghodousi(2) I Iannucci(1) F Saluzzo(1,2) A Yepes(3) D M Cirillo(1,2)

1:Universita Vita-Salute San Raffaele, Milan, Italy; 2:IRCCS San Raffaele Scientific Institute, Milan, Italy; 3:Global Health Medicines R&D, GlaxoSmithKline, Tres Cantos, Madrid 28760. Spain

GSK656 is a novel Benzoxaborole targeting Mycobacterium tuberculosis complex (MTBC) leucyl-tRNA synthetase. The proposed use of GSK3036656 in Phase 2 combination regimen studies calls for the establishment of a robust phenotypic drug susceptibility testing (DST) method and a properly set breakpoint. A first step in this direction is the establishment of an in vitro standardized test based on the EUCAST protocol and the identification of the MIC distribution for the reference strain H37Rv ATCC 27294 and phylogenetically diverse clinical isolates.

To reach this objective, we tested H37Rv ATCC 27294 (31 independent replicates) and a panel of 25 phylogenetically diverse clinical isolates, showing different phenotypic resistant pattern to 1st and 2nd line drugs, using serial 2-fold dilutions from 0,004 to 0,5 mg/mL. Isoniazid was used as control drug against the H37Rv strain (serial 2-fold drug dilutions 0,008 to 1 mg/mL).

We observed a similar distribution of GSK656 MICs for H37Rv and the phylogenetically diverse clinical isolates, without no verifiable associated lineage effect. The MIC mode and the provisory ECOFF were identified respectively at 0.06 and 0.125 μg/mL (Fig 1).

Moreover, the two-laboratory derived low/high level resistant isolates showed an MIC of 0,12 and 2 mg/ml as expected. 

In conclusion, based on the identified MIC distribution we propose a provisory critical concentration of 0.125 μg/mL for GSK656.

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