In vitro effect of three-drug combinations of antituberculous agents against multidrug-resistant Mycobacterium tuberculosis isolates

Multidrug resistance has become a problem in the management of tuberculosis, leading to an urgent need for research related to new regimens including the currently available drugs. The objectives of this study were: (i) to study the effect of the following second-choice three-drug combinations against multidrug-resistant (MDR) and drug-susceptible clinical isolates (levofloxacin, linezolid and ethambutol; levofloxacin, amikacin and ethambutol; and levofloxacin, linezolid and amikacin); and (ii) to compare the effect of these combinations with an isoniazid, rifampicin and ethambutol combination against drug-susceptible clinical isolates. A total of 9 MDR clinical and 12 drug-susceptible isolates (11 clinical isolates and the H37Rv reference strain) were studied using an adaptation of the chequerboard assay. The fractional inhibitory concentration index (FICI) was calculated as follows: FICI = FIC A + FICB + FICC = A/MICA + B/MICB + C/MICC, where A, B and C are the minimum inhibitory concentrations (MICs) of each antibiotic in combination and MIC A, MICB and MICC are the individual MICs. The FICI was interpreted as synergism when the value was <0.75. The FICI of all the combinations ranged from 1.5 to 3, showing indifferent activity. No differences were found between MDR and drug-susceptible isolates, or between the second-choice combinations and the fourth combination against drug-susceptible isolates. In conclusion, the second-choice drugs are equally effective as the combination of isoniazid, rifampicin and ethambutol. © 2013 Elsevier B.V. and the International Society of Chemotherapy.