The New Antitubercular Drugs SQ109 and TMC207 Act Synergistically In Vitro to Kill M. tuberculosis

A number of initiatives to discover and develop new therapeutic agents that not only cure TB but also shorten the duration of treatment are ongoing (5, 13). SQ109 and TMC207 are among new drugs being developed for therapy of TB, and both are currently in human clinical trials. SQ109 is a N-Geranyl-N’-(2adamantyl) ethane-1,2-diamine discovered as best-in-class of a 63,000 combinatorial chemistry library of diamine antimicrobial agents (8,10). It is most active against M. tuberculosis (MTB) complex bacteria, and is as active on multi-drug resistant (MDR) MTB as on drug sensitive strains. SQ109 inhibits cell wall biosynthesis, although the exact site of action is not yet known (7). TMC207 (also known as R207910) is a derivative of the diarylquinoline class of antimicrobial drugs. TMC207 is highly specific for Mycobacteria, and is equally effective against susceptible and MDR MTB (1). Both SQ109 and TMC207 are bactericidal. In combination drug studies with existing front-line antituberculars, SQ109 acts synergistically with isoniazid (INH) and rifampin (RIF) (3), whereas TMC207 is synergistic with pyrazinamide (PZA) (6). In this study we investigated, in vitro and in macrophages, the efficacy of combinations of TMC207 and SQ109 against MTB. MIC against MTB (H37Rv): By microdilution, the MIC of SQ109, TMC207 and RIF against H37Rv were 0.5, 0.25 and 0.0078 μg/ml, respectively. By BACTEC method the MIC of these drugs against H37Rv were 0.25, 0.062 and 0.062 μg/ml, respectively. The MIC of SQ109 and TMC207 were one or two dilutions lower by BACTEC compared to microdilution method; the reverse was true for RIF. In vitro synergy between SQ109, TMC207, and RIF: Two drug combinations: Combination of SQ109 with TMC207 was synergistic with H37Rv with ΣFIC of 0.375 in one experiment and 0.5 in the second. Against two clinical isolates of MTB, SQ109+TMC207 combination was additive displaying better activity against MTB8330 than MTB8642 with ΣFIC 0.625 and 2.0, respectively. SQ109+TMC207 combination displayed additive effect against MSMG (ΣFIC=1.0). SQ109+RIF combination, on the other hand, was synergistic against MTB (H37Rv) as well as MSMG with ΣFIC of 0.094 and 0.132, respectively. In contrast TMC207+RIF combination had additive effects against H37Rv as well as two other clinical isolates of MTB with ΣFIC of 2.0 against all the isolates. None of the combinations was antagonistic (Table 1). Three drug combinations tested for interactions on MTB-H37Rv: The two drugs (TMC207 and SQ109) were titrated in checkerboard titration in microtiter plates containing different concentrations of RIF: 0.5, 0.2, 0.1 and 0.05 x MIC. This study was designed to determine the lowest concentration (sub-MIC) of RIF that interacts with TMC207 and SQ109 to induce the greatest synergistic activity. Table 2 shows three drug combination data from one of the two experiments. RIF at 0.5 and 0.2 MIC caused 2 fold decrease in MIC of SQ109 (Plate 1 & 2), but had no effect at 0.1 and 0.05 MIC (Plates 3 & 4). RIF did not contribute to already existing synergy between SQ109 and TMC207 in combination (Plate 5). Rate of killing of MTB: Rate of killing is the time required to kill ≥90% of initial organisms in the original culture at a given concentration of drug. By RLU, the drugs individually at 2x MIC ranked SQ109>RIF>>TMC207, but the drugs at 4x and 8x MIC ranked as follows: TMC207>>SQ109>RIF (Table 3). In drug combinations, SQ109+TMC207 was more effective, followed by TMC207+RIF and SQ109+RIF. Combinations of RIF with either of the drugs failed to enhance the rate of killing significantly over the individual drugs.