Could high-concentration rifampicin kill rifampicin-resistant M. tuberculosis? Rifampicin MIC test in rifampicin-resistant isolates from patients with osteoarticular tuberculosis

PURPOSE Several studies have shown that the intralesional concentration of rifampicin in osteoarticular tuberculosis is typically at a subtherapeutic level. Sustained or controlled release by novel drug delivery systems has been investigated to maintain an effective rifampicin concentration, but the local administration of rifampicin remains controversial. Additionally, it is still unclear whether high-dose rifampicin could kill rifampicin-resistant Mycobacterium tuberculosis. The aim of this study was to assess the in vitro killing effect of high-concentration rifampicin on rifampicin-resistant M. tuberculosis isolated from patients with osteoarticular tuberculosis. METHODS A set of 18 rifampicin-resistant M. tuberculosis isolates by the BACT/MGIT 960 system from patients with osteoarticular tuberculosis was collected for further study. The detection of rpoB gene mutations was performed using non-fluorescent, low-density DNA microarrays to determine the resistant mechanism. Following secondary culture, susceptibility to gradient concentrations of rifampicin (2 to 256 μg/ml) was tested; these concentrations are attainable for prolonged periods of local chemotherapy. The relationship between microbial killing by high-dose rifampicin and rpoB gene mutations was analyzed. RESULTS Mutations in the rifampicin resistance-determining region (RRDR) of the rpoB gene were identified in 17 isolates (94.4%); one strain exhibited no mutations in this region. The most prevalent mutation sites were in codons 531 (55.56%), 516 (16.67%), 526 (11.11%), and 513 (11.11%). Isolates with mutations in the rpoB gene were highly resistant to rifampicin, 11 of which had minimal inhibitory concentrations (MICs) exceeding 256 μg/ml (not determined). The MICs for the remaining seven resistant isolates were between 32 and 256 μg/ml. Particularly in less rifampicin-resistant M. tuberculosis strains, growth was inhibited at high concentrations. CONCLUSION Increasing the rifampicin concentration may optimize this drug's antituberculous effect, even against some rifampicin-resistant isolates, if systemic and local toxic effects can be minimized.