Activity of rifapentine and its metabolite 25-O-desacetylrifapentine compared with rifampicin and rifabutin against Mycobacterium tuberculosis, Mycobacterium africanum, Mycobacterium bovis and M. bovis BCG

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Abstract
The in vitro activity of rifapentine and its metabolite, 25-O-desacetylrifapentine, as compared with that of rifampicin and rifabutin, was determined against Mycobacterium tuberculosis, Mycobacterium africanum, Mycobacterium bovis and M. bovis BCG. MICs were determined radiometrically and by the 1% proportional method using Middlebrook 7H11 agar. The bactericidal effect of the drugs was determined in parallel at selected concentrations. For drugsusceptible isolates of M. tuberculosis, the Bactec MICs of rifapentine and 25-O-desacetylrifapentine were 0.03–0.06 mg/L and 0.125–0.25 mg/L, respectively. Similar MICs were obtained for M. africanum (0.03–0.125 and 0.125–0.50 mg/L, respectively), and M. bovis (0.063–0.25 and 0.125–1.0 mg/L, respectively), but MICs were considerably lower for M. bovis BCG (0.008–0.063 mg/L for rifapentine and 0.016–0.125 mg/L for its metabolite). In general, MICs determined using 7H11 agar medium were usually one or two dilutions higher than those obtained using Bactec broth. When compared with rifampicin and rifabutin, the inhibitory activity of rifapentine for drug-susceptible isolates was roughly equal to that of rifabutin, and the inhibitory activity of 25-O-desacetylrifapentine was comparable to that of rifampicin; however, rifapentine was somewhat more bactericidal than rifabutin at equal concentrations. Clinical isolates of M. tuberculosis with a high degree of resistance to rifampicin (MIC ≥ 32 mg/L) were also highly resistant to rifabutin, rifapentine and 25-O-desacetylrifapentine, although the MICs of rifabutin in this case were somewhat lower than the MICs of rifapentine.