High-Dose Rifamycins Enable Shorter Oral Treatment in a Murine Model of Mycobacterium ulcerans Disease

Abstract

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is a neglected tropical skin and soft tissue infection that is associated with disability and social stigma. The mainstay of BU treatment is an 8-week course of rifampin (RIF) at 10 mg/kg of body weight and 150 mg/kg streptomycin (STR). Recently, the injectable STR has been shown to be replaceable with oral clarithromycin (CLR) for smaller lesions for the last 4 weeks of treatment. A shorter, all-oral, highly efficient regimen for BU is needed, as the long treatment duration and indirect costs currently burden patients and health systems. Increasing the dose of RIF or replacing it with the more potent rifamycin drug rifapentine (RPT) could provide such a regimen. Here, we performed a dose-ranging experiment of RIF and RPT in combination with CLR over 4 weeks of treatment in a mouse model of M. ulcerans disease. A clear dose-dependent effect of RIF on both clinical and microbiological outcomes was found, with no ceiling effect observed with tested doses up to 40 mg/kg. RPT-containing regimens were more effective on M. ulcerans All RPT-containing regimens achieved culture negativity after only 4 weeks, while only the regimen with the highest RIF dose (40 mg/kg) did so. We conclude that there is dose-dependent efficacy of both RIF and RPT and that a ceiling effect is not reached with the current standard regimen used in the clinic. A regimen based on higher rifamycin doses than are currently being evaluated against tuberculosis in clinical trials could shorten and improve therapy of Buruli ulcer.

Keywords: Buruli ulcer; Mycobacterium ulcerans; clarithromycin; high-dose rifamycins; rifampin; rifapentine.

FIG 1

Footpad swelling grade of infected mouse footpads in response to treatment with high-dose rifamycins and clarithromycin. Treatment was initiated 6 weeks after infection, when swelling approached swelling grade 2. Swelling grade 0 corresponds to no clinically visible pathology. Swelling grade 1 infers redness of the footpad, grade 2 edematous swelling of the footpad, and grade 3 ascending swelling of the leg and impeding necrosis. Data points represent medians per treatment group. Data were normalized to day 0 (beginning of treatment) by subtracting from the median swelling grade of all mice at D0 and assuming the total median as group mean for that time point. All regimens reduced swelling grade compared with untreated mice. There was a visible dose-dependent effect; with escalating doses of rifampin (A) and rifapentine (B), swelling grade was reduced more drastically. Minimum swelling grade values of 0.23 and 0.15 were recorded at week 4 for the highest-dose regimens RIF40CLR100 and RPT20CLR100, respectively. Numbers after drugs indicate doses in mg/kg. D, day; RIF, rifampin; STR, streptomycin; CLR, clarithromycin; RPT, rifapentine.

FIG 2

Mean relative light unit (RLU) counts from infected mouse footpads in response to treatment with escalating doses of rifamycins. Mice were infected with an autoluminescent strain of M. ulcerans that emits light when metabolically active. The mean per group RLU values are displayed, compared with day 0 when the mean (±SD) RLU was 35.09 (±6.89). As observed in previous experiments, RLU counts stagnate at 7- to 9-weeks postinfection (i.e., here, 3 weeks of treatment) when a static bacterial growth phase is reached, as seen in untreated mice. Treatment with rifamycins radically reduced RLU counts compared with untreated and CLR-only-treated mice. RIF, rifampin; STR, streptomycin; CLR, clarithromycin; RPT, rifapentine.

FIG 3

Microbiological outcome after 2 (A) and 4 (B) weeks of treatment with rifamycin-containing regimens. Mice were infected with 4.56 log₁₀ CFUs of M. ulcerans into hind footpads. After 6 weeks of incubation, treatment was initiated (D0). At this time point, the CFU mean (±SD) equaled 5.77 (±0.60). Groups of mice (n = 5) were sacrificed at week 2 and week 4, and footpads were dissected, minced, and plated on 7H11 agar for colony counting and CFU analysis. There was a dose-dependent reduction in CFU with elevating rifamycin doses. No ceiling effect was observed for RIF. At week 4, all rifapentine-containing regimens were culture negative, as was RIF40CLR100. Numbers after drugs indicate doses in mg/kg. RIF, rifampin; STR, streptomycin; CLR, clarithromycin; RPT, rifapentine.