Effects of Increasing Concentrations of Rifampicin on Different Mycobacterium tuberculosis Lineages in a Whole-Blood Bactericidal Activity Assay

Abstract

High-dose rifampicin improved bactericidal activity and culture conversion in early-phase tuberculosis (TB) trials, done mainly in Africa. We performed a whole-blood bactericidal activity (WBA) study to determine whether the effects of high-dose rifampicin differ across globally relevant TB strains and whether effects are similar in dormant bacilli that will be required for enhancing cure. Whole blood from healthy volunteers was spiked with rifampicin (range, 0.63 to 60 mg/L) and incubated with one of four Mycobacterium tuberculosis clinical strains (Haarlem, Latin American-Mediterranean [LAM], East African-Indian [EAI], and Beijing lineages) or a dormant strain (streptomycin-starved 18b [ss18b]). Change in bacterial CFU was estimated after inoculation of WBA cultures in MGIT. WBA increased with higher concentrations of rifampicin in all strains. At rifampicin concentrations up to 5 mg/L, the rates of increase in WBA per unit increase in rifampicin concentration were similar in all 4 clinical strains (P > 0.51). Above 5 mg/L, EAI (P < 0.001) and Beijing (P = 0.007) strains showed greater increases in WBA than did LAM; Haarlem was similar to LAM. The dormant strain showed a lower rate of increase in WBA than clinical strains at rifampicin concentrations up to 5 mg/L; above 5 mg/L, the rate of increase was similar to those in the LAM, Beijing, and Haarlem strains. Increasing rifampicin concentration enhanced WBA in all strains; the greatest effects were seen in strains common in Asia, suggesting that early-phase trial findings may be generalizable beyond Africa. Similar effects of high concentrations of rifampicin on the dormant strain support the concept that this intervention may enhance sterilizing activity.

Keywords: Mycobacterium tuberculosis; rifampicin; whole-blood bactericidal activity.

FIG 1

(a) Mean WBA at various concentrations of rifampicin against different M. tuberculosis strains. Error bars represent 1 standard deviation at each side (values represent total bactericidal kill after a 72-h assay). (b) Fitted curve at various concentrations of rifampicin against different M. tuberculosis strains (values represent total bactericidal kill after a 72-h assay). Strains tested included Haarlem, EAI (East-African Indian), Beijing, LAM (Latin American Mediterranean), and ss18b (streptomycin-starved 18b strain).