Effects of increasing concentrations of rifampicin on different Mycobacterium tuberculosis lineages in a Whole blood Bactericidal Activity Assay

Objectives: High-dose rifampicin improved bactericidal activity and culture conversion in early-phase 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 strains; and whether effects are similar in dormant bacilli that will be required for enhancing cure. Methods: Whole blood from healthy volunteers was spiked with rifampicin (range 0.63-60mg/L) and incubated with one of 4 Mycobacterium tuberculosis (Mtb) clinical strains (Haarlem, Latin American-Mediterranean [LAM], East African-Indian [EAI] and Beijing lineages) or a dormant strain (Streptomycin-Starved Mtb 18b strain [ss18b]). Change in bacterial cfu was estimated after inoculation of WBA cultures in MGIT. Results: WBA increased with higher concentrations of rifampicin in all strains. At rifampicin concentrations up to 5mg/L the rate of increase in WBA per unit increase in rifampicin concentration was similar in all 4 clinical strains (p>0.51). Above 5mg/L, EAI (p<0.001) and Beijing (p=0.007) strains showed greater increase 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 5mg/L; above 5mg/L the rate of increase was similar to LAM, Beijing and Haarlem strains. Conclusions: 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.