Relationship between plasma and intracellular concentrations of bedaquiline and its M2 metabolite in South African patients with rifampin-resistant TB.

Bedaquiline is recommended for the treatment of all patients with rifampin-resistant tuberculosis (RR-TB). Bedaquiline accumulates within cells, but its intracellular pharmacokinetics have not been characterized, which may have implications for dose optimization. We developed a novel assay using high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) to measure the intracellular concentrations of bedaquiline and its primary metabolite M2 in patients with RR-TB in South Africa. Twenty-one participants were enrolled and underwent sparse sampling of plasma and peripheral blood mononuclear cells (PBMCs) at months 1, 2 and 6 of treatment and at 3 and 6 months after bedaquiline treatment completion. Intensive sampling was performed at month 2. We used non-compartmental analysis to describe plasma and intracellular exposures and a population pharmacokinetic model to explore the relationship between plasma and intracellular pharmacokinetics and the effects of key covariates. Bedaquiline concentrations from month 1 to month 6 of treatment ranged from 94.7 - 2540 ng/mL in plasma and 16.2 - 5478 ng/mL in PBMCs and concentrations of M2 over the six-month treatment period ranged from 34.3 - 496 ng/mL in plasma and 109.2 - 16764 ng/mL in PBMCs. Plasma concentrations of bedaquiline were higher than M2, but intracellular concentrations of M2 were considerably higher than bedaquiline. In the pharmacokinetic modeling, we estimated a linear increase in the intracellular-plasma accumulation ratio for bedaquiline and M2, reaching maximum effect after 2 months of treatment. The typical intracellular-plasma ratio 1 and 2 months after start of treatment was 0.61 (95%CI: 0.42-0.92) and 1.10 (95%CI: 0.74-1.63) for bedaquiline and 12.4 (95%CI: 8.8-17.8) and 22.2 (95%CI: 15.6-32.3) for M2. The intracellular-plasma ratio for both bedaquiline and M2 was decreased by 54% (95%CI: 24-72%) in HIV-positive patients compared to HIV-negative patients. Bedaquiline and M2 were detectable in PBMCs 6 months after treatment discontinuation. M2 accumulated at higher concentrations intracellularly than bedaquiline, supporting in vitro evidence that M2 is the main inducer of phospholipidosis.