We evaluated the possible reasons for the large intersubject differences of rifampin concentrations among persons receiving standard treatment dosages. The geometric mean rifampin AUC0-24 and peak concentration were not significantly different between patients with tuberculosis and healthy controls or between African and non-African patients with tuberculosis. However, there were substantial intersubject differences of rifampin exposures (greater than a 10-fold range of rifampin AUC0-24 values) (Fig. ). We found that a SNP in the SLCO1B1 gene was associated with rifampin exposure.
In multivariate ANCOVAs that were adjusted for rifampin dosage (in mg/kg), the adjusted mean rifampin AUC0-24
was significantly lower in patients with tuberculosis from Africa than in healthy controls from North America. The similarity of rifampin (arithmetic and geometric) mean AUC0-24
values among African and non-African patients and controls in univariate analyses in part was due to lower body weight in African versus non-African patients and controls and, as a result, higher rifampin dosage in mg/kg in Africans patients (10.9, 9.8, and 7.3 mg/kg, respectively). Previous studies have shown regional differences in rifampin preparations (2
). van Crevel et al. attributed low rifampin concentrations in 70% of patients in Indonesia to the decreased bioavailability of local drug preparations (29
). To ensure a uniformly bioavailable formulation in this pharmacokinetic study, 93% of patients (from the United States and Africa) used rifampin manufactured by a single U.S. manufacturer. Further, potency of the study rifampin used in the trial was independently confirmed by testing of 12 sample tablets obtained from the drug supply used at the Uganda trial site.
The rifampin AUC0-24
was significantly affected by the SNP of SLCO1B1
c.463C>A. Peptide products of the SLCO
family of genes, organic anion transporter peptides (OATP), mediate transport of xenobiotics in the gastrointestinal tract and liver (11
). Chronic rifampin use induces OATP and P-glycoprotein (MDR1) via activation of the nuclear pregnane X receptor (28
). Further, rifampin is a potent inhibitor of OATP1B1
). Thus, there potentially are alternate ways for SNP of OATP1B1
c.463 to affect rifampin pharmacokinetic parameters. In this study, the adjusted mean rifampin AUC0-24
was 36% lower in 15 people with the SLCO1B1
c.463CA genotype than in 71 people with the c.463CC genotype (P
= 0.001). Effects on rifampin absorption and/or clearance by the c.463CA polymorphism are suggested by the decreased GM rifampin exposure and peak concentration. Opposing effects of increased apparent oral clearance and volume of distribution resulted in no change in half-life. Subjects of black race (most were enrolled from African sites) were more likely to have the SLCO1B1
c.463CA polymorphism associated with lower rifampin exposure. In a previous study, the SLCO
c.521T>C polymorphism did not significantly affect the pharmacokinetics of single-dose rifampin in Asian subjects (15
). With multidose therapy, we showed that the SLCO
c.521TC and -TT genotypes were associated with longer rifampin half-life in univariate but not multivariate analyses.
In this study, plasma rifampin concentrations were measured, but concentrations in lung or other body compartments were not. Conte et al. compared rifampin concentrations in lung epithelial lining fluid, plasma, and alveolar cells (8
). Considerable interpatient variability was found, and rifampin was not detected in epithelial lining fluid samples of 20% of subjects, despite detection in plasma samples (8
In multivariate ANCOVA, significant effects on the rifampin AUC0-24
were demonstrated by tuberculosis disease by geographic region after adjustments for subject rifampin dosage and SLCO
polymorphisms. This suggests that additional processes may affect rifampin exposure. For example, data from a prior study suggested a decrease in absorptive capacity with tuberculosis because patients with low rifampin blood concentrations also demonstrated decreased excretion of orally administered mannitol and lactulose (23
). Differences in rifampin exposures also could be caused by different rates of metabolism. Jamis-Dow and colleagues demonstrated in human liver microsomes that rifampin is metabolized to 25-desacetyl rifampin by B-esterases (16
). However, the effect of B-esterase gene polymorphisms on rifampin pharmacokinetic parameters has not been evaluated.
The substantial intersubject differences in rifampin exposure (Fig. ) are noteworthy. The important metric for comparing pharmacokinetics among populations, however, may not be the mean (or median) value; it may be the proportion of participants with a value below some threshold rifampin concentration. In this regard, it is notable that a number of patients with active tuberculosis had low rifampin concentrations. There is currently great interest in higher dosages and more frequent dosing of rifamycins to increase the potency of tuberculosis treatment regimens and allow treatment shortening. Improved therapeutic efficacy may result from increased rifamycin dosage not only by increased mean (and median) rifamycin concentrations but also by greater concentrations above a minimum threshold among patients with the lowest exposures.
In TBTC studies 27 and 28, from which the patients in this pharmacokinetic substudy were recruited, patients who enrolled at two African sites had much lower 2-month sputum culture conversion than did patients who enrolled at sites in North America, Spain, and Brazil. For example, the proportion of patients with negative cultures after 2 months of treatment in study 27 was 60% (108/179) among Africans patients, compared to 84% (91/108) among patients recruited elsewhere (P < 0.0001). However, the difference in 2-month culture conversions between African and non-African patients was not simply explained by a difference in the unadjusted mean rifampin AUC or peak concentration. More likely, the lower response to intensive-phase therapy among patients enrolled at African sites is multifactorial.
Our study has several limitations. First, patients with tuberculosis were recruited as a convenience sample into the pharmacokinetic substudy during two clinical trials. However, differences in the individual drugs in these regimens did not appear to significantly influence rifampin exposure or other factors in multivariate analyses. Second, we did not evaluate rifampin pharmacokinetics in tissue compartments (e.g., alveolar lining fluid) that may be important in its action (8
). Because drug transporters mediate uptake in the intestine, liver, and brain, further investigation is needed to assess the role of SNPs of drug transporters (SLCO1B1
, and MDR1
) in other tissue compartments. Third, the sample size (and power) of the study was not sufficient to evaluate subgroup differences (e.g., only three patients with HIV coinfection were sampled). Fourth, study subjects were sampled after a greater mean number of daily rifampin doses were administered to patients with tuberculosis than to controls. However, steady-state conditions are reported to be achieved after the sixth daily dose of rifampin (1
), and in this study, all study subjects were sampled after at least nine rifampin doses and after at least three consecutive daily doses. Also, in multivariate analyses, the number of rifampin doses received at the time of pharmacokinetic sampling was not significantly associated with rifampin exposure or peak concentration. Fifth, although we did not adjust for the multiple comparisons made in the analysis, the effect of multiple comparisons was assessed using the false discovery rate. Finally, residual confounding by measured or unmeasured variables is possible.
The strengths of this pharmacokinetic study are the comparison of healthy controls to well-characterized patients who were receiving directly supervised therapy during carefully conducted clinical trials, the enrollment of patients with tuberculosis from three continents, and the use of pharmacokinetic sampling at 7 time points to capture complete pharmacokinetic data for study subjects.
In summary, marked intersubject variations of rifampin AUC0-24 values was observed, but mean AUC0-24 values did not significantly vary between patients with tuberculosis and healthy controls. Lower rifampin exposure was associated with the polymorphism of the SLCO1B1 c.463C>A gene. When adjusted for rifampin dosage and transporter gene polymorphisms, rifampin exposure was significantly lower with tuberculosis disease in Africans, which suggests that additional absorption or metabolic processes affect rifampin exposure.