Estimating optimal therapeutic drug levels of anti-tuberculosis medications based on treatment safety and effectiveness.

Background: Therapeutic drug ranges (TDR) for standard anti-tuberculosis (TB) treatment have been determined based on expected drug levels at least 2 hours after taking the dose. In this study we constructed TDR for TB drug levels based on minimizing drug toxicity and maximizing treatment effectiveness.
Methods: Participants were followed prospectively in the Regional Prospective Observational Research in Tuberculosis (RePORT)-Brazil observational cohort study. We focused on participants with culture-confirmed drug-susceptible pulmonary TB who underwent standard TB therapy. TDR were estimated for each TB drug separately: isoniazid (INH), rifampin (RIF), ethambutol (EMB), and pyrazinamide (PZA). TDR were defined as drug concentrations that were both safe and effective: safety was defined as the probability of having an ADR of at most 5%, while effectiveness was defined as a probability of at least 95% of not having either TB treatment failure or recurrence.
Results: There were 765 plasma samples from 448 patients; 110 (24.6%) were people with HIV, 9 (2.0%) had a grade 3 or higher ADR, and 15 (3.3%) had treatment failure/recurrence. Higher drug concentrations of INH, RIF and EMB were associated with increased odds of having ADR. High concentrations of INH suggested protection against treatment failure/recurrence. Estimated therapeutic drug range for INH (2.3-8.2 µg/ml) and for RIF (0.5-7.5 µg/ml) differed from the currently recommended drug ranges (3-5 µg/ml and 8-24 µg/ml, respectively). Estimates for PZA and EMB were similar to the currently recommended values.
Conclusions: Our estimated upper end TDR were higher for INH and lower for RIF compared to currently recommended ranges.