Standards for model-based early bactericidal activity analysis and sample size determination in tuberculosis drug development

Early bactericidal activity of sitafloxacin against pulmonary tuberculosis

Sitafloxacin is a quinolone broad-spectrum antimicrobial agent, and its pharmacologic properties and in vitro data demonstrate that sitafloxacin has a potent killing effect against Mycobacterium tuberculosis, including drug-resistant strains, which is superior to that of other available quinolones. However, its efficacy in patients with primary-sensitive tuberculosis is unclear. This study aims to evaluate the early bactericidal activity (EBA) of sitafloxacin in patients with primary drug-susceptible tuberculosis. In this early bactericidal activity study, 30 patients with primary smear-positive tuberculosis were randomized to the once-daily oral administration of 200 mg sitafloxacin, 500 mg levofloxacin, or 300 mg isoniazid (INH) for 7 days. Sputum for quantitative culture was collected 2 days before the study of drug administration, followed by 16 hours of overnight sputum collected daily for 7 days of monotherapy. Colony-forming units (CFU) of Mycobacterium tuberculosis were counted from the collected overnight sputum on agar plates to calculate the EBA, defined as log10 CFU/mL sputum/day. The bactericidal activity was measured by measuring the first 2 days (early bactericidal activity 0-2) and the last 5 days (prolonged early bactericidal properties 2-7) of study drug administration. The EBA 0-2 of INH (0.39 ± 0.22 log10CFU/mL/day) was higher than that of levofloxacin (0.26 ± 0.27 log10CFU/mL/day) and sitafloxacin (0.22 ± 0.25 log10CFU/mL/day), with no statistically significant difference (P = 0.08). EBA 0-2 was similar for the three drugs. INH prolonged early bactericidal activity (2-7) (0.17 ± 0.16 log10CFU/mL/day) was higher than levofloxacin (0.14 ± 0.10 log10CFU/mL/day) and lower than sitafloxacin (0.26 ± 0.31 log10CFU/mL/day), with no statistically significant difference (P = 0.59). The EBA 2-7 of sitafloxacin showed higher activity than INH and levofloxacin. Sitafloxacin exhibits comparable early bactericidal activity and higher extended early bactericidal activity relative to levofloxacin. In addition, this novel fluoroquinolone has a good safety profile. The study data highlights the potential of sitafloxacin in the clinical management of drug-susceptible tuberculosis, as well as drug-resistant tuberculosis.IMPORTANCESitafloxacin is a quinolone broad-spectrum antimicrobial agent, and its pharmacologic properties and in vitro data demonstrate that sitafloxacin has a potent killing effect against Mycobacterium tuberculosis. However, its efficacy in patients with primary-sensitive tuberculosis is unclear. We investigated the early bactericidal activity of sitafloxacin in primary susceptible tuberculosis. The results showed that sitafloxacin exhibited comparable early bactericidal activity and higher extended early bactericidal activity relative to levofloxacin. In addition, this novel fluoroquinolone has a good safety profile. Our study data highlights the potential of sitafloxacin in the clinical management of drug-susceptible tuberculosis, as well as drug-resistant tuberculosis.

Power to identify exposure-response relationships in phase IIa pulmonary tuberculosis trials with multi-dimensional bacterial load modeling

Adequate power to identify an exposure-response relationship in a phase IIa clinical trial for pulmonary tuberculosis (TB) is important for dose selection and design of follow-up studies. Currently, it is not known what response marker provides the pharmacokinetic-pharmacodynamic (PK-PD) model more power to identify an exposure-response relationship. We simulated colony-forming units (CFU) and time-to-positivity (TTP) measurements for four hypothetical drugs with different activity profiles for 14 days. The power to identify exposure-response relationships when analyzing CFU, TTP, or combined CFU + TTP data was determined at 60 total participants, or with 25 out of 60 participants in the lowest and highest dosing groups (unbalanced design). For drugs with moderate bactericidal activity, power was low (<59%), irrespective of the data analyzed. Power was 1.9% to 29.4% higher when analyzing TTP data compared to CFU data. Combined analysis of CFU and TTP further improved the power, on average by 4.2%. For a drug with a medium-high activity, the total sample size needed to achieve 80% power was 136 for CFU, 72 for TTP, and 68 for combined CFU + TTP data. The unbalanced design improved the power by 16% over the balanced design. In conclusion, the power to identify an exposure-response relationship is low for TB drugs with moderate bactericidal activity or with a slow onset of activity. TTP provides the PK-PD model with more power to identify exposure-response relationships compared to CFU, and combined analysis or an unbalanced dosing group study design offers modest further improvement.