The pyrazinamide susceptibility breakpoint above which combination therapy fails

Abstract

Objectives: To identify the pyrazinamide MIC above which standard combination therapy fails.

Methods: MICs of pyrazinamide were determined for Mycobacterium tuberculosis isolates, cultured from 58 patients in a previous randomized clinical trial in Cape Town, South Africa. The MICs were determined using BACTEC MGIT 960 for isolates that were collected before standard treatment with isoniazid, rifampicin, pyrazinamide and ethambutol commenced. Weekly sputum collections were subsequently made for 8 weeks in order to culture M. tuberculosis in Middlebrook broth medium. Classification and regression tree (CART) analysis was utilized to identify the pyrazinamide MIC predictive of sputum culture results at the end of pyrazinamide therapy. The machine learning-derived susceptibility breakpoints were then confirmed using standard association statistics that took into account confounders of 2 month sputum conversion.

Results: The pyrazinamide MIC range was 12.5 to >100 mg/L for the isolates prior to therapy. The epidemiological 95% cut-off value was >100 mg/L. The 2 month sputum conversion rate in liquid cultures was 26% by stringent criteria and 48% by less stringent criteria. CART analysis identified an MIC breakpoint of 50 mg/L, above which patients had poor sputum conversion rates. The relative risk of poor sputum conversion was 1.5 (95% CI: 1.2-1.8) for an MIC >50 mg/L compared with an MIC ≤ 50 mg/L.

Conclusions: We propose a pyrazinamide susceptibility breakpoint of 50 mg/L for clinical decision making and for development of rapid susceptibility assays. This breakpoint is identical to that identified using computer-aided simulations of hollow fibre system output.

Keywords: MICs; anti-tuberculosis drugs; drug susceptibility; pharmacokinetics; sputum culture.

© The Author 2014. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy.

Figures

Figure 1.

Pyrazinamide MIC distribution in 58 tuberculosis patients. Distribution of MICs, based on inputting all values >100 mg/L as 200 mg/L. As a result, the range goes up to 400 mg/L, which would have a value of 0% of clinical isolates.

Figure 2.

CART analysis for primary outcome. The primary node, based on the stringent definition of 2 month sputum conversion, shows that the MIC above which therapy fails is 75 mg/L.

Figure 3.

CART analysis for secondary outcome. The MIC above which therapy fails, based on a less stringent definition of 2 month sputum conversion was >37.5 mg/L.