BACKGROUND: PZA resistance in multidrug-resistant tuberculosis (MDR-TB) is common and it is not clear how it affects interim and treatment outcomes. Although rarely performed, phenotypic drug susceptibility testing (pDST) is used to define PZA resistance but genotypic DST (gDST) and minimum inhibitory concentration (MIC) could be beneficial. We aimed to assess the impact of PZA gDST and MIC on time to sputum culture conversion (SCC) and treatment outcome in patients with MDR-TB.

METHODS: Clinical, microbiological and treatment data was collected in this cohort study for all patients diagnosed with MDR-TB in Sweden 1992-2014. MIC, pDST and whole genome sequencing of the pncA, rpsA and panD genes were used to define PZA resistance. A Cox regression model was used for statistical analyses.

RESULTS: Of 157 patients with MDR-TB, 56.1% (n=88) had PZA resistant strains and 49.7% (n=78) were treated with PZA. In crude and adjusted analyses, PZA gDST resistance was associated with a 29-day longer time to SCC (hazard ratio [HR] 0.57, 95% confidence interval [CI] 0.36-0.89, p=0.013 and HR 0.49, 95% CI 0.29-0.82, p=0.007, respectively). A two-fold decrease in dilutions of PZA MIC for PZA susceptible strains showed no association with SCC in crude or adjusted analyses (HR 0.98, 95% CI 0.73-1.31, p=0.89). Genotypic DST and MIC for PZA were not associated with treatment outcome.

CONCLUSION: In patients with MDR-TB, gDST PZA resistance was associated with a longer time to SCC. Rapid PZA gDST is important to identify patients who may benefit from PZA treatment.

Background: Therapeutic drug monitoring (TDM) could improve current TB treatment, but few studies have reported pharmacokinetic data together with MICs. Objectives: To investigate plasma concentrations of rifampicin, isoniazid, pyrazinamide and ethambutol along with MICs. Methods: Drug concentrations of rifampicin, isoniazid, pyrazinamide and ethambutol were analysed pre-dose and 2, 4 and 6 h after drug intake at week 2 in 31 TB patients and MICs in BACTEC 960 MGIT were determined at baseline. The highest plasma concentrations at 2, 4 and 6 h post-dose (C-high) were determined, as well as estimates of C-high/MIC and area under the concentration-time curve (AUC(0-6))/MIC including the corresponding ratios based on calculated free-drug concentrations. This trial was registered at www.clinicaltrials.gov (NCT02042261). Results: After 2 weeks of treatment, the median C-high values for rifampicin, isoniazid, pyrazinamide and ethambutol were 10.0, 5.3, 41.1 and 3.3 mg/L respectively. Lower than recommended drug concentrations were detected in 42% of the patients for rifampicin (amp;lt;8 mg/L), 19% for isoniazid (amp;lt;3 mg/L), 27% for pyrazinamide (amp;lt;35 mg/L) and 16% for ethambutol (amp;lt;2 mg/L). The median Chigh/MIC values for rifampicin, isoniazid, pyrazinamide and ethambutol were 164, 128, 1.3 and 2.5, respectively, whereas the AUC(0-6)/MIC was 636 (range 156-2759) for rifampicin and 351 (range 72-895) for isoniazid. Conclusions: We report low levels of first-line TB drugs in 16%-42% of patients, in particular for rifampicin. There was a wide distribution of the ratios between drug exposures and MICs. The future use of MIC determinations in TDM is dependent on the development of a reference method and clinically validated pharmacokinetic/pharmacodynamic targets.