A double-blind study to determine the maximum tolerated dose of ethionamide, when administered twice-weekly to patients with pulmonary tuberculosis

Ethionamide population pharmacokinetics/pharmacodynamics and therapeutic implications in South African adult patients with drug-resistant tuberculosis

INTRODUCTION

Ethionamide is part of the drug-resistant tuberculosis regimen whose pharmacokinetic (PK) and pharmacodynamic (PD) information is limited. The aim of the study was to describe the PK and simulate doses to assess PD attainment.

METHODS

This was an observational population PK study of patients admitted for drug-resistant tuberculosis at a hospital in South Africa. Nonlinear mixed-effects modelling implemented in Monolix 2019R2 was used to estimate population pharmacokinetic parameters. We performed Monte Carlo simulations to assess and optimise the dose regimen. The target C_max range was 2.5-5 μg/mL, which is within the minimum inhibitory concentration (MIC) range. The target AUC_0-24h was 140.5 μg*h/mL, which corresponds to the PK/PD target ratio AUC_0-24h /MIC of 56.2.

RESULTS

A one-compartment pharmacokinetic model with a lag-time, first-order absorption and elimination best described the PK of ethionamide. The lag-time, absorption rate constant (ka), volume of distribution (V/F) and clearance (Cl/F) were 0.66 hours, 0.434 h^-1 , 180 L and 99.5 L/h, respectively, for a typical individual weighing 52.6 kg. Between-subject variability in lag-time, ka, V/F and Cl/F were 38%, 92%, 168% and 120%, respectively. Simulation of the recommended doses of 15-20 mg/kg, 500 mg, 750 mg and 1000 mg for patients in the weight bands <33, 33-50, 51-70 and >70 kg resulted in <17% and 3% of the patients achieving the target C_max and AUC_0-24h , respectively.

CONCLUSION

There is high variability in ethionamide PK and very few patients attain the desired target exposure at standard or optimised doses. We propose individualised dose regimen optimisation.

General considerations in intermittent drug therapy of pulmonary tuberculosis

The advantages of intermittent chemotherapy for pulmonary tuberculosis are discussed and the results of several controlled trials summarized. Once-weekly therapy now appears a definite prospect.

The most appropriate approach to therapy at both national and individual physician level now seems to be a combination of unsupervised daily and supervised intermittent regimens.