Ethambutol disposition in humans: Challenges and limitations of whole-body physiologically-based pharmacokinetic modelling in early drug development

Whole-body physiologically based pharmacokinetic (WB-PBPK) models have become an important tool in drug development, as they enable characterization of pharmacokinetic profiles across different organs based on physiological (systems-specific) and physicochemical (drug-specific) properties. However, it remains unclear which data are needed for accurate predictions when applying the approach to novel candidate molecules progressing into the clinic. In this work, as case study, we investigated the predictive performance of WB-PBPK models both for prospective and retrospective evaluation of the pharmacokinetics of ethambutol, considering scenarios that reflect different stages of development, including settings in which the data are limited to in vitro experiments, in vivo preclinical data, and when some clinical data are available. Overall, the accuracy of PBPK model-predicted systemic and tissue exposure was heavily dependant on prior knowledge about the eliminating organs. Whilst these findings may be specific to ethambutol, the challenges and potential limitations identified here may be relevant to a variety of drugs, raising questions about (1) the minimum requirements for prospective use of WB-PBPK models during the characterization of drug disposition and (2) implication of uncertainty for dose selection in humans.

Standardization of the method for estimation of ethambutol in pharmaceutical preparations and biological fluid

A simple column chromatographic method for determination of ethambutol (EMB) in pharmaceutical preparations containing EMB in combination with other anti-TB drugs is presented. The method involved extraction of EMB into an organic solvent, followed by basification and column chromatographic separation on Amberlite CG 50 (100-200 mesh) and elution with suitable eluants and estimation at a wavelength of 270 nm. The assay was linear from 25 to 400 microg/ml. The relative standard deviations of intra and inter day assays were lower than 5%. Ethambutol was recovered from human urine quantitatively and stable for a period of at least one week in urine stored at -20 degrees C.

Can guardians supervise TB treatment as well as health workers? A study on adherence during the intensive phase

SETTING

In sub-Saharan Africa, tuberculosis (TB) has increased over the last two decades due to the human immunodeficiency virus pandemic. In Malawi, 20630 new TB patients were notified to the National Tuberculosis Programme in 1996, a fourfold increase since 1986. Due to this increase in cases and lack of resources (both human and monetary) it is becoming more difficult to ensure directly observed treatment (DOT) in the TB wards.

METHODS

In Ntcheu district, Malawi, a new TB regimen was introduced from April 1996 in which patients received supervised treatment by either a health worker or a guardian (i.e., family member). Adherence to the different treatment options was measured by form checks, tablet counts, and tests for detecting isoniazid in the urine. Adherence was measured at 2, 4 and 8 weeks after onset of TB treatment.

RESULTS

Overall adherence rate was 95-96%. Inpatients showed the highest adherence rates. Patients on guardian-based DOT (GB-DOT) (n = 35) showed 94% adherence, while patients on health centre based DOT (n = 40) showed more non-adherent behaviour: 11% according to monitoring forms, 14% according to tablet counts and 16% according to urine tests.

DISCUSSION

The results suggest that decentralised care is a feasible option for anti-tuberculosis treatment and that guardians can supervise TB treatment just as well as health workers during the intensive phase of TB treatment.

The potential use of urinary excretion data for assessing the relative bioavailability of rifampicin in fixed dose combination anti-tuberculosis formulations

SETTING

The perceived need for simple, non-invasive methods of assessing the relative bioavailability of rifampicin in fixed-dose combination (FDC) anti-tuberculosis formulations.

OBJECTIVE

To compare the performance of methods based on urinary excretion data with those utilising plasma concentration-time profiles to assess the relative bioavailability of rifampicin in combined and single-drug formulations.

DESIGN

A two-period randomised crossover bioequivalence study in healthy male volunteers with a 1 week washout period between treatments. Plasma rifampicin concentrations were measured at 0, 1, 2, 4, 6 and 8 hours after each drug administration using a high performance liquid chromatography (HPLC) method. The rifampicin and desacetylrifampicin content of complete urinary collections made from 0-4 and 4-8 hours after dosage were determined using both the HPLC and a much simpler colorimetric procedure.

RESULTS

There was good agreement between the relative bioavailability of the formulations using plasma and urinary excretion data, although the precision of the urinary-based estimates was slightly less than those derived from the plasma findings. There was also good agreement between the HPLC and colorimetric estimates of the combined urinary excretion of rifampicin plus desacetylrifampicin.

CONCLUSIONS

Urinary excretion data may be used for ongoing quality control to confirm that commercial combined rifampicin-containing formulations that were initially shown to be satisfactory continue to be so.

Effect of rifabutin on ethambutol pharmacokinetics in healthy volunteers

The effect of repeated administration of rifabutin on the pharmacokinetics and metabolism of ethambutol was evaluated in ten healthy volunteers. The subjects received a single oral administration of 1200 mg ethambutol on days 1 and 10 and a single daily oral dose of 300 mg rifabutin from days 3 to 9. No statistically significant difference was found in plasma pharmacokinetics (C(max), t(max), AUC, half-life and MRT) and in the renal clearance, whereas a significant decrease in the amount of unchanged ethambutol excreted in urine was observed. The decrease observed in ethambutol urinary excretion may be accounted for by taking into consideration the variability of the urinary excretion of ethambutol reported in the literature. However, a slight, likely not clinically relevant, induction or activation of kidney alcohol and/or aldehyde dehydrogenase isoenzymes by rifabutin cannot be ruled out at present. Evidence exists in the present study for autoinduction of rifabutin metabolism; this is shown by the lower plasma concentrations obtained 24 h after the seventh dose as compared to the theoretical concentrations.

Determination of ethambutol in human plasma and urine by high-performance liquid chromatography with fluorescence detection

A sensitive and selective HPLC method for the determination of ethambutol in human plasma and urine was developed. Ethambutol was extracted from basified plasma samples (0.2 ml) with diethyl ether, back-extracted into 0.01 M phosphoric acid and derivatized with 4-fluoro-7-nitrobenzo-2-oxa-1, 3-diazole. After 30 min at 80 degrees C and elimination of the reactive excess, the compound was determined by reversed-phase liquid chromatography. urine was analysed for ethambutol after dilution 1:200 with distilled water and derivatization as described for plasma. Quantification in plasma and urine was achieved by fluorescence detection of the eluate. The linearity, precision and accuracy of the method were evaluated. No interference from the constituents of human plasma and urine was observed. The limit of quantification was 10 ng/ml in plasma and 10 micrograms/ml in urine. The suitability of the method for in vivo samples was checked by analysis of plasma and urine samples drawn from healthy volunteers who had received a 1200-mg oral dose of the test compound.

Effect of gastric surgery on the gastrointestinal drug absorption in man

The effect of gastric surgery on the absorption of quinidine, ethambutol, and sulphafurazole was studied in 14 male patients, all serving as their own controls. Antrectomy with gastroduodenostomy (ABI) and selective vagotomy lowered the serum levels of all drugs significantly during the 6-hour test period. Excretion of drugs in 6-hour urine also decreased. Three patients showed practically no absorption up to 2 hours, and even therafter the absorption was lowered. Over one year after operation the urinary excretion of ethambutol, but not of the other drugs, was improved. ABI alone did not modify absorption. Preoperative gastric retention seemed to delay absorption.