Pharmacokinetic parameters of 960 mg Co-trimoxazole once daily in patients with tuberculosis

Multidrug-resistant (MDR) tuberculosis (TB) is thought to have high mortality
rate. The proportion of MDR-TB among new TB cases has nearly tripled to about
3% since 2004, with levels of 22% in former Soviet republics. The spread of
MDR-TB causes new challenges for the prevention, treatment and control of this
deadly disease. The number of drugs to which Mycobacterium tuberculosis (the
causative pathogens of tuberculosis) is susceptible has decreased.
Subsequently, the treatment of MDR-TB becomes less effective, increasing the
chance of treatment failure.

Therefore, there is continuing need for new drugs with minimum toxicity that is
effective against M. tuberculosis. However, the management of MDR-TB could be
rapidly improved by expanding the indications of established drugs. Although
Co-trimoxazole (SXT) is not registered for TB, it could be a promising drug for
the treatment of MDR-TB.

Co-trimoxazole (SXT) is a synergistic combination of two antimicrobial agents;
trimethoprim (TMP) and sulfamethoxazole (SMX). SXT is predominantly used for
the treatment of urinary tract infections and prophylaxis and treatment of
Pneumocystis jiroveci pneumonia (PCP) in HIV patients.

In available literature, no drug interactions are found between
SXT/trimethoprim and other frequently used drugs in the treatment of
tuberculosis (isoniazid, rifampicin, ethambutol, pyrazinamide). To evaluate the
pharmacokinetic and pharmacodynamic parameters, drug -drug interactions, safety
and tolerability of SXT in TB treatment we earlier performed a retrospective
chart review for all patients receiving 480 and 960 mg of SXT as drug with un
clear role for the treatment of (MDR) tuberculosis at the Tuberculosis Centre
Beatrixoord, University Medical center Groningen, The Netherlands between (10th
July 2006 to 1st July 2012).

SXT exhibits concentration independent (time dependent killing), thus 24 h AUC
/MIC would be the important PK/PD parameter determining efficacy, besides the
time during which the drug concentrations exceeded the MIC (T>MIC). Due to lack
of information about PK-PD parameters of SXT in TB, these parameters were
collected from infections with other pathogens. Improvement of treatment of
melioidosis caused by Bukholderia pseudomallei by SXT is achieved when the time
period in which the concentration exceeds the MIC 90 is more than 60% of the
interdose interval (8). The ratio of area under the free concentration-time
curve (AUC) from 0 to 24 h relative to the minimal inhibitory concentration
(MIC) had to exceed 25. Depending on the PK parameters of SXT from published
data and MIC of SXT against TB, we therefore assume that a suitable dose of
TMP/ SMX of 160-800 mg to achieve (f AUC0-24 h) /MIC of SXT ratio should exceed
25. This, and considering the fact that Mycobacterium tuberculosis replicates
significantly slower than other pathogenic bacteria, supports our hypothesis
that 960 once daily should be sufficient in treating tuberculosis.

Primary Objective:
The primary objective of this prospective clinical trial is to determine the
pharmacokinetic variability of SXT (960 mg) in patients receiving TB treatment.
With these pharmacokinetic parameters, a population model and limited sampling
model can be developed retrospectively.

Secondary Objective(s):
The second objective is to calculate the AUC/MIC ratio and T>MIC and to
validate drug concentration measurement using DBS by comparing the results of
blood samples routinely withdrawn from venous blood versus withdrawn by finger
prick and transferred to filter paper to make dried blood spots.

In a prospective clinical trial the pharmacokinetic parameters of 960 mg
co-trimoxazole will be evaluated. For this trial patients will be eligible for
inclusion who are referred for TB-treatment to Tuberculosis Centre Beatrixoord,
University Medical Center Groningen, The The patients will receive 960 mg once
a day for four to six consecutive days. This specific range of three days is
for practical reasons, since it is preferred to perform this study during
office hours.

Blood samples will be collected before the start of this study (1x 2 ml) and (8
x 2 ml) on the 4th, 5th or 6th day before administration and from start of
administering SXT at intervals of 1h, 2h, 3h, 4 h, 5h, 6h and 8h hours.
Parallel with the sampling of venous blood finger prick (0.1 ml) will be taken
to make a DBS at 1h, 5h and 8h after administration.

There are no published data that showed whether protein binding of
sulfamethoxazole is concentration or non- concentration dependent. Therefore
the unbound concentration in plasma ultra filtrate will be measured in the
sample at three different time points. The plasma concentrations of
sulfamethoxazole can be determined using a validated liquid
chromatography-tandem mass spectrometry (LS-MS/MS). sulfamethoxazole and
metabolite were measured by protein precipitation, followed by LC with tandem
mass detection.

The area under the concentration-time curve up to 24 h post dosage (AUC 0-24h)
for plasma will be determined with a standard non- compartmental
pharmacokinetic method using KINFIT module of MW Pharm 3.60 (Mediware, The
Netherlands). The AUC 0-24h will be calculated according to the log-linear
trapezoidal rule. In addition, several limited sampling strategies will be
evaluated based on a population one -compartment model, with first-order
absorption pharmacokinetics without lag time using the sulfamethoxazole dose,
the body surface area of the participants and the observed SXT plasma
concentrations, using an iterative two-stage Bayesian procedure. The population
pharmacokinetic model will be cross validated by developing a model based on
n-1. Limited sampling model (LSM) will be calculated using Monte Carlo data
simulation. The correlation between predicted SXT AUC0-24h and observed
AUC0-24h will be investigated by Bland-Altman analysis. The predictive
performance of the final model will be prospectively tested with SXT profiles
of TB patients receiving 960 mg once daily. AUC0-24h/MIC ratio of bound and
unbound drug and T>MIC will be calculated. To calculate MIC values, the drug
susceptibility test of the available M. tuberculosis isolates is performed with
the Middlebrook 7H10 agar dilution method at the Dutch national Mycobacterium
Reference Laboratory (National Institute for Public Health and the Environment,
RIVM).

The results from the analysis of DBS and the analysis of venous blood will be
compared to evaluate whether DBS method is reliable for routine TDM and
clinical pharmacokinetics. A DBS will be produced from venous samples (about
0.05 ml) to evaluate the differences between the drug concentration in venous
blood and capillary blood.

After exclusion of patients that cannot participate in this study, evaluation
of medical chart (including age, sex, weight, length, ethnicity, co-morbidity,
type of diagnosis, localization of TB, medical history, dose and duration of TB
co-administered medications) will be done at day 1 of the study. After
evaluation of baseline parameters, the attending physician (pulmonologist) will
start with 960 mg for four to six consecutive days in the participating TB
patients.

There are low risks from using SXT in this study because of the short study
time and low dose of SXT. Mild, no serious side effects may occur because SXT
in general is a safe and well tolerated drug. The patients could have mild
discomfort during the use of an indwelling IV catheter or due to the finger
prick to make a DBS.