Influence of Rifampicin on Tinidazole Pharmacokinetics
785
any medication for at least 15 days prior to the
administration of tinidazole in the study. Volun-
teers were excluded from the study if they had food
allergies or were allergic to tinidazole or rifampi-
cin.
rpm for 10 minutes. 20μl of the supernatant was
injected onto the column. A linearity calibration
curve in the range of 0.08 to 100 mg/L was also
established (r2 = 0.99) in serum matrix. Interassay
variability was determined at three different con-
centrations, 1, 10 and 30 mg/L, with coefficients
of variance of 8.13, 5.92 and 3.18%, respectively.
The pharmacokinetic parameters peak plasma
concentration (Cmax), time to reach peak concen-
tration (tmax), area under the plasma concentration-
Methods
After an overnight fast (approximately 12
hours), each volunteer received a tinidazole 500mg
tablet (Tiniba 500®; Cadila Healthcare Limited,
Ahmedabad, India) with 200ml of water.
1
time curve (AUC), elimination half-life (t ⁄ β), ap-
2
parent volume of distribution (Vd/F) and apparent
Venous blood samples of approximately 5ml
were drawn from the antecubital vein at 0, 1, 2, 3,
4, 6, 8, 12, 24, 36 and 48 hours after drug admin-
istration. The blood was allowed to clot and centri-
fuged for 10 minutes at 3000 rpm (R8C; Remi
Instruments, Mumbai, India). Serum was separated
into amber-coloured vials and stored at −20°C until
the analysis was performed. A once-daily dose of
rifampicin 600mg (2 × 300mg capsules; R-cin
300®; Lupin Laboratories, Mumbai, India) was
given for 5 consecutive days (from day 4 to 8)
under direct observation. On day 9, tinidazole
500mg was given again and the sample collec-
tion was repeated.
Tinidazole in the serum samples was estimated
by reverse-phase high performance liquid chroma-
tography (HPLC) [Chaluvadi et al., unpublished
work]. The HPLC system (Shimadzu, Japan)
consisted of an LC-10AT solvent delivery module
and a SPD-10A UV-Visible Spectrophotometric
Detector. The mobile phase consisted of methanol
systemic clearance (CL/F) for tinidazole were
obtained for each subject by using the computer
program RAMKIN (DR Krishna, unpublished
work) meant for calculation of model-independent
parameters. In the present study, AUC0-t refers to
the AUC from 0 to 48 hours and AUC0-∞ refers to
the AUC from 0 to infinity. The AUC0-t value is
more than 80% of the AUC0-∞ in the present study
and hence the extrapolation to ∞ is valid. AUC0-∞
was calculated using the formula AUC0-t + (Cl-
ast/Kel), where Clast is the concentration in mg/L at
the last timepoint and Kel is the elimination rate
constant. Because tinidazole was administered
orally, estimates of volume of distribution and
clearance are uncorrected for the fraction of drug
absorbed (F). However, F was assumed to be 1.0
on the basis of earlier reports on the pharmaco-
kinetics of tinidazole.[7,8]
Statistical Analysis
The mean pharmacokinetic parameters ob-
tained when tinidazole was given alone were com-
pared with those obtained after rifampicin pre-
treatment using Student’s t-test (paired data). A
value of p < 0.05 was considered to be statistically
significant.
:
:
acetonitrile 0.002 mol/L potassium dihydrogen
:
:
orthophosphate buffer (7.5 7.5 85) with a flow
rate of 1 ml/min. The column used was Altech C-18
(stainless steel column of length 25cm and internal
diameter of 4.6mm packed with porous silica
spheres of 5μm diameter, 100Å pore diameter) and
the eluent was monitored at 320nm.
Results
Metronidazole (6μl of 100 mg/L) was added to
0.3ml of serum as the internal standard and the
mixture was shaken well on a vortex-mixer. An
equal volume (0.3ml) of acetonitrile was added for
protein precipitation, the mixture was shaken on a
vortex-mixer for 1 minute and centrifuged at 3000
The mean standard deviation (SD) serum con-
centrations of tinidazole at different timepoints
before and after rifampicin pretreatment are shown
in figure 1. The pharmacokinetic parameters of
tinidazole are presented in table I.
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Clin Drug Invest 2001; 21 (11)