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Basic toxicokinetics

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basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: scientifically acceptable and well documented

Data source

Reference Type:
Systemic availability and pharmacokinetics of thymol in humans
Kohlert C, Schindler G, Maerz RW, Abel G, Brinkhaus B, Derendorf H, Graefe EU, Veit M
Bibliographic source:
J Clin Pharmacol 42, 731-737

Materials and methods

Objective of study:
Principles of method if other than guideline:
Twelve healthy male volunteers were recruitedfor the study after complete clinical examination. Routine blood and urine laboratory tests were performed. Mean age was 29.5 ± 6.74 years (mean ± SD), and mean body mass index was 24.6 ± 2.0 kgm² (mean ± SD). Subjects were not allowed to use any medicine during the study.
Each subject received a single dose of Bronchipret® TP tablets containing 60 mg of primrose dry extract (6.0-7.0:1; extracted by ethanol 47% (v/v)) and 160 mg of thyme dry extract (5.9-10.0:1, extracted by ethanol 50% (m/m)), which was batch-specific equivalent to 1.08 mg of thymol. The subjects were fastedat the time they received the medication. They stayed in the clinic for the first 15 hours of the study and returned to the
clinic for regular blood sampling visits. Prestudies indicated that long sampling times would be necessary to cover the elimination phase completely. Therefore, a sampling period of 72 hours was chosen.
Collection of Blood and Urine Samples
Venous blood samples (9 ml per bloodsample) were collected into EDTA tubes once before subjects were administered the medication and 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 24, 31, 38, 48, 55, 62, and 72 hours after administration. Blood was centrifuged for 10 minutes at 4000 × g. The supernatant plasma was transferred into reaction cups in aliquots of 0.5 ml, and 20 µl acetic acid 0.58 M were added to each aliquot for stabilization. The plasma was stored at -20°C until analysis. Urine was collected in plastic bottles in intervals of 0 to 3, 3 to 6, 6 to 9, 9 to 14, 14 to 24, 24 to 31, 38 to 48, 48 to 55, 55 to 62, and 62 to 72 hours.
An aliquot of 50 ml of each sample was mixed with 0.1 g ascorbic acid as antioxidant and stored at -20°C until analysis.
GLP compliance:

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Details on test material:
Bronchipret® TP tablet, which is equivalent to 1.08 mg thymol

Test animals

not specified

Administration / exposure

Route of administration:
oral: capsule
other: Bronchipret® TP tablets containing 60 mg of primrose dry extract (6.0-7.0:1; extractedby ethanol 47% (v/v)) and160 mg of thyme dry extract (5.9-10.0:1, extracted by ethanol 50% (m/m)), which was batch-specific equivalent to 1.08 mg of thymol.
Duration and frequency of treatment / exposure:
single dose
Doses / concentrations
Doses / Concentrations:
1.08 mg of thymol
No. of animals per sex per dose / concentration:
Control animals:
not specified

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Thymol was absorbed quickly. Considerable plasma concentrations of thymol sulfate could already be detected after 20 minutes. This fast absorption indicates that thymol is mainly absorbed in the upper part of the gut.
Details on distribution in tissues:
Maximum plasma concentrations of 93.1 ng/ml were reached after 1.97 hours
Details on excretion:
Thymol plasma concentrations - after enzymatic hydrolysis of thymol sulfate - showed a biphasic profile. The terminal elimination phase set in after 10 to 12 hours, and thymol could be detected up to an average of 38 hours. Elimination half-life was determined to be 10.2 hours. Although plasma levels were detectable up to an average of 38 hours, the renal elimination of thymol conjugates was completedwithin 24 hours. This discrepancy might be due to the fact that very small amounts of renally eliminated thymol conjugates could not be quantified any more after 24 hours (< LOQ).
The fact that thymol sulfate is eliminated slowly can also be deduced from the small clearance (Cltot/f) of 1.2 L/h. The renal clearance of 0.271 L/h indicates high protein binding and/or reabsorption in the kidney, respectively.
The volume of distribution (Vdss/f) of 14.7 L indicates that thymol sulfate stays mainly in the extracellular space. The bioavailability of
thymol sulfate after administration of thymol is at least 16%, because 16% of the dose administered was excreted into
the urine as thymol conjugates. However, since Vdss/f is only 14.7 L, a much larger number for f is likely.
Toxicokinetic parameters
Toxicokinetic parameters:
half-life 1st: 10.2 hours

Metabolite characterisation studies

Metabolites identified:
Details on metabolites:
Thymol was present in human plasma only as thymol sulfate.

Any other information on results incl. tables

Free thymol could not be detected in human plasma. By means of LC-MS/MS analysis, only thymol sulfate but not the glucuronide was identified in human plasma. No free thymol was found in urine either. Instead, two phase II conjugates were identified by LC-NS/MS as thymol sulfate and glucuronide. The ratio of peak areas of thymol sulfate and thymol glucuronide was constant over the different urine fractions. Thymol was rapidly absorbed. Thymol sulfate could be detected in plasma 20 minutes after application. Maximum plasma levels of 93.1 +/-24.5 ng/ml were reached after 1.97 +/-0.77 hours. The plasma concentration was biphasic, subdivided into a distribution phase and a slow terminal elimination phase beginning at about 10 hours after administration and lasting up to an average of 38 hours.
Pharmcokinetic Data of total thymol absorption and elimination in human plasma after single oral dose administration of one Bronchipet TP tablet:
Dose (mg) = 1.08
Cmax (ng/ml) = 93.11
tmax (h) = 1.97
t1/2 (h) = 10.2
AUC (ng h/ml) = 837.3
MRT (h) = 12.6
MAT (h) = 0.53
CL/f (L/h) = 1.2
Vdss/f(L) = 14.7
Vd/f (L) = 17.7

Applicant's summary and conclusion

Interpretation of results: no bioaccumulation potential based on study results.
Executive summary:

To determine the systemic availability and the pharmacokinetics of thymol after oral application to humans, a clinical trial was carried out in 12 healthy volunteers.

Each subject received a single dose of a Bronchipret® TP tablet, which is equivalent to 1.08 mg thymol. No thymol could be detected in plasma or urine. However, the metabolites thymol sulfate and thymol glucuronide were found in urine and identified by LC-MS/MS. Plasma and urine samples were analyzed after enzymatic hydrolysis of the metabolites by headspace solid-phase microextraction prior to GC analysis and flame ionization detection. Thymol sulfate, but not thymol glucuronide, was detectable in plasma. Peak plasma concentrations were 93.1 ± 24.5 ng/ml and were reached after 2.0 ± 0.8 hours. The mean terminal elimination half-life was 10.2 hours. Thymol sulfate was detectable up to 41 hours after administration. Urinary excretion could be followed over 24 hours. The amount of both thymol sulfate and glucuronide excreted in 24-hour urine was 16.2% ± 4.5% of the dose.