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Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics
Type of information:
experimental study
Adequacy of study:
key study
Study period:
September 1987 to June 1988
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Comparable to guideline study
Cross-referenceopen allclose all
Reason / purpose:
reference to same study
Reason / purpose:
reference to other study

Data source

Referenceopen allclose all

Reference Type:
publication
Title:
Metabolism and disposition of hydroquinone in Fischer 344 rats after oral or dermal administration.
Author:
English JC, Deisinger PJ
Year:
2005
Bibliographic source:
Food Chem Toxicol 43, 483 - 493
Reference Type:
study report
Title:
Unnamed
Year:
1988
Report Date:
1988
Reference Type:
other: abstract
Title:
Toxicokinetics studies with hydroquinone (HQ) in Fischer 344 rats.
Author:
English JC, Deisinger PJ, Murphy SJ, Blacker AM
Year:
1991
Bibliographic source:
Toxicologist 11, 181
Reference Type:
secondary source
Title:
Development of a physiologically based pharmacokinetic model for hydroquinone.
Author:
Corley RA, English JC, Hill TS, Fiorica LA, Morgott DA
Year:
2000
Bibliographic source:
Toxicol Appl Pharmacol 165, 163-174

Materials and methods

Objective of study:
absorption
distribution
excretion
metabolism
toxicokinetics
Test guideline
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 417 (Toxicokinetics)
Principles of method if other than guideline:
After single or repeated oral dosing of 14C-labelled hydroquinone quantitative investigations of absorption, tissue distribution, plasma clearance, excretion and urinary metabolites were performed.
GLP compliance:
yes

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
- Name of test material (as cited in study report): Hydroquinone, HQ
- Analytical purity: > 99%
- Radiochemical purity (if radiolabelling): 98.9%
- Specific activity (if radiolabelling): 22.2 mCi/mmol
- Locations of the label (if radiolabelling): U-14C-HQ
- Expiration date of radiochemical substance (if radiolabelling): not specified
- Stability under test conditions: stability in aqueous dose solutions determined by HPLC with radiochemical detection and UV absorption was > 99% for at least 48 h at about 30 °C
- Storage condition of test material: not specified
Radiolabelling:
yes
Remarks:
U-14C-HQ

Test animals

Species:
rat
Strain:
Fischer 344
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Charles River Laboratories, Wilmington, MA, USA
- Age at study initiation: 7-9 w
- Weight at study initiation: males 149 - 178 g, females 117 - 136 g
- Fasting period before study: no
- Individual metabolism cages: yes, glass metabolism chambers with efficient urine-faeces separator
- Diet: Agway Prolab RMH 3000 or 3200, ad libitum
- Water: ad libitum
- Acclimation period: at lest 3 d

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 23-27
- Humidity (%): 31-70
- Air changes (per hr): dehumdified air was drawn through the glass metabolism chambers at a rate of approximately 500 mL/min
- Photoperiod (hrs dark / hrs light): 12 / 12

Administration / exposure

Route of administration:
oral: gavage
Vehicle:
water
Details on exposure:
PREPARATION OF DOSING SOLUTIONS:
radiolabeled dose solutions prepared in degassed, distilled water to contain nominally 10 µCi

VEHICLE
- Concentration in vehicle: 12.5 (low dose) or 50 mg/kg (high dose)
- Amount of vehicle (if gavage): 2 mL/kg (low dose) or 7 mL/kg (high dose)

HOMOGENEITY AND STABILITY OF TEST MATERIAL: > 99% for at least 48 hrs at 30 °C
Duration and frequency of treatment / exposure:
single application of 14C-HQ with 7 days post-observation period
repeated application: 14 d, daily with unlabeled HQ, followed by a single dose of 14C-HQ
Doses / concentrations
Remarks:
Doses / Concentrations:
single dosing: 25, 50 or 350 mg/kg bw (LDS, MDS, HDS)
repeated dosing: 25 mg/kg bw/d (LDR)
No. of animals per sex per dose:
8 males and 8 females (N=4 for different endpoints) for doses of 25 or 350 mg/kg bw
3 males for dose of 50 mg/kg bw
Control animals:
no
Details on study design:
- Dose selection rationale: 25 mg/kg as NOAEL, 25 and 50 mg/kg are the dosages used in the lifetime bioassay (see Section 7.5.1 and 7.7), 350 mg/kg as toxic but sublethal dose
Details on dosing and sampling:
PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled: urine, faeces, tissues (liver, kidney, lungs, spleen, heart, brain, testes and ovaries; samples of dorsal skin, bone, abdominal fat, femoral muscle), carcass, blood, plasma, metabolism chamber washes
- Time and frequency of sampling: excreta and chamber washes from 0-8, 8-24, followed by 24 hr intervals up to 7 days; blood at 5, 10, 30, 60 min, and 3, 8, 24, 48 and 72 hrs p.a. for 25 and 350 mg/kg-doses; blood and plasma at 10, 20, 40, 60, 120 and 240 min p.a. for 50 mg/kg-dose
- Method type(s) for identification: 14C was measured by LSS; aliquots of urine and chamber washes directly, samples of faeces after oxidative combustion; blood and plasma samples after chemical digestion; tissues either after oxidative combustion, chemical digestion and bleaching (bone), or homogenization with water and oxidative combustion of aliquots. Carcasses were homogenized with acetone and refluxed for 24 h and acetone extracts were analyzed by LSS. Carcass residues were combusted before analysis.
- Limits of detection and quantification of 14C (lower limits of quantification as % of administered 14C): urine 0.01%, chamber wash 0.03%, faeces 0.005%; precision (determined with urine samples) 99.1%; blood 0.02 to 0.03 % of dose corresponding to 0.12 to 1.0 microgramm equivalents/g blood at the 25 mg/kg dose and 1.35 to 1.71 microgramm equivalents/g blood at the 350 mg/kg dose

METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: urine
- Time and frequency of sampling: 24 hr interval
- From how many animals: 4
- Method type(s) for identification: HPLC-MS, HPLC-radioactivity monitor
- Limits of detection and quantification (lower limit of quantification): 0.045% of a 10 µCi dose; precision 95.2%

TREATMENT FOR CLEAVAGE OF CONJUGATES (if applicable): enzymatic cleavage by treatment with ß-glucuronidase or arylsulfatase, or hydrolysis with hydrochloric acid

TOXICOKINETIC EVALUATION
Toxicokinetic parameters derived for total 14C label in blood using a computer program for nonlinear regression analysis (NONLIN84). (Further details see: Any other information on methods)

Results and discussion

Main ADME resultsopen allclose all
Type:
absorption
Results:
Peak blood concentrations within 10-14 min for LDS or RDS, within 20-34 min for HDS
Type:
distribution
Results:
Levels of 14C: 0.15-0.96% in combined tissues and carcass with highest amounts in livers (0.075-0.24%) and kidneys (0.01-0.04%); 0.1-0.4% in blood at 8 h p.a.
Type:
metabolism
Results:
metabolites in urine: HQ-glucuronide 45 - 53%, HQ-O-sulfate 19 - 33%, unmetabolized HQ < 3%, mercapturic acid conjugate 0.16 - 4.7% (mainly in HDS), p-benzoquinone < 1%
Type:
excretion
Results:
14C in urine: after 8 hrs ca. 77-87% in LDS and LDR, 45-45% in HDS; after 24 hrs ca. 89-93% in LDS and LDR, 82-85% in HDS; in faeces: 1-3% in total

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Fast and extensive absorption from gastro-intestinal tract
Peak blood concentrations as microg equ/g blood: LDS 7.62 at 14 min in m, 7.19 at 20 min in f; HDS 39.2 at 34 min in m, 45.8 at 48 min in f; LDR 6.61 at 10 min in m, 6.03 at 14 min in f (calculated with toxicokinetic parameters)
Details on distribution in tissues:
Blood levels: at 8 hr 0.1-0.4% of 14C, after 24 hrs ca. 0.03%; biphasic decline
Tissue levels: 0.15 - 0.96% of 14C in combined tissues and carcass; concentrations in females > males; highest concentrations in livers (0.075-0.24%) and kidneys (0.01-0.04%); testes <= 0.0036% , ovaries < 0.001%
Details on excretion:
14C in urine: after 8 hrs ca. 81% in LDS m and f, 54 and 45% in HDS m and f, 77 and 87% in LDR m and f; after 24 hrs ca. 89% in LDS m and f, 85 and 82% in HDS m and f, 91 and 93% in LDR m and f
Faeces: 1-3% in total
Toxicokinetic parametersopen allclose all
Test no.:
#1
Toxicokinetic parameters:
half-life 1st: 0.58 hr in males with repeated application of 25 mg/kg bw/d
Test no.:
#2
Toxicokinetic parameters:
half-life 1st: 0.29 hr in males with single application of 25 mg/kg bw/d
Test no.:
#3
Toxicokinetic parameters:
half-life 1st: 1.72 hr in males with sinlge application of 350 mg/kg bw/d
Test no.:
#4
Toxicokinetic parameters:
half-life 1st: 0.23 hr in females with repeated application of 25 mg/kg bw/d
Test no.:
#5
Toxicokinetic parameters:
half-life 1st: 0.91 hr in females with single application of 25 mg/kg bw/d
Test no.:
#6
Toxicokinetic parameters:
half-life 1st: 1.35 hr in females with single application of 350 mg/kg bw/d
Test no.:
#1
Toxicokinetic parameters:
half-life 2nd: 4.02 hr in males with repeated application of 25 mg/kg bw/d
Test no.:
#2
Toxicokinetic parameters:
half-life 2nd: 3.20 hr in males with single application of 25 mg/kg bw/d
Test no.:
#4
Toxicokinetic parameters:
half-life 2nd: 2.77 hr in females with repeated application of 25 mg/kg bw/d
Test no.:
#5
Toxicokinetic parameters:
half-life 2nd: 10.52 hr in females with single application of 25 mg/kg bw/d
Test no.:
#1
Toxicokinetic parameters:
Tmax: 0.16 hr in males with repeated application of 25 mg/kg bw/d
Test no.:
#2
Toxicokinetic parameters:
Tmax: 0.24 hr in males with single application of 25 mg/kg bw/d
Test no.:
#3
Toxicokinetic parameters:
Tmax: 0.56 hr in males with single application of 350 mg/kg bw/d
Test no.:
#4
Toxicokinetic parameters:
Tmax: 0.24 hr in females with repeated application of 25 mg/kg bw/d
Test no.:
#5
Toxicokinetic parameters:
Tmax: 0.32 hr in females with single application of 25 mg/kg bw/d
Test no.:
#1
Toxicokinetic parameters:
Cmax: 6.61 µg equivalents/g in males with repeated application of 25 mg/kg bw/d
Test no.:
#2
Toxicokinetic parameters:
Cmax: 7.62 µg equivalents/g in males with single application of 25 mg/kg bw/d
Test no.:
#3
Toxicokinetic parameters:
Cmax: 39.2 µg equivalents/g in males with single application of 350 mg/kg bw/d
Test no.:
#4
Toxicokinetic parameters:
Cmax: 6.03 µg equivalents/g in females with repeated application of 25 mg/kg bw/d
Test no.:
#5
Toxicokinetic parameters:
Cmax: 7.19 µg equivalents/g in females with single application of 25 mg/kg bw/d

Metabolite characterisation studies

Metabolites identified:
yes
Details on metabolites:
in urine: HQ-glucuronide 45 - 53%, HQ-O-sulfate 19 - 33%, unmetabolized HQ < 3%, mercapturic acid conjugate 0.16 - 4.7% (mainly in HDS), p-benzoquinone < 1%

Any other information on results incl. tables

Clinical observations:

350 mg/kg: males showed reduced activity; in females mild tremors with chewing and minimal reduced activity were noted during 15 to 45 min after HQ administration; animals appeared normal at 3.5 to 4 hrs post-treatment.

25 mg/kg: no effects

Table 2: Cumulative percentage of the administered radioactivity recovered from male and female F344 rats

Test group
Application, dose

Sampling time (h)

Percentage of dose (mean±standard deviation) in

Urine

Cage rinsesa

Faeces

Tissues and carcass

Total

Males

Repeated,
25 mg/kg bw/d

LDR

8

70.29±5.14

7.01±2.64

0.01±0.01

24

82.05±5.95

8.58±3.32

0.53±0.17

48

82.75±5.65

8.91±3.47

0.92±0.26

0.56±0.18

93.14±3.95

Males

Single,
25 mg/kg bw

LDS

8

65.76±2.67

15.14±1.74

0.30±0.36

24

72.36±4.43

16.70±1.90

0.98±0.43

48

73.35±4.52

17.25±1.85

1.18±0.45

72

73.96±4.53

17.71±1.89

1.26±0.45

0.51±0.07

93.43±2.69

Males

Single,
350 mg/kg bw

HDS

8

46.15±3.07

7.43±4.67

0.05±0.04

24

75.11±5.88

9.80±4.76

1.18±1.30

48

76.57±5.63

10.49±5.07

2.03±1.29

0.96±0.02

90.06±2.14

Females

Repeated,
25 mg/kg bw/d

LDR

8

78.59±5.82

8.02±2.18

0.23±0.29

24

83.83±3.21

8.79±2.16

0.97±0.44

48

84.37±3.05

9.17±2.27

1.11±2.27

0.53±0.13

95.18±1.49

Females

Single,
25 mg/kg bw

LDS

8

67.24±7.21b

14.87±3.11

3.31±4.40a

24

73.10±5.90

15.59±3.05

4.39±4.41

48

73.86±6.01

16.14±3.43

4.64±4.33

0.77±0.10

95.42±0.64

Females

Single,
350 mg/kg bw

HDS

8

39.62±2.89

5.52±2.14

0.51±0.48

24

72.80±4.96

8.94±2.27

2.40±1.460

48

78.66±3.03

9.38±2.30

3.10±1.54

72

79.15±3.02

9.77±2.44

3.23±1.55

0.95±0.14

93.09±1.76

a assumption that 14C recovered form in the chamber washes was derived from urinary radioactivity adhering to the glass surfaces of the metabolism chambers

b at the 8-hr sampling, the faeces of one rat was contaminated by urine as a result of malfunction in the separation mechanism resulting in higg faeces-associated radioactivity of 10.85% for this rat. The mean value for the faeces of the other three rats was 0.79%.

Table 3: Time course of 14C concentration (mean±standard deviation) in blood of male and female F344 rats

Test group
Application, dose

                                                                         Time point

5 min

10 min

30 min

1 hr

3 hrs

8 hrs

24 hrs

48 hrs

Males

Repeated,
25 mg/kg bw/d

LDR

µg equ/g blood

6.51±1.10

6.93±1.57

5.28±0.58

3.80±0.73

1.77±0.60

0.93±0.42

-

-

% dosea

1.53±0.278

1.63±0.383

1.24±0.212

0.89±0.185

0.42±0.148

0.22±0.097

-

-

Males

Single
25 mg/kg bw

LDS

µg equ/g blood

5.47±0.72

7.47±1.23

7.12±2.23

4.21±1.18

1.89±0.26

0.72±0.30

-

-

% dosea

1.27±0.17

1.74±0.29

1.65±0.50

0.99±0.27

0.44±0.06

0.17±0.07

-

-

Males

Single
350 mg/kg bw

HDS

µg equ/g blood

24.25±4.20

39.97±2.02

44.18±3.74

35.22±3.13

16.73±3.24

16.05±0.51

2.50±0.16

3.45±0.75

% dosea

0.44±1.00

0.72±0.071

0.82±0.042

0.60±0.051

0.30±0.067

0.29±0.012

0.05±0.002

0.06±0.015

Females

Repeated,
25 mg/kg bw/d

LDR

µg equ/g blood

5.34±2.62

7.57±3.58

5.35±1.80

5.13±0.42

1.86±0.54

0.48±0.17

-

-

% dosea

1.29±0.63

1.53±0.86

1.29±0.43

0.75±0.128

0.45±0.127

0.11±0.035

-

-

Females

Single
25 mg/kg bw

LDS

µg equ/g blood

7.08±1.54

8.63±1.76

7.17±1.25

3.98±0.46

1.66±0.24

0.40±0.047

0.15±0.019

-

% dosea

1.69±0.36

2.06±0.41

1.71±0.30

0.95±0.10

0.40±0.059

0.09±0.010

0.04±0.004

-

Females

Single
350 mg/kg bw

HDS

µg equ/g blood

39.2±5.71

54.9±5.81

52.3±0.63

39.6±2.77

26.8±6.56

8.33±3.11

11.9±2.96

3.45±0.75

% dosea

0.66±0.096

0.92±0.96

0.88±0.009

0.67±0.046

0.45±0.11

0.14±0.053

0.20±0.049

0.06±0.015

a The percentage of the administered14C in the blood volume was approximated assuming the blood volume of the rat to be 6% of body weight.

Table 4: Relative amounts of HQ and metabolites in urine of male and female F344 rats by 24 hrs after oral administration (means ± standard deviation)

Test group
Application, dose

Sex

% of total dose

HQ glucuronide

HQ sulfate

HQ

HQ mercapturate

p-benzo quinone

Repeated,
25 mg/kg bw/d

LDR

Male

45.45±3.86

33.42±1.79

1.62±1.09

0.16±0.10

0.24±0.15

Female

53.19±7.90

22.45±3.14

7.12±6.92

0.77±0.77

< LOQ

Single
25 mg/kg bw

LDS

Male

44.83±2.68

25.91±1.95

1.17±0.60

< LOQ

0.46±0.29

Female

45.30±4.52

21.89±2.07

2.80±2.60

2.66±2.06

0.44±0.77

Single
350 mg/kg bw

HDS

Male

46.11±4.13

22.94±1.66

1.94±1.27

2.34±0.64

0.84±0.31

Female

48.83±4.09

19.03±1.51

0.25±0.44

4.68±1.39

< LOQ

LOQ = limit of quantification


Table 5: Distribution of HQ-derived14C-activity in tissues of male F344 rats (means±standard deviation)

Sex

Males

Females

Test group
Application, dose

Repeated dosing,
25 mg/kg bw/d

LDR

Single
25 mg/kg bw

LDS

Single
350 mg/kg bw

HDS

Repeated dosing,
25 mg/kg bw/d

LDR

Single
25 mg/kg bw

LDS

Single
350 mg/kg bw

HDS

Timepoint
h p.a. of14C-HQ

48 h

72 h

48 h

48 h

48 h

72 h

Whole tissues

dpm/ g tissue

% dose

dpm/ g tissue

% dose

dpm/ g tissue

% dose

dpm/ g tissue

% dose

dpm/ g tissue

% dose

dpm/ g tissue

% dose

Liver

1603±213

0.075±0.010%

1686±135

0.075±0.0050%

4417±315

0.24±0.015%

4905±1183

0.16±0.043%

7517±533

0.23±0.023%

6899±1068

0.040±0.0072%

Kidneys

1198±108

0.0089±0.00073%

1403±43

0.010±0.00028%

3311±362

0.029±0.0028%

3185±244

0.020±0.0023%

3418±444

0.020±0.0024%

5910±307

0.18±0.014%

Heart

434±47

0.0013±0.00031%

< LOQ

640±153

0.0020±0.00051%

997±345

0.0023±0.00076%

< LOQ

971±263

0.0025±0.00075%

Spleen

247±23

0.00040±0.000041%

< LOQ

1241±180

0.0023±0.00011%

681±120

0.0010±0.00019%

675±156

0.00092±0.00019%

1526±156

0.0023±0.00011%

Brain

< LOQ

< LOQ

455±264

0.0031±0.0019%

625±74

0.0037±0.00064%

< LOQ

1331±132

0.0085±0.00086%

Lungs

< LOQ

< LOQ

754±92

0.0048±0.0011%

838±52

0.0036±0.00028%

787±104

0.0027±0.00016%

1240±108

0.0055±0.00095%

Testes or ovaries

< LOQ

< LOQ

472±68

0.0036±0.00061%

859±225

0.00052±0.000051%

747±126

0.00031±0.000046%

1594±216

0.00099±0.00023%

Carcass

736±304

0.46±0.18

733±126

0.42±0.061%

1086±23

0.67±0.016%

669±199

0.33±0.10%

1134±245

0.51±0.099%

1586±289

0.70±0.13%

Tissue samples

dpm/ g tissue

dpm/ g tissue

dpm/ g tissue

dpm/ g tissue

dpm/ g tissue

dpm/ g tissue

Bone

831±122

< LOQ

473±93

446±41

440±13

1014±59

Skin

547±210

390±103

867±250

533±276

1839±978

3338±836

Muscle

170±90

< LOQ

299±43

237±24

147±14

444±37

Fat

< LOQ

< LOQ

1277±515

696±150

526±56

1521±406

< LOQ: mean values were lower than the limit of quantification which was estimated to be twice the background

Table 6: Pharmacokinetic parameters of total 14C after oral administration of [U-14C]hydroquinone to F344 rats

Sex

Males

Females

Test group
application, dose

Repeated dosing,
25 mg/kg bw/d

LDR

Single
25 mg/kg bw

LDS

Single
350 mg/kg bw

HDS

Repeated dosing,
25 mg/kg bw/d

RDS

Single
25 mg/kg bw

LDS

Single
350 mg/kg bw

HDS

Dose of14C-HQ (mg/kg bw/d)

25.2

25.8

333

24.9

25.1

356

Tmax(hr)

0.16

0.24

0.56

0.32

0.24

0.80

Cmax(g equiv/g)

6.61

7.62

39.2

7.19

6.03

45.8

alphaT1/2(hr)

0.58

0.29

1.72

0.91

0.23

1.35

ß T1/2(hr)

4.02

3.20

-b

2.77

10.52

-b

AUC (hr-g/kg)

0.019

0.019

0.328

0.015

0.021

0.558

aData from 4 rats of each group were fit to computer-generated non-linear regression lines that minimized the sums of squares of the residuals.

bDue to inflections in the blood concentration vs. time curves, the parameter ß T1/2could not be accurately determined

Tmax(hr)            time to peak blood concentration

Cmax(g equiv/g)           peak blood concentration

alpha T1/2(hr)            half-life of the alpha (0.693/alpha)

ß T1/2(hr)            half-life of the ß (0.693/ß)

AUC (hr-g/kg)            area under the blood concentration-time curve extrapolated to infinity

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): no bioaccumulation potential based on study results
After oral administration, hydroquinone is rapidly absorbed from the gastro-intestinal tract, followed by extensive first-pass metabolism and rapid excretion primarily via the urine. Elimination from blood was biphasic, attributable to first-pass metabolism followed by enterohepatic cycling of a small fraction of the dose. The biotransformation of HQ occurred mainly by phase II metabolism to glucuronides and sulfates. Saturation of elimination was indicated at the high dose of 350 mg/kg. No marked sex-differences in the metabolism and disposition of HQ were found. Repeated dosing of 25 mg/kg bw/d did not result in tissue accumulation of HQ-derived material. About 90-95% of the applied dose was recovered from excreta and cage rinsings, and only 0.51–0.96% of the administered dose of 14C was recovered in the tissues and carcass at 48 to 72 h following application.
Executive summary:

In a toxicokinetic study with a protocol comparable to OECD Guideline 417, groups of 8 male and 8 female F344 rats were exposed to single doses of 25 or 350 mg/kg bw14C-HQ (LDS, HDS) or repeated doses of 25 mg/kg bw/d (14 days of dosing with unlabeled HQ followed by a single dose of14C-HQ) (LDR) by oral gavage application (Vehicle water). Blood was sampled from 4 rats per sex and dose at 5, 10, 30, 60 min, and 3, 8, 24, 48 and 72 hrs after the last administration. After 48 or 72 h following the adminstration of14C-HQ, rats were euthanized and tissue distribution of14C was measured in liver, kidneys, lung, spleen, heart, brain, testes and ovaries, and in samples of dorsal skin, bone, abdominal fat, femoral muscle, and the remaining carcass. Urine, faeces, and metabolism chamber washes from the other 4 rats per sex and dose were collected for time intervals from 0-8 h, and 8-24 h, followed by 24 hr intervals up to 7 days p.a.. 14C-activity was measured in excreta, blood and tissues by liquid scintillation spectroscopy after adequate processing. Metabolites in urine were analysed after enzymatic or acid hydrolysis of conjugates with HPLC-MS and HPLC combined with radioactivity monitor detection. Additionally, groups of 3 male rats were exposed to single doses of 50 mg/kg bw for monitoring the time course of14C in blood and plasma levels (sampling at 10, 20, 40, 60, 120 and 240 min p.a.).

Absorption into the blood was rapid after oral dosing. The maximum concentration of total14C were attained within 20 min after a single dose of 25 mg/kg (LDS), within 14 min after a repeated dose of 25 mg/kg (LDR), and within 50 min after a single dose of 350 mg/kg (HDS), respectively. Maximum blood concentration at the indicated time points were 7.6 and 7.2, 6.6 and 6.0, and 39.2 and 45.8 µg equivalents HQ/g blood in males and females of the LDS, LDR and HDS groups, respectively.

Elimination from blood was biphasic, with alpha T1/2 values of ca. 0.23-0.29, 0.58-0.91, 1.35-1.72/h for the LDS, LDR, and HDS groups. Beta T1/2 values ranged from 2.8-10.5/h in the low dose studies, no accurate estimation was possible for the HDS group. Dose-related differences in elimination from blood were observed suggesting saturation of elimination at the high dose of 350 mg/kg. This was also indicated from comparisons of applied doses and AUC-values, as a 13-fold higher mean dose resulted in a 17-fold higher AUC in male rats, and a 14 -fold higher mean dose resulted in a 26 -fold higher AUC in female rats. Since most of the dose was excreted by 8 hrs, elimination of HQ was associated mainly with the alpha elimination phase, having half times of 0.23- 1.72 hrs. This early elimination phase may represent conjugation of HQ to readily excretable metabolites by extensive first pass metabolism. The ß elimination phase may be attributable to enterohepatic recycling of a small fraction of the dose.

Parent compound represented less than 1% of total14C in blood, indicative of extensive first-pass metabolism. Less than 3% of the dose was excreted as parent compound in urine.The biotransformation of HQ occurred mainly by phase II metabolism to glucuronides and sulfates, which represented 45-53% and 19-33%, respectively, of the administered dose recovered in urine. The presence of the mercapturic acid conjugate (less than 5%) in the urine is consistent with the in vivo formation of a sulfhydryl reactive metabolite and subsequent conjugation with glutathione. No marked sex-differences in the metabolism and disposition of HQ were found except for approximately doubled amounts of mercapturic acid-HQ conjugate in the urine and of14C-activity in the livers and kidneys of female rats in comparison to male rats. Repeated dosing of 25 mg/kg bw/d did not result in tissue accumulation of HQ-derived material. Only 0.51–0.96% of the administered dose of14C was recovered in the tissues and carcass at 48 or 72 h p.a.. 

Excretion was via the urine and faeces, primarily within the first 8 h of gavage. Typically, 87-94% of the administered dose was excreted in urine, and 0.92-2.0% in the faeces.