Hydroquinone

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

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:

comparable to guideline study

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

reference to same study

Cross-reference 2

Reason / purpose for cross-reference:

reference to other study

Data source

Reference

Reference 1

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 2

Reference Type:

study report

Title:

Unnamed

Year:

1988

Report date:

1988

Reference 3

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 4

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

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

Test material information

Constituent 1

Reference substance name:

Hydroquinone

EC Number:

204-617-8

EC Name:

Hydroquinone

Cas Number:

123-31-9

Molecular formula:

C6H6O2

IUPAC Name:

hydroquinone

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 or test system 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 / concentration:

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 results

Main ADME results 1

Type:

absorption

Results:

Peak blood concentrations within 10-14 min for LDS or RDS, within 20-34 min for HDS

Main ADME results 2

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.

Main ADME results 3

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%

Main ADME results 4

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 parameters

Toxicokinetic parameters 1

Test no.:

#1

Toxicokinetic parameters:

half-life 1st: 0.58 hr in males with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 2

Test no.:

#2

Toxicokinetic parameters:

half-life 1st: 0.29 hr in males with single application of 25 mg/kg bw/d

Toxicokinetic parameters 3

Test no.:

#3

Toxicokinetic parameters:

half-life 1st: 1.72 hr in males with sinlge application of 350 mg/kg bw/d

Toxicokinetic parameters 4

Test no.:

#4

Toxicokinetic parameters:

half-life 1st: 0.23 hr in females with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 5

Test no.:

#5

Toxicokinetic parameters:

half-life 1st: 0.91 hr in females with single application of 25 mg/kg bw/d

Toxicokinetic parameters 6

Test no.:

#6

Toxicokinetic parameters:

half-life 1st: 1.35 hr in females with single application of 350 mg/kg bw/d

Toxicokinetic parameters 7

Test no.:

#1

Toxicokinetic parameters:

half-life 2nd: 4.02 hr in males with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 8

Test no.:

#2

Toxicokinetic parameters:

half-life 2nd: 3.20 hr in males with single application of 25 mg/kg bw/d

Toxicokinetic parameters 9

Test no.:

#4

Toxicokinetic parameters:

half-life 2nd: 2.77 hr in females with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 10

Test no.:

#5

Toxicokinetic parameters:

half-life 2nd: 10.52 hr in females with single application of 25 mg/kg bw/d

Toxicokinetic parameters 11

Test no.:

#1

Toxicokinetic parameters:

Tmax: 0.16 hr in males with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 12

Test no.:

#2

Toxicokinetic parameters:

Tmax: 0.24 hr in males with single application of 25 mg/kg bw/d

Toxicokinetic parameters 13

Test no.:

#3

Toxicokinetic parameters:

Tmax: 0.56 hr in males with single application of 350 mg/kg bw/d

Toxicokinetic parameters 14

Test no.:

#4

Toxicokinetic parameters:

Tmax: 0.24 hr in females with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 15

Test no.:

#5

Toxicokinetic parameters:

Tmax: 0.32 hr in females with single application of 25 mg/kg bw/d

Toxicokinetic parameters 16

Test no.:

#1

Toxicokinetic parameters:

Cmax: 6.61 µg equivalents/g in males with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 17

Test no.:

#2

Toxicokinetic parameters:

Cmax: 7.62 µg equivalents/g in males with single application of 25 mg/kg bw/d

Toxicokinetic parameters 18

Test no.:

#3

Toxicokinetic parameters:

Cmax: 39.2 µg equivalents/g in males with single application of 350 mg/kg bw/d

Toxicokinetic parameters 19

Test no.:

#4

Toxicokinetic parameters:

Cmax: 6.03 µg equivalents/g in females with repeated application of 25 mg/kg bw/d

Toxicokinetic parameters 20

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 rinses^a	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±7.21^b	14.87±3.11	3.31±4.40^a
	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	-	-
	% dose^a	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	-	-
	% dose^a	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
	% dose^a	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	-	-
	% dose^a	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	-
	% dose^a	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
	% dose^a	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. of^14C-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 of^14C-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 T_1/2could not be accurately determined

T_max(hr)            time to peak blood concentration

C_max(g equiv/g)           peak blood concentration

alpha T_1/2(hr)            half-life of the alpha (0.693/alpha)

ß T_1/2(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: 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.