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Fischer 344 rat, males, 103 w gavage (5 d/w) at doses of 0, 25 and 50 mg/kg bw/d, combined carcinogenicity and chronic toxicity study similar to OECD Guideline 453: NOAEL 25 mg/kg bw/d, LOAEL 50 mg/kg bw/d (decreased body weight, increased severity of chronic progressive nephropathy, significant increase of renal tubuli adenoma; renal findings as sex- and species-specific effect of no toxicological relevance for human risk assessment) (Kari et al., 1992; NTP, 1989)
HQ has been classified in Carcinogenicity Category 2 (suspected human carcinogen) according to C&L of the GHS. However, the indications that HQ may cause carcinogenic effects in humans are rather questionable as the critical evaluation of the total evidence from the cancer bioassays and epidemiological studies together with more recent reevaluations of the carcinogenic potential of HQ as well as recent studies on the mechanisms of the induction of renal neoplasms show. Additionally, principally HQ was found to inhibit the neoplastic responses of other carcinogenic compounds, and to show no tumor promoting activity. Toxicokinetic studies demonstrated a significant metabolic capacity of humans for the detoxification of HQ and an existing background exposure to HQ, e.g. from food or endogenous production. HQ was not found to exert a genotoxic activity in vivo under exposure conditions relevant for human exposure although a genotoxic potential principally exists. Consequently, in this special case a threshold approach seems to be justified and a DNEL for repeated dose toxicity can be derived.
Carcinogenicity: via oral route
25 mg/kg bw/day
There were 2 carcinogenicity studies showing kidney adenomas in male F344 rats. The other neoplastic or non-neoplastic findings were not consistent between studies or genders.
Carcinogenicity: via inhalation route
Carcinogenicity: via dermal route
With HQ two long-term studies with oral application in F344 rats and B6C3F1 mice have been performed by NTP (1989) and Shibata et al. (1991).
Studies with rats:
In the NTP study (1989), HQ was administered to male and female F344 rats over 103 weeks on 5 days per week via gavage at doses of 0, 25, and 50 mg/kg bw/d (vehicle water). The study protocol was similar to OECD Guideline 453 with several minor deviations (only 2 instead of 3 dose groups, food consumption and urinalysis not examined). In the study performed by Shibata et al. (1991), male and female F344 rats were dosed continuously with 0 or 0.8 % HQ via diet (m: ca. 351 mg/kg; f: ca. 368 mg/kg) over 104 w. In this study only tumour incidences for renal adenomas and hepatocellular carcinomas were given and only one dose was tested. Therefore, it is not possible to make any conclusions concerning dose-response-relationship. The individual data are summarized in the Tables 1 and 2.
Table 1: Long-term studies with F344 rats
0, 25 or 50 mg/kg via gavage on 5 d/w over 103 w
males: 50 mg/kg: slight, but statistically significant increased incidences of renal adenomas (0/55, 4/55, 8/55)*
females: >= 25 mg/kg: statistically significant increased incidences of mononuclear cell leukaemia (9/55, 15/55 and 22/55 [16, 27 or 40 %])
NTP (1989); Kari et al. (1992)
0 or 0.8 % via diet over 104 w (m: ca. 351 mg/kg; f: ca. 368 mg/kg)
males: 351 mg/kg: statistically significant increased incidences of renal adenomas (0/30 and 14/30 [0 or 47 %])
females: no indication for carcinogenic effects
Shibata et al. (1991)
* re-evaluated data (Hard et al., 1997): 0/44, 2/49 and 4/51
NTP (1989) / Kari et al. (1992): Kidneys of males showed slightly increased incidences of renal tubular hyperplasia in the high-dose group (0/55, 0/55, 2/55), and dose-related increased incidences of renal tubular adenomas. Chronic renal disease was diagnosed in the high-dose group after 65 and 103 w of administration, and in the low-dose group only at the 66-week interim sacrifice. In females a significant dose-dependent increase of mononuclear cell leukaemia was found at >= 25 mg/kg.
Shibata et al. (1991): In treated males, the finding of significantly increased incidences of renal tubular hyperplasia (100 % vs. 3 % in controls) and of renal tubular adenomas (47 % vs. 0 % in controls) was associated with an exacerbation of chronic nephropathy.
Studies with mice:
In the NTP study (1989), HQ was administered to male and female B6C3F1 mice for 103 weeks on 5 days per week via gavage at doses of 0, 50, and 100 mg/kg bw/d (vehicle water). The study protocol was similar to OECD Guideline 453 with several minor deviations (only 2 instead of 3 dose groups, food consumption and urinalysis not examined). In the study performed by Shibata et al. (1991), male and female B6C3F1 mice were dosed continuously with 0 or 0.8 % HQ via diet (m: ca. 1046 mg/kg; f: ca. 1486 mg/kg) over 96 weeks. In this study only tumour incidences for kidneys, liver and forestomach were given and only one dose was tested. Therefore, it is not possible to make any conclusions concerning dose-response-relationship.
Table 2: Long-term studies with B6C3F1 mice
0, 50 or 100 mg/kg via gavage on 5 d/w over 103 w
males: no indication for carcinogenic effects
females: >= 50 mg/kg: statistically significant increased incidences (no dose-response relationship) of hepatocellular adenomas (2/55, 15/55 and 12/55 [4, 27 or 22 %]) and of combined incidences of hepatocellular adenomas or carcinomas (3/55, 16/55 and 13/55 [5, 29 or 24 %])
0 or 0.8 % via diet over 96 w
(m: ca. 1046 mg/kg; f: ca. 1486 mg/kg)
males: 1046 mg/kg: statistically significant increased incidences of hepatocellular adenomas (6/28 and 14/30 [22 or 47 %]); no statistical evaluation given concerning combined incidences of hepatocellular adenomas or carcinomas
NTP (1989) / Kari et al. (1992): In both dose groups of males, increased incidences ofhepatocellularadenomas were offset by decreased incidences ofhepatocellularcarcinomas so that the combined incidences ofhepatocellularadenomas or carcinomas were not significantly different from the vehicle control group. However, in dosed females, both incidences ofhepatocellularadenomas and of combined incidences ofhepatocellularadenomas or carcinomas were significantly increased.
Shibata et al. (1991): In treated mice of both sexes small nodular lesions were grossly visible in the forestomach lumen, which was associated with a statistically significant increase of forestomach hyperplasia (m: 37 % vs. 4 % in controls; f: 47 % vs. 10 % in controls), indicating a local irritant effect of HQ that had also been observed in high incidence in the NTP study with gavage application. In treated males, significantly increased incidences of renal tubular hyperplasia (30 vs. 0 % in controls) were observed without a significant increase of renal tubular adenomas (10 % vs. 0 % in controls). There was no indication of alpha-2µ-globulin nephropathy. In dosed males, there was a significant increased incidence of hepatocellular adenomas compared to controls together with significant increased incidences of centrilobular hypertrophy (26 % vs. 0 %) and of foci of cellular alteration (47 % vs. 14 %). In contrast, there was a single case (3 %) of liver adenomas in HQ-treated females and no increase of altered liver foci. Principally, the applied doses appear to be very high as in a 13-week gavage study with B6C3F1 mice a dose of 200 mg/kg bw induced increased lethality (NTP, 1989; see section 7.5).
Evaluation of the interaction of renal non-neoplastic and neoplastic lesions
Studies with long-term exposure of F344 rats to HQ indicated the simultaneous appearance of neoplastic and non-neoplastic changes in the kidney (see Table 3).
Table 3: Comparison of the incidences of renal adenomas and CPN in 2-year studies with F344 rats
Rats graded (n)
Number of rats according to CPN grades
Renal adenomas
Low- to High-Moderate
End-stage
Data from NTP (1989) and Kari et al. (1992): 2 year study (gavage on 5 d/w over 103 w)
0/55*
Interim sacrifice (w 66)
Data from Shibata et al.(1991): 0 or 0.8 % via diet over 104 w (m: ca. 351 mg/kg; f: ca. 368 mg/kg)
* Data from NTP (1989) and Kari et al. (1982)
In the NTP study, high-dosed male rats showed both significant increases of relative kidney weights and increased severity of CPN compared to vehicle controls after 65 and 103 w, while in the low-dose group an increased severity of CPN occurred at the 66-week interim sacrifice only. Advanced renal disease was characterized by varied degrees of degeneration and regeneration of tubular epithelium, atrophy and dilatation of some tubules, hyaline casts in the tubular lumen, glomerulosclerosis, interstitial fibrosis, and chronic inflammation, cysts (dilated tubules in the renal cortex), and increased papillary hyperplasia of the transitional epithelium overlying the renal papillae. Additionally, in male rats kidneys showed a slight increase of the incidences of renal tubuli hyperplasia in the high-dose group (0, 0 and 3.6 %), and a dose-related increase of the incidences of renal tubuli adenomas (0, 7.3 and 14 %; significant increase at 50 mg/kg). There was no indication of renal lesions in female rats (NTP, 1989; Kari et al., 1992).
In the study of Shibata et al. (1991), CPN in treated male F344 rats appeared with higher severity and significantly increased incidence compared to control males (80 % vs. 57 %), which was also associated with a significantly increased incidence of epithelial hyperplasia in the renal papilla. Further, significantly increased incidences of renal tubular hyperplasia (100 % vs. 3 %) and of renal tubular adenomas (47 % vs. 0 %) existed in dosed males vs. controls. In female rats, a significantly increased incidence of CPN compared to control females (27 % vs. 3 %) was seen, but effects were only graded as slight. Renal tubular hyperplasia was observed in 7 % of female rats only, and there were no adenomas. Renal tubular hyperplasia was composed of tubules with stratified epithelial cells which partially or fully filled the tubular lumen, often with cystic forms. Renal tubular adenoma also exhibited cystic or solid forms, in both cases being composed of relatively uniform epithelial cells with clear or pale basophilic cytoplasm and round nuclei with prominent nucleoli.
Re-evaluation of renal lesions
Hard et al. (1997) published a re-evaluation of the renal histopathology of the NTP cancer bioassay on HQ with emphasis on evaluation of incidences, severity and location of CPN and of proliferative non-neoplastic and neoplastic lesions. Characterization of proliferative lesions followed the criteria of the Society of Toxicologic Pathologists (STP) and of the International Agency for Research on Cancer (IARC). There was no evidence of HQ-induced nephrotoxic changes or of alpha-2µ-globulin nephropathy. At terminal sacrifice high-dose males exhibited a statistically significant increase in the grade of CPN. Of the rats surviving into the last 10 w of the study, 49 % had end-stage CPN and 40 % had severe CPN. The re-evaluation of proliferative lesions deviated from the original NTP diagnoses, as at 25 mg/kg the diagnosis of originally 4 adenomas changed to 2 adenomas and 1 incipient adenoma with additional diagnosis of 2 foci of atypical tubule hyperplasia and at 50 mg/kg, instead of originally 8 adenomas the diagnosis was 4 adenomas (including 1 cystadenoma) and 3 incipient adenomas, and 14 foci of atypical tubule hyperplasia (in 11 rats). At 0, 25, and 50 mg/kg, 0/44, 4/49, and 15/51 male rats had either atypical tubule hyperplasia or adenomas, all being in the outer stripe of the medulla(OSOM), the junction of cortex and OSOM, or in the cortex within areas of severe or endstage CPN,and being statistically associated with CPN grade. Consequently, HQ not only exacerbated spontaneous CPN in high-dose male F344 rats, but also exerted a stimulatory effect on the proliferative potential of the advanced stages of CPN at both doses leading to an increased frequency of renal adenomas through this indirect mechanism. As a minimal renal tumour response is linked to an interaction with a rodent-specific spontaneous renal disease, these findings may have little relevance for humans.
A further reanalysis of renal findings was published by Whysner et al. (1995). The finding of renal adenomas appearing in HQ-treated male Fischer 344 rats but not in control groups or in dosed female rats was consistent between the two studies. The higher incidence in the study of Shibata was discussed to be explainable by the use of a larger daily dose administered by diet on 7 days per week compared to a 5 days per week gavage in the NTP study. In the NTP study, an increase in the severity of CPN was found to be related to treatment with HQ only in male rats with almost all of the adenomas and hyperplastic lesions appearing within foci of CPN. In the NTP study, there was no indication of CPN in female rats. In the Shibata study, CPN of a slight degree was increased in HQ-exposed female rats (27 % vs. 3 % in controls), whereas 17 %, 30 % and 33 % of male rats exhibited severe, moderate and slight degrees of CPN, respectively. No data of the locations of CPN and possible interactions with renal tumours are available for this study. There was no evidence of HQ-induced alpha-2µ-globulin nephropathy in either study.
Specific investigations on the mechanisms of the nephrotoxic action of HQ after acute to subchronic oral application (see IUCLID Section 7.9.3) show a strain- and species-specific susceptibility of male F344 rats. Renal lesions induced by exposure to HQ or by the critical metabolite 2,3,5-TriGSylHQ are characterized by tubular cell necrosis followed by cell regeneration. Cell proliferation distal to the site of toxicity was absent. Thus renal cell proliferation presumably is an attempt to compensate for proximal tubular cell loss rather than representing a direct mitogenic effect of HQ. The site-selectivity of nephrotoxicity at the outer stripe of the outer medulla (OSOM, corresponding to S3M segment of the nephron or P3 region) may be a consequence of the susceptibility of this area to oxidative stress, and to the high concentrations of gamma-glutamyl transpeptidase (GGT) in proximal tubular cells. Mechanistically, nephrotoxicity and hyperplasia in the kidney induced by HQ administration could predispose the male F344 rats to kidney tumour formation by an interaction with regenerative cell proliferation associated with CPN. This mode of tumour formation is not expected to operate in the absence of the renal disease process or at subtoxic exposure levels (Boatman et al., 1992, 1993, 1996; English et al., 1994; Peters et al., 1997).
Measurement of levels of 8-hydroxydeoxyguanosine adducts and of HQ-specific or BQ-specific DNA adducts in kidneys of HQ treated F344 rats indicated that hydroquinone does not induce oxidative DNA damage or covalent binding to DNA after multiple gavage doses (see Section 6.6: English et al., 1994, 1997).
In total, as also recently discussed by McGregor (2007),the increased incidence of renal adenomas in HQ-dosed male F344 rats, which is dependent onthe specific susceptibility of this strain and species to the nephrotoxic action of HQ and to the spontaneous development of CPN, does not represent a risk for humans to develop renal cancer.
Re-evaluation of hepatic lesions
In a critical evaluation, Whysner et al. (1995) pointed out that the findings of hepatic adenomas in mice were not consistent between the two bioassays (NTP, 1989; Shibata of al., 1991; see Table 4).
No increases of hepatocellular carcinomas were seen in either study in any sex. In the NTP study, compared to controls, HQ-treated male mice showed no significant change of total liver cancer incidence (adenomas and carcinomas combined) while high-dose males exhibited a lower incidence than low-dose males. Although, in both dose groups, the incidence of liver adenomas showed a similar, statistically significant increase in adenomas, the incidence of liver carcinomas was lower than in the vehicle controls. This means, in the presence of HQ there was a higher incidence of both altered liver foci and benign liver neoplasms while a simultaneous shift from malignant to benign liver tumours occurred in male mice compared to controls. In female mice, the NTP reported a statistically significantly increased incidence of adenomas and of adenomas and carcinomas combined for both dose groups. These incidences showed an inverse dose-relationship, as it had also been found in male mice in the same study. Based on these findings, the NTP concluded on some evidence of carcinogenic activity of HQ in female B6C3F1 mice.
In the study of Shibata et al. (1991), at a distinctly higher dose of 1046 mg/kg bw/d administered via feeding for 96 w, male mice demonstrated a statistically significant increase (P<0.05) in adenomas at the single dose tested together with a slight decrease in carcinomas. In contrast, no increases in either adenomas or carcinomas existed for female mice at a distinctly higher dose of 1486 mg/kg bw/d.
Importantly, liver adenomas and carcinomas are tumours known to occur with a high background incidence in B6C3F1 mice. Historical incidences in male B6C3F1 mice derived from NTP studies are up to 44% for adenomas and up to 58% for adenomas or carcinomas, combined (N=2032) in untreated mice of this strain, and up to 20% and 38%, respectively, in water gavage vehicle controls (N=347). Consequently, in both studies, liver tumour incidences in HQ-treated male mice were all within the upper range of historical incidences specified by NTP (NTP, 1989) for untreated controls of up to 58% (N=2032). Historical incidences in female B6C3F1 mice derived from NTP studies are up to 18% for adenomas and up to 20% for adenomas or carcinomas combined (N=2032) in untreated mice of this strain, and up to 12% and 14%, respectively, in water gavage vehicle controls (N=347). In female mice of the NTP study the observed maximal incidences of liver tumours were in the low dose group, with 27% for adenomas and 29% for combined adenomas or carcinomas, respectively, and were higher than the upper range of historical controls. This finding is contrasted with a single case (3%) of liver adenomas in HQ-treated females in the study of Shibata, which were exposed to a distinctly higher dose.
Based on the overall evidence from cancer bioassays with B6C3F1 mice and F344 rats, or from human exposure experience (see below), there is no indication that the liver is a target of a possible neoplastic action of HQ. In male mice, liver tumour incidences were within historical control ranges and were not dose-related. In female mice, there was no increase of liver tumours in one study and in the other study the total liver tumour incidence decreased from 29% to 24% from the low dose to the high dose, without associated reduced survival. Additionally, liver histopathology in rats demonstrated a significant reduction in bile duct hyperplasia in dosed compared to control males (P<0.05) as well as a significant reduction of the number of altered liver foci per cm2 liver tissue in both sexes (P<0.05 in males, P<0.01 in females) (data of Shibata et al., 1991).
Table 4: Overview on incidences of liver neoplasia and altered liver foci in mice after long-term treatment with HQ
Dose (mg/kg bw/d)
Basophilic foci (%)
Eosinophilic foci (%)
Adenomas (%)
Hepatocellular carcinomas (%)
Adenomas or hepatocellular carcinomas (%)
NTP (1989); Kari et al. (1992)****
21/54 (39)*
20/55 (36)**
Altered foci (%)
Shibata et al. (1991)*****
15/55 (27)***
1/30 (3)**
* significantly different, P<0.01;** significantly different, P<0.05;*** significantly different, P<0.001;****administered by gavage on
5 d/w for 103 w;*****administered via diet on 7 d/w for 96 w
Re-evaluation of mononuclear cell leukaemia
NTP (1989) concluded that there is some evidence of leukaemia in female rats at a dose level of >= 25 mg/kg (dosing via gavage on 5 d/w over 103 w). However, in a critical re-evaluation the finding of mononuclear cell leukaemia only in female rats from one study at an incidence within historical control incidences, was judged to be unrelated to HQ exposure (Whysner et al., 1995).
Both in controls and in dosed male rats high rates of mononuclear cell leukaemia were found, which are not indicating an effect due to HQ exposure. Although the high-dose group of females exhibited a statistically significant increased incidence of this neoplasm compared to controls (incidences 16 %, 27 %, 40 %), incidences were still within the range of historical controls from NTP studies (up to 31 % in untreated controls, N=1983, up to 47 % in water gavage vehicle controls, N=299). Furthermore, no increase in leukaemia incidence was found in either sex of F344 rats in the study of Shibata (personal communication cited in Whysner et al., 1995) even though the dose in this study was 10-times that of the NTP study when calculated on a daily basis. In addition, in the two studies with mice (NTP, 1989; Shibata et al., 1991) there was no increased incidence of leukaemia or lymphoma. Analysis of the incidence of spontaneous tumour in NTP studies later showed mononuclear cell leukemia to be among the highest rates of spontaneous neoplasms in Fischer F344 rats, and were found related to housing and dietary conditions (Haseman et al., Toxicol. Pathol., 26(3):428-441, 1998). The relevance to human cancer risk of this neoplasm commonly observed in aged F344 rats is debated (Williams et al., Principles of testing carcinogenic activity, in: Principles and Method of Toxicology, Fifth edition, Ed. A.W. Hayes, pp. 1265 -1316, 2007; Thomas et al., Toxicol. Sci. 99:3 -19, 2007).
A retrospective study was performed in a well-characterized 1942-1990 cohort of 879 workers (858 men and 21 women) employed principally in the manufacture and use of HQ at a large chemical plant comprising 22,895 person-years of exposure during the 50-year observation period. Mean duration of tenure in a HQ environment was 13.7 years, mean follow-up from first exposure was 26.8 years; 2205 person-years (10%) were represented by subjects with ages of >= 65 years. From 1949-1990 average exposure concentrations ranged from 0.1 to 6.0 mg/m3for HQ dust and less than 0.1 to 0.3 mg/m3for benzoquinone vapour. 20% of cohort members had presumably experienced higher exposures to HQ dust, e.g. up to 35 mg/m3in the packaging area, at the period up to 1949, before measures to reduce exposure had been installed. Causes of mortality were followed up to 1991 and were compared with vital statistics from the general population of the State of Tennessee, and to an occupational reference group of 30,000 hourly wage personnel employed at Kodak's Rochester facilities from 1964 to 1992, few of whom had potential HQ exposure.
There was no evidence of excess mortality in the investigated cohort. In contrast, the number of deaths from all causes of death, and from all forms of malignant cancer was statistically significantly below expectation compared with both general population and occupational controls. There was also a lower than expected death rate from circulatory diseases. Additionally, there were fewer than expected deaths from respiratory cancer, non-malignant pulmonary diseases, and genitourinary system illness, including nephrotoxicity. There was no evidence of a dose-relationship with respect to lifetime exposure or latency. This study did not support the findings of animal cancer bioassays showing an increased prevalence of predominately benign neoplasms of the kidney and liver and of mononuclear cell leukaemia in F344 rats or B6C3F1 mice (Kari et al., 1992; NTP 1989; Shibata et al., 1991). In the cohort of HQ workers, cancer mortality was not statistically significantly different from expectation, and no liver tumour or leukaemia deaths were observed. The two observed cases of malignant kidney tumours (1.3 expected, no significant effect) were confounded by an excessive smoking history, which is recognized as major cause of renal cancer in humans, and by workplace exposure to other chemicals (Key study: Pifer et al., 1995).
A cohort of 837 Danish lithographers, born in 1933 - 1942, was followed in the Danish Cancer Register from 1974 to 1989. Information on exposure to organic solvents and other chemicals is only available from questionnaires (no quantitative data). About 200 of the cohort members reported an exposure to HQ from photochemicals besides a lot of other chemicals. Due to the insufficient characterisation of the existing exposures, the study can be only be used as supporting information. There was no indication of an increased cancer incidence in the kidneys or the liver or of leukaemia in the total cohort including the subcohort with HQ-exposure. A discussed association between HQ exposure and appearance of malignant melanoma in lithographers is considered to be highly questionable as only two of the five melanoma cases reported exposure to HQ. Further, location of the melanoma at the trunk was not at the primary site of dermal contact with HQ during manual photo processing, and possible confounding factors contributing to the development of malignant melanoma, as exposure to a multitude of other chemicals, life-style and sun-exposure had not been controlled in the study (Supporting study: Nielsen et al., 1996).
Based on these epidemiological studies in HQ-exposed workers there are no indications, that kidneys, livers, or blood system are specific target organs of a possible carcinogenic effect of HQ, even at the elevated exposure levels of the early years of HQ production before 1950. This is in accordance with the known high metabolic capacity of humans for detoxification of HQ, which results in distinctly lower levels of possibly reactive critical metabolites compared to rats (see IUCLID Section 7.1.1).
SUPPORTING INFORMATION FROM OTHER EXPERIMENTAL STUDIES RELATED TO CARCINOGENICITY
HQ was found to show no tumor promoting activity in the urinary bladder, liver, forestomach and glandular stomach of F344 rats, in the liver of Wistar rats, and in the pancreas, liver and gallbladder of Syrian Golden hamsters when, after treatment with site-specific initiators, HQ was fed at concentrations of 0.05 to 2% for up to 51 weeks depending on the corresponding test protocols (for details see Table). Investigations included histopathological examination of hyperplastic and neoplastic lesions as wells as assays for cell proliferative activity (DNA synthesis). In the kidney of Wistar rats, HQ treatment enhanced formation of renal tumors after initiation with N-ethyl-N-hydroxyethyl-nitrosamine. In contrast, HQ was found to inhibit tumor formation in liver or pancreas initiated by treatment with DEN or BOP, respectively (Supporting studies: Hirose et al., 1989; Kurata et al., 1990; Maryama et al., 1991; Okazaki et al., 1993; Williams et al., 2007). There were no early proliferative responses in forestomachs of F344 rats or Syrian Golden hamsters after feeding of 0.5% HQ for 20 w or 0.8% HQ for 8 w (Hirose et al., 1986; Shibata et al., 1990). HQ was inactive in two short-term liver foci assays on enhancement of GST positive foci in male F344 rats after initiation with AAF or DEN (Hasegawa and Ito, 1992; Williams et al., 2007) while a weak enhancement of GGT-positive liver foci was reported in male Sprague-Dawley rats after initiation with DEN. In this study, HQ alone induced no altered liver foci. HQ-induced GSH depletion and cytotoxicity were events found to be critical for development of the altered liver foci (Stenius et al., 1989).
After dermal treatment of female Swiss mice with 5 mg HQ, 3 times weekly, for 368 d, there were no skin papilloma or skin carcinoma. Inhibition of the carcinogenic effect of BaP after coadministration of HQ was reflected in a cocarcinogenesis assay by skin tumor incidences decreased by at least 50%. Also HQ showed no promoting activity on mouse skin after intiation with BaP (Van Duuren et al., 1976).
Table 5: Overview on further studies related to carcinogenicity (all studies with Reliability 2)
Tumor promotion in urinary bladder
F344 rats
Initiation: 0.05% BBN in drinking water for 4 w,
promotion: 0.8% HQ in diet from day 31 until end of week 36
Control: without BBN treatment but exposure to HQ
Kurata et al., 1990
Tumour promotion in kidney and liver
Wistar Crj rat
Initiation: 0.3% EHEN in drinking water for 3 w
promotion: 0.8% HQ in diet from week 5 until end of week 36
Control: without EHEN treatment but exposure to HQ
Neg: in liver
Pos: in kidney (enhancement of EHEN-induced renal tumours)
Okazaki et al., 1993
Tumor promotion in liver
Short-term liver foci (GST pos foci) assay with and without initiation with AAF
Initiation: 3 mg/kg AAF by gavage 3 times a week for 12 w from week 1 to 13
Promotion: 0.05 and 2% HQ in diet from week 1 to 13 (ca. 25 and 100 mg/kg bw/d)
Neg: no effect of HQ alone
Inhibition of AAF-induced effects
Williams et al., 2007
Short-term liver foci (GST pos foci) assay after initiation with DEN
Initiation: 200 mg/kg DEN / hepatectomy
promotion: 2% HQ in diet from week 3 to 6
Neg:
inhibition of DEN-induced effects
Hasegawa and Ito, 1992
Short-term liver foci (GGT pos foci) assay without and with initiation with DEN
Sprague-Dawley rat
Initiation: 30 mg/kg DEN i.p. / hepatectomy
promotion: 100 or 200 mg/kg bw/d HQ in diet from week 2 to 8
promotion: 100 or 200 mg/kg bw/d HQ by gavage 5 d/w from week 2 to 8
Controls: treated with HQ alone
enhancement of GGT-pos. liver foci induced by DEN
Stenius et al., 1989
Tumour promotion in pancreas, liver and gallbladder
Syrian Golden hamsters
Initiation: 70 mg/kg BOPtwice
Promotion: 1.5% HQ in diet from week 4 to 20
Control: only HQ in diet
Neg: pancreas
inhibition of BOP-induced effects
Neg: liver and gallbladder no significant changes
Maryama et al., 1991
Early proliferative changes in forestomach
0.5% HQ in diet for 20 w
Hirose et al., 1986
Tumour promotion in forestomach and glandular stomach
F344 rat
Initiation: 150 mg/kg MNNG by gavage
0.8% HQ in diet for 8 w
Shibata et al., 1990
Carcinogenicity skin
Swiss mice ICR/Ha
Dermal treatment
5 mg HQ for 3 times weekly, 368 d
Van Duuren et al., 1976
Cocarcinogenic activity on skin
Coadministration of 5 µg BaP + 5 mg HQ for 3 times weekly, 368 d
Inhibition of carcinogenic effects of BaP
Tumor promotion skin
Initiation with 150 µg BaP
Promotion: 5 mg HQ for 3 times weekly, from day 15 to 409
AAF: acetylaminofluorene; BaP: benzo[a]pyrene; BBN: N-butyl-N-(4-hydroxybutyl)nitrosamine; BOP: N-nitroso-bis(2-oxopropyl)amine;
EHEN: N-ethyl-N-hydroxyethyl-nitrosamine; MNNG: N-methyl-N'-nitro-N-nitrosoguanidine; TBMP: 2-tert-buty-4-methylphenol
HQ has been classified in Carcinogenicity Category 2 (suspected human carcinogen) according to C&L of the GHS, mainly based on the kidney tumors observed in male F344 rats. However, the indications that HQ may cause carcinogenic effects in humans are rather questionable as the critical evaluation of the total evidence from the cancer bioassays and epidemiological studies together with more recent reevaluations of the carcinogenic potential of HQ as well as recent studies on the mechanisms of the induction of renal neoplasms. Additionally, principally HQ was found to inhibit the neoplastic responses of other carcinogenic compounds, and to show no tumor promoting activity. Toxicokinetic studies demonstrated a significant metabolic capacity of humans for the detoxification of HQ and an existing background exposure to HQ, e.g. from food or endogenous production. HQ was not found to exert a genotoxic activity in vivo under exposure conditions relevant for human exposure. Consequently, in this special case a threshold approach seems to be justified and a DNEL for repeated dose toxicity can be derived.
Justification for selection of carcinogenicity via oral route endpoint:
Renal tubular hyperplasia and renal tubular adenomas were reported in male F344 rats in 2 different carcinogenicity studies, via oral gavage or in the feed. Numerous mechanistic investigations have indicated that the likely mechanism is related to exacerbation of Chronic Progressive Nephropathy which seems to occur with a particular sensitivity in Fischer F344 male rats, possibly related to a higher kidney exposure to toxic metabolites in that rat strain, but not related to direct DNA damage. This was comforted by negative results in in vivo TGR assay in mice and in vivo Comet assay in F344 rats. This mechanism of action is not relevant to human. As the in vivo comet assay also showed no genotoxic activity in the germ cells, a DNEL for threshold effects is derived for the risk assessment.
Justification for selection of carcinogenicity via dermal route endpoint:
No carcinogenic effects observed in mice following dermal treatment, and no cocarcinogenic effects or tumor promoting effects.
Carcinogenicity: via oral route (target organ): urogenital: kidneys
HQ has been classified in Carcinogenicity Category 2 (suspected human carcinogen) according to C&L of the GHS based on the presence of renal tubular adenomas and hyperplasia in male Fischer F344 rats. Numerous mechanistic investigations have indicated that the likely mechanism is related to exacerbation of Chronic Progressive Nephropathy which seems to occur with a particular sensitivity in Fischer F344 male rats, possibly related to a higher kidney exposure to toxic metabolites in that rat strain, but not related to direct DNA damage nor to DNA adducts. This was supported by the lack of DNA binding activity in kidneys, and by negative results in in vivo TGR assay in mice and in vivo Comet assay in F344 rats. The mechanism of action identified in Fischer F344 rats is not relevant to human. A more severe classification is therefore not justified. As the in vivo comet assay showed also no genotoxic activity in the male gonads, as a reliable surrogate of germ cells, a DNEL for threshold effects is derived for the risk assessment.