Chromium

Endpoint summary

Key value for chemical safety assessment

Genetic toxicity in vitro

Description of key information

Chromium metal exists in several valence states, depending on surrounding conditions (pH, medium, form, etc…), its state and what other atoms it is bound to (organic or inorganic forms). The most stable and common forms are metallic chromium, under trivalent valence (chromium III), and hexavalent valence (chromium VI). While chromium VI has been shown to cause cancer in other animal studies, chromium III which is an essential trace element and is ingested in food or dietary supplements, which has a different chemical properties and profile. Additionally, chromium (III) has been evaluated as non-carcinogenic for human by IARC (International Agency for Research on Cancer) in 1990. Thus as by definition, genotoxicity describes thepropertyof chemical agents that damages the genetic information by processes which alter the structure, information content or segregation of DNA and which may lead tocancer, the genotoxicity assessment discussed in this section follows the same logic.

 

A lot of reports and references state that unlike chromium (VI), Chromium (III) is poorly absorbed and taken up by cells and as a consequence, trivalent chromium presents low toxicity. Even if Cr(III) compounds have the potential to react with DNA in a cellular system, water-insoluble Cr (III) unable to cross cell membrane as it’s not a substrate and can only be intake by pinocytosis, which highly limit is intracellular concentration (Salnikow and Zhitkovich 2007 ; EFSA, 2014 ; Junaid et al., 2016). Accordingly, chromium (III) metal genotoxicity can be assessed and discussed as trivalent chromium.

Several in vitro and in vivo studies about the potential of genotoxicity of trivalent chromium have been published with variable results, quality and conclusions. Most of these studies have been assessed in official reviews (WHO, 2009 ; ATSDR, 2012 ; EFSA, 2014) which described the lack of evidence of genotoxic effect for trivalent chromium compounds. Despite, the mutagen form of chromium that binds DNA is the trivalent form produced intracellularly from Cr(VI), the apparent lack of activity of Cr(III) is solely due to its poor cellular uptake.

 

In vitro genotoxicity results suggest that the data for genotoxicity are equivocal, considered that for the majority of the bacterial and mammalian cell assays, results were negative, and where positive results are recorded, these assays are only weakly positive.

Considering the quality score (Klimisch quotation), the main results obtained by in vitro genotoxicity studies on Cr(III) are discussed below:

 

A lot of research tested chromium picolinate whichis one of a number of compounds that contain chromium in the trivalent state (Cr III), and which is used as dietary supplements.

 

- Stearns et al. (2002) investigated the mutagenicity of Cr(III) picolinate in CHO AA8 cells, using solutions of Cr(III) picolinate up to 1 mM and of picolinic acid up to 3 mM. Cr(III) picolinate was found positive for HPRT mutations. The HPRT mutations were increased up to 40-fold compared to control. Picolinic acid was more cytotoxic than the corresponding Cr(III) picolinate complex but there was no evidence of mutation induction.

 

- Negative results were obtained by Slesinski et al. (2005) by performing a HPRT assay in CHO cells exposed to concentrations of Cr(III) picolinate up to 1.43 mM (15.6-500 μg/ml) with and without metabolic activation for 5- and 48 hour periods.

 

- Whittaker et al. (2005) tested the mutagenic potential of Cr(III) picolinate and its component compounds, Cr(III) chloride and picolinic acid in bacteria (Ames test) and L5178Y mouse lymphoma cells. Results were negative in the in vitro AMES test. Cr(III) picolinate induced mutagenic effects with and without the addition of S9.

Cr(III) chloride was negative in the absence of metabolic activation but questionable results were observed with metabolic activation and after exposure to picolinic acid due to high cytotoxicity at mutagenic doses. Indeed, according to the OECD 476 guideline about in vitro mammalian cell gene mutation test, positive results which do not reflect intrinsic mutagenicity may arise from changes in pH, osmolality or high levels of cytotoxicity.

 

- Coryell and Stearns (2006) evaluated the mutagenic effects of Cr(III) picolinate in the HPRT mutation assay in CHO AA8 cells after 48 h exposure using either acetone or dimethyl sulfoxide (DMSO) as solvents. Cr(III) picolinate increased the mutation frequency under both treatement conditions but it was 3.5-fold more mutagenic when dissolved in acetone. The authors hypothesized that this effect is due to the radical scavenger properties of DMSO suggesting that the free radical production by Cr(III) picolinate contributes to genotoxicity. It should be noted that the increases in mutation frequency were observed at cytotoxic doses too.

 

El-Yamani et al. (2011) compared the genotoxicity of chromium (III), as chromium chloride, and chromium (VI), as sodium chromate, in human lymphoblastoid (TK6) cells exposed to chromium concentrations of 0, 0.2, 0.4, 0.6, 0.8 or 1 mM by the COMET assay. The authors also assessed the type of DNA occurring in these cells by treatment with enzymes that recognise oxidised bases, formamidopyrimidine (FPG) and endonuclease III (endoIII) at concentrations of 0.8 or 1 mM, and conducted a kinetic repair study at a concentration of 1 mM.

In the COMET assay, the percentage of DNA in the tail was significantly increased in with both chromium compounds at the top four doses. Treatment with FPG and endoIII revealed a higher degree of DNA damage with both chromium compounds at both doses indicating the induction of oxidized bases. The kinetic repair study demonstrated that DNA damage was removed after 8 hours, with the damage more rapidly repaired or removed following exposure of the cells to chromium (III).

Genetic toxicity in vivo

Description of key information

The evidence in these official reviews (WHO, 2009 ; ATSDR, 2012 ; EFSA, 2014) also suggest that chromium (III) compounds are not genotoxic in vivo.

 

- In an in vivo study, Sprague-Dawley rats (5/sex/dose) were orally administered a single dose of chromium (III) as chromium picolinate at doses of 0, 33, 250 or 2000 mg/kg bw. Bone marrow cells in metaphase were examined for interstitial deletions, chromatid and chromosome gaps, breaks or other anomalies. No evidence of genotoxic effects were noted at any dose (Komorowski et al., 2008).

 

- Chromium picolinate and niacin-bound chromium (III) complex also did not cause DNA damage, or increased frequencies of micronuclei in rats exposed in vivo (NTP 2008; Shara et al. 2007).

 

In 2008, a review of studies on the genotoxicity of chromium (III) compounds has been conducted (Eastmond et al., 2008) and these data indicate that chromium (III) compounds are not genotoxic in vivo. Additional two years NTP studies on Cr(III) picolinate also observed and concluded on the negative results:

 

- In a Cr(III) picolinate NTP study (NTP, 2010) the in vivo micronucleus assay was performed in male F344/N rats (156 to 2500 mg/kg b.w.) by oral gavage three times at 24-hour intervals.

Negative results were observed in bone marrow erythrocytes of male rats.

 

- Another NTP study (NTP, 2010) studied the in vivo micronucleus assay, performed in male and female B6C3F1 mice administered Cr(III) picolinate monohydrate (80 to 50000 mg/kg diet corresponding to 2-1419 and to 1.7-1090 mg Cr(III)/kg b.w. per day for male and female respectively) in feed for 3 months.

Negative results were observed in peripheral blood erythrocytes of the male mice. The weak increases in the micronuclei frequency observed in erythrocytes of female mice were considered equivocal findings as the anhydrous form was inactive.

 

 

Chromium has a wide range of industrial uses including in the leather tanning industry, in the manufacture of catalysts, paints, fungicides, ceramics and glass, photography, chrome plating (such as taps), and as a metal in alloys. Many studies refer and assessed the genotoxicity potency of trivalent chromium on human exposed in their working place. However, studies in humans were limited in several aspects: mixed pollutants, chemical valence analysis, statistical validation, etc… Generally, the levels of exposure to chromium (VI) were not known and co-exposure to other potentially active compounds (namely ultraviolet rays and other potentially genotoxic metals) occurred in several studies. Considering the quality criteria of these in situ studies, the main references and results, are as follow:

 

- Increases in DNA damage were found in tannery workers exposed to chromium (III) (Zhang et al. 2008). Significant associations between DNA damage and blood and urinary chromium levels were observed; blood chromium levels ranged from 13.10 to 68.30 μg/L (median of 22.95 μg/L) and urinary chromium levels ranged from 1.50 to 42.20 μg/L (median of 10.60 μg/L) in the high-exposure group and 4.30–64.3 μg/L (median of 22.95 μg/L) and 1.50–18.00 μg/L (median of 2.25 μg/L), respectively, in the low-exposure group. Compared to the COMET assay guideline (OECD 489) deviation should be underlined in this study, mainly the number observed cells which is below the OECD guidance.

 

- Another study (González Cid et al., 1991) reported elevated frequency of chromosomal aberrations in the exposed tannery workers but not statistically different from the frequency seen in the controls. In this study the urinary Cr concentrations did not differ between the exposed and control workers.

 

The literature on genotoxicity of Cr(III) compounds, reviewed in several official reports, gave largely negative results in or weakly positive, results in mammalian cell assays (although often at cytotoxic doses). In vivo tests for genotoxicity were all negative. Cr(III) compounds have the potential to react with DNA in acellular systems however in intact cells restricted access limits or prevents genotoxicity. At high concentrations, multiple studies showed that Cr(III) compounds might cause DNA damage which is potentially mutagenic but trivalent chromium not considered / classified as carcinogenic by any official reports / review (IARC, 1990 ; WHO, 2009 ; ATSDR, 2012 ; EFSA, 2014).

 

Considering that limited intake of trivalent chromium which highly limited reaction with DNA, the absence of genotoxic effects in vivo, the very limited information from few case studies was not suitable to assess human toxicity after oral exposure to Cr(III) compounds, the definition of a genotoxic (as given by the REACH regulation) and accordingly to the assessment of official reviews, trivalent chromium is not expected to present genotoxic properties.

Additional information

Limited data were available on gentoxicity testing with chromium metal. Therefore read-across to studies using chromium(III) familly such like chromium(III) oxide, stainless steel and chromium(III) chloride was applied.

Negative results were observed in a single bacterial mutagenicity assay, whereas somein vitro studies with mammalian cells gave positive results. Based on these studies in vitro mutagenicity is inconclusive.

The mouse guideline study using a single high dose of chromium(III) oxide showed no induction of micronuclei in bone marrow polychromatic erythrocytes. However, a decrease in the ratio of normochromatic to polychromatic eythrocytes was observed, which may hamper the detection of micronuclei. Lack of clear in vivo genotoxicity is, however, supported by one negative bone marrow micronucleus study in mice with soluble chromium(III)chloride, as well as a negative chromosomal aberration test with stainless steel.Thus, although the genotoxic potential of metallic chromium and chromium(III) oxide is uncertain based on available test data, the bioavailability of chromium is low and it is unlikely that this practically insoluble substance could be systemically distributed in concentrations high enough to cause genotoxicity.

When comparing the incidence of micronuclei in nasal cells of ferrochromium workers (Cr(III) exposure) and control persons, no differences were observed.

Short description of key information:
An in vivo test carried out according to the OECD 427 guideline showed no indications of clastogenic effects after one intraperitoneal dose of 10000 mg/kg of chromium(III) oxide in mice, as indicated by unchanged number of micronucleated polychromatic erythrocytes.
Chromium(III) oxide in vitro studies resulted in both positive and negative results. The lack of clear in vivo mutagenicity is supported by a negative chromosomal aberration test with stainless steel and a negative micronucleus study with chromium(III)chloride.

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

An in vivo mouse guideline test showed no clastogenic effects with chromium metal or chromium(III) oxide. These results are supported by the negative results of a chromosomal aberration test with stainless steel, as well as a negative bone-marrow micronucleus study in mice with chromium(III) chloride. Some in vitro tests signal positive results, but however these do not give enough evidence for classification. The poor solubility of chromium suggests that it is unlikely that the substance could be distributed systemically in concentrations high enough to cause genotoxicity.

Considering that limited intake of trivalent chromium which highly limited reaction with DNA, the absence of genotoxic effects in vivo, the very limited information from few case studies was not suitable to assess human toxicity after oral exposure to Cr(III) compounds, the definition of a genotoxic (as given by the REACH regulation) and accordingly to the assessment of official reviews, trivalent chromium is not expected to present genotoxic properties.

No classification is suggested for chromium (III).