Hydrocarbons, C6-C7, isoalkanes, cyclics, <5% n-hexane

Endpoint summary

Description of key information

Short description of key information on absorption rate: 
Percutaneous absorption values are less than 2 ug/cm2/day or less than 1% of applied dose

Additional information

In a study of the metabolism of commercial hexane, n-hexane was metabolized and excreted within 168 h of iv bolus administration, inhalation exposure or dermal application. Exhaled breath and urine were the two primary routes for the excretion and its metabolites. n-Hexane was widely distributed to the body tissues but neither n-hexane nore its metabolites were concentrated significantly by any of those tissues. It was extensively metabolized and a number of radio labeled metabolites were excreted in the urine. n-Hexane and its radio labeled metabolites disappeared from the blood of rats with a half-life of approximately 9-10 hours. No significant differences between males and females were noted in the rates and routes of metabolism and excretion of the test compounds. Repeated inhalation exposure had no apparent effect on the rates or routes of excretion of either of the test compounds or their metabolites.

 

The toxicokinetics of cyclohexane have been studied in rat (RTI, 1984) and rabbit (Elliott et al., 1959).

 

Male Fisher-344 rats were administered single oral doses by gavage of 100, 200, 1000 or 2000mg [14C]-cyclohexane per kg bodyweight, an additional group of rats received a single intravenous dose of 10 mg [14C]-cyclohexane per kg bodyweight. Exhaled volatiles, urine and faeces were collected at intervals up to 72 hours after dosing. Blood, plasma and selected tissues were retained at termination and analysed for total 14C; metabolite profiles were determined by HPLC.

 

Female rabbits were administered single oral doses by gavage of 0.3, 100 or 350 - 400mg [14C]-cyclohexane per kg bodyweight in a number of small experiments. Exhaled volatiles, urine, faeces and tissues were retained and analysed for total 14C; metabolite profiles were determined by paper chromatography or derivatisation. In an additional experiment, doses of cyclohexane and putative metabolites were administered individually and the degree of conjugation with glucuronic acid was determined.

Mice were exposed for 1 hour by inhalation to cyclohexane vapour derived from an adhesive; vapour concentrations of 8000, 14000 and 17500ppm were used. Blood concentrations of cyclohexane were determined during and after exposure.

The ability of cyclohexanol to induce the activity of specific male Wistar rat cytochrome (CYP) P450 isozymes was investigated (Espinosa-Aguirre et al., 1996 & 1997). Rats were exposed orally to cyclohexanol (2.5% v/v in drinking water, ad libitum) for 5 days prior to determination of isozyme activity using Western blot analysis.

 

Absorption

Although absorption was not determined specifically, minimum values can be estimated from the total of the amounts excreted in exhaled volatiles and urine and that retained in tissue. Using this approach, estimates for absorption of cyclohexane after gavage dosing are approximately 91% for rats (100 - 2000mg/kg bodyweight) and 95% for rabbits (0.3 - 400mg/kg bodyweight).

 

Distribution

Following administration of 200mg [14C]-cyclohexane per kg bodyweight to rats, concentrations of total 14C in whole blood and plasma were similar; there was considerable inter-animal variation in concentrations when the same dose was administered on three separate occasions to different rats. Peak concentrations of total 14C in blood and plasma were attained by 6 - 12 hours after dosing. Concentrations of total 14C in all studied tissues were greatest 6 hours after dosing and were significantly lower by 72 hours after dosing. Tissue residues of total 14C at 72 hours after administration accounted for approximately 0.4% of a dose of 200mg [14C]-cyclohexane per kg bodyweight. Following dosing by either intravenous (10mg/kg) or oral (200, 1000 or 2000mg/kg) routes, the highest concentrations of 14C at 72 hours after dosing were in adipose tissue. The ratios of the concentrations of 14C in adipose to those in blood ranged from approximately 16:1 for the 10mg/kg intravenous and 100mg/kg oral doses to 41:1 and 47:1 for the 1000 and 2000mg/kg oral doses (RTI, 1984).

The tissue distribution of [14C]-cyclohexane was not determined in rabbit tissues; total residues in tissue at termination (3 - 6 days after administration) were approximately 2.5% of the dose (Elliott et al., 1959).

 

Metabolism

Following an intravenous dose of 10mg [14C]-cyclohexane per kg bodyweight to rats, 79.5% of the dose was exhaled unchanged during the initial 24 hours after dosing with a further 1.27% and 1.43% exhaled during the 24 - 48 and 48 -72 hour periods respectively. 14C exhaled as either cyclohexanone or cyclohexanol only accounted for a total of 0.22% of the dose over the 0 - 72 hour period after dosing. Following oral administration of 100, 200 or 1000mg/kg, unchanged cyclohexane exhaled during the 72 hour period following dosing accounted for 59.4, 59.8 and 92.1% of the doses respectively. The corresponding values for 14C exhaled as either cyclohexanone or cyclohexanol were 0.09, 0.62 and 0.24% of the dose respectively. The report authors commented that the high value for cyclohexane after a dose of 1000mg/kg may have been due to an error in estimating the dose. No significant amounts of 14CO2 were detected after any of the doses. Similar urinary metabolite profiles were observed after both intravenous and oral administration and at all oral dose levels. Only trace quantities of cyclohexane, cyclohexanone and cyclohexanol were present, the majority of the 14C was present as four unidentified polar metabolites. Plasma contained minor amounts of unchanged cyclohexane along with cyclohexanone, cyclohexanol and five unidentified metabolites that accounted for at least half the total 14C present.

 

Unchanged cyclohexane in adipose accounted for 79 and 94% of the total 14C present 72 hours after the 10mg/kg intravenous and 1000mg/kg oral doses respectively; small amounts of cyclohexanone and cyclohexanol were also detected. The majority of the total 14C present in muscle, liver and skin was not extractable; small amounts of cyclohexane, cyclohexanone and cyclohexanol were detected (RTI, 1984).

 

In rabbits unchanged [14C]-cyclohexane in exhaled volatiles accounted for 25 - 38% of a dose of 360 - 390mg [14C]-cyclohexane per kg bodyweight; an additional 8 - 10% of the dose was eliminated as 14CO2; after a low dose of 0.3 mg[14C]-cyclohexane per kg bodyweight all exhaled radioactivity (6% of dose) was present as 14CO2. No cyclohexanone or cyclohexanol was detected in exhaled volatiles. The major urinary metabolites were cyclohexanol accounting for 35 - 50% of the high dose and 60% of the low dose and (±) trans-cyclohexane -1, 2 diol accounting for 3 – 8% of the high dose and 17% of the low dose; both metabolites were present as the glucuronic acid conjugate (Elliott et al., 1959).

 

Excretion

In rats the major route of excretion of 14C was in exhaled volatiles accounting for 83.2, 63.4, 61.8, 96.8* and 78.5% of the dose following 10mg/kg intravenous, 100, 200, 1000 or 2000mg/kg oral doses respectively; the corresponding values for urinary excretion were 13.5, 28.8, 28.6, 19.3* and 12.0% of the dose (* the high values after the 1000mg/kg oral dose may be due to an error in estimating the dose). A preliminary experiment showed that as only minor amounts of 14C(<0.3% of the dose) were excreted in faeces; samples were not analysed for the remainder of the studies. Elimination half lives for total 14C from plasma and tissues were 10 - 15 hours with a slightly longer value for skin (RTI, 1984).

 

In rabbits administered 360 - 390mg [14C]-cyclohexane per kg bodyweight, 35 – 47% of the dose was eliminated in exhaled volatiles; at 0.3mg [14C]-cyclohexane per kg bodyweight the corresponding values was 6% of the dose. Urinary excretion of radioactivity at the high and low dose levels was 33 - 56% and 87% of the doses respectively. Faecal excretion of radioactivity (0.1-0.2% of dose) was minimal (Elliott et al., 1959).

Discussion on absorption rate:

OVERVIEW OF PERCUTANEOUS ABSORPTION OF HYDROCARBON SOLVENTS

There are no studies of repeated dose toxicity of hydrocarbon solvents using the dermal route of administration. Accordingly, where it is necessary to calculate dermal DNELs, systemic data from studies utilizing other routes of administration, normally inhalation but also oral data, can be used in some situations.  In accordance with ECHA guidance, read across from oral or inhalation data to dermal should account for differences in absorption where these exist (R8, example B.6). In fact, hydrocarbon solvents are poorly absorbed in most situations, in part because some are volatile and do not remain in contact with the skin for long periods of time and also because, due to their hydrophobic natures, do not partition well into aqueous environments and are poorly absorbed into the blood. 

 

           If these differences in relative absorption are introduced into the DNEL calculations to calculate external doses, the DNELs based on systemic effects are highly inflated. This seems potentially misleading as it implies that substances have different intrinsic hazards when encountered by different routes whereas in fact the differences are due ultimately to differences in absorbed dose. Accordingly, it is our opinion that it would be more transparent if the differences in absorption were taken into account in the exposure equations rather than in DNEL derivation. 

 

           Shown below is a compilation of percutaneous absorption information for a number of hydrocarbon solvent constituents covering carbon numbers ranging from C5 to C14 as well as examples of both aliphatic and aromatic constituents. The low molecular weight aliphatic hydrocarbons (n-pentane, 2-methylpentane, n-hexane, n-heptane, and n-octane) were tested by Tsuruta (1982) using rat skin in an in vitro model system. As shown (Table 1), the highest percutaneous absorption value was 2 ug/cm2/hr for pentane. Lower values (< ~ 1 ug/cm2/hr) were reported for aliphatic hydrocarbons ranging from hexane to octane. Several authors have assessed the percutaneous absorption of higher molecular weight aliphatic constituents including Baynes et al. (2000), Singh and Singh (2003), Muhammad et al. (2005), and Kim et al., (2006). The first three of these authors used porcine skin models and reported that, except for one anomalous result with tridecane, the percutaneous absorption values for aliphatic constituents ranging from nonane to tetradecane were well below 1 ug/cm2/hr. Rat and human skin are considered to be more permeable than human skin (Kim et al., 2006), so these numbers can be considered conservative. 

 

           Kim et al. (2006) reported results of percutaneous absorption studies with human skin under in vivo conditions. In this case, the assessment method was based on tape stripping. The authors reported percutaneous absorption values ranging from 1 – 2 ug/kg/day for decane, undecane and dodecane. These values are higher than those reported by other authors, most likely because this technique measures absorption into the skin but not through the skin as was done in the studies listed above. Accordingly, it seems likely that these numbers are conservative as well.

 

           With respect to aromatic hydrocarbons, most of the reported percutaneous absorption values [Baynes et al. (2000); Singh and Singh (2003); Mohammad et al. (2005); and Kim et al. (2006)] are less than 2 ug/cm2/day. The only exceptions are the values for naphthalene from Mohammad et al. (2005) which range from 4.2-6.6 ug/cm2/hr. 

 

           After considering all of the above, it seems reasonable to assume apparent that across the entire range of hydrocarbon solvent constituents, percutaneous absorption values are less than 2 ug/cm2/day or less than 1% of applied dose. Accordingly, when systemic dermal DNELs are calculated using route to route extrapolations, the values will not be corrected for differences in absorption. Rather, 2 ug/cm2/hr or 1% of applied dose will be used as a common percutaneous absorption rate for all hydrocarbon solvents for which dermal exposure estimates are provided. 

           

Table 1: Summarized information on percutaneous absorption of hydrocarbon solvent constituents (C5-C16). 

 

Constituent	Molecular Weight	nmol/min/cm2	nmol/hr/cm2	ug/cm2/hr	Reference
Aliphatic Constituents
Pentane	72	0.52	31.2	2.2	Tsuruta et al. 1982
2-methyl pentane	86	0.02	1.2	0.1	Tsuruta et al., 1982
n-hexane	86	0.02	0.6	0.5	Tsuruta et al., 1982
n-heptane	100	0.02	1.2	0.1	Tsuruta et al., 1982
n-octane	114	0.08 x 10-3	0.005	0.0005	Tsuruta et al., 1982
Nonane	128			0.03	Muhammad et al., 2005
Nonane				0.38	McDougal et al., 1999
Decane	142			2	Kim et al., 2006
Decane				1.65	McDougal et al., 1999
Undecane	156			0.06-0.07	Muhammad et al., 2005
Undecane				1.0	Kim et al., 2006
Undecane				1.22	McDougal et al., 1999
Dodecane	170			0.02-0.04	Muhammad et al., 2005
Dodecane				2	Kim et al., 2006
Dodecane				0.3	Singh and Singh, 2003
Dodecane				0.51	McDougal et al., 1999
Dodecane				0.1	Baynes et al. 2000
Tridecane	184			0.00-0.02	Muhammad et al., 2005
Tridecane				2.5	Singh and Singh, 2003
Tridecane				0.33	McDougal et al., 1999
Tetradecane	198			0.3	Singh and Singh, 2003
Hexadecane			7.02 x 10E-3	0.00004	Singh and Singh, 2002
Aromatic Constituents
Trimethyl benzene	120			0.49 - 1.01	Muhammad et al., 2005
Trimethyl benzene				1.25	McDougal et al., 1999
Naphthalene	128			6.6 - 4.2	Muhammad et al., 2005
Naphthalene				0.5	Kim et al., 2006
Naphthalene				1.4	Singh and Singh 2002
Naphthalene				1.8	Baynes et al. (2000)
Naphthalene				1.0	McDougal et al., 1999
1 methyl naphthalene	142			0.5	Kim et al., 2006
Methyl naphthalene				1.55	McDougal et al., 1999
2-methyl naphthalene				0.5	Kim et al., 2006
2-methyl naphthalene				1.1	Singh and Singh, 2002
Dimethyl naphthalene	156			0.62 – 0.67	Muhammad et al., 2005
Dimethyl naphthalene				0.59	McDougal et al. 1999

 

 

 

Table 2. Estimated percentages of various hydrocarbon solvent constituents absorbed

 

Based on the information provided below, an overall estimate of 1% for all hydrocarbon solvents seems reasonable. 

 

 

Category	Representative Substance	Estimate of Percent absorption	Proposal for category	Reference for percent value
1	Trimethyl benzene	0.2%	0.2%	Based on data in Muhammad et al. (2005)
2	Naphthalene	1.2%	1.2%	Riviere et al. 1999
3	Dodecane (75%)	0.63%	0.5%	Riviere et al., 1999
	TMB (25%)	0.2%		Muhammad et al., 2005
4	Hexadecane (70%)	0.18%	0.5%	Riviere et al., 1999
	Naphthalene (30%)	1.2%		Riviere et al., 1999
5	Pentane
6	Hexane
7	Heptane	0.14%	0.14%	Singh et al. 2003
8	Dodecane	0.63%	0.63%	Riviere et al. 1999
9	Hexadecane	0.18%	0.18%	Riviere et al., 1999

 

Kim, D., Andersen, M., and Nylander-French (2006). Dermal absorption and penetration of jet fuel components in humans. Toxicology Letters 165:11-21.

 

Muhammad, F., N. Monteiro-Riviere, R. Baynes, and J. Riviere (2005). Effect of in vivo jet fuel exposure on subsequent in vitro dermal absorption of individual aromatic and aliphatic hydrocarbon fuel constituents. Journal of Toxicology and Environmental Health Part A. 68:719-737.

 

Singh Somnath, Zhao Kaidi, Singh Jagdish. (2002). In vitro permeability and binding of hydrocarbons in pig ear and human abdominal skin. Drug and chemical toxicology, (2002 Feb) Vol. 25, No. 1, pp. 83-92.

 

Singh, S. and Singh, J. (2003). Percutaneous absorption, biophysical and macroscopic barrier properties of porcine skin exposed to major components of JP-8 jet fuel. Environmental Toxicology and Pharmacology 14:77-85.

 

Singh, S., Zhao, K., Singh, J. (2003). In vivo percutaneous absorption, skin barrier perturbation and irritation from JP-8 jet fuel components. Drug Chem. Toxicol 26:135-146.

 

McDougal, J., Pollard, D., Weisman, W., Garrett, C., and Miller, T. (2000). Assessment of skin absorption and penetration of JP-8 jet fuel and its components. Toxicological Sciences 25:247-255.

 

Muhammad, F., N. Monteiro-Riviere, R. Baynes, and J. Riviere (2005). Effect of in vivo jet fuel exposure on subsequent in vitro dermal absorption of individual aromatic and aliphatic hydrocarbon fuel constituents. Journal of Toxicology and Environmental Health Part A. 68:719-737.

 

Riviere, J., Brooks, J., Monteiro-Riviere, N., Budsaba, K., and Smith, C. (1999). Dermal absorption and distribution of topically dosed jet fuels jet A, JP-8 andJP-8(100). Toxicology and Applied Pharmacology 160:60-75.

 

Tsuruta, H. et al. (1982). Percutaneous absorption of organic solvents III. On the penetration rates of hydrophobic solvents through the excised rat skin. Industrial Health 20:335-345.