Hexadecylnaphthalene

Endpoint summary

Description of key information

Toxicokinetic Assessment of Hexadecyl Naphthalene

 

There are no studies available in which the toxicokinetic behaviour of hexadecyl naphthalene has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) No 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012), assessment of the toxicokinetic behaviour of the substance hexadecyl naphthalene is conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012). As data on the toxicity of hexadecyl naphthalene are based on read-across to alkylated naphthalene (a dodecyl naphthalene), data on that substance will be included.

 

Hexadecyl naphthalene is liquid at room temperature, has a molecular weight of 352 g/mol and a water solubility of < 10µg/L at 20 °C. The log Pow is > 7.2 and the vapour pressure was measured to be 7.1E-4 Pa at 25°C.

 

Absorption

 

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters providing information on this potential are the molecular weight, the octanol/water partition coefficient (log Pow) value and the water solubility. The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2012).

 

Oral:

In general, molecular weights below 500 are favourable for oral absorption (ECHA, 2012). As the molecular weight of hexadecyl naphthalene is <500 g/mol, some absorption of the molecule in the gastrointestinal tract is in general anticipated.

The log Pow of > 6 suggests that hexadecyl naphthalene is favourable for absorption by micellar solubilisation, as this mechanism is of importance for highly lipophilic substances (log Pow > 4), which are poorly soluble in water (1 mg/L or less).

The results of acute oral studies performed on the analogue alkylated naphthalene (MW <500 for a significant part of the component, and logKow >8.2), did not reveal any adverse systemic effects at 2000 mg/kg bw. In the available OECD 422 study in rats on alkylated naphthalene, at 1000 mg/kg/day no adverse effects were seen related to parental and offspring toxicity. No effects on reproduction became apparent at any of the dose levels in the study.

The findings in these oral studies are indicative either for lack of oral absorption or lack of toxicity of the substance.

In view of the structural similarity, systemic bioavailability of hexadecyl naphthalene in humans is considered likely after oral uptake of the substance, but toxicity is expected to be very low.

 

Dermal:

The smaller the molecule, the more easily it may be taken up. In general, a molecular weight below 100 favours dermal absorption, above 500 the molecule may be too large (ECHA, 2012). As the molecular weight hexadecyl naphthalene is <500 g/mol, dermal absorption of the molecule cannot be excluded.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2012). As, based on the data on alkylated naphthalene, hexadecyl naphthalene is not expected to be skin irritating, enhanced penetration of the substance due to local skin damage can be excluded.

For substances with a log Pow above 4, the rate of dermal penetration is limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. For substances with a log Pow above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin, and the uptake into the stratum corneum itself is also slow. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis (ECHA, 2012). As the water solubility of hexadecyl naphthalene is less than 10µg/L and the log Pow is >7.2, dermal uptake is likely to be (very) low.

Furthermore, an acute toxicity study with the analogue alkylated naphthalene is available indicating an LD50 value > 2000 mg/kg bw after dermal application. In the study, no systemic signs of toxicity were noted in any of the animals, thus supporting the assumption of a low dermal absorption. The relevant skin irritation study on alkylated naphthalene indicated that this analogue is not skin irritating. Skin sensitisation studies showed no skin sensitisation properties of alkylated naphthalene. A QSAR calculation on hexadecyl naphthalene using DERMWIN 2.02 resulted in a low Dermal Flux of 1.84 x 10^-5mg/cm² per h.

Taking the physiochemical parameters, the relevant toxicological test data and the QSAR calculation together, the dermal absorption potential of hexadecyl naphthalene is presumably low.

Inhalation:

Hexadecyl naphthalene has a vapour pressure of 7.1E-4 Pa at 25 °C and decomposes before boiling at atmospheric pressure. The substance is thus considered to be of low volatility and under normal use and handling conditions, inhalation exposure and thus availability for respiratory absorption of the substance in the form of vapour or gas is considered negligible. No exposure to aerosols of the substance is expected based on the uses as described and therefore exposure via the inhalation route is considered not relevant.

 

Distribution and Excretion

 

Distribution within the body through the circulatory system depends on the molecular weight, the lipophilic character and water solubility of a substance. In general, the smaller the molecule, the wider is the distribution. If the molecule is lipophilic, it is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration particularly in fatty tissues (ECHA, 2012).

Excretion of hexadecyl naphthalene will occur mainly via the urine. Based on its high partition coefficient (>7.2 measured at 20°C), it can generally be assumed that hexadecyl naphthalene after uptake can distribute into cells and accumulate in adipose tissue. However, for highly hydrophobic substances, experimental data demonstrate that both the bioconcentration factor and the bioaccumulation factor tend to decrease with increasing log Kow (above 6) therefore the likelihood of bioconcentration and accumulation is relatively low.

 

Metabolism

 

The metabolism of hexadecyl naphthalene is mainly contingent on both the nature of the alkyl groups and the number of naphthalene ring substitutions. There are currently no metabolism studies available on this substance and therefore no conclusion on the extend of metabolism can be drawn.

 

 

 

Conclusion

 

After oral exposure hexadecyl naphthalene can be taken up, but no wide distribution is expected. The substance if taken up is expected to be excreted via the urine.

Uptake via the skin is limited by the physico-chemical properties of the substance.

 

No bioaccumulation is expected.

 

 

References (not included in IUCLID)

ECHA (2012). Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

10

Absorption rate - dermal (%):

10