A mixture of isomers of: mono-(2-tetradecyl)naphthalenes; di-(2-tetradecyl)naphthalenes; tri-(2-tetradecyl)naphthalenes

Toxicity to reproduction: other studies

Administrative data

Endpoint:

toxicity to reproduction: other studies

Adequacy of study:

weight of evidence

Applicant's summary and conclusion

Executive summary:

In accordance with Annex X section 8.7.2, the registrant has considered the need to perform a prenatal developmental toxicity study in a second species for the registered substance.

Based on a weight of evidence consideration of the relevant data for the registered substance, the developmental hazards of the registered substance are already adequately and reliably characterized, therefore a prenatal developmental toxicity test in a second species is not scientifically justified. The elements and studies taken into consideration for this assessment are outlined below:

1-     

Data source	Study guideline	Duration of treatment	Species	Test material	Exposure route	Klimisch rating	Findings
Kojima, 1984	na		Rat	2-isopropylnaphthalene	Oral gavage	4	Total urinary excretion of unconjugated and conjugated metabolites was approximately 23% after 24h. Four primary metabolites and their conjugates were characterized in the urine and bile, corresponding to hydroxyl and/or carboxylic acid substitutions on the isopropyl side chain.
Honda, 1987	na	Daily for 4 days, or single dose	Rabbit	2-isopropylnaphthalene	Oral gavage	4	The total urinary excretion of unconjugated and conjugated metabolites of 2-IPN in 24 h after oral administration in rabbits was almost the same as that in rats (about 23%). These results suggest that the isopropyl sidechain of 2-IPN was oxidized according to the metabolic pathway of 2-IPN in the rat as reported previously.
Cnubben, 2013	OECD 417	Single oral dose	Rat	Naphthalene, reaction products with tetradecene	Oral gavage	1	Oral systemic absorption of MCP 2484 is very low (0.55 to 1.30% of total radioactive dose administered). Although MCP 2484 was distributed to the tissues after oral absorption, the amounts remaining in most tissues were extremely low at the end of 144 hours. Although MCP 2484 was distributed to the tissues after oral absorption, the amounts remaining in most tissues were extremely low at the end of 144 hours.
Cnubben, 2013	OECD 427	Single dermal dose	Rat	Naphthalene, reaction products with tetradecene	Dermal	1	% of Radioactive Dose Absorbed Dermally - 0.040% in 6 hrs to 0.159% in 120 hrs. The absorbed radioactivity appears to be mainly excreted in the faeces (<0.001%, 0.003% and 0.038% in 6, 24 and 120h group animals, respectively) with smaller percentage excreted in the urine (<0.001%, 0.001% and 0.009% in 6, 24 and 120h group animals, respectively). Based on radioactivity recovered in blood, internal tissues, urine and faeces, there was very little indication that radiolabeled MCP 2484 is absorbed systemically into the blood stream through the skin.
Huntingdon Life Sciences, Inc., 2002	OECD 417	Single oral dose	Rat	Reaction products of 1-hexadecene and naphthalene	Oral gavage	2	Radiolabelled sec-hexadecylnaphthalene (100 mg/kg and 7.5 microcurie per animal) was orally adminstered to male rats in a limited toxicokinetic study to evaluate the ADME of the test substance (i.e., primarily to assess oral absorption and metabolism). Oral absorption/ bioavailability was evaluated to be low (10%). Residue carass radioactivity (~2.1%) indicated a low bioaccumulation potential in the tissues. More importantly, the test substance was extensively metabolized to a large number of metabolites (28 discrete components) which were excreted in the urine.

 

2-     

Data source	Study guideline	Duration of treatment	Species	Test material	Exposure route	Klimisch rating	Findings
HLS, 2011	OECD 414	GD 6-19	Rat	Naphthalene, reaction products with tetradecene	Oral gavage	1	Maternal (systemic) and fetal NOAEL 1,000 mg/kg/d (highest dose tested)
LPT, 1993	OECD 414	GD 6-19	Rat	2,6-diisopropylnaphthalene	Oral gavage	4	Maternal (systemic) and fetal NOAEL 100 mg/kg/d (decreased body weight gain in 250, 625 mg/kg dose groups). Fetal skeletal ossification retarded in same dose groups, probable link to maternal toxicity
Nemec, MD	EPA OPPTS 870.3700	GD 6-19	Rat	2,6-diisopropylnaphthalene	Oral gavage	4	Maternal NOAEL: 50 mg/kg/d (decreased body weight and food consumption). Fetal NOAEL 150 mg/kg/d (decreased fetal body weight, possible cartilage anomaly)

 

3- Results from reproductive toxicity tests

Data source	Study guideline	Duration of treatment	Species	Test material	Exposure route	Klimisch rating	Findings
Stamp, 2012	OEC 421	F0 M: 57d, F0 F: >57d, F1: lifetime to PND26-30	Rat	Naphthalene, reaction products with tetradecene	Dietary administration	1	F0 NOAEL: 12,000ppm ; F1 NOAEL: 12,000ppm (approx. 1,000 mg/kg/d; highest doses tested)
Stamp, 2013	OECD 416		Rat	Naphthalene, reaction products with tetradecene	Dietary administration	1	F0 NOAEL: 12,000ppm ; F1/2 NOAEL: 12,000ppm (approx. 1,000 mg/kg/d; highest doses tested)
Kawai, 1977	OECD 416		Rat	2,6-diisopropylnaphthalene	Dietary administration	4	F0 NOAEL: >192 mg/kg/d; F1/2 NOAEL: >192 mg/kg/d; highest doses tested)

 

4- Results from repeated dose studies in rats

Data source	Study guideline	Duration of treatment	Species	Test material	Exposure route	Klimisch rating	Findings
MEHSL, 1991	OECD 410	28d	Rat	Naphthalene, reaction products with tetradecene	Dermal	1	NOAEL >= 500 mg/kg/d
ExxonMobil, 1991	OECD 408	90d	Rat	Reaction products of 1-hexadecene and naphthalene	Dietary administration	1	NOAEL >= 500 ppm (~36 mg/kg/d)
ExxonMobil, 1994	OECD 411	90d	Rat	Reaction products of 1-hexadecene and naphthalene	Dermal	1	NOAEL >= 125 mg/kg/d)

 

Species-dependent differences in susceptibility to toxicants is due to differences in toxicokinetics and/or toxicodynamics. For industrial chemicals shown to be of low bioavailability and bioactivity in the rat, it is generally expected that the same would hold true in the rabbit. The following is a brief summary of the evidence to support the non-necessity of testing the registered substance for developmental toxicity in a second species as scientifically unjustified.

Toxicokinetics:

Due to high molecular weight and high lipophilicity (log Pow >6), GI tract absorption is likely to be low in any species tested. ADME studies using¹⁴C -radiolabelled 1-tetradecene, reaction products with naphthalene demonstrated a maximum oral absorption rate of 1.3% after gavage dosing (TNO, 2013). The test substance was extensively metabolized to a large number of metabolites (28 discrete components) which were excreted in the urine. Low bioavailability is further supported by an overall low degree of systemic toxicity in several study designs conducted by multiple routes of exposure. Taken together, the low bioavailability, the extensive metabolism and negligible systemic effects support that the registered substance has low bioaccumulation potential. 

Similarly, based on QSAR modeling and the physico-chemical properties of the registered substance (high molecular weight, high Log P, low water solubility), dermal absorption is expected to be minimal. This is supported by a recently completed in vivo percutaneous absorption study in rats on the registered substance that only reported 0.159% dermal absorption over 120h (TNO, 2013). This is further supported by a low degree of systemic toxicity observed in repeat dose toxicity studies with dermal exposures. Therefore, the registered substance has both low absorption and low bioaccumulation potential. This is expected to be consistent across species. 

Generally, as reviewed by Hoke (1998), it has been shown that as alkyl side chain length increases, the degree of ring oxidation decreases. Nearly 100% of naphthalene metabolism is through ring oxidation, whereas introduction of a methyl group at the 2- position reduces ring oxidation products to only approximately 15-20%, whereas the rest of the metabolites are formed through methyl group oxidation. It is estimated that less than 5% of the metabolites of isopropylnaphthalene are formed through ring oxidation. There is no evidence of ring oxidation in studies with di-isopropylnaphthalene (Hoke, 1998).

Ring oxidation has long been implicated in the toxicity of naphthalene and its structural analogs. Comparative metabolism studies exist for 2-isopropylnaphthalene that can inform us on the likelihood of the formation of uniquely different metabolites in both rats (Kojima, 1980; 1984) and rabbits (Honda, 1987). In the rat, no ring oxidized metabolites could be identified in the urine and bile of the rat after oral administration. More than 40% of the total dose applied was recovered after 24h as products oxidized at the side chain (Kojima, 1984). In the rabbit, the alkyl-oxidized portion in the 24h urine amounted to about 28% of the total dose (bile not analyzed), while approximately 1.5% was identified as a dihydrodiol derivative—the result of ring oxidation. However, one small phenolic fraction could not be quantified (Honda, 1987). These findings indicate that the identity and ratio of metabolites formed between the rat and rabbit are conserved for 2-isopropylnaphthalene. While there is a small presence of ring oxidation that was not detected in the rat, it is emphasized that this is for a 3-carbon substituted naphthalene molecule with molecular weight of 170. For a C16/32-substituted naphthalene with an average molecular weight of 379, it is considered highly unlikely that ring oxidation would occur. Instead, side chain oxidation would occur by the well-conserved mechanism that has been shown to be similar between rats and rabbits for isopropylnaphthalene. 

Importantly, as a lubricant, the 'most relevant route' is dermal. Rat and rabbit dermal absorption are unlikely to be different, particularly because there is evidence demonstrating a lack of dermal irritation in rabbits, which could potentially increase dermal bioavailability. Therefore, due to probable extremely low bioavailability at the most relevant route of exposure owing to its physicochemical properties (High LogP (11) and low water solubility [< 0.0005 mg/L]), the conduct of a study in rabbits is not scientifically justified.  Supporting this, dermal absorption of Naphthalene, reaction products with tetradecene was found to be < 0.159% after a single dermal dose. 

In conclusion, based on a toxicokinetic behaviour assessment of the available data, we conclude the following:

1)      This is how the registered substance is metabolized in the rat. \

2)      Due to its low bioavailailability, with absorption expected to be considerably below the ~10% oral absorption rate based on its physicochemical properties and low (0.159%) absorption in the rat, it is considered unlikely that the registered substance could cause any systemic effects. 

Developmental toxicity information:

The developmental toxicity endpoint for the registered substance has been adequately assessed by a high quality study with the registered substance. In a study with the registered substance, no effects were observed in either dams or offspring at all doses tested.  This study supports the conclusion that Naphthalene, reaction products with tetradecene is either not bioavailable, has low toxicity, or that its metabolites are of low toxicity.

Reproductive toxicity information

Additionally, the existence of two reproductive toxicity studies conducted with Naphthalene, reaction products with tetradecene by the oral route, one of which is a 2-generation reproductive toxicity study, provides further information about the overall biological inertness of the registered substance. For both studies, the parental, F1 and F2 NOAELs were the highest doses tested, 12,000ppm in the diet which is approximately 1,000 mg/kg/d. This supports the overall conclusion that the registered substance is biologically inert, and that species-specific differences are unlikely to provide further useful information to inform human health risk assessment.

Conclusion:

The assessment of prenatal developmental toxicity in a second species has become standard practice in the pharmaceutical and pesticide industries. However, unlike the products of those industries, the registered substance was not designed for biological activity, and would only be subject to incidental, not deliberate exposure. True biological activity would be evident in the abundant available data. Therefore, because human exposures are likely to be low, these substances are of a low order of toxicity, and the lack of bioavailability would be expected to be conserved across species, it is not foreseen that testing in a second species would further inform risk assessment.

Finally, it is well known that rabbits are coprophagic. Therefore, a study conducted by the oral route would not provide useful information for human health risk assessment due to double-dosing of the test material. Because of the physicochemical and experimental data indicating low dermal absorption (<1%), a study conducted via dermal exposure in the rabbit would achieve only a fraction of the internal dose achieved in the oral route studies in the rat (this substance is not a dermal irritant). For this reason, in the unlikely event the rabbit is of greater susceptibility to this inert chemical, due to a 10-fold lower absorption rate it is considered unlikely that the rabbit dermal study would provide greater insight to inform risk assessment.

Factoring all of these considerations into account with the collective goal of minimising animal suffering, developmental testing in a second species is not scientifically justified.