Mono-, and di-(sec-hexadecyl)naphthalene

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 study substance.

Based on a weight of evidence consideration of the relevant data for the study substance, the developmental hazards of the study 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.

Huntingdon Life Sciences, Inc., 2002

OECD 417

Single oral dose

Rat

Mono-, and di-(sec-hexadecyl)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 study 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 study substance was extensively metabolized to a large number of metabolites (28 discrete components) which were excreted in the urine.

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.

 

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 repeat 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

Mono-, and di-(sec-hexadecyl)naphthalene

Dietary administration

1

NOAEL >= 500 ppm (~36 mg/kg/d)

ExxonMobil, 1994

OECD 411

90d

Rat

Mono-, and di-(sec-hexadecyl)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 study 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. ADME studies using C14 -radiolabelled sec-hexadecylnaphthalene indicate that oral absorption occurs to a low extent (~10% of radioactive dose) in rats (Huntingdon, 2002). Similarly, a recently-completed toxicokinetics study on the structural analog 1-tetradecene, reaction products with naphthalene demonstrated a maximum oral absorption rate of 1.3% after gavage dosing (TNO, 2013). It is noted that this structural analog, due to it having a greater proportion of di- and tr-substituted naphthalene, is of greater average molecular weight, therefore explaining the markedly lower oral bioavailability. The study 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 study substance has low bioaccumulation potential. 

Similarly, based on QSAR modeling and the physico-chemical properties of the study 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 structural analog 1-tetradecene, reaction products with naphthalene that only reported 0.159% dermal absorption over 120h (TNO, 2013). It is noted that the analog has a higher average molecular weight which explains greater relative oral bioavailability, but nonetheless, dermal absorption would still be expected to be extremely low for the study substance. This is further supported by a low degree of systemic toxicity observed in repeat dose toxicity studies with dermal exposures. Therefore, the study 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)      

2)      Due to its low bioavailailability, with absorption expected to be considerably below the ~10% oral absorption rate based on its physicochemical properties and read across to Naphthalene, reaction products with tetradecene (0.159% absorption), it is considered unlikely that the study substance could cause any systemic effects. 

Developmental toxicity information:

The developmental toxicity endpoint for the study substance has been adequately assessed by read across to studies on the analogs Naphthalene, reaction products with tetradecene and 2,6-diisopropylnaphthalene. 

The existing developmental toxicity studies support the conclusion that Naphthalene, reaction products with tetradecene is either not bioavailable, has low toxicity, or that the metabolites of Naphthalene, reaction products with tetradecene are of low toxicity. In a study with Naphthalene, reaction products with tetradecene, no effects were observed in either dams or offspring at all doses tested.

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 study 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 study 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 study 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.