Reaction mass of 3a,4,5,6,7,7a-hexahydro-4,7-methano-1H-inden-5-yl isobutyrate and 3a,4,5,6,7,7a-hexahydro-4,7-methano-1H-inden-6-yl isobutyrate

Endpoint summary

Description of key information

The oral, dermal and inhalation absorption is considered to be 50, 50 and 100%

The substance is not bioccumulating in view of the experimental BCF of 309 l/kg.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

Toxico-kinetic assessment of Cyclabute

Introduction: 

Cyclabute (Cas no 93941-73-2) is an isobutyl ester attached to a tricyclodecenyl fused ring structure. It is a clear colourless liquid with a molecular weight of 220 that does not preclude absorption. The test material may show some hydrolysis in alkaline conditions rather than in acidic conditions because it is an ester. The substance has a low volatility: 0.61 Pa. 

Absorption

Oral: The results of the repeated dose oral toxicity studies from the read across substances Cyclacet and Cyclobutanate (IUCLID section 7.5) show that the substance is being absorbed by the gastro-intestinal tract following oral administration, because non-adverse alpha-hydrocarbon nephropathy specific for the male rate was seen. The relatively low molecular weight and the moderate octanol/water partition coefficient (Log Kow 5.1) and water solubility (16 mg/l) would favour absorption through the gut. According to Martinez and Amidon (2002) the optimal log Kow for oral absorption falls within a range of 2-7. This shows that Cyclabute is likely to be absorbed orally and therefore the oral absorption is expected to be > 50%. 

Skin: Based on the physico-chemical characteristics of the substance, being a liquid, its molecular weight (220), log Kow (5.1) and water solubility (16 mg/L), indicate that (some) dermal absorption is likely to occur. The optimal MW and log Kow for dermal absorption is < 100 and in the range of 1-4, respectively (ECHA guidance, 7.12, Table R.7.12-3). Cyclabute is outside the optimal range and therefore the skin absorption is not expected to exceed the oral absorption. 

Lungs: Absorption via the lungs is also indicated based on these physico-chemical properties. Cyclabute is a low volatile substance because of its low vapour pressure of 0.61 Pa, but the octanol/water partition coefficient (5.1), indicates that inhalation absorption is possible. The blood/air (BA) partition coefficient is another partition coefficient indicating lung absorption. Buist et al. (2012) have developed a BA model for humans using the most important and readily available parameters: 

Log PBA = 6.96 – 1.04 Log (VP) – 0.533 (Log) Kow – 0.00495 MW. 

For Cyclabute the BA partition coefficient would result in: 

Log P (BA) = 6.96- 1.04 x (-0.21) – 0.533 x (5.1) – 0.00495 x 220= 3.37 

This means that Cyclabute has a tendency to go from air into the blood. It should, however, be noted that this regression line is only valid for substances which have a vapour pressure > 100 Pa. Despite Cyclabute being somewhat out of the applicability domain and the exact BA may not be fully correct, it can be seen that the substance will be readily absorbed via the inhalation route and therefore 100% absorption will be used. 

Distribution: 

The moderate water solubility of the test substance would limit distribution in the body via the water channels. The log Kow would suggest that the substance would pass through the biological cell membrane. Due to its metabolisation the substance as such would not accumulate in the body fat. In view of the BCF of Cyclabute in fish (309 l/kg) it is expected that the substance does not bioaccumulate.

Metabolism: 

There are no actual data on the metabolisation of Cyclabute. Hydrolysis is limitedly seen; at pH4 and pH7 no hydrolysis occurs when using read-across from the structural similar substance Cyclobutanate (which has a straight butyl group instead of an isobutyl group). At pH 9 the half-life of this read-across substance was 13 days at 25°C. Small chain straight alkyl esters, which are not hindered by adjacent bulky groups, are likely to be fully metabolised by micro-organisms and and/or human carboxylesterase (hCE-2) (, see Fig 1). This can be done by micro-organisms in the gut (Imai and Ohare, 2010) and/or human carboxylesterase (hCE-2) in the liver (WHO, 2006, White et al. 1990 as presented by EFSA (2008, 2012). EFSA uses Cyclandalate (456-59-7) metabolism as an example for de-esterification and formation of a secondary alcohol. 

The Cycla-alcohol metabolite found in the biodegradation study with Cyclabute (isobutyl-Cycla-ester) support the degradation of the ester by micro-organism (see biodegradation section, additional information). The “Cycla-alcohol” is too bulky to be further cleaved and needs glucuronidation for excretion (EFSA, 2008) (Fig. 1). Another pathway of excretion is seen in male rat which is via alpha 2u-globulin. 

 

Fig. 1 Cyclobutanate metabolises into the alcohol (3385-61-3) and isobutanoic acid (79-31-2). Thereafter it will be glucuronidated. 

 

Air-breathing organisms: These organisms may bioaccumulate substances which are limitedly metabolised. This would leave excretion via the lungs as the key route. For substances with log Kow > 2, this may result in bioaccumulation. The presented metabolism for Cyclabute results in ester cleavage, glucuronidation, and excretion via the urine. The glucuronidated Cycla-alcohol has a low log Kow because glucuronic acid has a low log Kow – 1.87. This log Kow presents absence of bioaccumulation in air-breathing organism. Gobas et al., 2020, figure 6, D, show that oxygen containing substances in general are no concern for air-breathing organism due to Phase 1 and/or Phase 2 (conjugation e.g. by glucuronidation) pathways.

Excretion 

The metabolites presented in Fig. 1 will be more easily excreted. Cycla-alcohol will more easily be excreted via glucuronidation. Effects seen in the kidney of the rats show that excretion is through the urine. Isobutanoic acid is expected to be further metabolised into innocuous products (WHO, 1999). For isobutanoic acid also an OECD SIDS (2003) is available and for this substance no systemic toxicity is anticipated. Any unabsorbed substance will be excreted via the faeces. 

Discussion 

The substance is expected to be absorbed, orally, dermally and via inhalation, based on the human toxicological information and physico-chemical parameters. The MW and the log Kow are higher than the favourable range for dermal absorption but significant absorption is likely. 

The IGHRC (2006) document of the HSE and mentioned in the ECHA guidance Chapter 8 will be followed to derive the final absorption values for the risk characterisation. 

Oral to dermal extrapolation: In view of the absence of adverse effects, route to route extrapolation is not needed. Based on the information above it can be assumed that the oral absorption will equal dermal absorption. Using the asymmetric handling of uncertainty the oral absorption will be considered 50% (though likely to be higher) and the dermal absorption will be considered also 50% (though likely to be lower).

Oral to inhalation extrapolation: In view of the absence of adverse effects, route to route extrapolation is not needed. For the oral absorption 50% has been used and also for the dermal route. For inhalation absorption 100% will be used because this will be precautionary for the inhalation route. 

Conclusion: Cyclabute is expected to be readily absorbed via the oral and inhalation route and somewhat lower via the dermal route based on toxicity and physico-chemical data. The final absorption percentages derived are: 50% oral absorption, 50% dermal absorption and 100% inhalation absorption. 

References 

Buist, H.E., Wit-Bos de, L., Bouwman, T., Vaes, W.H.J., 2012, Predicting blood:air partition coefficient using basic physico-chemical properties, Regul. Toxicol. Pharmacol., 62, 23-28. 

EFSA, 2008, Scientific opinion on Flavouring groups evaluation, 47, (FGE.47), Bicyclic secondary alcohols, ketones and related esters form chemical group 8, Scientific opinion of the panel on food additives, flavouring, processing aids and materials in contact with food, https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2008.743: See Annex 3 for references on carboxylesterases; Heymann, 1980 and White et al, 1990. 

EFSA, 2012, (FGE.87Rev1): Consideration of bicyclic secondary alcohols, ketones and related esters evaluated by JECFA (63rd meeting) structurally related to bicyclic secondary alcohols, ketones and related esters evaluated by EFSA in FGE.47 (2008), EFSA Journal 2012, 10(2), 2564. Follow up of the more elaborated EFSA, 2008 review.

Gobas, F.A.P.C., Lee, Y-S, Lo, J.C., Parkerton, T.F., Letinskid, D.J., 2020, A Toxicokinetic Framework and Analysis Tool for Interpreting Organisation for Economic Cooperation and Development Guideline 35 dietary bioaccumulation test, Environ. Toxicol. Chem., 39, 171-188. 

Imai and Ohare, 2010, The role of intestinal carboxylesterase in the oral absorption of prodrugs, Curr Drug Metab,11, 793-805. 

Martinez, M.N., And Amidon, G.L., 2002, Mechanistic approach to understanding the factors affecting drug absorption: a review of fundament, J. Clinical Pharmacol., 42, 620-643.

IGHRC, 2006, Guidelines on route to route extrapolation of toxicity data when assessing health risks of chemicals, http://ieh.cranfield.ac.uk/ighrc/cr12[1].pdf 

White, D.A., Heffron, F., Miciak, A., Middleton, B., Knights, S., Knight, D., 1990. Chemical synthesis of dual radiolabelled cyclandelate and its metabolism in rat hepatocytes and mouse J774 cells. Xenobiotica 20(1), 71-79. 

WHO, 2006, Food Additive Series 54, Safety evaluation of certain food additives, http://www.inchem.org/documents/jecfa/jecmono/v54je01.pdf; page 385 and 399 and 400, of the report.