A mixture of: 4-(2,2,3-trimethylcyclopent-3-en-1-yl)-1-methyl-2-oxabicyclo[2.2.2]octane; 1-(2,2,3-trimethylcyclopent-3-en-1-yl)-5-methyl-6-oxabicyclo[3.2.1]octane; spiro[cyclohex-3-en-1-yl-[(4,5,6,6a-tetrahydro-3,6',6',6'a-tetramethyl)-1,3'(3'aH)-[2H]cyclopenta[b]furan]; spiro[cyclohex-3-en-1-yl-[4,5,6,6a-tetrahydro-4,6',6',6'a-tetramethyl)-1,3'(3'aH)-[2H]cyclopenta[b]]furan]

Endpoint summary

Description of key information

No experimental toxico-kinetic data are available for assessing adsorption, distribution, metabolisation and excretion of the substance. Based on non-adverse effects seen in the human health toxicity studies and physico-chemical parameters Cassiffix is expected to be readily absorbed via the oral and inhalation route and somewhat lower via the dermal route. In view of the "no hazard identified" for systemic toxicity route to route extrapolation is not needed. The final absorption percentages derived are: 50% oral absorption, 50% dermal absorption and 100% inhalation absorption.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

Cassiffix toxico-kinetic assessment

Introduction: The test material Cassiffix (Cas no. 139539-67-6) has tricyclic-ring consisting of a fused ring and one pentyl ring separated by a methyl group. In the fused bicyclic ring an ether bond is present. To the pentyl ring methyl groups are attached to each C atom and a double bond is present. The substance is a liquid. Its melting point is -25°C and it has a molecular weight of 234 that does not preclude absorption. The test material is not likely to hydrolyse under physiological circumstances and has a low volatility (1.5 Pa).

Absorption: Oral: The results of the 28-day repeat oral dose (gavage) and oral (dietary) reproductive toxicity show that the substance is being absorbed by the gastro-intestinal tract following oral administration because minimal non-adverse liver effects were seen at the highest dose (1000 and 550 mg/kg bw, respectively). The relatively low molecular weight and the moderate octanol/water partition coefficient (Log Kow 4.72) and water solubility (11.1 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 the substance is likely to be absorbed orally and therefore the oral absorption is expected to be > 50%.

Skin: The substance is a skin and eye irritant but not a skin sensitiser, which indicates that absorption occurs. Also based on the physico-chemical characteristics of the substance being a liquid, its molecular weight (234), log Kow (4) .72and water solubility (11.1 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). The substance is just outside this range optimal range and therefore the skin absorption is not expected to exceed oral absorption.

Lungs: Absorption via the lungs is also indicated based on these physico-chemical properties. Though the inhalation exposure route is thought minor, because of its low volatility (1.5 Pa), the octanol/water partition coefficient (4.72), 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 B/A model for humans using the most important and readily available parameters:

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

For Cassiffix the B/A partition coefficient would result in:

Log P (BA) = 6.96 – 1.04 x Log 1.5 – 0.533 x 4.72 – 0.00495 x 234 = 6.96 – 0.19-2.13-1.16 = 3.12

This means that the substance 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 Cassiffix 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 could be close to 100%.

This logP(BA) equation is based on experimental data and an equivalent to the calculated log Koa. The Koa is a measure for the substance retaining in fat (octanol) versus exhaling the substance into air. A log Koa of 5 may indicate bioaccumulation in air-breathing organisms in absence of metabolism (Gobas et al., 2020). This log P(BA) shows that the screening criteria for such bioaccumulation are not met.

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. The log Kow of 4.72 indicates some bioaccumulation. Due to the expected metabolisation the substance as such would limitedly accumulate in the body fat.

Metabolism: The metabolisation of Cassiffix is assessed using OECD Toolbox 3 liver metabolism simulator (Fig. 1). The metabolites are presented below in two print screens (please note there is overlap between these two figures). OH groups may be attached to every methyl group and/or an acid may be formed. Also the formation of an epoxide and a ketone group is presented by this Toolbox. These metabolites are expected to be more water soluble, have a lower Log Kow values and will therefore be more easily excreted. These alcohol metabolites will be glucuronidated during Phase 2 conjugation to facilitate excretion via kidneys.

In addition, in the water simulation study (OECD TG 309) the metabolite Cassiffix-Lactone was found, which is an oxidised carbon next to the ether bond and an additional bond in the same ring. Because this is a simple oxidation (and reduction) such a product is also expected in mammalian systems (Fig. 2). The lactone may be de-esterified and turn into an acid or it may be reduced into an alcohol. Both are expected to be conjugated in the Phase 2 pathway.

 

 Fig. 1 The theoretical metabolisation of Cassiffix according to the QSAR toolbox 3.0 liver metabolism is expected to result via hydroxylation of the methyl groups of the pentyl-ring and/or an acid group can be formed at these OH groups. One of the metabolites is an epoxide and also a ketone may be formed.

 

Fig. 2 Cassiffix-Lactone is also a likely metabolite; this metabolite is an oxidised carbon next to the ether bond forming a lactone. This was found in the water simulation test according to OECD TG 309 and is expected to occur in mammalians as well.

Distribution route after metabolism, important for air-breathing organisms: Based on the expected metabolism and on the experimental observed Cassiffix-Lactone, which has a measured log Kow of 3, these metabolites will be conjugated on an alcohol or acidic fragment and distributed to the kidneys. Due to this metabolism, kidneys being the key excretion route and the limited or non excretion via air for air-breathing organisms, the bioaccumulation for air-breathing organism is not relevant for Cassiffix. This is further indicated by Gobas et al. (2020, fig. 6) in more general terms in which he shows that oxygen containing substances will be distributed and excreted via the kidneys and therefore unlikely are accumulating in air breathing organisms .

Fig. 3 Snapshot from Gobas et al. 2020 where he presents an overview of substances that may and may not be of concern for air-breathing organisms.

Excretion: Slight alpha-hydrocarbon nephropathy is seen in the kidney of the rats indicate that one route of excretion is through the urine. Any unabsorbed substance will be excreted via the faeces.

Discussion: The substance is expected to be readily absorbed, orally and via inhalation, based on the human toxicological information and physico-chemical parameters. The substance is expected to be absorbed dermally due to the observed skin irritation properties. 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: Cassiffix 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 basis physico-chemical properties, Regul. Toxicol. Pharmacol., 62, 23-28.

Gobas, A.P.C, Lee, Y-S, Lo, J.C, Parkerton, T., Letinski, D.J., 2020, Toxicokinetic framework and analysis tool for interpretating Organisation for Economic Cooperation and Development Guideling 305 Dietary bioaccumulation testing, Environmental Toxicology and Chemistry, 39, 171–188, 2020, https://setac.onlinelibrary.wiley.com/doi/pdf/10.1002/etc.4599

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

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.