Nonanoic acid

Bioaccumulation: aquatic / sediment

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

bioaccumulation in aquatic species: fish

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
[Describe why the read-across can be performed (e.g. common functional group(s), common precursor(s)/breakdown product(s) or common mechanism(s) of action]
The target substance n-nonanoic acid differs from the source substance 3,5,5-trimethylhexanoic acid only regarding structural isomerism (same molecular weight). While the target is straight chain, the source substance 3,5,5-trimethylhexanoic acid has three methyl substituents on a straight chain C6 backbone. Both compounds share the carboxylic acid moiety responsible for the acidic character. The read-across from source to target is based on the hypothesis of approximately the same uptake rate and theoretical equilibrium concentration inside the organism (molecular size, dissociation behaviour, log Kow) for source and target, but a slower metabolism of the source 3,5,5-trimethylhexanoic acid due to slower metabolization and therefore - effectively - a higher bioconcentration compared to the target. Accordingly, reading across from source to target n-nonanoic acid must be regarded as conservative.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
[Provide here, if relevant, additional information to that included in the Test material section of the source and target records]
The source substance is a mono-constituent substance of high purity (99.4% (w) 3,5,5-trimethylhexanoic acid). Also, the target substance is mono-constituent and of high purity (>=95.5%). Accordingly, both, source as well as target must be considered of high purity and high molecular similarity (structural isomers).

3. ANALOGUE APPROACH JUSTIFICATION
[Summarise here based on available experimental data how these results verify that the read-across is justified]
As outlined under paragraph 1 above, the target substance n-nonanoic acid differs from the source substance 3,5,5-trimethylhexanoic acid only regarding structural isomerism (same molecular weight). While the target is straight chain, the source substance 3,5,5-trimethylhexanoic acid has three methyl substituents on a straight chain C6 backbone. Both compounds share the carboxylic acid moiety responsible for the acidic character.
Based on a) the same molecular weight and b) very similar log Pow-values (source: log Kow = 3.2; target: log Kow = 3.4) and c) practically identical dissociation behaviour as a function of pH (pKa 4.8), source and target compound will be taken up to nearly the same extent with very similar kinetics (rate of equilibration). However, while the target is a straight chain (unbranched) fatty acid and thus will rapidly be metabolically transformed via ß-oxidation and further metabolized via citric acid cycle, the source compound is branched with methyl groups at C-ß (i.e. position 3) and C5. Due to the methyl group at C-ß, direct metabolic breakdown via ß-oxidation will not be possible and thus a slower metabolism compared the target compound is probable. Based on comparable uptake but slower metabolism, the effective bioconcentration of the source compound 3,5,5-trimethylhexanoic acid will be higher compared to the target compound n-nonanoic acid. Accordingly, the read-across from source substance 3,5,5-trimethylhexanoic acid to target substance n-nonanoic acid must be considered to be conservative and can be performed with high confidence.

4. DATA MATRIX AND CONCLUSIONS
Elemental physico-chemical parameters are similar for both, source and target substance. Slight deviations are due to the branched structure of the source substance 3,5,5-trimethylhexanoic acid, prohibiting the close molecular alignment possible for linear carbohydrate chains and thus leading to lower intermolecular forces (van der Waals forces), which becomes especially evident in the significant difference of melting points. However, density and boiling point are only slightly lower, vapour pressure and water solubility only slightly higher compared to the target n-nonanoic acid (all data from the most recent registration dossiers of the submitter):

Melting point 3,5,5-trimethylhexanoic acid: -77 °C
Melting point n-nonanoic acid: 13 °C

Relative density 3,5,5-trimethylhexanoic acid: 0.8996
Relative density n-nonanoic acid: 0.9046

Boiling point 3,5,5-trimethylhexanoic acid: 236 °C
Boiling point n-nonanoic acid: 249 °C

Vapour pressure (20 °C) 3,5,5-trimethylhexanoic acid: 4.6 Pa
Vapour pressure (20 °C) n-nonanoic acid: 0.12 Pa

Water solubility (20 °C) 3,5,5-trimethylhexanoic acid: 0.7 g/L (pH 3.8)
Water solubility (20 °C) n-nonanoic acid: 0.27 g/L (pH 4.2)

log Kow 3,5,5-trimethylhexanoic acid (25 °C): 3.2 (pH 3.0)
log Kow n-nonanoic acid (25 °C): 3.4 (pH 3.0)

pKa 3,5,5-trimethylhexanoic acid (20 °C): 4.8
pKa nonanoic acid (20 °C; read across): 4.8

Biodegradability 3,5,5-trimethylhexanoic acid: readily biodegradable (OECD 301A)
Biodegradability n-nonanoic acid: readily biodegradable (OECD 301 B)

Organic carbon adsorption coefficient 3,5,5-trimethylhexanoic acid: Koc = 123 L/kg (pH 6.5)
Organic carbon adsorption coefficient n-nonanoic acid: Koc = 141 L/kg (pH 6.5)

Based on these similar physico-chemical and environmental fate properties, together with the considerations given under sections 1 through 3, the read-across from the valid experimental bioconcentration study in fish performed with the source substance 3,5,5-trimethylhexanoic acid to the target substance n-nonanoic acid must be considered conservative, reliable and relevant. A study on bioconcentration of n-nonanoic acid in fish - if performed - would result in lower or at maximum comparable bioconcentration as observed for the source substance 3,5,5-trimethylhexanoic acid. Concluding, the read-across for endpoint bioconcentration in fish can be performed with high confidence.

Reason / purpose for cross-reference:

read-across source

Radiolabelling:

no

Lipid content:

3.5 %

Time point:

start of exposure

Conc. / dose:

>= 0.932 - <= 0.936 mg/L

Temp.:

25 °C

pH:

7

Type:

BCF

Value:

>= 0.5 - <= 1.7 L/kg

Basis:

whole body w.w.

Remarks:

based on individual concentration results for fish (n= 2 at each time point)

Time of plateau:

2 wk

Calculation basis:

steady state

Remarks on result:

other: based on arithmetic mean values for each time point, steady state was reached between 2 and 3 weeks.

Remarks:

Nominal exposure concentration: 1 mg/L

Key result

Conc. / dose:

>= 0.092 - <= 0.094 mg/L

Temp.:

25 °C

pH:

7

Type:

BCF

Value:

>= 4.1 - <= 7 L/kg

Basis:

whole body w.w.

Remarks:

based on individual concentration results for fish (n= 2 at each time point)

Time of plateau:

4 wk

Calculation basis:

steady state

Remarks on result:

other: based on arithmetic mean values for each time point, steady state was reached between 3 and 4 weeks.

Remarks:

Nominal exposure concentration: 0.1 mg/L

Validity criteria fulfilled:

yes

Conclusions:

No significant bioaccumulation potential is to be expected for n-nonanoic acid based on the measured BCF of <= 7.0 (steady state) determined according to the MITI protocol, which corresponds to OECD 305 C (NITE, 2000).

Executive summary:

The study used as source investigated the aquatic bioaccumulation potential of the source substance isononanoic acid (3,5,5-trimethylhexanoic acid). The study results of the source substance were considered applicable to the target substance. Justification and applicability of the read-across approach (structural analogue) is outlined in section "Justification for type of information" of this endpoint study record.

Result:

The bioconcentration factor for freshwater fish was determined using carp (Cyprinus carpio; NITE, 2000). The valid test was performed compliant with GLP according to OECD 305C (1981), following the steady state method. Based on preliminary acute toxicity testing (LC50 (Oryzias latipes; 48 h) = 160 mg/L), two exposure concentrations of 1.0 and 0.10 mg/L had been applied in a flow through system using glass aquaria (25 °C).

Aquatic test item concentrations were determined 2 times a week, and concentrations in fish were determined after two, three, four and six weeks. At each time point 2 fish per concentration level were used and extracted separately after weighing (wet weight). A mean lipid content of 3.5% had been determined at the start of exposure for the fish used.

Steady state (plateau) was reached between 2 and 3 weeks for 1 mg/L nominal concentration (>= 0.932 mg/L <= 0.936 mg/L measured) and between 3 and 4 weeks for 0.1 mg/L nominal concentration (>= 0.0925 mg/L <= 0.0943 mg/L measured). The highest BCF determined based on individual concentration results for fish (n= 2 at each time point) was 7.0 L/kg (whole body wet weight).

Concluding, these valid results convincingly demonstrate that there is no relevant aquatic bioaccumulation potential for the source substance 3,5,5-trimethylhexanoic acid. Consequently, for the target substance n-nonanoic acid any relevant bioaccumulation potential can be reliably excluded as well based on these data.

Reference 2

Endpoint:

bioaccumulation in aquatic species: fish

Type of information:

(Q)SAR

Adequacy of study:

supporting study

Study period:

October 2020

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with limited documentation / justification

Justification for type of information:

BCFBAF (v3.02, April 2015) according to Meylan et al. has been used for BCF calculation.
The model is very well documented in the BCFBAF help file distributed together with the software, including test and training set compounds. The methodology is published (Meylan et al., 1999). It is characterized by a wide applicability domain including ionic compounds.
The applied estimation methodology is published and described in Meylan et al. (1999) for BCFWIN, an earlier version of BCFBAF. The regression methodology applied for BCFWIN and described in this publication was kept unchanged for BCFBAF. BCFBAF, however, was improved compared to BCFWIN by using an updated and better evaluated BCF database for selecting training and validation datasets, as described in detail in the BCFBAF Help File associated with the program.

In brief, the model was developed by linear regression of log BCF against log KOW for the training set compounds. The algorithm classifies substances as non-ionic or ionic (carboxylic acids, sulfonic acids, quaternary N compounds). Separate equations are generated for log KOW ranges log KOW <1.0, log KOW 1.0 to 7.0 and log KOW > 7.0. For 11 functional groups, specific correction factors are applied. Ionic substances are assigned categorical BCF values according to the log KOW range assignable:
log BCF = 0.50 (log Kow < 5.0)
log BCF = 1.00 (log Kow 5.0 to 6.0)
log BCF = 1.75 (log Kow 6.0 to 8.0)
log BCF = 1.00 (log Kow 8.0 to 9.0)
log BCF = 0.50 (log Kow > 9.0)
Further, if the number of -CH2- groups contained within the molecule is ≥ 11, the following equation is used:
Log BCF = 1.85 (Ionic; 11 or more -CH2- groups)
For more details see BCFBAF help file and Meylan et al. (1999).

Applicability domain
Currently there is no clearly defined model domain but molecular weight and log KOW ranges of the training set are given to be compared to the target compound. The submission substance is well within these limits.

References:
Meylan, W.M.; Howard, P.H.; Boethling, R.S.; Aronson, D.; Printup, H.; Gouchie, S. (1999)
Improved method for estimating bioconcentration/bioaccumulation factor from octanol/water partition coefficient
Environmental Toxicology and Chemistry, 18, 664-672

Qualifier:

no guideline followed

Principles of method if other than guideline:

Modelling of BCF with BCFBAF program (v3.02, April 2015) on the basis of the measured log Pow and chemical structure

GLP compliance:

no

Radiolabelling:

no

Details on sampling:

Not applicable - QSAR

Details on preparation of test solutions, spiked fish food or sediment:

Not applicable - QSAR

Test organisms (species):

other: Fish, not further specified (QSAR)

Details on test organisms:

Not applicable - QSAR

Route of exposure:

aqueous

Justification for method:

aqueous exposure method used for following reason: Carboxylic acid of moderate solubility

Test type:

other: QSAR

Details on estimation of bioconcentration:

BASIS FOR CALCULATION OF BCF
- Estimation software: BCFBAF (v3.02, April 2015)
- Result based on measured log Pow of: 3.4 (measured)
- equation used to make BCF estimate: log BCF = 0.50 (Ionic; Log Kow dependent)

Key result

Type:

BCF

Value:

3.162 L/kg

Basis:

whole body w.w.

Calculation basis:

other: QSAR

Remarks:

BCFBAF v3.02 (2015)

Remarks on result:

other: log Pow (experimental): 3.4

Details on results:

SMILES : O=C(O)CCCCCCCC

CHEM : Nonanoic acid

MOL FOR: C9 H18 O2

MOL WT : 158.24

--------------------------------- BCFBAF v3.02 --------------------------------

Summary Results:

Log BCF (regression-based estimate): 0.50 (BCF = 3.16 L/kg wet-wt)

Biotransformation Half-Life (days) : 0.853 (normalized to 10 g fish)

=============================

BCF (Bioconcentration Factor):

=============================

Log Kow (estimated) : 3.52

Log Kow (experimental): 3.42

Log Kow used by BCF estimates: 3.40 (user entered)

Equation Used to Make BCF estimate:

Log BCF = 0.50 (Ionic; Log Kow dependent)

Estimated Log BCF = 0.500 (BCF = 3.162 L/kg wet-wt)

===========================================================

Whole Body Primary Biotransformation Rate Estimate for Fish:

Please see attached illustration for details!

Validity criteria fulfilled:

not applicable

Conclusions:

BCFBAF (v3.02, April 2015):
BCF = 3.152 L/kg wet weight;
biological half-life normalized to 10 g fish at 15 °C: 0.853 days.

Executive summary:

The bioconcentration factor in fish was estimated using QSAR (BCFBAF, v3.02, April 2015).

The Meylan et al. model contained within BCFBAF v. 3.02 is recommended according to ECHA guidance document R.7c (ECHA, 2017), Appendix R.7.10-3 for ionized substances. n-Nonanoic acid is within the applicability domain of the model. Based on molecular structure and experimental log Kow determined at pH 3.0 (AQura, 2009) of 3.4, the following result was determined:

Estimated Log BCF = 0.500 (BCF = 3.162 L/kg wet-wt)

Further, biotransformation half-life is estimated by the programme according to the method of Arnot et al. (2008). n-Nonanoic acid is within the applicability domain of the model (molecular weight range; fragment type and number of occurrence). The biological half-life normalized to 10 g fish at 15 °C was estimated to 0.853 days. This corroborates the fast metabolic transformation which must generally be assumed for linear (unbranched) fatty acids.

Concluding, the regression based low BCF estimate of 3.162 L/kg wet weight is corroborated by a short metabolic half-life estimate of 0.853 days (normalized for 10 g fish at 15 °C).

Description of key information

BCF of <= 7.0 (steady state) determined according to the MITI protocol, which corresponds to OECD 305 C (NITE, 2000)

Key value for chemical safety assessment

BCF (aquatic species):

7 L/kg ww

Additional information

The study used as source (key study) investigated the aquatic bioaccumulation potential of the source substance isononanoic acid (3,5,5-trimethylhexanoic acid). The study results of the source substance were considered fully applicable to the target substance n-nonanoic acid. Justification and applicability of the read-across approach (structural analogue) is outlined in section "Justification for type of information" of the respective IUCLID endpoint study record.

Result:

The bioconcentration factor for freshwater fish was determined using carp (Cyprinus carpio; NITE, 2000). The valid test was performed compliant with GLP according to OECD 305C (1981), following the steady state method. Based on preliminary acute toxicity testing (LC50 (Oryzias latipes; 48 h) = 160 mg/L), two exposure concentrations of 1.0 and 0.10 mg/L had been applied in a flow through system using glass aquaria (25 °C).

Aquatic test item concentrations were determined 2 times a week, and concentrations in fish were determined after two, three, four and six weeks. At each time point 2 fish per concentration level were used and extracted separately after weighing (wet weight). A mean lipid content of 3.5% had been determined at the start of exposure for the fish used.

Steady state (plateau) was reached between 2 and 3 weeks for 1 mg/L nominal concentration (>= 0.932 mg/L <= 0.936 mg/L measured) and between 3 and 4 weeks for 0.1 mg/L nominal concentration (>= 0.0925 mg/L <= 0.0943 mg/L measured). The highest BCF determined based on individual concentration results for fish (n= 2 at each time point) was 7.0 L/kg (whole body wet weight).

Concluding, these valid results convincingly demonstrate that there is no relevant aquatic bioaccumulation potential for the source substance 3,5,5-trimethylhexanoic acid. Consequently, for the target substance n-nonanoic acid any relevant bioaccumulation potential can be reliably excluded as well based on these data.

Supporting study:

This result was further corroborated via reliable QSAR using BCFBAF v.3.02 (April 2015; part of US EPA EPI Suite v.4.11; FoBiG, 2020):

The Meylan et al. model contained within BCFBAF v. 3.02 is recommended according to ECHA guidance document R.7c (ECHA, 2017), Appendix R.7.10-3 for ionized substances. n-Nonanoic acid is within the applicability domain of the model. Based on molecular structure and experimental log Kow determined at pH 3.0 (AQura, 2009) of 3.4, the following result was determined:

Estimated Log BCF = 0.500 (BCF = 3.162 L/kg wet-wt)

The estimated BCF of 3.16 L/kg wet-weight is within the range determined experimentally in the key study (NITE, 2000; 1.7 - 7 L/kg wet weight) and thus corroborates the read across approach.

Further, biotransformation half-life is estimated by the programme according to the method of Arnot et al. (2008). n-Nonanoic acid is within the applicability domain of the model (molecular weight range; fragment type and number of occurrence). The biological half-life normalized to 10 g fish at 15 °C was estimated to 0.853 days. This corroborates the fast metabolic transformation which must generally be assumed for linear (unbranched) fatty acids.

Concluding, the regression based low BCF estimate of 3.162 L/kg wet weight is corroborated by a short metabolic half-life estimate of 0.853 days (normalized for 10 g fish at 15 °C). Both results are in full support of the read-across hypothesis developed for the experimental key study.