2-hydroxy-4-(methylthio)butyronitrile

Endpoint summary

Additional information

Two reliable studies are available for HMTBN covering two trophic levels, i.e. aquatic invertebrates and algae.

 

In the first study (Cellier P., 1996), the ability of HMTBN to immobilise Daphnia magna was tested in an acute toxicity study according to the EU Directive 92/69/EEC C.2. The following concentrations were tested :16, 22, 32, 48, 70, 100, 140 and 200 ppm (weight/weight) of test item (aqueous solution 10% HMTBN). No analytical monitoring has been performed. No other information is available on the test conditions.

There was 5% mortality of daphnia magna in the controls at the end of the exposure period (48h). The EC50(48 h)-value was estimated to be 33 ppm (w/w) with a 95% CL of 25 -36 ppm (w/w) based on the test item. This result correspond to an EC50(48 h)-value of 3.3 ppm (w/w) based on HMTBN non diluted.

Based on the results of this study, HMTBN is considered as Toxic to the aquatic organisms tested in accordance with the Directive 67/548/EC.

In the second study (Höger S., 2010), the influence of HMTBN on the growth of the freshwater green algal species Pseudokirchneriella subcapitata was investigated in a 72‑hour static test according to OECD TG 201 (2006), the EU Directive 92/69/EC, C.3 (1992) and the Commission Regulation (EC) No 440/2008, C.3. The study was compliant with the GLP. The nominal concentrations of the test item of 0.10, 0.32, 1.0, 3.2 and 10 mg/L were tested in parallel with a control. The measured concentrations of the test item in the test media of the nominal test concentrations of 0.32 to 10 mg/L were between 40 and 56% of the nominal values at the start of the test. The measured concentration of the test item in the test medium of the nominal test concentration of 0.10 mg/L was below LOQ (Limit of quantification,LOQ_bio: 0.0668 mg/L).Based on results of the pre-experiment on the stability of HMTBN (discussed below) the effect concentrations are expressed in nominal concentration in order to cover both, the toxicity of the parent product and of the degradation products.

The results obtained were as follows:

72h-ErC50 = 1.3 mg/L (growth rate)

72h-EbC50 = 0.88 mg/L (biomass)

72h-NOErC = 0.32 mg/L (growth rate and biomass)

Based on these results, HMTBN is considered as toxic to the aquatic organisms tested in accordance with the Directive 67/548/EC.

 

Discussion on the stability of HMTBN during ecotoxicological tests:

 

In the frame of the algal toxicity test (Höger S., 2010), pre-experiments on stability of the test item have been performed to determine the kinetic of degradation of HMTBN to its known degradation product Aldehyde Methylthiopropionique (MMP) in the algal medium. The results obtained suggest that the degradation kinetics of HMTBN is concentration dependent. The higher the concentration, the more stable HMTBN seems to be under the tested conditions. At test start, mostly HMTBN was found in the samples from all three concentration levels (i.e. 1, 10 and 100 mg/L). MMP was only found at the highest tested concentration. During the next hour, the MMP concentration steadily increased, especially in the samples from the two lower concentration levels. After 72 hours, no HMTBN and hardly any MMP was found in the sample from the lowest tested concentration. Instead, a second degradation peak was observed. At 10.1 mg/L, a mixture of about three partially co-eluting degradation products was found, one of them most likely MMP. At the highest test concentration, mostly MMP and HMTBN were detected

 

MMP seems not to be the only degradation product, because up to three co-eluting degradation peaks were observed. Based on the information provided by the Sponsor, it is known that the HMTBN in water and at pH 7 is degraded back to the raw material used for its manufacture (i.e. MMP and HCN). HMTBN is stable in water at pH < 5. At pH between 7 and 11, there are HCN and ions CN- and polycyanides (coloured products). At pH 11 there is a majority of HCN.

Considering the toxicity profile of the degradation products (i.e MMP toxic to algae and HCN is classified N; R50/53), the toxicity observed during the test can be caused by the degradation products. However, toxicity of the parent substance cannot be excluded. Therefore, and considering the high percentage of degradation of the test substance also at test initiation in the main test, the effect concentrations selected for the characterisation of the hazard profile of the test substance are based on nominal concentrations, in order to cover the toxicity of the parent substance and the degradation products.

Based on this conclusion, even if no analytical monitoring has been performed in the toxicity study on Daphnia magna (Cellier P., 1996), the results based on nominal concentration are considered as reliable.