Butane, 2-ethoxy-2-methyl-

Neurotoxicity

Description of key information

TAEE causes transient CNS effects. The inhalation NOAEC of the structure analogue TAME, 1060 mg/m3, will be used for the derivation of an acute inhalation DNEL for TAEE.

Additional information

In the sub-acute oral toxicity study with TAEE (GLP-compliant OECD 422 guideline), neurobehavioural examination was performed (including a functional observation battery, locomotor activity assessment and functional tests) (Covance Laboratories Ltd., 2009d). Male and female rats received 0, 100, 375 or 750 mg/kg bw/day by oral gavage in corn oil. Males were dosed for up to 8 weeks (56 days), females for approx. 6 weeks (41-46 days). The examination showed that males given 375 and 750 mg/kg bw/day, and some females given 750 mg/kg bw/day, were slightly ataxic and less active relative to controls. Based on these findings a sub-acute (28-days) NOAEL for CNS effects (reversible ataxia) of 100 mg/kg bw/day in males and 375 mg/kg bw/day in females was derived.

 

Inhalation toxicity studies for structural analogues TAME (2-methoxy-2-methylbutane) and ETBE (2-ethoxy-2-methylpropane) will be used for derivation of an acute inhalation DNEL for TAEE (see Appendix H for Reporting Format).

 

TAME:

A neurotoxicity study with rats has been conducted as a part of a 90-day repeated dose inhalation toxicity study (Huntington Life Sciences, 1997). In F-344 rats exposed to TAME vapour concentrations of 250, 1500 and 3500 ppm (1060, 6360 and 14840 mg/m³) for 6h/day, 5 days/week, signs of acute toxicity were prominent in the high dose group. Most animals were prostrate during exposure. Also the animals in the mid dose group were prostrate or lethargic during the first month of the investigation. During the latter half of the study (exposure days 45-90) a few animals in this group had laboured breathing and lethargy. The low dose group (250 ppm) showed no abnormal signs during the study. The observations in the high dose group during the recovery period were comparable to the control group.

 

As a part of the study described above, a satellite group of 10 rats/sex/group was evaluated using a Functional Observation Battery (FOB) 1, 6 and 24 hours after a single 6-hour exposure. The FOB was also performed on another group of 10 rats/sex/group in weeks 2, 3, 5, 9 and 14. A modified version of Schulze’s procedure was used to monitor motor activity in weeks 5, 9 and 14 with a group of 10 rats/sex/group. The FOB was performed on all animals before evaluation of motor activity. A neuropathology examination was performed on 6/10 animals. After a single 6 hour exposure to 3500 ppm, animals from the satellite group of 10 rats/dose/sex exhibited concentration related effects on the central nervous system and neuromuscular junction 1 hour post-exposure. The effects included depression of the central nervous system and neuromuscular junction impairment. The effects were no longer evident 6 or 24 hours after the acute exposure ended, nor were they seen after repeated exposure to TAME. In the 1500 ppm dose group, these effects were only present in male rats. There were no neuropathological changes at any exposure level. The NOAEC for acute neurobehavioral effects of TAME was 250 ppm in males and 1500 ppm in females.

 

Using the same protocol mice were exposed to the same concentrations (Huntingdon Life Sciences, 1997). Due to high mortality in the 3500 ppm exposure group, TAME air concentration was lowered and a new high exposure group was established at 2500 ppm (10600 mg/m³) together with a concomitant control group. Control and high dose (3500 and 2500 ppm) groups had initially 46 animals/sex and 250 and 1500 ppm dose groups 36 animals/sex. No neurobehavioral or neuropathological studies were conducted on the mice. Ten male and 12 female mice died during the first week of the study in the highest concentration group. The clinical signs in the high exposure animals included mainly prostration, lethargy and decreased activity. At lower concentrations, death incidences were similar to the control group. In the 1500 ppm group, TAME caused lethargy and some prostration during the study.

 

Effects on the CNS were also observed in a 28-day study with rats (ITT Research Institute, 1992). Ten Sprague-Dawley rats were exposed 6 hours daily to target TAME vapour concentrations of 0, 500, 2000 and 4000 ppm (0, 2120, 8480 and 16960 mg/m³), 5 days per week for 4 weeks. Body weights were measured at study initiation and termination and weekly during the study. In addition to the daily general toxicity assessment, the animals were evaluated with a FOB for neuromuscular function and sensory perception one week before exposure and after 1, 5 or 20 exposures. Four high dose females and three high dose males died. The probable cause of death was severe central nervous system depression. Other observations in the 4000 ppm group which were also seen in the 2000 ppm dose group included sedation, coma, ataxia, coldness to touch, ptosis, hyperirritability, hypoactivity and effect on posture. The FOB evaluation performed 1 hour after exposure confirmed the clinical observations: the 4000 ppm rats exhibited reductions in tail pinch response, righting reflex and negative geotaxis together with reduced body temperature, impaired rotorod performance and increased hind limb splay. The signs of CNS depression were absent in animals examined 18 hours after the end of the study.

In conclusion, the NOAEC for acute neurobehavioral effects of TAME was set to 250 ppm (1060 mg/m³).

 

ETBE:

A neurotoxicity study with rats has been conducted as a part of a 90-day repeated dose inhalation toxicity study (CIIT, 1996a) (highest tested concentration 5000 ppm (21000 mg/m³)). There were no gross macroscopic changes in brain or nervous tissue. The FOB evaluation found no evidence of sensorimotor dysfunction, neuromuscular dysfunction, ataxia, piloerection, excessive vocalization, muscle tremors or spasms, clonic or tonic seizures, increased salivation, abnormal respiration or abnormal pupil reflex. Decreased mean hindlimb grip strength and increase in mean hindlimb splay were observed in low exposure group males at first exposure, with a statistically significant increase in mean forelimb grip strength in high exposure males on day 10. A statistically significant decrease in mean forelimb grip strength was observed in low dose females following 65 exposures. The absence of any consistent dose-related trend suggests these observations are of doubtful toxicological significance. There was no treatment-related effect on overall motor activity during any observation period. No gross, functional or microscopic abnormalities were observed in males and female rats exposed to 5000 ppm (21000 mg/m³) ETBE vapour for up to 13 weeks.

 

In the available 28 days inhalation study with rats (IIT Research Institute, 1991b), ETBE caused transient signs of CNS depression. Signs of general sedation and reduced motor activity were noted in rats exposed to 4000 ppm (16720 mg/m³) ETBE vapour, with some animals exhibiting mild to moderate ataxia. Complete recovery from these signs of CNS depression was observed within 15 minutes following exposure termination. Body temperature was reduced after the fifth exposure in the 4000 ppm males and a significantly different trend in hindlimb splay was observed in both males and females of the 4000 ppm exposure group. Both of these alterations were most likely associated with the residual effect of the sedation observed during exposure since the FOB was performed approximately one hour following exposure termination. No other indications of CNS depression or neurotoxicity were observed in any of the other FOB parameters. Another treatment-related clinical sign included salivation. In conclusion, the NOAEC for transient signs of CNS depression in this study was 2000 ppm (8400 mg/m³).

 

Using the precautionary principle, the (lower) inhalation NOAEC reported for TAME, 1060 mg/m³, will be used for the derivation of an acute inhalation DNEL for TAEE.

Justification for classification or non-classification

TAEE should be classified with R67 (Vapours may cause drowsiness and dizziness). According to the EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008 this should be translated into Specific Target Organ toxicity – Single exposure, Cat. 3 (H336).