2-Propenoic acid, methyl ester, reaction products with 2-ethyl-1-hexanamine and sodium hydroxide

Specific investigations: other studies

Description of key information

LOAEL (ocular toxicity) intraperitoneal = 5000 mg/kg bw for rats (no NOAEL identified)

Additional information

In a study by Eells et al. (2000), rats were intraperitoneally dosed with methanol. In all of these animals, the folate-dependent formate-oxidation was inhibited. After the initial dosage of 4000 mg/kg bw, 12 hours later an injection of 1000 or 2000 mg/kg bw followed. Formic acidemia, metabolic acidosis and visual toxicity occurred (Eells et al., 2000). Histopathology demonstrated vacuolation in the photoreceptors, mitochondrial swelling and mitochondrial disruption in the retinal pigment epithelium, which were dependent on blood formate levels. However, functional changes could already be demonstrated by electroretinogram (ERG) and flash evoked cortical potential (FEP) in animals not showing morphological changes, 72 hours of recovery. These functional tests provide functional evidence of retinal toxicity in methanol poisoning at stages not yet pronounced as histopathological changes. The authors stated the hypothetical mechanism that formic acid binds to cytochrome aa3 and inhibits cytochrome oxidase activity with inhibition constants between 5 and 30 mM, which is in the range of concentrations found in the retina and vitreous humour of methanol-administered rats. This may explain the effect on mitochondria and resulting visual dysfunction (Eells et al., 2000).

Formate oxidation was found to be about 50% lower in human than in rat retina (Eells et al., 1995). This is in line with the finding that lower folate levels in human retina may limit the conversion of formate into CO2 and contribute to the high ocular toxicity in humans.

A subacute oral toxicity study in monkeys indicated that repeated methanol dosing caused ocular lesions after a high initial dose of 2000 mg/kg bw followed by lower doses for up to 6 days, depending on the animal´s acidotic response in blood (Martin-Amat et al., 1977), while acute methanol toxicity did not yield signs of ocular toxicity (McMartin et al., 1975). The only detectable ocular change was optic disc edema (of the optic papilla) which was similar to that seen in raised intracranial pressure in humans, but without this pressure after methanol (Hayreh et al., 1977). The primary sites of ocular injury were the optic nerve heads and the anterior segment of the optic nerve rather than the retinal ganglion cells themselves. It appears that interference with oxidative phosphorylation causes mitochondrial damage; thus, disruption of active axoplasmic flow in the retrolaminar optic nerve (Baumbach et al., 1977; Hayreh et al., 1977). Mechanistically, there is a close causal relationship between the prolonged increase in formic acid resulting from methanol and the development of optic edema. Similar effects can be produced by intravenous administration of formate without acidosis (Martin-Amat et al., 1978).