cis-4-[3-(p-tert-butylphenyl)-2-methylpropyl]-2,6-dimethylmorpholine

Neurotoxicity

Description of key information

Fenpropimorph did not cause neurotoxic effects in a neurotoxicity study after acute (gavage) oral feeding exposure in Wistar rats. After subchronic oral feeding effects on parameters of a functional observational battery (FOB) were observed (for details please refer to the chapter on repeated dose toxicity). In the acute study the clinical symptoms are in line with other acute studies in rats. The primary effect was a reduction on plasma cholinesterase and a minor reversible effect on brain cholinesterase but no effect on erythrocyte cholinesterase. In the subchronic neurotoxicity study there was only an effect on plasma cholinesterase as already known from subchronic studies in rats. A reduction of brain cholinesterase was observed only in a single oral study and could never been reproduced, this effect is of rather questionable certainty. After acute intraperitoneal administration in rats plasma cholinesterase (the only cholinesterase measured) was reduced as expected. No clear assessment on liver function/hepatotoxicity was possible due to concurrent peritonitis caused by the mode of administration.

In domestic hens, known to be very sensitive to neurotoxic agents no effect as determined by clinical symptoms and histopathology of the spinal cord/sciatic nerve was found even at doses ranging up to an LD50 in this species. There was no protective effect of PAM or atropine sulphate on mortality, indicating that death was not related to cholinesterase activity.

The toxicological significance and relevance of the decrease of cholinesterase in rats for the risk assessment for man should be addressed taking into account studies on kinetics and metabolism, standard toxicology studies and special neurotoxicity studies together. 

From acute studies in rats it is known, that the onset of toxicity is delayed by 2 - 3 days. This does not indicate a direct effect of Fenpropimorph on cholinesterase inhibition but rather suggests the involvement of metabolites, which are formed after biotransformation of the parent compound. Several structural properties such as ammonium ions, a hydrophobic alkylarylmoiety and an ethanolamine group might be responsible for cholinesterase inhibition.

In the metabolic pathway of Fenpropimorph, a cleavage of the dimethylmorpholine ring system to form such structural properties is well known. It was demonstrated in in vitro experiments that structurally similar compounds to metabolites in which the dimethylmorpholin portion of the molecule has been opened and partly metabolised led to a decrease in plasma cholinesterase (see IUCLID chapter 7.9.4). Cis-Dimethylmorpholine itself, i.e. the intact structure of Fenpropimorph does not have this effect.

It is of importance to observe that the inhibition of plasma-acetyl-cholinesterase in vivo was only seen in rats. Since this species dependency may be caused by species differences in metabolism, an in vitro metabolism study has been carried out. In this study (for the RSS of the study, please refer to IUCLID chapter 7.1.1), a metabolic toxification could be proven by an increase of inhibition of plasma-acetyl-cholinesterase activity in the active incubates of microsomes versus the metabolically inactive control samples. Samples of purified incubates from liver slices demonstrated a metabolic toxification of fenpropimorph in rat liver slices only. This effect was not observed for mice and dogs. In the in vitro study, the main phase I metabolite in micsrosomes that was formed in all investigated species could be shown to be an inhibitor of plasma-acetyl-cholinesterase and therewith this metabolite may be responsible for the inhibitory effects of purified in vitro incubates. However, incubations in liver slices demonstrated that the main metabolite BF 421-1 is species dependently glucuronidized. The glucuronide BF 421-1-Gluc was predominantly formed in dogs and mice and only to a low extent in rats. Since BF 421-1 but not fenpropimorph itself inhibits plasma-acetyl-cholinesterase activity, the formation of BF 421-1 has to be considered a metabolic toxification. The further glucuronidation in mice and dogs can be concluded to be a subsequent detoxification, which reflects the species dependent enzyme inhibition in vivo.

The inhibition in vivo, either directly or indirectly observed, was not seen in mice, dogs, rabbits and hens in contrast to rats. The extreme species specific effect on the inhibition, combined with the fact that the cholinesterase inhibition is not related to a property of the technical active ingredient but rather to specific metabolites indicates that the metabolic toxification may be rat specific.

 

Thus the following conclusion can be drawn:

 

·        there is no indication that Fenpropimorph or its metabolites have neurone damaging properties

·        the effects on cholinesterase can be described as neuropharmacological

·        the effect on plasma cholinesterase is reversible and comparable to the effect of choline

·        the effect on plasma cholinesterase is only seen in rats

·        rat specific metabolic toxification may cause this effect

·        the effect on plasma cholinesterase seen in rats may not be relevant for risk assessment for man.