Description
Vamidothion is a colorless crystalline substance,. It is readily soluble in
water (4 kg/L) and most organic solvents except aliphatic
hydrocarbons. Log Kow = 0.12. It decomposes in strong
alkaline and acidic media.
Uses
Vamidothion is a phosphorothioic insecticide for apples and potatoes.
Uses
Vamidothion is a systemic insecticide used to control Homoptera
in cotton, fruit and rice.
Definition
ChEBI: An organic thiophosphate that is N-methyl-2-[(2-sulfanylethyl)sulfanyl]propanamide in which the thiol group has been converted to the corresponding O,O-dimethyl thiophoshate. Formerly used as an inse
ticide and acaricide, it is no longer approved for use within the European Union.
Pharmacology
Pyriproxyfen formulations demonstrate persistent efficacy. For example, a water-based 5.3%
pyriproxyfen spot-on formulation applied to cats was
reported to completely prevent the hatching of flea eggs for
at least 46 days after treatment and continued to provide
greater than 96% control until day 60 (57). Because
pyriproxyfen is efficacious at very low concentrations, trace
amounts of the chemical, when transferred from treated
pets to their environments, are sufficient to inhibit the
development of larvae.
Metabolic pathway
Vamidothion is mainly metabolised via oxidation to its sulfoxide; further
oxidation to the corresponding sulfone has been observed in houseflies
but occurs much less readily than with other thioether-containing
organophosphates (e.g. phorate). The sulfoxide is then hydrolysed via
P-S and C-S bond cleavage to give the thiol or hydroxyl derivatives and
dimethyl phosphate and O,O-dimethyl phosphorothioate respectively.
O-Demethylation apparently occurs as a major degradation process
in plants but has not been observed in soil or in animals. N-Demethylation
and hydrolysis to the corresponding carboxylic acid, such as
occurs with dimethoate, does not apparently happen in the case of
vamidothion.
Degradation
Vamidothion is decomposed in strongly acidic or alkaline media
(PM). Barceld et al. (1993) examined the photolysis of vamidothion in
water containing 2-4% methanol and 5% acetone as a photosensitiser
irradiated by a xenon arc (suntest) lamp. Metabolites were analysed
and characterised by HPLC, thermospray MS and UV (diode array)
spectra. The main degradation product identified was vamidothion
sulfoxide (2). This product of vamidothion photolysis is shown in
Scheme 1.
Toxicity evaluation
The acute oral
LD
50 for rats is 64–105 mg/kg. Inhalation LC50 (4 h) for
rats is 1.73 mg/L air. ADI is 8 μg/kg b.w.
