CH3O oh 0,0-Dimethyl Phosphorothioate Figure 17.18 • Comparison of metabolism of malathion by mammals and insects.

however, enzymes present in liver microsomes and insect tissues convert parathion (p/50 <4) to paraoxon, a more potent inhibitor of cholinesterase (pi50 >8).64 Parathion is also metabolized by liver microsomes to yield ^-nitrophenol and diethylphosphate; the latter is inactive as an irreversible cholinesterase inhibitor.65

Schradan. Schradan, octamethyl pyrophosphoramide (OMPA), bis[bisdimethylaminophosphonous] anhydride (Pestox III), is a viscous liquid that is miscible with water and soluble in most organic solvents. It is not hydrolyzed by alkalies or water but is hydrolyzed by acids. Schradan is used as a systemic insecticide for plants, being absorbed by the plants without appreciable injury. Insects feeding on the plant are incapacitated.

Schradan is a weak inhibitor of cholinesterases in vitro. In vivo, it is metabolized to the very strong inhibitor hydrox-ymethyl OMPA. Hydroxymethyl OMPA is not stable and is metabolized further to the N-methoxide, which is a weak inhibitor of cholinesterase.66

Pralidoxime Chloride, USP. Pralidoxime chloride, 2-formyl-1-methylpyridinium chloride oxime, 2-PAM chloride, or 2-pyridine aldoxime methyl chloride (Protopam chloride), is a white, nonhygroscopic, crystalline powder that is soluble in water, 1 g in less than 1 mL.

Pralidoxime chloride is used as an antidote for poisoning by parathion and related pesticides. It may be effective against some phosphates that have a quaternary nitrogen. It is also an effective antagonist for some carbamates, such as neostigmine methylsulfate and pyridostigmine bromide. The mode of action of pralidoxime chloride is described in Figure 17.16.

The biological half-life of pralidoxime chloride in humans is about 2 hours, and its effectiveness is a function of its concentration in plasma, which reaches a maximum 2 to 3 hours after oral administration.

Pralidoxime chloride, a quaternary ammonium compound, is most effective by intramuscular, subcutaneous, or intravenous administration. Treatment of poisoning by an anticholinesterase will be most effective if given within a few hours. Little will be accomplished if the drug is used more than 36 hours after parathion poisoning has occurred.

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