Singapore Drugs Database

Indication

For patients infected with Pediculus humanus capitis (head lice and their ova) of the scalp hair.

Mechanism of Action

Malathion is a nonsystemic, wide-spectrum organophosphate insecticide. It inhibits acetylcholinesterase activity of most eukaryotes. Malathion is toxic to aquatic organisms, but has a relatively low toxicity for birds and mammals. The major metabolites of malathion are mono- and di-carboxylic acid derivatives, and malaoxon is a minor metabolite. However, it is malaoxon that is the strongest cholinesterase inhibitor. Cholinesterases catalyze the hydrolysis of the neurotransmitter acetylcholine into choline and acetic acid, a reaction necessary to allow a cholinergic neuron to return to its resting state after activation. Because of its essential function, chemicals that interfere with the action of cholinesterase are potent neurotoxins, causing muscle spasms and ultimately death.

Pharmacokinetics

Absorption

Malathion in an acetone vehicle has been reported to be absorbed through normal human skin only to the extent of 8% of the applied dose. Absorption may be increased when malathion is applied to damaged skin. Malathion is rapidly and effectively absorbed by practically all routes including the gastrointestinal tract, skin, mucous membranes, and lungs. However, it is readily excreted in the urine, and does not accumulate in organs or tissues.

Distribution

Metabolism

The major metabolites of malathion are the diacid and monoacid metabolites, namely, malathion dicarboxylic acid (DCA) and malathion monocarboxylic acid (MCA). Malaoxon, the active cholinesterase-inhibiting metabolite of malathion, is a minor metabolite. Both malathion and malaoxon are detoxified by carboxyesterases leading to polar, water-soluble compounds that are excreted.

Elimination

Toxicity

Malathion is slightly toxic via the oral route, with reported oral LD₅₀ values of 1000 mg/kg to greater than 10,000 mg/kg in the rat. It is also slightly toxic via the dermal route, with reported dermal LD₅₀ values of greater than 4000 mg/kg in rats. Moderate poisoning can result in chest tightness, difficulty breathing, bradycardia, tachycardia, tremor/ataxia, blurred vision, and confusion. Severe, life-threatening signs include coma, seizures, respiratory arrest, and paralysis. Malathion may also be irritating to the skin and eyes.

Active Ingredient/Synonyms

[(dimethoxyphosphinothioyl)thio]butanedioic acid diethyl ester | Carbophos | diethyl (dimethoxyphosphinothioylthio)succinate | Karbofos | Maldison | Mercaptothion | O,O-dimethyl S-(1,2-bis(ethoxycarbonyl)ethyl) | O,O-dimethyl S-(1,2-dicarbethoxyethyl) dithiophosphate | O,O-dimethyl S-(1,2-dicarbethoxyethyl)phosphorodithioate | O,O-dimethyl S-1,2-di(ethoxycarbamyl)ethyl | O,O-dimethyldithiophosphate diethylmercaptosuccinate | Malathion |