Pharmacokinetics of Metrifonate and its Rearrangement Product Dichlorvos in Whole Blood

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Abstract
Six male healthy volunteers were given single oral doses of 7.5 mg/kg of metrifonate and concentrations of metrifonate and dichlorvos were determined in whole blood using a standardized sampling procedure. Blood was collected in test tubes containing equal volumes of 0.74 M phosphoric acid for the determinations of metrifonate and dichlorvos with gas chromatography and mass spectrometry at different time points for up to 24 hr. Cholinesterases were also determined in blood haemolyzed with water. Metrifonate was quickly absorbed with a Cmax of 50.5±18.9 μmol/l which was obtained between 0.17 to 1 hr after drug intake. Mean whole blood t1/2, Cl0 and AUC were 2.07±0.24 hr, 0.34±0.06 l/hr/kg and 89.2±16 μmol.hr/l respectively. The concentrations of dichlorvos closely followed those of metrifonate with a constant ratio of 0.01 to 0.02. The concentrations of metrifonate were detectable for up to 8 hr but those of dichlorvos had fallen below the level of determination by this time. Both plasma and red blood cell cholinesterases were readily inhibited and were still low after 24 hr. None of the volunteers complained of side effects.