Value of ED50 testing in assessing hazards of acute poisoning by carbamates and organophosphates.

It is shown from the kinetics of inhibition of cholinesterase by N-methylcarbamates and organo-phosphates that the LD50 dose is likely to be a much greater multiple of the dose causing signs of poisoning in 50%-of the animals (the ED50) for the carbamates than for the organophosphates. The expected difference was demonstrated by a comparison of the LD50s and ED50s, intravenous and intramuscular, of five carbamates (2-isopropoxyphenyl N-methylcarbamate, 3-isopropylphenyl N-methylcarbamate, 6-chloro-3,4-xylyl N-methylcarbamate, 3,4,5-trimethylphenyl N-methyl-carbamate, and 3-methyl-5-isopropylphenyl N-methylcarbamate) and two organophosphorus compounds (diethyl 4-nitrophenyl phosphate and dimethyl 4-nitrophenyl phosphate). The slightest evoked tremor was chosen as the most reliable sign of poisoning from which to estimate the ED50 values. Carbamates gave much greater LD50/ED50 ratios than organophosphorus compounds. It is likely that occupational exposure to carbamates will produce incapacitating symptoms at doses well below lethal levels. Two groups of insecticides with anticholinesterase action are in widespread use-organophosphorus compounds and N-methylcarbamate esters. There are good theoretical reasons for expecting that the carbamates will produce the first distinct signs of poisoning at very much smaller fractions of their LD50 values than will the organophosphorus compounds. Thus organophosphorus compounds can be regarded as irreversible inhibitors, so that inhibition of cholinesterase is progressive, as described by Aldridge and Davison (1952). For such inhibitors the activities of the uninhibited and partially inhibited enzymes are related by the equation: inee volvi = kicit (1) where vo and vi are the activities before and after inhibition, ci is the concentration of inhibitor acting for time t, and ki is the rate constant for the reaction between inhibitor and enzyme. In the case of carbamates, interaction with the enzyme involves both inhibition and re-activation (Wilson, Hatch, and Ginsburg, 1960), so that after a certain time an equilibrium is reached, i.e., the rates of inhibition and re-activation are equal. The equilibrium is described by the equation (Reiner, unpublished): K = vici/(v,-vi) (2) where K stands for the equilibrium constant. This equation is equally applicable whether equilibrium is reached through carbamylation of the enzyme or by the formation of a reversible complex between enzyme and inhibitor. In the Figure the inhibition of cholinesterase by carbamates and organophosphorus compounds is presented at various concentrations of inhibitor. The degree of inhibition was calculated from equation 1 for organophosphorus compounds and from equation 2 for carbamates. In Figure a the curves were constructed for the case where both a carbamate and an organophos-phate produced 12-5 …