INHIBITION OF ACETYLCHOLINESTERASE IN THE BRAIN AND DIAPHRAGM OF RATS BY A TERTIARY ORGANOPHOSPHOROUS ANTICHOLINESTERASE AND ITS QUATERNARY ANALOGUE; IN VIVO AND IN VITRO STUDIES

Abstract— Pinacolyl S -(2-dimethylaminoethyl)methylphosphonothioate (compound I) and its quaternary analogue (compound II), are potent anticholinesterases, that form a very stable phosphonylated AChE and differ in their in vitro anticholinesterase potency by a factor of two, but have widely differing lipid solubilities.
In vitro , compound I diffused through a cerebral cortex slice when applied to the intact surface at twelve times the rate of compound II and through a diaphragm segment at four times the rate. When applied to the intact surface of a cerebral cortex slice or a diaphragm segment for 10 min, compound I gained access to AChE sites more readily than compound II but the difference was much less than the difference in their lipid solubilities. There was no discontinuity in the percentage AChE inhibition versus logarithm of the concentration of compound II, indicating that there was no clear separation of AChE into two fractions which differed greatly in their accessibility to quaternary compounds. Both compounds gained access to AChE sites in cerebral cortex slices more readily than in diaphragm segments.
In vivo , the peak plasma levels and the rates of removal from the plasma of free inhibitor were similar for both compounds, given subcutaneously in equimolar amounts. Compound I in high doses inhibited over 90 per cent of the AChE in the cerebral cortex and the diaphragm; compound II even in lethal doses produced only marginal inhibition of AChE in the cerebral cortex and only 50–60 per cent inhibition of AChE in the diaphragm.
These results indicate that the in vivo distribution of quaternary compounds is different from that observed in vitro . The implications of this are discussed.