Some structure activity relationships of phencyclidine derivatives as anticholinergic agents in vitro and in vivo

Phencylidine derivatives exhibit multiple interactions with cholinergic systems: they block nicotinic and muscarinic receptors,and inhibit both acetyl and butyrylcholinesterase. In peripheral tissue, the net pharmacological effects of the phencyclidines is antiacetylcholine activity. The dissociation constants measured in isolated smooth muscle and from competition experiments for the muscarinic high-affinity binding sites in brain homogenates (Kd = 10(-5) - 10(-6) M) are 3--4 orders of magnitude lower than those of anticholinergic glycolate esters. However, phencyclidines have comparable potency to that of d-tubocurarine in blocking the nicotinic receptor in the isolated frog rectus abdominis (Kd = 10(-6) M). Brain uptake experiments of (3H) labeled phencyclidine showed that during the time period in which central effects are observed with these drugs their concentration in brain reaches values close to the Kd (10(-5) - 10(-6) M). This finding, and the cross tolerance observed in vivo between phencyclidine and other centrally acting cholinergic drugs supports the possible involvement of cholinergic interactions in the psychotropic action of phenyclidine derivatives. Quantum chemical calculations of the interaction pharmacophores of drugs in the phencyclidine series have indicated the molecular determinants for the interaction of these drugs with the muscarinic receptor. The calculations revealed that these drugs can match the reactivity characteristics of ACh and the semi-rigid muscarinic agonist 3-acetoxyquinuclidine, but their rigid molecular frame will be conductive to antagonistic rather than agonistic activity when the drug-receptor complex is formed. The identification of a "cholinergic interaction pharmacophore" for these drugs by quantum mechanical calculations made possible the suggestion of other active phencyclidine derivatives, e.g. p-NH2 and p-OH analogs which proved to be equipotent to phencyclidine. The inactivity of the p-NO2 derivative was also predicted on this basis and served as an additional confirmation of the theoretical criterion for activity; the difference between the activities of the ethynyl and cyano derivatives was explained by the modification of the cholinergic interaction pharmacophore. On the basis of these theoretical predictions, electrophysiological studies were carried out by the others and the results prompted the suggestion that "physostigmine is of potential value in the treatment of post-operative patients emerging from ketamine anesthesia and in the treatment of phencyclidine overdosed patients".