LECTURE 4

INTRODUCTION Irreversible type of indirect cholinomimetics are phosphate esters which are very stable to hydrolysis. Cholinomimetics Direct Indirect Reversible Irreversible

2- Irreversible AChEIs Mechanism of action: -They inhibit AChE by the same mechanism as the carbamate inhibitors except that they leave the enzyme esterified as phosphate esters. -The rate of hydrolytic regeneration of the phosphorylated enzyme is much slower than that of the carbamylated enzyme (in hours).

Indirect Cholinomimetics Inactive

2- Irreversible AChEIs Why referred to as irreversible inhibitors? 1- because the duration of action is very long >>> death occurs before regeneration takes place. 2- because the phosphorylated ACHE can undergo a process known as aging.

2- Irreversible AChEIs

Applications 1- Medical (Isofluorophate) 2- Insecticides (Parathion) 3- Warfare agents (Sarin)

2- Irreversible AChEIs Aging is the result of cleavage of one or more of the phosphoester bonds while the AChE is phosphorylated. Phosphorus atom become much less electrophilic >>> will not undergo hydrolytic regeneration to give the active form of AChE. Only those phosphorus-derived AChEIs that possess at least one phosphoester group undergo this aging process.

Isofluorophate -It is an irreversible AChEI & a powerful miotic agent which can effectively reduce eye pressure. Uses: Chronic glaucoma(topical).

Parathion - It contains Sulfur atom bonded to the Phosphorous atom. - It is a very weak AChEI - Parathion is rapidly bioactivated by microsomal oxidation in insects (but not in human)to afford the corresponding oxo dv. which is a powerful AChEI:

Isofluorophate Disadvantage: Its vapor is highly toxic (nerve gas)and it is recommended that only solutions in arachis oil can be used therapeutically.

SARIN - It is a colorless, odorless heavy lipophilic liquid - It is an illegal chemical warfare - After phosphorylation, only one aging reaction takes place, and then the enzyme becomes completely refractory to regeneration.

SARIN - It is lethal even at very low conc. -People who absorb a non-lethal dose, but do not receive immediate medical treatment, may suffer of permanent neurological damage

SARIN Symptoms of sarin exposure could be summarized in what is known as SLUDGEM syndrome: Which is a summary of the pathological effects indicative of massive discharge of peripheral nervous system.

SARIN SLUDGEM (cont.) 1-Salivation: stimulation of salivary gland 2-Lacrimation: stimulation of lacrimal gland 3-Urination: relaxation of int. sphincter of urethra 4-Defecation: relaxation of int. anal sphincter 5-GIT upset: diarrhea 6-Emesis: vomiting 7-Miosis: stimulation of pup. constrictor muscles or 8-Muscle spasm: Stimulation of skeletal muscle

Antidote for irreversible AChEIs The problem required the design of reagents capable of: 1- efficiently catalyzing phosphate ester hydrolysis (strong nucleophile)and regenerating active AChE. 2- being safe enough for use as therapeutic agents.

PRALIDOXIME - Pralidoxime (PAM) is an oxime derivative 2-pyridinealdoxime chloride - It is a strong nucleophile and safe at the same time.

CHEMICAL SYNTHESIS PAM is synthesized by reacting picolinaldehyde (2-formyl pyridine) with hydroxylamine, giving pyridine-2-aldoxime, which is further reacted with an alkyl halide, giving the desired pralidoxime:

MODE OF ACTION 1-In organophosphate poisoning, an organophosphate binds to just one end of the acetylcholinesterase enzyme [ the anionic site ], blocking its activity. Pralidoxime is able to attach to the other half [ the unblocked, esteric site ] of the acetylcholinesterase enzyme.

MODE OF ACTION 2-It then binds to the organophosphate, the organophosphate changes conformation, and loses its binding to the acetylcholinesterase enzyme.

MODE OF ACTION 3-The conjoined poison / antidote then unbinds from the site, and thus regenerates the enzyme, which is now able to function again.

LIMITATIONS 1- It must be given within a short period of time, after enzyme phosphorylation.. Why? 1- because of the aging process. 2- It is only effective in organophosphate toxicity 2- (i.e. it does not have an effect if the AChE is carbamylated )

LIMITATIONS 3- It can not cross the blood-brain barrier to regenerate phosphorylated AChE in the brain, 3- this is why atropine, is concomitantly administered with pralidoxime during the treatment of organophosphate poisoning