LECTURE 4. INTRODUCTION Irreversible type of indirect cholinomimetics are phosphate esters which are...

LECTURE 4

INTRODUCTION

Irreversible type of indirect cholinomimetics are phosphate esters which are very stable to hydrolysis.

Cholinomimetics

DirectIndirect

ReversibleIrreversible

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

Inactive

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

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 :

NO2OPO

O

S

Parathion

NO2OPO

O

O

Paraoxon

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