Anticholinergics

Acetylcholine (Ach)

Drugs that directly inhibit pharmacological response of Ach

Muscarine antagonist/antispamodics

• These drugs block the response of Ach in the muscarine receptor by competitively binding to it and inhibiting any response.• They have opposite pharmacological

response of Ach ie if Ach agonist slows heart rate then Ach antagonist speeds heart rate or if Ach relaxes bladder then it antagonist constricts it

• Their medical use is in– in Sooth muscle spasm – in cold n flu (to reduce nasal secretion)– previously in ulcer (but now replaced by H2

antagonist and proton inhibitors)– Overactive bladder (to much urination)– Motion sickness– Treat organophosphate poisoning (still doesn’t

work in aging and doesn’t treat respiratory failure)– Parkinson (brain disease where nerves start

degrading and person slowly goes crazy)

SAR of anticholinergics

• Atropine was the first drug of this type and was used to generate SAR. It was noted that unlike Ach, the terminal ester carbon in Atropine had a bulky substituent. This was considered important and modifications were done there

1) The R1 or R2 groups must be carbocyclic or heterocyclic, but if both are cyclic it gives maximal antagonist potency. The rings may be same or different. One of is generally aromatic and other is saturated ring or olefinic group (ie it has a C-C double bond)

•Rings may be same or different•The benzene could be any typeCyclohexane (non-aromatic carbocyclicor pyridine (aromatic heterocyclic)or Pyrrolidine (non-aromatic heterocyclic

General framework of Anticholinergics

C X (CH2) N substituent

R1

R3

R2 n

C X (CH2) N substituent

R1

R3

R2

Detect R1-R3 and X

2)The R3 group can be hydrogen, hydroxyl (-OH), hydroxymethyl (-CH2OH), amide ( ) or a component of the R1 and R2 group. Best potency is seen with hydroxyl or hydroxymethyl (this hints that the oxygen group must be participating in H bond)

Hydrogen Hydroxyl Hydroxymethyl

Amide

Component of R2 and R3

3) The X is mostly ester in most potent derivatives but it can be a ether oxygen or absent completely

Mostly ester Or else ether Or absent completely

4) The N substituent cab be both quaternary ammonium salt or tertiary amine with different alkyl groups. Most potent derivatives have quaternary ammonium salt. The alkyl group is not restricted to only methyl (as in SAR of Ach agonist). It can be ethyl, propyl or isopropyl.

Quaternary form is most potent Alkyl = methyl

Alkyl = ethyl Alkyl = isopropyl

5) The distance between the ring substituted carbon and nitrogen is not fixed ie it can vary The no. of alkyl units between that carbon and nitrogen can vary from 2-4, with most potency in case of two CH2 units.(this is also unlike SAR of Ach agonist where the CH2 units should not be more than 2).

2 CH2 units is best distance

2 CH2 units (don’t count O or N, just C)

3 CH2 (don’t count double or triple bonded carbon. Only count single carbons)

summary

• The R1 or R2 groups must be carbocyclic or heterocyclic• The R3 group can be hydrogen, hydroxyl (-OH),

hydroxymethyl (-CH2OH), amide or a component of the R2 and R3 group

• The X is mostly ester in most potent derivatives but it can be a ether oxygen or absent completely

• The N substituent cab be both quaternary ammonium salt or tertiary amine with different alkyl groups

• The distance between the ring substituted carbon and nitrogen is not fixed but maximum potency requires about 2 carbon units

• (Note: The SAR does not say anything about selectivity for muscarinic subtypes)

C X (CH2) N substituent

R1

R3

R2

What happens during muscular spam?

• A muscle spasm, or muscle cramp, is an involuntary contraction of a muscle. Muscle spasms occur suddenly, usually resolve quickly, and are often painful. Both skeletal muscle and smooth muscle are effected by it.

• Spasms may occur when a muscle is overused and tired, particularly if it is overstretched or if it has been held in the same position for a prolonged period of time. The muscle becomes hyperexcitable, resulting in a forceful contraction

Contrast between SAR of Ach agonist and antagonist

A) Nitrogen group• In agonist the N can only be quanternary but • In antagonist N can be both quanternary or

tertiary

Methacholone

B) Ethylene group• In agonist the no of ethylene is fixed at only 2

but• In antagonist no of ethylene can range from 2-

4

Bethanecol

carbamate

C) Selectivity• In agonist the methyl substitution in ethylene

group controls selectivity of muscarinic or nicotinic but

• In antagonist no such feature is present. Still It only antagonizes muscarinic only

MethacholoneMuscarinic selective

D) Ester group or X group• In agonist ester is not needed and can be

removed but an Oxygen must exist in it’s place• In antagonist ester is not needed and can be

removed but an Oxygen need not exist in it’s place

H3C O CH2 CH2 N(CH3)3

H3C CH2 CH2 N(CH3)3

O

H2N O

O

CH2 CH2 N(CH3)3

Mostly ester

Oxygen in place on ether

Oxygen absent

Ester

Ether

Ketone

E) Rule of five and terminal carbonIn agonist rule of five is followed and terminal

carbon is bonded to HydrogensIn antagonist, rule of five is not followed and

the terminal carbon is bonded to two bulky ring groups

Methacholone

Spot the associated functional groups

Specific Muscarinic antagonists

• Aminoalcohol esters– Atropine– Scopolamine

• Aminoamide– Tropicamide

• Aminoether– Benztropine– Orphenadrine

• Micellneous– Solifenacin– Darifenacin

While all of these are selective to muscarinic receptor they are not selective to a specfic subtype eg M1, or M2 etc. Only the last two Solifenacin and Darifenacin show selectivity to M3.

NCH3O C

O

CH

CH2OH

NCH3O C

O

CH

CH2OH

O

Atropine

Scopolamine

Dicyclomine Tropicamide

Atropine

• It is anticholinergic that blocks muscarinic receptors• It is an alkaloid extracted from Solanaceae plant and was the first

anticholinergic.• It is an ester of tropine and tropic acid and used as a sulphate Salt in

racemic from• At therapeutic does it can penetrate the brain and stimulate the CNS• Uses

– Treat Bardycardia– Reduce secretion before surgery– Treat Iritis (painful inflammation of eye)– Organophosphate poisoning (only to decrease muscarinic action, not an

antidote like PAM) • MOA – It competitively binds to muscarinic receptor and antagonizes it

thus blocking all cholinergic effects

NCH3O C

O

CH

CH2OH

Atropine synthesis

HC

NCH3

Esterification with HCl(-H20)

NCH3O C

O

CH

CH2OH

OH

CH2OH

CHO

O

Tropine Tropic Acid

Atropine

Scopolamine• It is anticholinergic that blocks muscarinic receptors• It is an alkaloid extracted from Solanaceae plant • It is used as salt hydrobromide salt in enantiopure (-)

form• At therapeutic does it depresses CNS • Uses

– Treat Iritis (painful inflammation of eye)– Treat Parkinson– Treat Motion sickness

• MOA - It competitively binds to muscarinic receptor and antagonizes it thus blocking all cholinergic effects

NCH3O C

O

CH

CH2OH

O

Dicyclomine

• It is an anticholinergic that blocks muscarinic receptors

• It is a weaker antagonist than atropine and doesn’t stimulate the brain

• Uses– treat intestinal hypermotility (causes constipation and

diarrhea and decreased opportunity for the absorption of nutrients)

– irritable bowel syndrome (a disorder in large intestine that causes cramping, abdominal pain, bloating, gas)

• MOA - It competitively binds to muscarinic receptor and antagonizes it thus blocking all cholinergic effects

Synthesis

CH2CN

Phenyl acetonitrile

H2C CH2 CH2 CH2 CH2 Br

Br

-2HBr

CN

Cyclohexane derivative

1,5-Dibromopentane

Saponification

COOH

HO CH2 CH2 N

C2H5

C2H5

HCli)

ii) Reduction

C O CH2 CH2 N

C2H5

C2H5

O

.HCl

Dicyclomine Hcl

Tropicamide

• It is an anticholinergic that blocks muscarinic receptors

• Its duration of action is shorter than Atropine• Uses

– Mydriatic (drug that dilates pupil)– Cycloplegia (to fix eye movement)

• MOA - It competitively binds to muscarinic receptor and antagonizes it thus blocking all cholinergic effects

Synthesis CH

COOH

CH2OH C Cl

O

H3C

Esterifcation

CH

COOH

CH2O C CH3

O

SOCl2

CH

C

CH2O C CH3

O

O

Cl

Tropic acid Acetyl Chloride

Tropic acid acetate

An acid chloride

NH2CHN

C2H5

CH

C

O

N CH2 N

C2H5

CH2O C

O

CH3

Tropicamide acetate

Saponification

CH

C

O

N CH2 N

C2H5

CH2OHTropicamide

Newer Muscarinic Antagonist

• Newer drugs focus on subtype selectivity and don’t have defined SAR

Older SAR based non-selective drug

Selective to M1 Selective to M2

Observe the use of tri-cyclic ring and how minor modification changed Selectivity from M1 to M2. This is true not just here in in every case

Acetylcholine and muscle contraction

Ach binds to it’s receptor in

muscle

Myosin cross-bridges with Actin leading

to contraction

Action potential

(AP)is generated

AP causes Release of

Ca2+ from SR

Muscle return back to

relaxed state

AP ends and Ca2+ is put

back into SR

Events during Muscle contraction

Events during an action potential

An action potential is a temporary “all or nothing” changes in cell membrane potential. During this period cell is taken from polarized to depolarized state

Depolarizing blockerskeep maintaining depolarized state

Nicotinic antagonist

• Nicotinic Antagonist competitively bind to nicotinic receptors and block nicotinic response which results in blockade of skeletal muscle contraction ie paralysis

• There are two types– Neuromuscular Blockers (not the same as skeletal

muscle relaxant that work by CNS depression)– Ganglionic Blockers

(We will only discuss the first)

Neuromuscular Blockers

• The first Neuromuscular Blockers was extracted from the plant cucare which contained Tubocuraine.

• It was noted that given in normal condition, they cause muscle paralysis.

• Tribesmen of Amazon used it in their arrows for hunting.

• The poison will cause respiratory failure in the prey

Therapeutic application• As an Adjunct to general Anesthetic ,(general

Anesthetic are drugs that makes you unconscious for surgery) they lower the dose of Anesthetic thus lowering side effects and promoting postanesthetic recovery time and

• Tracheal intubation (putting tube inside food pipe) or endoscopy (putting tube inside rectum)

• Correct bone dislocation• 2 types

– Non-depolarizing (desired property)– Depolarizing (undesired property)

SAR• Two quaternary ammonium salts separated by

10-12 carbon units is the only known general requirement. This follows from the observation that nicotinic receptor has two cationic site

N

H3C

H3C

H3C CH2 N

CH3

CH3

CH3

10

Decamethonium bromide

Succinylcholine Chloride• It is a depolarizing neuromuscular blocker. This depolarization

effect makes the muscle fiber resistant to further stimulation by Ach. This nature makes it therapeutically undesirable in comparison to the Non-depolarizing blockers

• It is a dimer (two same molecules joining each other) of acetylcholine molecules

• Like Ach, it’s is rapidly metabolized in blood and thus has short duration of action of about 6 to 8 mins

• Uses– Endotracheal insertions– Endoscopy

• MOA – It antagonizes nicotinic receptors at neuromuscular junction which causes skeletal muscle paralysis

Think of modification to improve duration of action of Succinylcholine Chloride

Hint- Rapid metabolism is due to hydrolysis of ester

O

O

O

O

CH2

CH2

CH2 N{CH3}3Cl

CH2 N{CH3}3Cl

synthesis

CH2

CH2

C

O

Cl

C Cl

OSuccinyl Chloride

-2HCl

Condensation

CH2

CH2

C

O

OCH2CH2N(CH3)2

C OCH2CH2N(CH3)2

O

CH3ClMethyl chloride

2HOCH2CH2N(CH3)2

CH2

CH2

C

O

OCH2CH2N(CH3)3

C OCH2CH2N(CH3)3

O.2Cl

Succinyl Choline Chloride

Dimethyl animoethanol

D-Tubocuraine

• It is a Non-depolarizing neuromuscular blocker. It doesn’t make the muscle fiber resistant to further stimulation by Ach.

• This nature makes it therapeutically desirable in comparison to the depolarizing blockers

• It is metabolically stable, only 1 % is degraded by liver, thus acts for a longer period of about 80-120 mins

• It’s preparation includes bisulfites (an anti-oxidant) which causes histamine release thus can cause allergic reactions

• Uses• MOA – It antagonizes nicotinic receptors at neuromuscular

junction which causes skeletal muscle paralysis

Point to be noted• Being metabolically allows drug to be active

for longer periods but it also means that the only way to get rid of their effect is excretion through the kidney.

• Thus in patients with kidney failure, such characteristic can cause trouble. An ideal drug must be readily expelled from body. In such case does might need to be lowered.

Thank You

Drugs from Plants and HTS

Drug Plant Class Chemical Type

Atropine Solanaceae Antimuscarinic

Alkaloid

Tubocuraine Curare Antinicotinic Alkaloid

Physostigmine

calabar beans

Reversible AChE inhibtor

Alkaloid

•Historically, Plants have always been the source of the first drugs in a class. Both Atropine and Tubocuraine were the first of their kind and notice they were all alkaloids, along with Physostigmine . •The only downside to this process is that there are thousands of plants and each plant typically contain a multitude of compounds, that testing for each and every one against a receptor/enzyme is a very exhaustive process. •Thankfully, the industry can test thousands of compounds in few days using High Throughput Screening(HTS) which are automatic and robotic system. To use it first we have to design as assay. This is now available in academia too.

• In these systems drugs/compounds are injected into plates that contain many hole which actually is a individual assay that contains receptor/enzyme on which we desire to find if the drug has any activity or not

• In each hole different drug concentration is injected• Successful drugs generally gives some kind of visible color

change (colorimetric assay) or change in fluroscence reading (florimetric assay) in every hole in the plate. All the data for every hole can be seen easily in a computer screen

• The intensity of color change and fluroscence reading allows to both qualify and quantify drug action at that concentration

• If a compound is having good activity, medicinal chemists alter it to improve potency/selectivity

HTS machine looks like this

For more info:Watch this video https://www.youtube.com/watch?v=EQC5MViYCtIRead : http://ajpcell.physiology.org/content/286/3/C465