PharmacologyA Selective Overview

Carl Rosow, M.D., Ph.D.

Disclosures

Consultant – Salix Pharmaceuticals

• Pharmacodynamics: The action of drugs.“What the drug does to the body”

• Pharmacokinetics: The absorption, distribution, metabolism, and excretion of drugs.“What the body does to the drug”

Topics for Today

• Pharmacodynamics– Receptors: “potency,” “efficacy”– Agonists, antagonists– The dose response curve

• Cholinergic Pharmacology– Nicotinic and muscarinic effects– Acetylcholinesterase– Cholinesterase inhibitors: effects and side

effects

Topics for Friday

• Pharmacokinetics– Volume of distribution– Clearance– Drug Metabolism

• Variability in Drug Response– Idiosyncratic variability– Pharmacogenetic variability

Case

• Sydney S. is a 62 year old male with a diagnosis of early-onset Alzheimer’s Dementia. One month ago, his dose of donepezil was increased to improve his increasing memory lapses. His wife says that he now seems a bit more alert and (possibly) less forgetful. Sydney doesn’t like it, because now the medicine gives him stomach cramps and diarrhea.

Sydney’s drug interacts with several types of CHOLINERGIC RECEPTORS

• Receptor: A macromolecular component of a cell that interacts with a drug and initiates an effect

• Drug-receptor interaction is characterized by1. Structural specificity/Stereoselectivity

2. Binding affinity which determines POTENCY

3. Ability to cause effect --EFFICACY (Intrinsic Activity)

4. Agonist or antagonist properties

1. Structural Specificity/Stereoselectivity

AcAc

CH=CH2

MORPHINE: An opioid analgesic

Change hydroxys to acetoxys → HEROIN: more potent analgesic

Change methyl to allyl → NALORPHINE: a hallucinogen and opioid antagonist

Change to dextro enantiomer → d-MORPHINE: cough suppressant, no analgesic effect [“Stereospecificity”]

2. Drug-Receptor Binding (a)• At the molecular level, drug binding is a

chemical interaction characterized by a DISSOCIATION CONSTANT KD.

• Strength of binding is AFFINITY (1/KD)

D + R RD, whereD x kon

koff

2. Drug-Receptor Binding (b)

Can be rearranged to

So, an increase in bound receptors [RD] can be

increase in [D] or

increase in [R]

KD is constant

Next slide

2. Drug-Receptor Binding (c)

D

D

D

tot KD

D

K

DR

K

DR

RDR

RD

R

RD

1

Assume total number of receptors Rtot = R + RD is constant, then the fraction of bound receptors is:

A Langmuir Isotherm

Ligand-Receptor Binding Curves

•Binding curve often covers 4 orders of magnitude•A semilogarithmic plot is useful•Drugs that bind to the same receptor have parallel curves

PotencyAssume Binding is Proportional to Response:

• Concentration-response curve looks like the binding curve.

• Why do we use the 50% effect?

3. EfficacyPotency and Efficacy are not the same!

More/less potent Full/partial agonist

Which of these (potency or efficacy) is more important to a clinician?

The Dose-Response Curve

• The Sine Qua Non of drug effect• Characterized by

– Emax – maximal effect (efficacy or intrinsic activity)

– EC50 or ED50 – dose for ½ maximal effect (potency)

– Slope or g (characteristic of particular ligand-receptor interaction – should be monotonic)

Which of These Drugs is the Most Potent?

“Affinity” vs. “Efficacy”

Ligand (D) binds to Receptor (R) causing a measurable effect.

D + R RD RD*Koff

Kon

F

Affinity = Potency Efficacy(Intrinsic Activity)

Agonists (Affinity vs. Efficacy)

The molecular de-activation equilibrium constant is a drug-specific factor, analogous to kD.

When f is a smaller number, it denotes more receptor activation (greater efficacy).

*][

][

RD

RD

k

k

activate

deactivate

Agonist Efficacy

10-4 10-3 10-2 10-1 100 1010.0

0.2

0.4

0.6

0.8

1.0

Frac

tion

Max

. Effe

ct

[D] x KD

= 0.01 = 0.1

= 1

= 10

= 3 Partial Agonists

Full Agonists

4. Antagonists

What does all of this have to do with …

• …Cholinergic pharmacology?• Sydney’s memory loss?• The drug used for his treatment? • The side effects of diarrhea and dizziness?

What are cholinergic neurons?• Nerves that use acetylcholine (ACh)

Where do we find them?1. Brain

2. Motor nerves in skeletal muscle

3. Autonomic ganglia

4. Parasympathetic post-ganglionic nerves

Cholinergic Neurons in the Brain

Nucleus Basalis of Meynert – ACh involved in memory, alertness

Pedunculopontine projection – ACh involved in sleep-wake cycle

Cholinergic receptors in the brain are “Nicotinic” and “Muscarinic”

ACh Action at Motor End-Plate

“Nicotinicm”Receptors

Synapse

1. ACh vesicles released into synaptic cleft2. ACh binds to receptors causes membrane depolarization3. Voltage gated Na channels on muscle propagate depolarization

wave and cause muscle contraction.4. ACh is hydrolyzed by AChE

Sympathetic Div.

Parasympathetic Div.

Ach Nicotinicn

Ach Nicotinicn

Ach Muscarinic

(a1, a2, b1, b2) DA, ACh

NE

Autonomic Nervous System

ANS End-Organ Effects

a b ACh(musc) ACh effect

Heart Contractility Ý ↓ --

Heart Rate Ý ß Slow HRBlood Vessel Tone Ý ß ↓ (NO) --

Salivary Glands ß Ý SecretionsGI Tone ß ß Ý PeristalsisGI Sphincter Ý ß Relax sphinct.Detrusor Contraction ß Ý Empty bladderUrine Sphincter Ý ß Relax sphinct.Pupil Radial Muscle Ý --

Pupil Circ. Muscle Ý Small pupilCiliary Muscle ß Ý Accommodate

Atropine!

Diphenhydramine (Benadryl) is an antihistamine with powerful muscarinic antagonist properties. These are likely to produce which of the following side effects?

1. Constipation

2. Sedation

3. Urinary retention

4. Dry mouth

5. Tachycardia

Nicotinic Cholinergic Receptor

Acetylcholine Action at Nicotinic Receptors1. 2 ACh bind and activate the receptor by inducing a

conformational change

2. When resting receptors (A2R) become active (A2R*), they allow inward Na flow which depolarizes the membrane

3. Active receptors then become inactive. **They must recycle to resting in order to work again.**

This happens when AChE hydrolyzes acetylcholine

Inactive A2R

Acetylcholinesterase (AChE)

COO -

COO -

COO -

(3)

Rate: 600,000/min!!

Problem

Succinylcholine is a compound that binds to the nicotinic receptor and activates it (i.e., it is an agonist). However, it is not a good substrate for AChE.Based upon this information, what would you predict its effect would be?

Acetylcholine Succinylcholine

The Muscarinic Receptor is a G-Protein Coupled Receptor

G-Protein Coupled Receptor

ACh inhibits Adenylyl Cyclase and Activates Phospholipase C

Muscarinic Receptors

Receptor Site Mechanism

M1 CNS, ganglia, glands

↑ PLC(Gq)

M2 Heart, periph nn., sm. Musc., CNS

↓ cAMP (Gk, Gi)

M3 CNS, Sm Musc, glands

↑ PLC(Gq)

M4 CNS ↓ cAMP (Gk, Gi)

M5 CNS ↑ PLC(Gq)

Muscarinic Receptors

Tissue Ionic Conductance Response

Ganglion (M1) ↑ gNa+, ↑gCa++ Depolarize

Heart (M2) ↑ gK+ Hyperpolarize

Sm Musc (M3) ↑ gNa+, ↑gCa++ Depolarize

Let’s get back to poor Sydney.… Alzheimer’s Disease = Loss of Cholinergic neurons

Degeneration of Nucleus Basalis of Meynert

Loss of neurons in pedunculopontine projection

How can we treat this deficiency?

Possible Treatments• Acetylcholine?

Cannot be administered exogenously. Why?• Other Agonists?

Nicotine and succinylcholine are “agonists,” but they cause twitch, then flaccid paralysis. Why? – Not hydrolyzed by Acetylcholinesterase– Nicotinic receptors cannot return to resting state– Membrane stays depolarized, but no Na+ current

This is called DEPOLARIZING BLOCK

How can we produce a cholinergic AGONIST effect?

Neostigmine (+)

Donepezil (+)

ChE inhibition: Neostigmine

COO -

COO -

COO -

(3)

• Form carbamylated enzyme intermediate resistant to hydrolysis

• But - does not enter CNS Neostigmine

Cholinergic Therapy

• Alzheimer’s Dementia – orally active AChE inhibitors – work by increasing

[Ach] in the synapse– must cross BBB – Symptomatic treatment: donepezil, rivastigmine, or

galantamine

What side effects do you expect from these AChEI?

Case

• Sydney S. is a 62 year old male with a diagnosis of early-onset Alzheimer’s Dementia. One month ago, his dose of donepezil was increased to improve his increasing memory lapses. His wife says that he now seems a bit more alert and (possibly) less forgetful. Sydney doesn’t like it, because now the medicine gives him stomach cramps and diarrhea.

ANS Effects

a b ACh(musc) ACh effect

Heart Contractility Ý ↓ --

Heart Rate Ý ß Slow HRBlood Vessel Tone Ý ß ↓ (NO) --

Salivary Glands ß Ý SecretionsGI Tone ß ß Ý PeristalsisGI Sphincter Ý ß Relax sphinct.Detrusor Contraction ß Ý Empty bladderUrine Sphincter Ý ß Relax sphinct.Pupil Radial Muscle Ý --

Pupil Circ. Muscle Ý Small pupilCiliary Muscle ß Ý Accommodate

Other Acetylcholinesterase Inhibitors

}

InsecticidesNerve Gases, (Irreversible

AChE block)

Relaxant Reversal, Myasthenia, Alzheimer’s(Temporary AChE block)

}

Organophosphates

• Powerful, irreversible AChE antagonists• “Nerve Gases”: Soman, VX

– Absorbed through skin, eyes, lungs– Synapse flooded with Ach – receptors can’t reset– ↓Nicotinic effect --- Lethal paralysis in seconds– ↑↑Muscarinic effects --- Bradycardia, secretions

• Treatment– Reactivate AChE to ↑Nicotinic receptors –

PRALIDOXIME (2-PAM)– Block Muscarinic receptors -- ATROPINE

Muscarinic ACh receptors are

1. Voltage gated ion channels

2. Ligand gated ion channels

3. G-protein coupled receptors

4. Intracellular receptors

5. Receptor tyrosine kinases

Donepezil changes the amount of ACh in cholinergic synapses, because it

1. Increases ACh release

2. Increases ACh storage

3. Increases ACh synthesis

4. Decreases ACh reuptake

5. Decreases ACh breakdown

Which of the following are muscarinic cholinergic effects?

1. Bladder contraction

2. Tachycardia

3. Pupil dilation

4. Constipation

5. Salivation

What did Dr. Stanley Goodspeed (Nicholas Cage) inject when he was exposed to VX?

1. Neostigmine

2. Pralidoxime

3. Curare

4. Atropine

5. Prozac

Test Question

• Why do all of the cholinergic “agonists” (except acetylcholine) produce weakness and paralysis?