MOLECULAR TARGETS FOR DRUG ACTION (and other topics – a review before the final test) Prof. M....

MOLECULAR TARGETS FOR DRUG ACTION

(and other topics – a review before the final test)

Prof. M. Kršiak

Department of Pharmacology, Third Faculty of Medicine, Charles University in Prague

Charles University in Prague, Third Faculty of Medicine

Cycle II, Subject: General Pharmacology2013-2014

http://vyuka.lf3.cuni.cz/

FOUR MAJOR TARGETS FOR DRUGS:

1. RECEPTORS

2. ION CHANNELS

3. CARRIER MOLECULES

4. ENZYMES

Molecular Targets For Drug Action

Cellular RECEPTORS

Cell Membrane

Intracellular - Receptors linked to gene transcription (nuclear receptors)

Channel-linked receptors

G-protein-coupled receptors

Proteinkinase-linked receptors

1. RECEPTORS

Dopamine receptors D1-5 (type D1,5, type D2,3,4 )

They differ in localization (occur mostly in the CNS, post- or pre-synaptically), they differ in mechanisms of transduction (some are coupled with Gs, some with Gi, some act via adenylyl cyclase, some via phospholipase C, or via ion channels – K, Ca)

Synthesis of dopamine: tyrosine → L-DOPA →dopamine → noradrenalin →adrenaline

Decarboxylase: L-DOPA→dopamine

Elimination of dopamine:extracellulary(in the synaptic cleft):

transport protein (reuptakes DA from synapt.cleft to the presynaptic nerve ending)

COMT catechol-O-methyl transferaseintracellulary: MAO monoamino oxidase

DOPAMINERGIC SYSTEMClinical potency of antipsychotics correlates with their

affinity for D2 receptors

Decarboxylase inhibitors in combination with levodopa →

antiparkinsonics

COMT inhibitors→ antiparkinsonics

Inhibitors of MAO (IMAO) → antidepressants

Inhibitors of DA, NA, 5-HT reuptake → antidepressants

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Ac, nucleus accumbens; Am, amygdaloid nucleus; C, cerebellum; Hip, hippocampus; Hyp, hypothalamus; P, pituitary gland;SN, substantia nigra; Sep, septum; Str, corpus striatum; VTA, ventral tegmental area; Reward

system

Chemoreceptor trigger zone

MAJOR DOPAMINERGIC PATHWAYS/SYSTEMS IN CNS

PHARMACOLOGY OF MAJOR DOPAMINERGIC SYSTEMS IN CNS

System Clinically most important drugs/ effects* Note

Mesocortical, mesolimbic

↓antipsychotics→antipsychotic effect ↑ e.g.. levodopa→ psychosis

Nigrostriatal ↓ antipsychotics → extrapyramidal adverse effects

↑antiparkinsonics (dopaminergic)

Tuberohypophyseal ↓ antipsychotics →hyperprolactinemia ↑ e.g.bromocriptine→therapy of hyperprolactinemia

Reward system(nc. accumbens)

↑addictive drugs e.g. metamphetamine, morphine, nicotine, etc.

Vomiting centre Chemoreceptor trigger zone in medulla, area postrema

↓ antiemetics → inhibition of nausea, vomiting - metoclopramide, domperidon

↑ e.g. apomorphine→ vomiting

↓ inhibition, ↑ stimulation

* Additional neuromediator systems may participate in these effects (e.g. serotonergic, glutamatergic systems in antipsychotic effects, cholinergic system in antiparkinsonic , antiemetic effects, etc.)

Antipsychotics D1 D2 alfa1 H1 mAch 5-HT2A Notes

1st generation

chlorpromazine ++ +++ +++ ++ ++ + + EPS, increased prolactin, hypotension, antimuscarinic effects

haloperidol + + ++ ++ - ± + As chlorpromazine but fewer antimuscarinic effects

2nd generation

(atypical)

clozapine ++ ++ ++ ++ ++ +++ Risk of agranulocytosis! Regular blood counts required. Weight gain. No EPS

olanzapine ++ ++ ++ ++ ++ +++ Weight gain. Without risk of agranulocytosis, No EPS

risperidone - ++ ++ ++ ++ +++ Weight gain. Significant risk of EPS

sulpiride - +++ - - - - Increased prolactin (gynaecomastia)

quetiapine - + +++ - + + Weight gain. No EPS

aripiprazole - +++ PA

+ + - ++ Fewer side effects [“Third generation?“- dopamine stabilizers]

EPS=extrapyramidal side effects, PA = partial agonist

Figure 45.1 Correlation between the clinical potency and affinity for dopamine D2 receptors among antipsychotic drugs. Clinical potency is expressed as the daily dose used in treating schizophrenia, and binding activity is expressed as the concentration needed to produce 50% inhibition of haloperidol binding. (From Seeman P et al. 1976 Nature 361: 717.)

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Correlation between the clinical potency and affinity for dopamine D2 receptors among antipsychotic drugs.

DRUG TREATMENT OF PARKINSON‘S DISEASE

Normal extrapyramidal system:Nigrostriatal dopaminergic neurons inhibit cholinergic neurones in striatum

Parkinson‘s disease:Death of nigrostriatal dopaminergic neurons → disinhibition of cholinergic neurons

The aim of pharmacotherapy is, therefore, to enhance the dopaminergic transmission and to reduce the cholinergic transmision

Dopaminergic antiparkinsonics:Levodopa (+ inhibitors of dekarboxylase in the periphery:carbidopa, benserazid)

IMAO (selegiline)

Agonists of dopamine (ropinirol, pramipexol)

Other: amantadine, inhibitors of COMTAnticholinergic antiparkinsonics: biperiden

ANTIPARKINSONICS

ANTIDOPAMINERGIC ANTIEMETICS:

metoclopramide, domperidone

Also gastroprokinetic effectcommon adverse reactions: extrapyramidal - akathisia, dystonia

Serotonin receptors 14 subtypes (!) in 7 classes (5-HT1-7)Almost all are metabotropic:They differ in localization (occur mostly in the CNS, post- or pre-synaptically), but also in the periphery. They differ in mechanisms of transduction (are coupled with various G proteins, some act via adenylyl cyclase, some via phospholipase C, or via ion channels –Ca)

Only 5-HT3 receptors are ionotropicSynthesis of serotonin/5-hydroxytryptamine(5-HT): tryptofan → 5-hydroxytryptofan →5-hydroxytryptamine

Elimination of serotonin:extracellular (in synaptic cleft):

transport protein (reuptakes 5-HT back in the nerve terminal)

intracelular: MAO monoamino oxidaseInhibitors of MAO (IMAO) → antidepressants

Reuptake inhibitors of 5-HT → SSRI and some other antidepressants

SEROTO(NI)NERGIC SYSTEM

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MAJOR SEROTONERGIC PATHWAYS/SYSTEMS IN CNS:

FUNCTION OF SEROTONERGIC SYSTEM

IN THE BRAIN: regulation of emotion (e.g. depression, anxiety), sleep, body temperature, eating, sexual functions, pain, perception (halucinations), nausea-vomiting

IN THE PERIPHERY: ↑ peristalsis in the GIT, vasoconstriction, ↑↓ BP, ↑platelet agregation

CLINICALLY IMPORTANT DRUGS ACTING VIA SEROTONERGIC SYSTEM:

TRIPTANS (5-HT1D agonists)- e.g. sumatriptan – ANTIMIGRAINE DRUGS

SSRI (selective serotonin reuptake inhibitors) e.g. fluoxetin, citalopram, sertralin,

effective as ANTIDEPRESSANTS and in ANXIETY DISORDERSSome other antidepressant can also inhibit reuptake of seotoninIMAO (inhibitors of MAO) – ANTIDEPRESSANTS e.g.. moclobemide

„SETRONS“ (5-HT3 antagonists)- e.g. ondansetron – ANTIEMETICS

SDA (serotonin dopamine antagonists)atypic antipsychotics e.g. risperidone

Histamine receptors, H1,H2, H3, (H4) All are metabotropic

They occur in the brain and in the periphery

Synthesis, elimination of histamine – not utilized in applied pharmacology

HISTAMINERGIC SYSTEM

Drugs producing release of histamine – morphine, atracurium

IN THE BRAIN:H1 –↑ vigility, H3 – presynaptic ↓ release of neuromediators

H1 antagonists 1. generation → sedation, drowseness, e.g. promethazine, antiemetics – dimenhydrinate in motion sickness

IN THE PERIPHERY:H1 – mast cells, vasodilatation, ↑ capilar permeability, alergic reactions (itching, urticaria, allergic rhinitis), bronchoconstriction

H2 – parietal cell in stomach mucose (↑ sekretion HCl)

H1 antagonists – drugs for allergic rhinitis, urticaria - H1 antagonists 2. generation (nonsedating) - cetirizin

H2 antagonists – drugs for peptic ulcer disease – ranitidine, famotidine

H3 antagonist betahistine→ vasodilatation in the inner ear – antivertigo drug ( Méniere‘s disease)

CLINICALLY IMPORTANT DRUGS ACTING VIA HISTAMINERGIC SYSTEM:


1. RECEPTORS

2. ION CHANNELS


4. ENZYMES


ION CHANNELS

VOLTAGE-DEPENDENT CHANNELS

LIGAND-GATED CHANNELS

Extracellular ligands

Calcium channels

Sodium channels

GABA-gated Cl- channels Nicotinic receptor

NMDA receptor

Intracellular ligands

ATP-sensitive potassium channels

VOLTAGE-DEPENDENT CHANNELS

Calcium channels - Ca++ flows into cells, necessary for contraction of cardiac and smooth muscles, blocked by CALCIUM CHANNEL BLOCKERS : amlodipine, verapamil –used in hypertension, angina pectoris, dysrytmias

Sodium channels - Na+ flows into cells, necessary for propagation of action potentials in excitable cells, blocked by LOCAL ANAESTHETICS : procaine, lidocaine, articaine, bupivacaine, some Antiepileptics: phenytoin, some Antidysrhytmics : lidocaine



GABA-gated Cl- channels –Benzodiazepines as modulators (ANXIOLYTICS) –, diazepam, alprazolam, midazolam

Cl-

Cl-

GABA-gated Cl- channels

GABAA receptor

Benzodiazep. receptor



Nicotinic receptor

NEUROMUSCULAR-BLOCKING DRUGS • Non-depolarising blocking agents, e.g. atracurium

act as competitive antagonists at the nicotinic receptors of the motor endplate

act by activating nicotinic receptors and thus causing persistent depolarisation of the motor endplate

• Depolarising blocking agents - suxamethonium

to Ca2+, as well as to other cations, so activation of NMDA receptors is particularly effective in promoting Ca2+ entry.

LIGAND-GATED CHANNELSExtracellular ligands

NMDA (N-methyl-D-aspartate) receptor glutamate receptor

Activation of NMDA receptors results in the opening of an ion channel

It requires co-activation by two ligands: glutamate and either d-serine or glycine

NMDA receptor antagonist – ketamine (General anaesthetic – intravenous)

produces 'dissociative' anaesthesia, in which the patient may remain conscious although amnesic and insensitive to pain . Sometimes psychotomimetic effects


Intracellular ligands

ATP-sensitive potassium channels (KATP channels)

In the presence of increased levels of ATP, or by action of sulfonylureas

(Antidiabetics) e.g. glimepiride

the KATP channels close, causing the membrane potential of the cell to depolarize, thus promoting insulin release

The KATP channels in pancreatic beta cells when open,

allow potassium ions to flow out the cell.

K+

K+

ATP

See also Fig. 30.3 Golan et al. 2012, p. 528


1. RECEPTORS

2. ION CHANNELS


4. ENZYMES



• „pumps“

sodium pump - Na+/K+ ATPase,

„pumps“ Na+ from the cell, inhibited by cardiac glycosides

proton pump - H+/K+ ATPase,

„pumps“ H+ from the cell , proton pump inhibitors

• transporters transporters for noradrenaline, serotonine inhibited by most antidepressants (RUI, TCA, SSRI etc)

TRANSPORTERS

„Pumps“

Transport proteinstransporters for noradrenaline (NA),

serotonin(5-HT), dopamine (DI)

P-glycoprotein (P-gp)

sodium pump

proton pump

sodium pump - Na+/K+ ATPase,

„pumps“ Na+ from the cell. This is inhibited by

cardiac glycosides - digoxin – which lowers

extrusion of Ca++ from cardiac muscle -> the intracellular concentration of Ca++ is increased -> force of cardiac muscle contraction is increased

proton pump - H+/K+ ATPase,

„pumps“ H+ from the cell in the stomach mucosa – increased production of HCl,

inhibited by,Proton pump inhibitors omeprazol used in peptic ulcer

„Pumps“

ION CHANNELS

VOLTAGE-GATED CHANNELS



Calcium channels

Sodium channels

GABA-gated Cl- channels

Nicotinic receptor

NMDA receptor

Intracellular ligands ATP-sensitive potassium channels

CALCIUM CHANNEL BLOCKERS

LOCAL ANAESTHETICS

Summary :

ANXIOLYTICS - Benzodiazepines

NEUROMUSCULAR-BLOCKING DRUGS

INTRAVENOUS ANAESTHETIC - ketamine

ANTIDIABETICS -sulfonylureas

Transporters for noradrenaline, serotonine, dopamine

inhibited by most Antidepressants – Reuptake inhibitors (RUI), TCA, SSRI etc)

Transport proteins

NERVE ENDING (presynaptic)

SYNAPTIC CLEFT

POSTSYNAPTIC NEURON

↓ ELIMINATION by MAO

moklobemid

↓ REUPTAKE imipramin

Almost all antidepressants increase supply of

monoamine transmitters at postsynaptic receptors

P-glycoproteinIt is an efflux pump capable of transporting a wide range of compounds from the intracellular space into the extracellular matrix.

Intestinal P-glycoprotein reduces effective drug absorption by actively transporting drugs back into the intestinal lumen. P-glycoprotein in the liver and kidneys promotes excretion of drugs from the blood stream into the bile and urine, respectively. In addition, P-glycoprotein is present at the blood–brain barrier, where it reduces drug access to the CNS.

P-glycoprotein can be induced and inhibited by other drugs

Transport proteins

Inhibition of P-glycoprotein [and CYP3A4]

Grapefruit juice inhibits P-glycoprotein [and CYP3A4]

GRAPEFRUIT-DRUG INTERACTIONS

The P-gp and CYP3A4 are located in the enterocytes (intestinal absorptive cells) → first-pass effect

Grapefruit juice by inhibition of P-glycoprotein [and CYP3A4] can markedly increase the bioavailability and toxicity of some drugs, particularly (most hazardous) in:

amiodarone (arrythmias)simvastatin, lovastatin (rhabdomyolysis)

TRANSPORTERS

„Pumps“

Transport proteinstransporters for noradrenaline (NA), serotonin(5-HT), dopamine (DI)

P-glycoprotein (P-gp)

sodium pump

proton pump

CARDIAC GLYCOSIDES -digoxin

PROTON PUMP INHIBITORS - omeprazol

ANTIDEPRESSANTS- Reuptake Inhibitors

GRAPEFRUIT-DRUG INTERACTIONS

Summary :


1. RECEPTORS

2. ION CHANNELS


4. ENZYMES


Other drug-enzymes interactions

Enzyme inhibition by drugs

Enzymes Inhibitors Therapeutic groups, indications

Cyclo-oxygenase aspirin, ibuprofen, diclofenacAntiinflammatory and antirheumatic

agents, analgesics Monoamine oxidase moclobemide Antidepressants

Acetylcholinesterase neostigmine, rivastigmin Parasympathomimetics, Anti-dementia-

drugsAngiotensin-converting

enzymeenalapril, ramipril Antihypertensives

HMG-CoA reductase simvastatin, atorvastatinLipid modifying agents; (hypercholesterolaemia)

Xanthinoxidase allopurinol Drugs inhibiting uric acid production

Phosphodiesterase type V

sildenafil Drugs used in erectile dysfunction

Dihydrofolate reductase trimethoprim

Antimicrobial agents

methotrexate Antimetabolites, folic acid analogues

Neuroamidase oseltamivir Antivirals ( influenza virus)

Thymidine kinase aciclovir Antivirals (Herpes virus)

HIV protease saquinavir Antivirals (HIV), protease inhibitors

Many drugs are targeted on enzymes and mostly act by inhibiting them:

Drugs can inhibit enzymes reversibly (usually a competitive inhibition by non-covalent binding) or irreversibly (enzyme is usually changed chemically by covalent binding)

An enzyme inhibitor is a molecule which binds to enzymes and decreases their activity

Irreversible inhibitors usually react with the enzyme and change it chemically (e.g. via covalent bond formation). These inhibitors modify key amino acid residues needed for enzymatic activity (e.g. aspirin, acting on cyclo-oxygenase)

Competitive inhibition is a form of enzyme inhibition where binding of the inhibitor to the active site on the enzyme prevents binding of the substrate and vice versa. Often, the drug molecule is a substrate analogue (e.g. captopril, acting on angiotensin-converting enzyme)

The active site of angiotensin-converting enzyme. [A] Binding of angiotensin I. [B] Binding of the inhibitor captopril, which is an analogue of the terminal dipeptide of angiotensin I.

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Reversible competitive inhibition of enzyme (inhibition of ACE by captopril)::

Irreversible non-competitive inhibition of enzyme (inhibition of COX-1 or COX-2 by aspirin):

This makes aspirin different from other NSAIDs (such as diclofenac and ibuprofen, which are reversible inhibitors).

Aspirin acetylates serine residue in the active site of the

COX enzyme

Irreversible inhibition of enzyme:

Recovery is possible only by synthesis of a new enzyme

Those of importance in the metabolism of psychotropic drugs are

CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4,

the last being responsible for the metabolism of more than 90% of psychotropic drugs that undergo hepatic biotransformation.

Cytochrome P450 (CYP) enzymes

Many psychotropic drugs have a high affinity for one particular CYP enzyme but most are oxidised by more than one

Drug - cytochrome P450 interactions

The most important enzymes involved in drug interactions are members of the cytochrome P450 (CYP) system that are responsible for many of the phase 1 biotransformations of drugs. These metabolic transformations, such as oxidation, reduction and hydrolysis, produce a molecule that is suitable for conjugation.

Genetic polymorphism

The CYP enzymes that demonstrate pharmacogenetic polymorphism include CYP2C9, CYP2C19 and CYP2D6.

In clinical practice, the polymorphism produces distinct phenotypes, described as poor metabolisers, extensive metabolisers (the most common type) and ultra-rapid metabolisers.

Genetic effects:

CYP enzymes can be induced or inhibited by drugs or other biological substances, with a consequent change in their ability to metabolise drugs that are normally substrates for those enzymes.

Drug effects:

Enzymatic inductionenzymatic induction can cause a decrease as well as an increase in the drug’s effect

The onset and offset of enzyme induction take place gradually, usually over 7–10 days

The most important are inducers of CYP3A4 and include carbamazepine, phenobarbital, phenytoin, rifampicin and St John’s wort (Hypericum perforatum). An example of an interaction in psychiatric practice is the reduced efficacy of haloperidol (or alprazolam) when carbamazepine is started, resulting from induction of CYP3A4.

Enzymatic inhibitionenzymatic inhibition can cause an increase as well as a decrease in the drug’s effect

Most hazardous drug interactions involve inhibition of enzyme systems,

which increases plasma concentrations of the drugs involved, in turn leading to an increased risk of toxic effects.

Inhibition of CYP enzymes is the most common mechanism that produces serious and potentially life-threatening drug interactions

Inhibition is usually due to a competitive action at the enzyme’s binding site. Therefore, in contrast to enzyme induction, the onset and offset of inhibition are dependent on the plasma level of the inhibiting drug

4. ENZYMESsites of action of about 30% of drugs

Drugs inhibiting the enzyme:

Cholinesterase Cholinesterase Inhibitors

Cyclo-oxygenase Non-Steroid Antiinflammatory Drugs

Monoamine oxidase IMAO

Angiotensin- converting enzyme

ACE Inhibitors

HMG-CoA reduktase Statins

and other - e.g. recently phosphodiesterase

neuroamidase

sildenafil (VIAGRA) oseltamivir (TAMIFLU)

degradating cGMP

stops the virus from chemically cutting ties with its host cell

G-protein coupled receptorsmembr.

Voltage gated- Calcium chan.

- Sodium chan.

„pumps“- sodium

- proton

transporters

cardiac glykosides

PP inhibitors

lok. anaesthetetics

Calcium ch. blockers

about 45% of drugs,e.g. beta-blockers

antidepressants

ACE inhibitors, IMAO

perif. muscle relaxants

Examples of drugs::

Proteinkinase-linked receptors

c.intracelul.

Ligand-gated, G-prot.,…

ACE, MAO, COX, HMG-CoA reductase

Channel-linked receptors

1. RECEPTORS

2. ION CHANNELS


Molecular mechanisms of drug effects - summaryFOUR MAJOR TARGETS FOR DRUGS:

4. ENZYMES

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