Molecular pharmacology/ Drug receptors and pharmacodynamicmechanisms /drug metabolism / Autonomic pharmacology

Pharmacology

Study of the actions and effects of drugs on living systems and interaction of drugs with living systems

Pharmacokinetics

How does the drug concentration change as it moves through different compartments of the body "what the body does to the drug"
From the time the drug is administered to the time it is metabolized, distributed, or eliminated

Pharmacodynamics

How does the drug exert its effects on the body "what does the drug do to the body?"
-after the drug is transported to the site of action

Affinity definition

Binding to the molecular target

Efficacy definition

Response to the drug

What does pharmacodynamics study?

Drug actions at target (receptor) sites and the physiological/ chemical/ behavioral effects produced by these actions
"The drugs mechanism of action"

Pharmacodynamics provides the bases for

Rational therapeutic uses and the design of therapeutic agents

Most drugs act by interacting with.....

A specific target (protein)

3 groups of functional proteins

Enzymes
Ion channels
Transporters
Receptors

How do drugs affect enzymes ?

Drugs can either increase or decrease the rate of enzyme mediated reactions

5 major types of drug effects / actions

Stimulation
Depression/ inhibition
Irritation/inflammation
Replacement
Cytotoxic effect

An example of a stimulation effect

Adrenaline on the heart

examples of inhibition /depression

Quinidine on the heart
Barbiturates on CNS
Omeprazole on gastric acid secretions

Examples of irritation/ inflammation

Epithelial cells
Necrosis and morphological changes

Examples of replacement drugs

Levodopa in Parkinson's
Insulin in diabetes
Iron in anemia

Examples of cytotoxic effect

Anti-parasitic drugs
Antibiotic drugs
Anti-cancer drugs

Magnitude of drug response dependent on ....

Concentration achieved at the site of action/ receptor (dosage, extent of absorption, metabolism/ distribution to the site, rate/ extent of elimination)

Receptors are ...

Proteins inside or on the surface of cells that mediate drug activity

Receptors respond to....

Specific ligands (neurotransmitters, hormones, antigens, chemicals, or substances)

A ligand bind to a specific site (blank) and triggers a response (blank) in the cells

Receptor and signal

Ligands generally function as

Either an agonist or an antagonist

Agonist

Mimics the endogenous /physiologic ligand to produce similar response
Morphine for opioid receptors

Antagonist

Blocks the usual ligand and inhibits the physiological response
Naloxone for opioid receptors

Receptors have a

High specificity and selective affinity for the ligand (drug molecule)

Specificity

The measure of a receptors ability to respond to single ligand

Low specificity results in

Physiological responses not targeted or intended by the drug (side effects)

Affinity

The strength of attraction between the drug and its receptor

High affinity is associated with

A lower dose requirement

Occupancy theory

The effect of a drug results the occupancy of its receptor (sometimes)
1. One drug molecule combines reversible with one receptor
2. All receptors are identical and equally accessible to drug
3. Magnitude of response is proportional to the # of receptors occupied
4. The effective drug concentration does not change during the reaction

Graded dose-response curve (A)

Plot of response versus the drug concentration or dose
Efficacy (Emax) and potency (EC50 or ED50)

Sigmoid curve (B)

Graded dose-response curve on a logarithmic concentration axis
Efficacy (Emax) and potency (EC50 or ED50)

EC50

Dose or concentration at which effect is half the maximal effect

Emax

Maximal effect

The smaller the EC50/ED50

The greater the potency (smaller dose is needed to reach 1/2 max)

Kd

dissociation constant, concentration at which 50% of receptors are bound, measure of affinity of the drug to it binding site

The smaller the Kd

The greater the affinity of the drug for its receptor

Bmax

maximal number of receptors bound

Binding curve

Percent of receptors bound by drug (Y)
Drug concentration unite (X)

Dose response curve

Effect (Y)
Drug concentration units (X)

Efficacy

The greatest effect (Emax) an agonist can produce if the dose is taken to the highest tolerated level

Potency

The amount of drug needed to produce a specified effect
Measured based on EC50
Determined by the affinity of receptors for the drug and the number of receptors available

Therapeutic window

Safe range between the minimum therapeutic contraction and the minimum toxic concentration of the drug

Full agonist

Drug capable of fully activating the effector system when it binds to the receptors
Has high affinity for the activated receptor conformation and sufficiently high concentration result in all the receptors achieving the activated states

Partial agonist

Produces less than the full effect when it has saturated the receptors

Inverse agonist

Have a higher affinity for the inactive state than for the active state (decrease or abolish any constitutive activity, activity in the absence of ligand)

Antagonists bind to receptors but

Produce no effects! Bind to receptors, prevent agonists binding and block their actions

Neutral antagonist bind with

equal affinity to the Ri (inactivated) and Ra (activated) states, prevent binding by an agonist and prevent any deviation from the level of constitutive activity

When effect and occupancy aren't the same graph

Competitive antagonist

Drugs that bind to or very close to the agonist receptor site in a reversible way without activating the effector system for that receptor

Noncompetitive antagonist

Acts at an allosteric site on the receptor may bind reversibility or irreversibly

Neutral antagonist

Bind the receptor without shifting the ratio of activated receptors to inactivated receptors

Combining competitive antagonist with an agonists

The agonist and the antagonist have to compete for the same binding site

Combining a non-competitive antagonist with an agonist

Since the noncompetitive binds at a different spot from the agonist it is more effective at stoping activity, it doesn't have to compete

What do we not normally combine

Full agonist with partial agonist

Full agonist (single large dose) + partial agonist (increasing dose) graph

Spare receptors

Exist when maximal drug response (Emax) is reached at less than 100% occupation of receptors (Bmax), EC50<Kd

What does it mean for EC50 < Kd ?

To achieve 50% of maximal effect, less than 50% of the receptors need to activated

What is the significance of spare receptors ?

Full effect can be reached with <100% occupancy
Duration of effector activation > duration of drug-receptor interaction
# of receptors > # of effector molecules

Endogenous receptor mechanisms visual

Intracellular nuclear hormone receptor

Drug must be small/ lipophilic to get inside and reach receptor, the receptor has a ligand binding domain where the hormone will bind and it has a DNA binding domain (bind to promoter on DNA) and transcription-activating domain will include TF/TA and the binding will initiate gene expression

Ligand-regulated kinase receptors

They are kinases that are regulated by the binding of a ligand, on the cell membrane, when the ligand binds it activated dimerization of the receptor and the receptor will start activating each-other by phosphorylation, the activated kinase will now phosphorylate things down stream
*EGF= epidermal growth factor

Cytokine receptors

Monomers on cell membrane when the ligand binds it forms a dimer, it will activate a protein kinase (JAK), JAK will then activate substrate (STAT), phosphorylation STAT will then translocate to the nucleus and activate gene expression

Ligand-gated ion channels

The channel will open and close to allow ions in/out of the cell that is regulated by the binding of the ligand, transmembrane proteins made of multiple subunits, binding of the ligand cause conformational change to receptor to open the gate that allow ions to move based on gradient

G-protein coupled receptors (GPCRs)

Couple the presence of a ligand/agonist to activate ion os a specific G protein, and this will induce and initiate important cellular functions and responses, transmembrane that snake through the cellular membrane 7 times (N terminus outside and C inside), when the ligand binds it will trigger a conformational change that triggers the the G protein to be activated and activate downstream effectors

more than 70% of drugs on the market target ...

GPCRs

What the 2 states of G proteins ?

GDP bound and GTP bound

GDP

guanosine diphosphate

GTP

guanosine triphosphate

G protein subunits

alpha, beta, gamma

When the G protein is the activated state

The G alpha subunit is bound to GDP

Once a ligand binds to G protein receptor ...

G proteins are activated by exchanging GDP for GTP and the beta and gamma subunits dissociate then GaGTP and BY go on to activate targets (effectors)

G-protein activation cycle image

How is activation of GTP turned off?

GTP can be turned off through hydrolysis of GTP to form GDP

What activates effectors in the G-protein activation cycle ?

GalphaGTP and B/Y

Inactive G protein

bound to GDP

Active G protein

bound to GTP

4 major classes of G-alpha proteins that can be activated by GPCRs

Gs, Gi, Gq, and G12 (cancer)

B/y dissociates from which G-alpha protein?

Gi

Gs effector

adenylyl cyclase (+) in the cell membrane

Gi effector

adenylyl cyclase (-) in the cell membrane

Gq effector

Stimulates phospholipases (enzymes on cell membrane)

B/Y subunits effects

Stimulatory effect on ion channels (calcium) and phospholipase C and adenosine cyclase (only when they leave the Gi)

Effectors

Targets of G proteins
Enzymes or ion channels

Enzymes generate

Second-messengers, hydrophilic soluble products that can diffuse in the cytoplasm and bind to their targets

What 2 enzymes can be activated or inhibited by G proteins ?

AC and PLC

What second messenger can adenylyl cyclase produce?

cAMP (cyclic adenosine monophosphate)

What second messenger can phospholipases produce?

IP3 (inositol 1,4,5-triphosphate)
DAG (diacylglycerol)

G-protein effect on ion channels

Modulate gating by the G proteins (inhibited or stimulated)

Adenylyl cyclase

Produces cAMP
Stimulated by Gs
Inhibited by Gi

cAMP activates

cAMP-dependent protein kinase (PKA) holoenzyme

PKA substrates

Transcription factors (cAMP response element CRE)
Ion channels (AChR, GluR, GABAA, ionotropic receptors,a den calcium channels)

What deactivates adeneylyl cyclase ?

Phosphodiesterases (PDE), cleave a phosphate from cAMP

adneylyl cyclase regulation image

C=catalytic
R=regulatory

Phospholipase C

Membrane bound protein
Multiple isoforms = different regulatory domains (activated by Gq and GBY from Gi)
Produces DAG and IP3
Hydrolyzes PIP2
DAG in membrane which activates PKC
soluble IP3 releases Ca++ which further activates PKC

What activates PKC?

DAG and Ca++

PKC substrates

Transcription factors (MAP kinases)
Ion channels (AchR, GluR, GABAAR)

Ca++ effects (IP3)

Calcium-dependent kinases: smooth muscle contraction

What terminates the signals triggered by phospholipase C?

DAG is phosphorylated / deacylated
IP3 is dephosphorylated
Calcium is sequestration

Phospholipase C regulation image

Voltage gated ion channels

Q-type calcium channels (voltage-activated)
Couple action potential to NT release