Medsci 204 foundations block

Drug Targets

Receptors

Ion channels

Transporters

Enzymes

• some drugs act on DNA (anti cancer and a tip optic drugs)

4 steps of neuro transmission

Neurotransmitter synthesis

Neurotransmitter release

Action on Receptors

Inactivation

Two sites of action in choline synthesis

Choline transporter-transports choline into nerve terminal

ChAT- enzyme involved in synthesis of Ach

endogenous

natural- neurotransmitter/hormone

exogenous

synthetic- Drug/chemical

4 different receptor families

Ligand gated ion channels

G protein coupled receptors

Tyrosine kinase receptors

Nuclear/steroid hormone receptors

Ligand gated ion channels

Multi proteins subunit that conducts ions through the otherwise impermeable membrane in a highly selective manner when bound by a specific ligand

Examples of ligand gated ion channels

Nicotinic Acetylcholine receptors

Nicotinic Acetylcholine receptors

Ach binds to both sides of channel at the interfaces formed by the peptide loops between one of the 2 alpha subunits and its neighbour

Opening and closing of Nicotinic acetylcholine receptors

transmembrane helices are sharply linked inwards forming a construction- when Ach binds confirmational change occurs opening the channel

Nicotinic acetylcholine receptor as a drug target

Nicotine, pancuronium used as a muscle relaxant during anaesthesia

G protein coupled receptors

Monomer if proteins with 7 trans membrane domains coupled to g proteins

VEGFR

Essential for angiogenesis

Activation regulates- Endothelial cell survival, Endothelial cell proliferation, Endothelial cell migration, NO and prostaglandin release and increase vascular permeability

Drug specificity and selectivity

Drug specificity- acts only at the desired drug target

Drug selectivity- acting preferentially at a drug target (even the best drugs tend to only act selectivly)

Receptor Tyrosine kinase

Extracellular component that ligand binds to and intracellular part that functions as a kinase

Kinases

Enzymes that transfer phosphate groups from ATP to a substrate

Function of RTK

mediate the actions of growth factors, cytokines and certain hormones (eg.insulin)

what is the function of the kinases in RTK

the phosphate groups are transferred to tyrosine amino acid residues on intracellular target proteins

VEGFR

Essential for angiogenesis

Activation regulates- Endothelial cell survival, Endothelial cell proliferation, Endothelial cell migration, NO and prostaglandin release and increase vascular permeability

activation of VEGFR2

Ach binds to both sides of channel at the interfaces formed by the peptide loops between one of the 2 alpha subunits and its neighbour

signal transduction pathway that drives proliferation

receptor activation leads to activation of PLCy which leads to PIP2 hydrolysing to DAG and IP3- DAG activated PKC which leads to activation of ERK which leads to increased gene transcription

Nuclear/steroid hormone receptors

Located intracellularly in the cytosol and nucleus

Functions to stimulate transcription of target genes

how do drugs bind to receptors

Van der Waals

Hydrogen binding

Ionic interactions

Covalent binding

Kd definition

The concentration of drug which occupies 50% of the existing receptor population

Bmax

Saturation- maximum occupancy of the receptors

higher affinity

lower the concentration at which it produces a given level of receptor occupancy

Agonist

Receptor is activated

Inverse agonist

Basal activity is decreased

Antagonist

Receptor is inhibited

EC50

effective concentration of a drug when you get 50% of max response

Emax

Maximum response that can occur with that agonist

Efficacy

the ability of a drug to bind to a receptor and cause a change in the receptors action

Potency

amount of drug required to produce a defined effect- measured by EC50

Opening and closing of Nicotinic acetylcholine receptors

transmembrane helices are sharply linked inwards forming a construction- when Ach binds confirmational change occurs opening the channel

Nicotinic acetylcholine receptor as a drug target

Nicotine, pancuronium used as a muscle relaxant during anaesthesia

G protein coupled receptors

Monomer if proteins with 7 trans membrane domains coupled to g proteins

Reversible competitive antagonism

binds to a receptor in a reversible manner to compete with agonist binding

Irreversible antagonism

bind covalently to the receptor- reduces number of receptors available for the agonist- only way to overcome is to insert more receptors into the membrane

Allosteric modulators

Bind to allosteric site to modulate the binding of endogenous ligand and/or signalling properties at orthosteric site

Allosteric binding site

non-overlapping and spatially distinct site that is conformationaly linked to the orthosteric binding site

Positive allosteric modulators

Increasing ligand association rate and/or decreasing ligand dissociation rate

Allosteric modulation ceiling effect

Allosteric modulators can influence the orthosteric binding site to a maximum after which altering the does will not create a greater effect

Advantages of Allosteric modulators

Ceiling effect

Increased selectivity

Maintenance of spatial and temporal signalling by endogenous ligand

Drugs that act on enzymes and transporters mainly inhibit

True

Two key mechanisms of enzymatic degradation

Enzymatic degradation

Transport back into presynaptic terminal

Inhibitors of acetylcholine esterase

Irreversible- eg.nerve gas

forms stable phosphorous bonds with AChE leading to a build up of ACh at synapses

Reversible-

binds breifly to increase duration of ACh at synapse

Serotonin transporter

involved in sleep, appetite, memory, sexual behavior, neuroendocrine function and mood

Synthesised from tryptofan and packed into vesicles which are released into synapses following an action potential

Reputable determines the extent and duration of receptor activation

Two things needed for carrying out the assay

Means of detecting ligand

Sample containing target of interest

Basic procedure of an assay

Incubate labelled compound with tissue/cell

Seperate bouncy ligand from free

Measure signal

Saturation binding

Gives information about labelled ligand

Non-specific binding

most compounds bind to sites in sample other than receptor of interest- eg. binding to lipid components or free ligand not completely washed away

How to determine receptor binding when non-specific binding occurs

Incubate labelled logan with high concentration of an equivalent non-labelled ligand that binds to receptor of interest and have no effect on non-specific binding

Total binding- non-specific binding= specific binding

Ki

dissociation constant of the inhibitor of labelled ligand

Unlabelled ligand affinity

adme

absorption

distribution

metabolism

excretion

Major organs involved in ADME

Gastrointestinal tract

Liver

Kidney

Lungs

Physiochemical properties that influence ADME

Solubility

Lipophilicity

Ionisation

Mechanisms of membrane transport

Passive diffusion

Facilitated diffusion

Active transport

Endocytosis

Filtration

Passive diffusion

Most common mechanism for drug absorption

Driven by concentration gradient

Non-selective

Which molecules travel by passive diffusion

lipophilic, uncharged, and small molecules pass easily

eg. aspirin

When is competitive binding used

When interested in an unlabelled compound

Competition binding procedure

labelled applied at approximate kd concentration

unlabelled ligand applied at range of concentration in presence of labelled ligand

Facilitated diffusion

Passive diffusion of drugs through transmembrane proteins

Requires recognition by carrier or channel protein

Rate is faster and can saturate

which molecules travel by facilitated diffusion

Sugars and amino acids eg. glucose through Glucose transporter

active transport

uses cellular energy to transport drugs across the membrane

requires recognition by membrane transporters

active transport allows

accumulate compounds essential for growth

remove waste products

be protected against toxins

where are active drug transporters highly expressed

liver, kidney, Blood brain barrier, gut epithelium

ATP binding cassette

present in GI tract and kidney

act to efflux from cell

wide range of substrates

Endocytosis

Drug is taken up by vesicles

energu dependant

types of drugs that travel through endocytosis

mainly for drugs with mw>1000Da

eg. hormones cytokines growth factors antibodies such as monoclonal antibodies immunoglobines

2 types of endocytosis

Pinocytosis

Receptor mediated endocytosis

most drugs pass through cells to cross biological barriers except

Blood capillaries- contain remedy rations that allow rapid interchange between blood and interstitial fluid

Glomerular capillaries- extremely porous allowing passage of all plasma constituents excoet macromolecules

some cells in the liver also contain fenestrations

enteral and paraental

enteral- gastric eg. oral, sublingual, buffaloes, rectal

paraental- around - avoid first pass metabolism eg. intravenous, subcutaneous, intramuscular, transdermal, respiratory tract, topical

define drug absorption

transfer of drug from administration site to the systemic circulation

Rate definition

how rapidly the drug gets from the site of administration to the systemic circulation

Extent definition

how much of the administered dose enters the systemic circulation

with an oral dose what happens to the drug

• some metabolised in gut wall

• some delivered to liver via portal vein

• some removed by hepatic first pass metabolism

• some pumped back by efflux transporters to be expelled by gut

• some arrives in systemic circulation

advantages of IV

very rapid

precise control

can be administered as a bolus infusion or both

no absorption involved

good for drugs with high irritation

disadvantages of IV

requires hospitalisation

careful preparation of injected material requires

most hazardous- no recall

oral advantages

safest

most convenient

economic

oral disadvantages

slow

unpredictable in regard to

• rate

• extent

• reproducibility

absorption sites in GI tract

oral mucosa- limited absorption

stomach - absorption site for weak acids and neutral drugs

small intestine - major site of drug absorption

large intestine - little absorption

drug characteristics effecting absorption of oral drugs

dosage form

dissolution rate

water and lipid solubility

ionisation

chemical stability

liability for metabolism

patient characteristics effecting absorption of oral drugs

gastric emptying rate

intestinal mobility

drug-food interactions in the gut

distribution

transfer of drug from the blood circulation to the various tissues in the body

exception to capillary permeability

brain capillaries have no pores and an additional later of glial cells- only lipid soluble drugs diffuse across brain capillaries into the brain unless they undergo active transport

what does rate of distribution depend on

relative blood flow

plasma protein binding

bound drug will remain in circulation

pharmacologically inactive, protected from metabolism and excretion and binding is reversible and rapid

binding to tissue proteins may also influence where drugs collect

elimination

metabolism + excretion

metabolism

biotransformation of drugs

often terminates activity but can also increase, cause no change and produce toxic or carcinogenic metabolites

possible sites of drug metabolism

Liver

intestinal wall

GI tract

plasma

lungs

what are the different metabolism reactions

oxidation

reduction

hydrolysis

conjunction

CYP450

most important oxidative enzymes

5 features of CYP oxidation

substrate binding

oxygen binding

oxygen splitting

inserting oxygen into substrate

release of the metabolite

factors influencing drug metabolism

organ function, diseases, other drugs, diet, cigarettes. alcohol. age, sex , pregnancy

excretion

process by which drugs are removed from the body

organs for drug excretion

kidney expelled in the urine

biliary excretion- into the bile out into faeces

faecal excretion of non-absorbed drug

tears, respiration, sweat, milk, saliva

the kidneys

regulate volume and composition of body fluids

conserve essential compounds and remove waste products

specialised transport systems

removes water soluble drugs and metabolites

lipophilic drugs and metabolites usually retained

factors that influence renal drug excretion

Age. pregnancy, disease, other medications

pharmacokinetics

is the study of the concentration time profile of a drug in the body