PHRM20001 MST 1

receptors

a biological macromolecule/complex located on the plasma membrane of cytoplasm that binds another molecule & initiates/modulates signalling/effector activity within a cell

• can be agonists or antagonists
• can be ligand-gated ion channels, GPCRs, kinase-linked, nuclear

Agonist

a molecule/drug/ligand that, by binding to a receptor site, stimulates a response

Antagonist

a molecule/drug/ligand that, by binding to a receptor site, inhibits a response

ion channels

a type of drug target where drugs block/modulate channel opening to allow passage of ions through the channel pore

• channel contracting allows ions in
• reduced channel contraction blocks ions from entering

carrier molecules

a type of drug target where drugs can inhibit the protein that functions in transport of molecules across a membrane → increase in activation/build up of molecules on 1 side of the membrane

• e.g. Fluoxetine blocks serotonin reuptake into nerves ∴ increase amount of serotonin available to bind to receptors

enzymes

catalysts for breakdown/synthesis of molecules
-acetylcholine is broken down through the enzyme acetylcholinesterase
-aspirin where it inhibits cyclooxygenase (COX) leading to a reduced synthesis of mediators of pain/fever/inflammation.

ligand-gated ion channel

A transmembrane protein containing a pore that opens or closes as it changes shape in response to a signalling molecule (agonist or antagonist ligand), allowing or blocking the flow of specific ions

• e.g. ACh binds to nicotinic receptor on skeletal muscle → Na+ channel opens → Na+ entry stimulates contraction
• takes miliseconds for drug action
• e.g. nicotinic Ach receptor

G protein-coupled receptors (GPCRs)

large family of integral membrane proteins involved in signal transduction; characterised by their 7 membrane-spanning alpha-helices; utilise G protein to transmit signals to effector cells → G-protein able to selectively interact with the effector protein (which may be an ion channel, enzyme)

• G proteins can be: Gs - Gs, Gi, Gq/11
  - takes seconds for drug action
  - e.g. muscarinic, Ach receptor

Gs protein

G stimulatory protein that increases adenylate cyclase which catalyses formation of cAMP from ATP

Gi protein

G inhibitory protein that decreases adenylate cyclase conversion of ATP → cAMP

Gq/11 protein

G protein that stimulates phospholipase C conversion of PIP2 → IP3 & DAG

Enzyme-linked receptors

contains large extracellular domain, single helical transmembrane region & an intracellular effector region that participate in cell signalling through extracellular ligand binding and initiation of second messenger cascades - intrinsic/linked enzymatic activity

• activates enzymatic activity of the protein's cytoplasmic domain

Kinase-linked receptors

Ligands bind to the receptor-binding site at the extracellular domain → causes phosphorylation of amino acids within the intracellular domain

• takes hours fro drug action
• e.g. cytokine receptors

nuclear receptors

type of receptor located within the nucleus whereby lipid-soluble agonists can enter the cell & bind/activate the intracellular receptor → regulates transcription & protein synthesis

• takes hours for drug action
• e.g. oestrogen receptor

Pharmacokinetics

How the organism affects the drug - the process by which drugs are absorbed, distributed within the body, metabolised, and excreted

Pharmacodynamics

The study of how the drug affects the organism

3 steps to drug response

1. Binding
2. Activation
3. Transduction

drug binding

Drug has to ____ in order for it to act

• concentration & affinity both matter for the ability of a ligand to ____
  [A]/KA - [A] is the concentration of the ligand, KA is the ligand's affinity
• Drugs w/ multiple effects typically _ to more than one receptor type.
• Drugs that _ receptors for a long time have a high affinity for that receptor.

affinity

ligand/drug's ability to bind

• differences contribute to potency differences

drug activation

• efficacy
• Agonists may act on the same receptor but effect is determined by whether the agonist is full or partial
• Continuous activation of receptors → desensitisation & down regulation (Desensitisation may be v fast w/ strong receptor activation)

efficacy

the ability of the drug to activate

• differences contribute to differences in potency & maximum effect
• high in full agonists to produce the largest possible effect
• low in partial agonists to produce less effects, typically not eliciting a strong activation

potency

index of how much drug is needed to elicit an action

ED50

an index of agonist potency & is the estimated dose for 50% of a response

EC50

the estimated concentration of the agonist that produces 50% of maximal effect

neuron

a signalling nerve cell which consists of the soma, dendrites, axons & presynaptic terminals

soma

cell body which contains the nucleus & is responsible for housekeeping functions (protein synthesis & processing)

dendrites

primary sites on neurons for synaptic input - receives signals

axon

neuron extension that propagates action potential to a specific target - conducts signal

presynaptic terminals

end point of the axon/neuron that converts electrical signals into chemical signals - transmit signals

sympathetic nervous system

the division of the autonomic nervous system that arouses the body, mobilising its energy in stressful situations

• consists of a short pre-ganglionic neuron & long post-ganglionic neuron
• Adrenal gland = an exception to the double neuron system since there's 1 neuron straight to adrenal gland to release adrenaline into the circulation
• release Ach to nicotinic receptors & NA to adrenergic receptors

parasympathetic nervous system

the division of the autonomic nervous system that calms the body, conserving its energy

• consists of long pre-ganglionic neuron & short post-ganglionic neuron
• release Ach to nicotinic receptors & Ach to muscarinic receptors

chemical transmission

Communication between neurons via release of chemical substances (neurotransmitters) that are contained in synaptic vesicles in the presynaptic terminals:

1. Action potential fires
2. N-type voltage-gated calcium channels open → influx of [Ca2+]i into the pre-synaptic neuron
3. Synaptic vesicles containing neurotransmitters dock and fuse with the membrane to release the neurotransmitters into the synaptic gap
4. Neurotransmitters bind to the binding site of receptors on the post-synaptic neuron
5. Binding of neurotransmitters to the receptors elicit a response in the effector tissue

Co-transmission

characteristic of neurons where they can utilise/release several different types of neurotransmitter at a single synapse - sometimes there's a dominant neurotransmitter

• advantage to enable local fine-tuning of physiological control mechanisms by neuromodulation (via pre- & post-junctional mechanisms) or by changing neurotransmitter composition

Cholinoceptors

Ach receptors; nicotinic and muscarinic

nicotinic receptors (nAChRs)

ligand-gated ion channel that binds to Ach to initiate ganglionic transmission & skeletal muscle responses

• fast response time

• pentamer structure consisting of 5 subunits around a central pore

• 2 subtypes:N1 & N2
  - agonists: ACh, nicotine
  - antagonists: hexamethonium, D-tubocurarine
  - mechanism of action:
  1. Binding of 2 ACh molecules to extracellular domain → channel opens
  2. nAChRs = permeable to both Na+ & K+ but Na+ entry dominates
  3. Overall inward flow of +ve ions → excitatory postsynaptic potential (transient membrane depolarisation)
  4. If excitatory postsynaptic potential (EPSP) reaches a threshold potential → AP is generated in postganglionic neuron

muscarinic receptors (mAChRs)

GPCR which binds to Ach to initiate post-ganglionic parasympathetic responses

• intermediate response time

• subtypes: M1, M2, M3
  - agonists: ACh, pilocarpine
  - antagonists: atropine

Adrenoceptors

GPCRs that bind to NA from sympathetic nerves, & bind to circulating adrenaline; alpha and beta

• tissues have differing expression profiles of
• All GPCRs couple to & activate a G-protein → selectively activate cellular enzyme → generates second-messenger → triggers intracellular signalling cascades - cellular modulation → provides selectivity to the effect produced

Catecholamines

adrenergic mediators that affect the sympathetic nervous system in stress response

• NA & adrenaline
• Made up a catechol (a benzene ring with hydroxyl groups) & an amine group
• synthesis occurs in the periphery of sympathetic varicosities & adrenal glands

Noradrenaline synthesis

Adrenaline synthesis

drugs affecting catecholamine synthesis

1. competitive inhibitor of DOPA decarboxylase → ≠ dopamine production
2. false transmitter - stored & released instead of NA → displaces NS in vesicles → limiting NA release
3. alpha-methylnoradrenaline - agonist at alpha2-adrenoceptors → negatively feedback on NA release
4. L-DOPS - NA prodrug which enhance release from enhanced synthesis to be used in hypotension associated with decreased NA synthesis
5. L-DOPA - Loss of DOPAminergic neurons → less dopamine available as a neurotransmitter within the NS
6. Carbidopa - DOPA decarboxylase inhibitor → ≠ dopamine production

VMAT

Vesicular Monoamine Transporter that transports dopamine into the secretory vesicles to produce NA & adrenaline

• inhibitors → NA depletion

neurotransmitter inactivation

Reuptake of transmitter back into neuron
Enzymatic transformation or degradation
MAO inside neuron
COMT outside neuron
Diffusion away from the receptor

neuronal uptake

neurotransmitter inactivation by going back into the pre-synaptic neuron

• high affinity

Extraneuronal uptake

neurotransmitter inactivation by going into the effector tissue/post-synaptic neuron

• low affinity

monoamine oxidase (MAO)

a class of enzymes present within the pre-synaptic neuron that destroy the monoamine neurotransmitters: dopamine, norepinephrine, and serotonin

COMT

enzyme present in both pre- & post-synaptic neurons to degrade catecholamines

indirectly acting sympathomimetics

drugs that mimic the effects of endogenous agonists of the sympathetic nervous system - cause the release of NA from sympathetic nerve terminals & activates the adrenergic receptors without directly binding to it → increases synaptic levels of neurotransmitters by inducing neurotransmitter release or blocking neurotransmitter reuptake

steps:

1. NET transports the drug into the nerve terminal since its structural similarity to NA ∴ limit neuronal uptake

2. VMAT takes up the drug into vesicles in exchange for NA → NA displaced into cytosol

3. Some displaced NA escapes via NET ∴ increasing amount of NA in junction

4. Increase of NA in junction activates adrenoceptors to elicit response in tissue ∴ SYMPATHOMIMETIC

5. Drug can be metabolised by MAO ∴ limit NA metabolism
   - e.g. amphetamine, tyramine, ephedrine

Directly acting sympathomimetics

drugs bind to adrenoceptors & mimic the actions/ responses observed during normal sympathetic ns activation - mimics NA, adrenaline & dopamine

alpha 1 adrenoceptors

localisation in vascular smooth muscles

• Gq protein + PLC → ↑IP3 + DAG

• smooth muscle contraction, vasoconstriction of blood vessels

• agonist: phenylephrine - nasal decongestant

• antagonist: prazosin - hypertension

alpha 2 adrenoceptors

localisation in the pre-junctional sympathetic nerve terminal where activation would inhibit responses mediated by cAMP/PKA pathway but only if expressed in that tissue

• 'auto-inhibitory feedback'

• Gi protein + AC → ↓cAMP

• decreases neurotransmitter release

• agonist: clonidine

• antagonist: yohimbine

beta 1 adrenoceptors

localisation in the heart, kidney where activation can increase contractile force & rate in cardiac muscle

• Gs + AC → ↑cAMP

• agonist: dobutamine for heart failure

• antagonist: atenolol for hypertension

beta 2 adrenoceptors

localisation in the blood vessels of bronchioles, gut, liver where activation increases smooth muscle relaxation

• Gs + AC → ↑cAMP

• agonist: salbutamol for asthma

ACh synthesis

• Choline transporter (ChT) brings Choline into cell (inhibited by hemicholinium) → Once in cell, Choline + Acetyl CoA → ACh (via choline acetyltransferase (CAT)) → ACh packaged & stored into the vesicular acetylcholine transporter (VAChT) (inhibited by Vesamicol)

Hemocholinium

inhibits choline transporter (ChT)

Vesamicol

inhibits vesicular acetylcholine transporter (VAChT) → no storage of ACh in vesicles to be released

neurotransmitter release

a calcium dependent vesicular exocytosis process

Steps:

1. Priming: Synaptobrevin forms SNARE complex with Syntaxin & SNAP-25
2. Priming: Ca2+ enters the nerve terminal & binds to Synaptotagmin on vesicle membrane
3. Fusion & exocytosis: Ca2+ bound Synaptotagmin interacts with SNARE complex, driving membrane fusion & exocytosis of neurotransmitters

Botulinum toxin

a neurotoxin produced by C. Botulinum that prevents the exocytotic release of ACh

• contain 2 chains:
  1. heavy chains which selectively & irreversibly binds presynaptic receptors on cholinergic neurons to be endocytosed & the light chain separates from the heavy chain
  2. light chain cleaves the specific SNARE proteins that are important for vesicular exocytosis process → ≠ formation of SNARE complex → ≠ fusion of vesicle to presynaptic membrane → no release of neurotransmitters
  
  - site of action at autonomic ganglia, NMJ, post-ganglionic parasympathetic & sympathetic nerve terminals that release ACh
  - symptoms: progressive parasympathetic & motor paralysis, muscle weakness

Anticholinesterase

enzyme which inhibits the breakdown of ACh → more ACh in the junction for long duration → greater activation of nAChR or mAChR to induce a response

• variable duration of action

Organophosphates

irreversible anticholinesterase found in pesticides that can cause cholinergic overstimulation

• binds & covalently modifies AChE
• recovery requires synthesis of new cholinesterase so they can break down the excess ACh

neostigmine

reversible anticholinesterase that acts peripherally only, has some direct ACh-like activity at NMJ → prolongs activity of endogenous ACh

• selective for NMJ
• medium duration
• e.g. Myasthenia Gravis - autoimmune disease where the body develops auto-antibodies for nAChR which blocks/destroys nAChRs

physostigmine

reversible anticholinesterase that is selective for parasympathetic junctions & increases ACh availability in neuroeffector junction → greater activation of M3 receptors on ciliary smooth muscle of eye → ciliary muscle contraction → facilitates removal of fluid

• medium duration
• e.g. glucoma

Acetylcholinesterase

the enzyme that metabolises ACh into choline & acetate

• the choline is taken up by the choline transporter to be used to produce more ACh

end plate potential (EPP)

local depolarisation that initiates AP in muscle fibre & subsequent skeletal muscle contraction if end-plate potential is sufficient & reaches certain threshold

N1 (Nicotinic)

nicotinic receptor located on the neuromuscular junction

N2 (Nicotinic)

nicotinic receptor located in autonomic ganglia (neuronal), postsynaptic membrane of sympathetic & postganglionic neuron of parasympathetic

M1 (muscarinic)

muscarinic receptor located in autonomic ganglia (but N2 receptors dominate), glands & the CNS

M2 (muscarinic)

muscarinic receptor present in cardiac muscle - inhibits AC activity → decrease in cAMP & PKA production → decrease in contractile heart rate & smooth muscle contraction in some tissues

• Gi
• decrease contractile rate & force

M3 (muscarinic)

muscarinic receptor present in glandular/smooth muscle which activates PLC to break down PIP2 into DAG & IP3, & increases calcium required for secretion from all glands & facilitates smooth muscle contraction

• Gq
• increase gastric, salivary secretion, smooth muscle contraction

d-tubocurarine

competitive reversible nicotinic antagonist for neuromuscular blocking of drug for surgical paralysis

hexamethonium

competitive reversible nicotinic antagonist used for antihypertensive to block sympathetic & parasympathetic ganglia

atropine

muscarinic antagonist used to reduce secretions during anaesthesia

local administration

method of administration where drugs are able to exert effects at/near site of administration

• limiting tissue access can limit side effects

Systemic administration

method of administration where drugs are able to enter the bloodstream to access many tissues around the body

• variety of dosage forms: mixtures, tablets, capsules

• Drugs given orally can be subject to first pass hepatic metabolism → can reduce the bioavailability of the drug

• routes that avoid first-pass metabolism: subcutaneous (sc), intramuscular (im), intravenous (iv)

first pass hepatic metabolism

The amount of metabolism that occurs to a drug prior to reaching the systemic circulation

• Following oral administration, drug enters hepatic portal vein & is transferred directly to liver
• If drug is extensively metabolised & rendered inactive by metabolism → amount of drug entering systemic circulation will be reduced

Bioavailability

A measure of the extent of drug absorption for a given drug and route - the proportion of a dose that enters the systemic circulation (0-100%)

• affected by: how much drug is absorbed & how much drug undergoes first-pass hepatic metabolism
  - oral bioavailability needs to be known to accurately give the correct dose
  - if low % → more dosage is required

Steps of Drug Distribution

1. dilution of drugs in blood
2. movement of small, non-protein bound drugs into the ECF
3. binding of drugs to cells to be uptaken by the cells

Volume of distribution (Vd)

A ratio used to estimate the distribution of a drug within the body relative to the total amount of fluid in the body. It is calculated as the amount of drug administered divided by the plasma concentration of the drug (Vd = X/C)

• tells you how much drug will give a particular concentration of drug in the plasma
• Vd = small if drugs bind to plasma proteins → ≠ taken up ∴ greater concentration appears in plasma
• Vd = larger if drugs bind to tissues & are taken up by cells → drugs are stripped from the plasma
• Drugs bound to plasma protein tend to have Vd close to plasma volume (0.06L/kg)
• Drugs escaping vasculature, evenly distributed & unable to cross cell membranes (0.2L/kg)
• Drugs escaping vasculature, evenly distributed & able to cross cell membranes (0.6L/kg)

processes of renal excretion

1. Glomerular filtration
2. Tubular secretion
3. Tubular reabsorption

glomerular filtration

The first step in renal excretion where drugs are passively filtered out of the blood

• not for drugs bound to plasma protein

tubular secretion

The second step in renal excretion where drugs are actively filtered out of the blood via active carriers

• can remove protein-bound drugs
• can be competitively inhibited

tubular reabsorption

process of renal excretion where drugs passively move across cell membrane and back into the blood

• pH dependent

renal clearance

volume of plasma cleared of a particular substance in a given time

drug metabolism

the enzymatic biotransformation of a drug into metabolites occurs in the liver

• End effect of drug metabolism = to yield water soluble metabolites that ≠ be readily reabsorbed → get excreted

• metabolites can be inactive, active, have new activity, or be toxic

• 2 types: phase 1 & 2

Phase 1 drug metabolism

phase of metabolism that involves creating functional groups on drugs (-OH, -NH2, -SH, -COOH)

Phase 2 drug metabolism

phase of metabolism that involves attaching a water soluble molecule to the functional group on the drug to make the drug more water soluble to be excreted easily

serendipity

drug discovery from luck, accidents, wrong hypotheses

• e.g. discovery of sulphanomides by Paul Ehrlich

drug screening

process of drug discovery which involves purifying compounds from naturally occurring source & then assay the compounds for activity

• compounds from plants & microorganisms etc.

rational drug discovery

the development of new therapeutics based on an understanding of the underlying disease mechanism to give best chance of drug discovery - more efficient

• steps:
  1. Choosing a disease
  2. Choosing a target agent
  3. Searching for new leads
  4. Optimising leads
  5. Preclinical development & toxicology
  6. Clinical trials
  7. Approval

Phases of clinical trials

Phase 1: carried out on a small number of healthy individuals to test whether the drug can be tolerated
Phase 2: carried out on a larger number of health individuals or small number of patients with diseases to test the appropriate dosage of drug
Phase 3: carried out on a large number of patients with disease to test whether the drug is effective
Phase 4: after drug's approval - monitoring the side effects & adverse effects of the wider population

pharmacogenomics

the study of genetically determined variations in the response to drugs - targeting particular genes of interest

DNA polymorphisms

variations in DNA sequences; used as a basis for comparing genomes

• often single nucleotide polymorphisms (SNPs)

• detected via dna sequencing, microarrays

• effects: change protein structure and/or function (direct & alternative splicing), change in gene transcription, lead to loss of proteins from misfolding

SNP effects on pharmacokinetics

can effect drug metabolising enzymes

• e.g. suxamethonium, thiopurine methyltransferase (TPMT), statins

Thiopurine Methyltransferase (TPMT)

metabolise & inactivated drugs (thiopurines)used to treat childhood leukemias, rheumatoid arthritis, or inflammatory bowel disease

• Most individuals have relatively large amount of TPMT ∴ a balance of TPMT & HPRT is required for an effective dose → less ADRs

statins

drugs used to lower cholesterol in the bloodstream by reducing cholesterol synthesis

• Can cause severe skeletal muscle damage in some patients
• SNPs in genes can reduce statin uptake in the liver for metabolism ∴ enhancing toxicity if high statin concentration in the blood

SNP effects on pharmacodynamics

SNPs effects on receptors, ion channels, enzymes

legitimate drugs used to treat injury

• local anaesthetic drugs to reduce sensation & pain
• analgesic drugs to reduce pain
• anti-inflammatory drugs to reduce inflammation (NSAIDs & Glucocorticosteroids)

Non-steroidal anti-inflammatory drugs (NSAIDs)

large group of anti-inflammatory, analgesic, antipyretic drugs

• Phospholipase A2 liberates the membrane phospholipids → arachidonic acid → converted to leukotrienes or prostaglandins via cyclooxygenase (COX) → mediators of inflammatory process in tissues - competitively inhibit the cyclooxygenase →inhibits formation of intermediate prostaglandins from which the prostanoids were derived from → anti-inflammation, antipyretic, & analgesic activities since no prostanoid production = lessen fever, pain sensitivity & inflammation

• Inhibited normal regulatory roles of the prostanoids which include: controlling appropriate blood flow, reducing acid secretion & enhancing mucus protection in the stomach, ensuring adequate blood supply to the kidney → potentially ↑ chance of GI ulceration/bleeding ∴ require selective COX2 inhibitors to reduce adverse effects of inflammation etc. but ≠ affect the normal roles of prostanoids (stomach acid secretion, protection of stomach mucosa)

Glucocorticosteroids

anti-inflammatory, immunosuppression, skin thinning, muscle wasting drugs etc. that stop phospholipase A2 from being made → ≠ convert membrane phospholipids into arachidonic acids → inhibition of the rest of the pathway → reduce inflammation

• delivered locally where side effects are greatly reduced as compared to delivering the drugs systemically → considerable & extensive side effects

• lipid soluble drugs that can diffuse through the PM to reach the receptors in the cytoplasm

• Once bound to receptor → dimerisation of the receptors & translocation to the nucleus to influence protein synthesis → induce protein synthesis of antiinflammatory mediators or suppress inflammatory mediators

• Agonist-receptor complex can interact directly with DNA or independently of DNA molecules through coactivator molecules & transcription factors to alter protein synthesis

Anabolic agents

drugs that increase skeletal muscle mass & strength in athletes

• Similar action to testosterone: Virilisation (↑ clitoral size, penis size in adolescents, body hair), Testicular atrophy, Heart alterations & other cardiovascular effects