Module 1: Principles of Drug Action
Introduction and Drug Targets
• Pharmacologists refer to drugs by their generic name (salbutamol), and not the trade name (Ventolin)
• First neurotransmitter was acetylcholine which binds with the nicotinic acetylcholine receptor
• Protein targets for drug action are RICE – Receptors, Ion channels, Carriers, Enzymes
o Receptors – proteins found in cell membrane that are responsible for receiving chemical information from hormones, transmitters, cytokines, growth factors to regulate cell function
§ Agonist: acts on a receptor by binding to a site causing a conformational change in the receptor e.g. opens an ion channel, DNA transcription
§ Antagonist: acts on a receptor by binding to a site but therefore prevents an agonist from binding so endogenous mediators are blocked producing no effect
Example: cannabis
§ The CB1 receptor is blocked by the main psychoactive constituent causing euphoria, anxiety, memory impairment and appetite stimulation
§ A cannabinoid receptor antagonist SR141716 selectively blocks the CB1 receptor o Ion channels: protein ‘gates’ found in the cell membrane that change membrane potentials of
cells when ions pass through
§ Blockers:
§ Modulators:
Example: local anaesthetics
§ Block Na+ channels in the membrane of nociceptor (a nerve cell) axons which blocks the production of action potentials and nerve communication à no pain experienced o Carriers: proteins found in the membrane of cells used to transport molecules (e.g. glucose) into
or out of the cell
Example: serotonin (5-HT)
§ 5-HT is released then acts on a 5-HT receptor where it is transported out of the synapse and back into the cell (recycling)
§ However if the channel is blocked then 5-HT can’t re-enter the synapse so 5-HT accumulates and therefore alleviates depression (increase serotonin = happiness) o Enzymes: proteins found in cells which catalyse biochemical reactions; the drug binds to the
enzyme
Sympathetic (fight or flight):
• Preganglionic fibres are short and all join the paravertebral sympathetic chain
• Postganglionic fibres are long and then join organs
• Black dots between pre and post are ganglia Parasympathetic:
• Long preganglionic fibres
• Short postganglionic fibres
• Ganglia lie in the organ itself
Medulla à cervical à thoracic à lumbar à sacrum - Drugs bind to proteins to produce a response
- Drugs act selectively by binding to certain proteins but not to others
- Binding sites and specificity are very important
Example: dopamine
§ Dopamine naturally gets released from our brain (L-dopa à dopamine à released to receptor) then acts on a D2 receptor
§ However dopamine as a drug cannot cross the blood-brain barrier in order to enter the brain so as a drug it inhibits the enzyme in the PNS not in the brain
• Four Receptor Superfamilies
o Ligand-gated ion channels (milliseconds therefore fast synaptic transmission of info)
§ Relatively few of these
§ 20 transmembrane segments that surround a central aqueous channel
§ Alters the electrical excitability of membranes to make action potentials (i.e. cations (depolarisation) or anions (hyperpolarisation) into cell) more or less likely = more or less neural messages
§ Examples: nACh receptor, GABAA receptor, LGICs
LGICs: nicotinic acetylcholine receptor has 2 binding points with different subunits
o G-protein coupled receptors (seconds so slower synaptic transmission for more subtle control on organs by the use of a cascade system)
§ Most abundant; 7 transmembrane domains linked to a G-protein (guanine nucleotide- binding proteins) which links to either an ion channel or a different enzymatic pathway
§ No subunits; N-terminus is outside cell while C-terminus is inside cell
§ Second messenger cascade: signals from receptors on the cell surface are relayed to target molecules inside the cell, amplifying the signal in the process
Levodopa is used to treat Parkinsons Disease
§ There are four main G-proteins:
• G alpha q à targets phospholipase C which increases [Ca2+] to contract muscle (this protein is found on smooth muscle)
• G alpha s à targets adenylyl cyclase to push Ca2+ back into the endoplasmic reticulum so muscle relaxes
• G alpha i à inhibits adenylyl cyclase therefore process above is inhibited
• G alpha o à similar to G alpha s
§ Promiscuity of G-proteins: receptors may couple to more than one type of G-protein o Kinase-linked receptors (minutes/hours)
§ Receptors for many growth factors and cytokines
§ Contain only one transmembrane helix
§ N-terminus is on the outside while C-terminus is on the inside
§ Activates intracellular kinase cascades
§ Example: growth factors which leads to increase in cell division, growth and differentiation
1. Binding of growth factor to receptor
2. Dimerisation where a conformational change leads to two of the receptors coming together
3. Autophosphorylation where P is added to each receptor in the dimer
4. Grb binds then activates a cascade of kinases leading to transcription factors o Nuclear/intracellular receptors (hours/days as it involves transcription and translation)
§ Not found in membranes (found in the cytosol) hence the drug must cross the plasma membrane in order to act on the nuclear receptor
§ Regulate gene transcription (e.g. steroid and thyroid hormones)
§ Example: corticosteroids 1. Bind to receptor 2. Receptor dimerises
3. DNA unwinds and exposes the promoter sequence 4. Transcription occurs and proteins are made
Drug Targets in Asthma
• Asthma is a chronic lung inflammatory disease which can be controlled but not cured and is defined by the presence of excessive variation in lung function and respiratory symptoms that vary over time
o 10% of adults and 15% of children are affected; 300 people per year die of asthma o More prevalent in indigenous, young boys, older women
• Diagnosis is based on history, physical examination, other diagnoses, measurement of variable airflow limitation but diagnosis is difficult especially in younger children
• Pathology of the lungs:
o Left lung has 2 lobes, right lung has 3 lobes
o Airways bifurcate until they become alveoli
o Tension in the alveoli is what keeps the airways open o Airway:
Airways are lined with epithelial cells while blood vessels are lined with endothelial cells
• Spirometry: amount of air able to be breathed out i.e. small airway = small volume of air
• The more severe the asthma the more medication is needed to stimulate a change in the airway
o Bronchial hyperresponsiveness is the sensitivity of the airways to stimuli and when measured can be correlated with severity of the condition
o Measuring BHR with intake of corticosteroids:
People with normal lungs can inhale as much of the drug as they like with no detrimental effect while an asthmatic should not go above 40% or they could die
• Asthma Treatment:
Primarily salbutamol (Ventolin): activates a pathway that works against the contractive pathway Salbutamol opens the airways by acting upon the β2 adrenoreceptor (β2-AR) which is coupled to the G- protein, activating the α-subunit to produce adenylyl cyclase. This catalyses the production of cyclic AMP, decreasing the intracellular Ca2+ concentration and inhibiting the contraction of smooth muscle in the bronchial passages.
o Relievers (bronchodilator aerosols) which mimic the effect of the endogenous ligand adrenaline
§ Short acting B2 AR agonists (SABA) e.g. salbutamol, fenoterol
§ Long acting B2 AR agonists (LABA) e.g. salmeterol, formoterol à not used as monotherapy Side effects of SABA and LABA: palpitations, tremors, increased BP, increased asthma severity
§ Anticholinergics
§ Theophylline
Selective agonists to act on β2AdR on surface of many cells, e.g. airway smooth muscle, epithelium, mast cells, inflammatory cells such as mast cells, skeletal muscle vasculature o Preventers (anti-inflammatory)
§ Cromones (but exact MOA isn’t known)
§ Inhaled corticosteroids to alleviate underlying inflammation à suppress the inflammation in the airways by acting on the glucocorticoid receptor, altering gene transcription to produce an anti-inflammatory protein
§ Anti-leikotriene drugs and anti-IgE
Responses to Agonists and Measurements of Antagonist Activity
• Agonist: a small molecule that activates a receptor that has a physiological regulatory function; can be endogenous (from body) or exogenous (not from body)
e.g. acetylcholine o small molecule
o endogenous and released mainly from nerves o activates several receptors
o changes physiological state of cell that has receptors that bind to Ach Binding forces between agonist and receptor are:
o Electrostatic (charge) o H-bonding
o Van der Waals o Hydrophobic
• Law of Mass Action: at equilibrium the rate of chemical reaction µ product of the conc. of reactants
• Affinity: indicates the strength of binding of the drug (i.e. its potency)
o Indicated by the dissociation constant KD à the lower the number the more potent the drug
§ KD = conc. of ligand that results in half occupancy of receptors
§ The lower the conc. the greater the affinity o As affinity increases, so too does binding specificity
o The graph of ligand conc. (x-axis) against amount of receptors bound (y-axis) is a rectangular hyperbola and never really reaches 100% occupancy \ is asymptotic
o Hill-Langmuir Equation: proportional occupancy = conc. of ligand / (conc. ligand + KD) o When a linear scale is used = rectangular hyperbola
When a log scale is used = sigmoidal
• A log scale is easier to compare concentration-occupancy relationships is clearer and easier to see the different potencies between agonists
o To measure a ligand binding to a receptor, use a radioactive ligand e.g. C14 or P32 which allows us to measure KD at extremely low concentrations à some ligands will be specifically bound to receptors and some non-specifically to tissues so add lots of non-radioactive ligand to displace the radioactive ligands bound to receptor so only non-specifically bound radioactive ligands remain à subtract this from the total and we will get a rectangular hyperbola
o Scatchard plot gives density of receptors:
§ Bmax = no. of receptors bound per amount of tissue
§ Slope = dissociation constant
o Concentration-Response (linear) gives us a rectangular hyperbola again:
§ We can measure EC50 in the same way as KD BUT KD ≠ EC50 as EC50 < KD
§ EC50 is the conc. which causes a half maximal response à max response can occur without full receptor occupancy
A larger Bmax means more
receptors and a smaller KD means greater affinity.
