PHAR3306 Notes - Week 1 2 PDF

PHAR3306 Notes Wk 1: Pharmacodynamics I Drugs An agent that interacts with specific target molecules in the body and produces a physiological effect - Until the 19th Century most drugs were naturally organic dried and...

PHAR3306 Notes Wk 1: Pharmacodynamics I Drugs An agent that interacts with specific target molecules in the body and produces a physiological effect - Until the 19th Century most drugs were naturally organic dried and fresh plant products Pharmacodynamics The mechanism by which drugs exert their effect on the body in order for a therapeutic action to occur. Pharmacodynamics encompasses: - Drug-receptor interactions. - General principle of drug action. Pharmacokinetics is what the body does to the drug. Drugs with activity at high Drugs acting at low concentrations Types of Drugs concentrations. - Structural specificity. - Little structural - Act by chemical rather than physical specificity. interaction (Isoprenaline). - Cause physical change (general anaesthetics). Receptors The site at which a ligand (agonist) can attach Intro Ligands (also known as drugs) may be neurotransmitters, hormones or local factors Activation of receptors by a ligand or agonist produces a response (effect). Affinity: The attraction of a drug for binding. Selectivity: Ligand - Gated ion channels G-protein coupled receptors: Types of Fast neurotransmitters Slow neurotransmitters Receptors e.g. nicotinic, GABA, NMDA, AMPA e.g. Ach, NA, chemokines, opioids Kinase-linked receptors Nuclear receptors Insulin, cytokines and growth factors Steroid hormones, thyroid hormone, retinoic acid and vit D Several subtypes; Receptor - Cholinergic - muscarinic; nicotinic Sub-types - Adrenergic receptor subtypes include 2 (lungs), 1 (heart) and 1 (blood vessels) Development of selective drugs which interact ONLY with specific receptor subtypes has revolutionised pharmacology. Side Effects Because: Receptors for a drug occur in Because: Of the non specificity/selectivity of drugs. several tissues, not just the target tissue. Regulations Until the 1930’s drugs did not need to be tested for safety or effectiveness. ‘Elixir of Sulfonamide Tragedy’ of 1937 led to the requirement that drugs be tested for safety before they reached t– 107 children died. Establishment of the Food, Drug andCosmetic Act of 1938 in the USA; requiring details of a drug’s uses and proof of its safety. Clinical trials were not required. New Regulations required proof of quality, safety and efficacy or effectiveness. The Australian government established: The Australian Drug Evaluation Committee [ADEC]: 1962 Registry of adverse drug reactions: 1962 Adverse Drug reactions Advisory Committee [ADRC]:1971 - It now takes many years of research before a new drug can be registered. - Adverse drug reactions still occur e.g. Rofecoxib Thalidomide Was introduced onto the European market as a safe sedative/hypnotic in the late 1950’s. - Many pregnant women took it for morning sickness. - Tested on male rats only; no teratogen testing. - Thalidomide was found to be a teratogen which causes birth defects. - All drugs capable of crossing the placenta are capable of affecting the foetus. General principles of Drug Action Types of Drug Action Agonists mimic endogenous ligands. They bind to a receptor and cause a secondary effect. Agonists and Antagonists An Antagonist binds to a receptor and prevents the action of an agonist. Most antagonists are competitive and reversible. (No affinity) Locke + Key Model Antagonist is the twig, can't do anything, just locks the effect of the agonist.  Receptor binding can be used to measure effect of drug by measuring efficacy or Drug-receptor effectiveness of drug in producing maximal response Interaction Assumed that the effect of a drug is proportional to the fraction of receptors occupied. Assume that the maximal effects occur when all receptors are occupied. These assumptions are not always true. Properties Efficacy is a measure of the effectiveness of a drug in producing a maximum response Dose ED50 (Effective Dose 50%): The dose of a drug response that produces 50% of its maximum effect in a population EC50 (Effective Concentration 50%): The concentration of a drug that produces 50% of its maximum effect in an individual or a tissue sample. KD (Dissociation Constant): The concentration of a drug at which half of the receptors are occupied. It measures how strongly a drug binds to its receptor. - Assumed: effect is proportional to receptor occupancy and maximal effect occurs when all receptors are occupied. - KD becomes the ED50 i.e. the dose that produces a 50% effect. - Drugs which are highly potent require low dosages The measure of the drug dosage needed to produce a Potency particular therapeutic effect. - It is determined by the strength of binding of a drug to a receptor or the receptor affinity for the drug. ED50: When all receptors are occupied. (For a spare receptor, the presence of spare receptors shifts the dose-response curve to the left of the KD.) Efficacy - The measure of the effectiveness of a drug in producing a maximum response. - Full agonists have high efficacy - Antagonists have no efficacy. Which drug is most potent? Which drug is the most efficacious? Examples Lower Kd = More affinity & potency Dose - Response measurement Determines accurate dosing needed when new drugs are formulated  Cannot use these measurements to calculate affinity only dosing → Agonists bind to receptors causing contractions which are plotted. → Increasing doses = Increasing response until 2 doses have an equal response → Divide everything by that response = max response Practice question  Drug Antagonism Competitive (or surmountable) Antagonism Types Non-Competitive (or irreversible) Antagonism Physiological Antagonism Competitive Occurs when agonists and Antagonism antagonists compete for the same receptor sites. Maximal effect unchanged (ie antagonism is surmountable). Parallel shift to the right leading to increase in dosage to overcome the antagonism Produce irreversible changes Non- Competitive by acting on the receptor itself Antagonism to change it and make binding impossible Cause a change in the receptor so that the agonist can no longer bind. A maximum effect is no longer produced Spare receptors Are important in non-competitive antagonism as a receptor reserve can allow a maximum response to be reached. Spare receptors occur because there are several amplification steps downstream from the initial drug-receptor interaction. ED50 for a drug effect with spare receptors may not be equal to KD and this shifts the dose response curve to the left of KD as in classical theory, half receptors bound should lead to half of the maximal effect but this is not the case with spare receptors Physiological Occurs when 2 agonists act on different receptors to produce opposite effects. Antagonism The drugs have different mechanisms of action. Eg bronchoconstriction due to histamine can be treated with adrenaline which acts as a vasodilator. Practice question i) T ii) F iii) T iv) F Agonists in-vivo: The effect of an antagonist relies solely on blocking the action of an agonist which is Concept of tone already producing a certain effect ie there must be an agonist induced tone Partial agonists are less ‘efficacious’ - Partials Agonists 1. Never achieve maximum effect. 2. Also act as an antagonist (locking full agonist from binding) Inverse Agonists Some receptors are constitutively active, even in the absence of any agonist. An Inverse agonist restores the receptor to its inactive state. Mechanism: 1. Receptors exist in equilibrium between active and inactive forms 2. The presence of an agonist will increase the proportion of active receptors 3. The presence of an inverse agonist will increase the proportion of inactive receptors 4. Mechanism of action of inverse agonists is thought to involve the destabilisation of G-protein receptor coupling. Potentiation of Occurs due to the decreased inactivation of an agonist i.e the breakdown of the drug is Agonists reduced. The drug will be able to accumulate to higher concentrations and have a more potent effect - Acetylcholine in the presence of anticholinesterase (neostigmine; physostigmine). - Noradrenaline in the presence of an uptake blocker (cocaine, tricyclic antidepressants). Allosteric These compounds bind to a separate site on the receptor from agonists called an Modulators allosteric site. Occupation of this site can either increase or decrease the response to endogenous agonists, depending on whether it is positive or negative modulation. Qualitative and Quantitative response: is measured in gradual steps, eg fall in blood pressure Quantal response Quantal response: is all or none e.g. responders or non- responders. Quantal dose - Response curve Tells us the therapeutic range and safety of a particular drug. - Smaller distance between curves indicating less safe drug - ED50 refers to dose that results in response in 50% of population - LD50 refers to the dose that results in death in 50% of the population. Therapeutic aka Toxic ratio A measure of the safety of the drug. The dose that is lethal in 50% of the population divided by the the effective dose, in 50% of the population. Higher ratio = safer - Problems with the therapeutic ratio is that it’s not ethical to kill 50% of animals and does not reflect sublethal toxicity and presence of differences in animal and human data Risk Vs Benefit: A contraindication is an existing condition or situation that alters the use of a drug Contraindications because of increased risk of adverse effects. Contraindications may be absolute (i.e. drug should not be used at all) or relative (i.e. drug can be used with caution – may require increased monitoring or dose adjustment) Molecular Aspects The number of receptors in a cell is not static but dynamic. of Drug Action: There is a high turnover of receptors as they are continuously removed or Receptor replaced. numbers Repeated drug treatment may up-regulate or downregulate the receptor numbers. Tolerance The same dose of drug, on repeated administration, produces less effect. - Tachyphylaxis: Tolerance which develops very rapidly. Desensitisation Desensitisation: Less effect is produced the longer the agonist remains in contact with the receptor. Causes of desensitisation / tachyphylaxis / tolerance Change in receptors (phosphorylation) Downregulation of receptors (internalisation /reduced expression) Depletion of mediator Increased metabolic breakdown Pharmacology Food as drugs – nutraceuticals and lifestyle - Food-drug interactions Exercise:  - Effect of exercise on drugs; pharmacokinetics and pharmacodynamics - Activation or induction of antioxidant systems - Effect of drugs on exercise The role of Provides information regarding dose/dosage regimen vs response. pharmacodynamics - Factors affecting pharmacodynamics together with pharmacokinetics are in pharmacology considered when a dose is individualised for special populations such as the elderly - Useful tool for introducing new indications, new dosages or new treatment populations contributing to valuable information for drug development. Practice question =D - Sites of Drug Action Receptor A protein that responds to a molecule (e.g., hormone or neurotransmitter) and causes a consequent change in cellular or biological function. The Locke and Key - Key is the Endogenous Model Agonist while Lockpick can be considered the Exogenous Agonist and both can open the lock and cause an effect - Object jammed in the lock is analogous to the antagonist which prevents the agonist from acting on it Sites of Drug Action Main Drug targets The estimated proportion of genes from different gene families that are targets for approved drugs, but you can see there are a number of ‘usual suspects’ Transporters Occurs when 2 agonists act on different receptors to produce opposite effects. - Not strictly defined as receptors as they don’t bind endogenous agonists - Transporters actively move substances from one side of a cell membrane to the other ATP-binding Primary active transporters: use ATP cassette (ABC) hydrolysis to actively pump substrates transporters against a concentration gradient - Examples include P-glycoprotein (P-gp; multidrug resistance protein 1) or cystic fibrosis transmembrane regulator (CFTR) Solute-linked carrier Secondary active transporters: One or more (SLC) transporters molecules travel down their concentration gradient so that the solute can travel against its concentration gradient - Examples include the monoamine transporters DAT (dopamine), NET (noradrenaline) and SERT (serotonin)  Receptors are classified into different Cellular location families, primarily based and structural on their structural similarities. features - Three of these receptor of receptors families are cell surface receptors, which means that they are embedded in the plasma membrane. - While the nuclear receptors are soluble proteins that are present either in the cytosol or the nucleus. Ligand-gated ion channels (are also receptors!) Ligand-gated ion - Essential for synaptic transmission channels: in the central and peripheral nicotinic nervous system in humans. acetylcholine (ACh) - Has 2 alpha subunits which ligand receptor binds between and requires 2 ligands to open the channel - Each subunit is made up of 1 polypeptide chain that has 4 transmembrane helices - 5 Receptor subunits form the active channel G protein-coupled 7 transmembrane helices with N-Terminus receptors that is Extracellular and a C-Terminus that is intracellular Largest family of cell surface proteins Conserved throughout evolution >800 GPCRs in human genome (~3% total genome) ~250 have been matched to a ligand and the rest are ‘orphans’ Most drugged family of proteins with >35% market share. Heterotrimeric protein with 3 parts, one alpha, beta and gamma subunit G protein-coupled When a ligand binds to the GPCR, a conformational change receptor signalling occurs that immediately disrupts the G protein heterotrimer, so it dissociates from the GPCR and the alpha subunit also dissociates from the beta and gamma subunits. - The alpha Subunit then goes down to have downstream signalling effects, and so does the beta and gamma heterotrimer Catalytic receptors AKA Kinase-linked/enzyme-linked receptors dimerise when bound to ligands to form the functional receptor. - Extracellular Binding Domain, 1 transmembrane helix and an intracellular catalytic domain forming the C Terminus with an attached enzyme which is where signalling occur - Cross Phosphorylation of the C-Terminus initiates signalling to allow Catalytic receptors to dimerise when bound to a ligand to form the functional receptor Types of Catalytic 1. Receptor Tyrosine Kinase (RTK): receptors Have intrinsic tyrosine kinase activity (EGF, VEGF, Insulin) 2. Receptor Serine/Threonine Kinase: Contain intrinsic serine/threonine kinase activity (TGF- ) 3. Cytokine Receptors: Receptors that associate with proteins that have tyrosine kinase activity (Interleukin Receptors) 4. Receptor Guanylyl Cyclases: Have intrinsic cyclase activity (ANP receptors) 5. Receptor Protein Tyrosine Phosphatases: Have intracellular phosphotyrosine phosphatase activity (PTEN, a tumour suppressor) Receptor tyrosine Activation of receptors leads to dimerisation that kinase signalling causes autophosphorylation of the C-Terminus of each receptor allowing other proteins to bind and initiate a kinase cascade Nuclear receptors - Live within the cytoplasm or nucleus and their ligands therefore need to be able to cross the plasma membrane and sometimes the nuclear membrane to get to their receptor meaning they need to be lipid soluble - Basic Structure contains Ligand binding domain, DNA binding domain and variable N-terminal activation domain and activation function 1 (AF1) - Bind inhibitory proteins (co-repressors) in their inactive conformation. - Agonist binding causes conformational change in the ligand-binding domain, → co-repressor(s) dissociation and coactivator recruitment, → increasing gene transcription. - Zinc fingers, dimer pair needed for functional receptors. Class I nuclear Are steroid receptors that act as Homodimers with receptors mainly endocrine ligands present in the Cytoplasm High affinity ligands Synthesised in inactive forms bound to Heat Shock Protein which upon ionisation will allow for ligand binding → conformational change that causes dissociation from the HSP complex and translocation into the nucleus with dimer pair binding to response element at gene to initiate gene modulation Class II nuclear Act as heterodimers with mainly lipid ligands. receptors Low affinity and are located within the nucleus Exist as retinoic acid receptor (RXR) In absence of Agonist, corepressors/inhibitory proteins bind to nuclear receptors resulting in inactivated conformation preventing transcription Binding of Agonist causes conformational change in ligand-binding domain causing corepressor to dissociate and coactivator protein to bind to receptor transcription activating domain leading to increased gene transcription This occurs as part of a slow process so we won’t target a nuclear receptor expecting a rapid effect Receptor signalling over time Practice question  Wk 2: Pharmacodynamics II Cell Signalling Three stages of Cell Signalling 1. Reception when the agonist binds to the receptor 2. Transduction when the message is relayed 3. Activation of cellular responses Stages - A signal is detected when the molecular signal (also known as a ligand) binds to a Reception receptor protein on the surface of the cell or inside the cell. - Receptor will transmit information from extracellular to intracellular environment by changing its shape which helps to propagate the signal into the cell - When an agonist induces a conformational change in the Transduction receptor it initiates the process of transduction. - Each molecule in the pathway acts upon the next molecule in the pathway. - These molecules are often enzymes, which can catalyse multiple reactions each. - This process leads to amplification of the signal. Response - Kinases play a central role in signal transduction. - Kinase cascades are triggered by GPCRs, guanylyl cyclase-linked (GC) receptors, or by catalytic (kinase-linked) receptors. - Kinase cascades regulate various target proteins, leading to numerous cellular events. Summary Smooth Muscle Found in - Throughout the body arteries and veins, bronchi, bladder, Iris, Urethra and Prostate, in Skin, Uterus and the Gastrointestinal tract - A muscle fibre contracts when the actin fibres are pulled and slide past the myosin Contraction fibres. This process requires ATP and calcium. How is Smooth Muscle Contraction & Relaxation Regulated? Heterotrimeric - G proteins bind to the intracellular G proteins surface of these G protein coupled receptors, which is the beginning of the intracellular signalling cascade. - G proteins are hetero trimeric proteins consisting of an alpha subunit, a beta subunit and a gamma subunit. - Both the alpha subunit and the beta gamma hetero dimer independently induce their own signalling outcomes. - There are multiple different subtypes of the alpha G protein that each have unique effects on second messenger molecules. Gαq signalling 1. GPCR activates Gαq – smooth 2. IP3 produced, and opens IP3R on SR muscle 3. Ca2+ released from SR contraction 4. Ca2+ binds and activates calmodulin (CaCM) 5. CaCM binds and activates the myosin light chain kinase (MLCK) 6. The activated MLCK catalyses the transfer of phosphate to myosin heads, activating myosin head ATPases 7. Phosphorylated myosin heads form cross bridges with actin, shortening occurs 8. Muscle contraction Steps 1-3 Gαq signalling Agonists bind to G protein coupled receptors, leading to the conformational change. Then GDP dissociates while GTP binds to Gαq, which causes the band γ and β to dissociate. The Gαq binds to the enzyme called PLCβ, which helps PIP2 change into DAG and IP3. DAG will activate enzyme call PKC, while the second messenger IP3 combined to the ligand channel in SR, leading to the gate open, which increase the intracellular calcium Steps 4-8 The intracellular calcium combines with calmodulin to form complexes. These complexes activate myosin light chain kinase (MLCK) to bring about phosphorylation of the myosin light chains, which generates ATPase activity to cause sliding of myosin over the actin fibrils and contracting the muscle. - Activating Gαq coupled receptors leads to increased cytoplasmic calcium. The calcium is released by IP3 ligand gated ion channels on the SR membrane. - Image 2: Adding the agonist for the M3 muscarinic receptor, causes calcium release from the sarcoplasmic reticulum. The release of calcium causes contraction of the  smooth muscle Gαq signalling Parasympathetic innervation of the sphincter muscle: - - Acetylcholine (ligand) activation of M3 muscarinic receptors on the iris sphincter smooth Pharmacology muscle resulting in smooth muscle contraction and consequent pupillary constriction. in the iris Sympathetic innervation of the radial muscle: - Noradrenaline (ligand) activation of α1 -adrenoceptors on the iris radial smooth muscle resulting in smooth muscle contraction and consequent pupillary dilation. Clinically used drugs: Pilocarpine (M3 receptor agonist), phenylephrine (α1-adrenoceptor agonist), tropicamide (M3 receptor antagonist). Gαs signalling 2 methods of relaxation – smooth 1. Gαs muscle 2. Nitric oxide relaxation 1.Gαs Signalling Agonists bind to Gαs coupled receptors, leading to the conformational change. Then GDP dissociates while GTP binds to Gαs, which causes the band γ and β to dissociate. Then Gαs bind to the enzyme called Adenylate cyclase which leads to the production of a second messenger cAMP from ATP. cAMP activates protein kinase A (PKA). - (This pathway is inhibited by Gαi-coupled GPCRs, which activate Gi to inhibit adenylyl  cyclase, thereby reducing the amount of cAMP in the cell) 2 ways for PKA lead to muscle relaxation - PKA catalyses the transfer of phosphate to the phosphatase enzyme. This activates the dephosphorylation of myosin light chain. - PKA catalyses the transfer of phosphate to myosin light chain kinase. This prevents the phosphorylation of the myosin light chain. - TOGETHER, THE BALANCE TIPS TOWARDS RELAXATION Gαq and Gαs Parasympathetic innervation of bronchial smooth muscle: signalling - Acetylcholine activation of the M3 muscarinic receptor causes smooth muscle pharmacology contraction and narrowing of the airways. in the lung Sympathetic innervation of bronchial smooth muscle: - Adrenaline activation of the β2 -adrenoceptor causes smooth muscle relaxation and widening of the airways. Clinically used drugs: - Salbutamol (short-acting β2 -adrenoceptor agonist), salmeterol (long-acting β2 -adrenoceptor agonist), ipratropium and tiotropium (M3 muscarinic receptor antagonists) Nitric oxide - Activation of the Gαq coupled receptor on the endothelium cell results in the production (NO) – smooth of nitric oxide (NO). NO travels to the smooth muscle cell where it binds to soluble muscle guanylyl cyclase (GC), this induces a conformational change resulting in the activation relaxation of the enzyme and conversion of GTP to cGMP. cGMP activates cGMP dependent kinase (PKG). Nitric oxide - NO activates soluble guanylyl cyclase (GC), converting GTP to (NO) causes cGMP. relaxation of - cGMP activates cGMP dependent kinase (PKG). smooth - PKG phosphorylates myosin light chain phosphatase (MLCP), muscle cells which activates this enzyme. via PKG - MLCP catalyses the removal of phosphate from myosin light chain, resulting in smooth muscle relaxation. - PKG also leads to MLCK inactivation. Summary Terminating Signalling GPCR - Reduction in smooth muscle contraction when the drug was washed out and the muscle Desensitisatio given time to recover stimulation with the same drug. n/Tachyphylax - The same concentration caused an is equivalent level of smooth muscle contraction as the first drug administration, so this process here, where our contractile response reduces upon re repeated administration of the same drug is known as desensitisation/tachyphylaxis. Causes Desensitisatio - Receptor modifications (phosphorylation/arrestin binding) n/Tachyphylax - Down-regulation of receptors (internalisation/reduced cell-surface expression) is - Depletion of mediators - Increased metabolic breakdown GPCR Desensitisatio n: turning off the signal Gao and Gai interact with its target enzyme to produce a second messenger. After a while, GRK activates GPCR and phosphorylates them, which means GPCR are less likely to bind G protein, leading to stop in signal. Also, β-arrest would bind to the complex and stop the binding of G protein. Which also stops signalling. Practice Question  Lecture 2: Pharmacological modulation of the ANS I Autonomic Nervous System - The autonomic nervous system controls the involuntary functions and influences the activity of internal organs Sympathetic and Parasympathetic Homeostasis Sympathetic vs When adrenaline acts on Sympathetic Nervous System, we get far vision, inhibition of Parasympathetic waste excretion and stimulates ‘Fight or Flight’ response whereas acting on the Parasympathetic nervous system, it stimulates near vision, reduced heart rate, increased food digestion and waste removal mechanisms as part of the ‘Rest and Digest’ functions of the body Site of drug action Each stage of Neurotransmission is a Potential Site of drug action - Places where a drug can be targeted Cholinergic Neurotransmission and drug targets Exocytosis of - SNARE proteins, SNAP-25, syntaxin, synaptobrevin (VAMP), acetylcholine (Ach) on the vesicle and the inner surface of the nerve terminal containing vesicles membrane mediate the fusing of the vesicles containing acetylcholine (Ach) to the membrane. - Release of Ach from the vesicles requires influx of calcium and is mediated by an action potential. - Calcium interacts with the SNARE protein complex on the vesicle membrane and triggers fusion of the vesicle membrane and release of the Ach into the synaptic cleft Inhibition of - A neurotoxin produced by Clostridium exocytosis of ACh botulinum, a bacterium that causes containing vesicles: food poisoning (botulism, lethal) Botulinum toxin - Botulinum toxin, taken up into (BOTOX) vesicles, cleaves the SNARE proteins, preventing assembly of the fusion complex and preventing the release of acetylcholine Botulinum toxin Mechanism of Action - Paralyses the muscles by blocking the release of acetylcholine at the neuromuscular junction Indications - Upper-limb spasticity in stroke patients, focal spasticity of the upper and lower limbs due to cerebral palsy, cervical dystonia in Parkinson's patients, strabismus, blepharospasm, overactive bladder (due to spinal cord injury or multiple sclerosis), prophylaxis of headaches in adults with chronic migraine, primary hyperhidrosis and cosmetic indications (temporary improvement in the appearance crow's feet and forehead lines) Contraindications - Myasthenia gravis or Eaton Lambert myasthenic syndrome (conditions in which the immune system attacks the neuromuscular junctions). Side Effects - weakness of adjacent muscles (this is expected for any injection procedure), urinary retention and urinary infections when used for overactive bladder and eyelid ptosis, dry eye & photophobia when used to treat strabismus and blepharospasm. Route(s) of administration Injection Cholinesterase inhibitors Effects after AChE inhibited: - Enhance the actions of ACh and producing actions similar to ACh agonists - Autonomic actions: enhancement of ACh activity at parasympathetic synapses (bradycardia, increased saliva secretion, increased smooth muscle contractility) - Neuromuscular junction: repeated firing of the muscle fibre leading to twitching and increased muscle contraction - If cross blood brain barrier (e.g. physostigmine), they can cause profound CNS effects AChE inhibitors - - Clinically used AChE inhibitors are medium duration, and their inhibitory actions clinical uses are reversible by adding muscarinic receptor antagonists. Myasthenia Gravis (MG) Neostigmine Cholinergic Receptors Acetylcholine - Nicotinic receptors are pentameric (5) Nicotinic Receptors- proteins. Ligand Gated Ion - Made up of combinations of α,β,γ,δ and ε Channels subunits - Muscle-types (NM) are formed with α,β,γ,δ,ε - Neuronal-types (NN) are formed with α or α &β subunits. Skeletal Muscle Neuromuscular Junction Neuromuscular Depolarising blocking agents blocking agents - Agonists at nicotinic receptors (Effects mostly due - Causes muscle twitching before paralysis to motor paralysis) - Maintains muscle depolarization. Non-depolarising agents Competitive antagonists at nicotinic receptors - Can block pre- and postsynaptic nicotinic receptors. Leading to tetanic fade. - The majority of clinically used neuromuscular blocking agents are non-depolarizing Non-depolarising blocking agents  - Sites of action of drugs that modulate the function of noradrenaline Uptake - Inhibiting NET-1 leads to an accumulation of NE in the synaptic cleft and 1/Norepinephrine increased activation of the postsynaptic adrenoceptors Transporter (NET) Inhibitors Indirectly Acting - Taken up into the nerve terminal via NET-1 and Sympathomimetic enter the synaptic vesicles via VMAT. Both these Amines transporters are usually unidirectional but the indirectly acting sympathomimetic amines alter their function converting them into bidirectional / exchange transporters leading to the removal of noradrenaline from the vesicle and then the synapse and its accumulation in the synaptic cleft activating post synaptic receptors. Uptake 1/Norepinephrine Transporter (NET) Inhibitors Monoamine Oxidase Inhibitors: Moclobemide Feedback control of noradrenaline release α2-Adrenergic Agonists: Clonidine Practice question  Lecture 3: Pharmacological modulation of the ANS II Muscarinic Receptors - GCPRS + Effects on tissues (Parasympathetic) Adrenergic Receptor Subtypes - GPCRs + Effects on tissues (Sympathetic) Signalling: Side effects Undesired effects that occur when a drug is (Adverse Effects) administered. - Unintended secondary effects that a drug will predictably cause - Side effects often stop people taking medication as the side effects are unpleasant. - Known to occur in a percentage of patients Side effects can be due to actions at the intended target (on-target) or an unintended target (off-target) Selectivity A drug’s ability to preferentially produce a particular effect and is related to the structural specificity of the drug binding to receptors. Selective drugs have greater affinity for one receptor over another. - At low concentrations, an agonist selective for receptor A will only activate receptor A and not receptor B. - At high concentrations, an agonist selective for receptor A will activate both receptor A and receptor B. - Agonist or antagonist drugs that are ‘selective’ for the intended receptor can still produce significant effects at other related receptors if a high enough dose is given. - Selectivity is useful in clinical practice only when the ratio of the affinity of a drug at the target receptor versus other related receptors is 100 x or more. When selectivity is lower, it is difficult to predict drug doses that will exploit the difference in subtype activity. Selectivity reduces off-target side effects. Adrenaline - Given during an asthma attack (Sympathetic) ASTHMA - To eliminate some of the side effects we can rely on beta receptor  Using a b2 agonist leads to relaxation → less side effects Turning off the Parasympathetic Nervous System  Less side effects: Autonomic Receptors - Eye Muscarinic Receptor - Mainly used in the treatment of glaucoma and to induce miosis Agonists Muscarinic receptor agonists stimulate muscarinic M3 receptors on ciliary muscle and GLAUCOMA sphincter pupillae to cause these smooth muscle contraction - In open angle glaucoma - increase trabecular outflow by contracting the longitudinal part of ciliary muscle - In angle closure - contraction of the sphincter pupillae causes miosis (pupil constriction), pulling iris away from trabecular meshwork and opens the angle  Reducing effects Sympathetic Diurnal variation: day 2.5 μl/min during day and 1.5 ul/min during the night Aqueous regulation of humour formation during rest (parasympathetic) is at basal levels and increases with aqueous humour activity (sympathetic) due to activation of β-adrenergic receptors (increased cAMP) and production inhibition occurs via α2-adrenergic receptors activation (decreased cAMP). Active ion transport mechanisms in ciliary epithelial cells result in fluid movement. This is dependent on HCO3−-dependent ion transport mechanisms, along with Na+/K+/Cl− cotransport and Cl− channels, which drive net Na+ and Cl− flux into the posterior chamber and the movement of fluid. cAMP production, has been shown to activate the Na+/K+/Cl− symporter and the Cl− channel. - Reducing IOP Angina Pectoris Practice question  Wk 3: Pharmacokinetics 1 Drug Absorption - Summary of Pharmacokinetics Absorption - Routes of administration / drug formulations - Oral bioavailability (F) Distribution - Factors impacting drug distribution - Volume of distribution (Vd) Metabolism - Phase I and Phase II drug metabolism - Induction and inhibition of drug metabolising enzymes Excretion - Clearance (CL) - Half life Clinical PK - Finding drug information - Single and two compartment models - Clinical PK calculations Why Pharmacokinetics is Vital for Clinical Use of Drugs The aim of clinical drug therapy is to achieve an efficacious response while avoiding toxicity - Adverse drug effects (side effects) to drug toxicity Requires achieving a plasma concentration (Cp) within a set range - Above the minimum effective concentration (MEC) - Below the minimum toxic concentration (MTC) - → the therapeutic range of Cp Plasma How the drug concentration in the bloodstream changes over time concentration (Cp) Staying within the therapeutic range ensures that drug concentrations are high enough to be effective but low enough to avoid toxicity, thereby maximising treatment benefits while minimising risks. Deviating from this range can lead to either ineffective treatment or harmful side effects. - Eg. Taking oral contraceptives once a day to not have too high of a dose.  - Routes of administration Administration: Parenteral = administration avoiding the GIT Parenteral IV Bolus (IVB) - Single dose administered intravenously - Achieve an immediate, fast-acting therapeutic effect - Quick, achieve Cp immediately, can therefore be very dangerous and cause overdose, resulting in toxicity IV Infusion (IVI) - Drug is dissolved in a large volume (saline) - Administered as a constant infusion (volume/time) - Reliable, controllable, maintain constant Cp Intramuscular (im) - Delivery of drug into muscle - Absorbed into systemic circulation fairly quickly - Used for vaccines, Epipen (adrenaline) Subcutaneous (sc) - Delivery of drug into layer between skin and muscle - Short fine needles (~30G) - Insulin administration Administration: Oral (po) Enteral - Vast majority of drug administration is via this route - Easy, self-administration - Absorption is slower, impacted by absorption rate and first-pass metabolism Rectal (r) - Useful when oral route is unavailable (vomiting, unconscious) - Stesolid (diazepam); suppositories Administration: Inhalation Other - Drugs administered directly to airways - Aerosolized particles useful for asthma treatment, gaseous anaesthetics - Given at metered doses, eg. 100ug per inhalation/spray in asthma - Local effects but some systemic absorption Sublingual - Sublingual migraine tablet due to migraine-induced gastric stasis - Useful for drugs that are destroyed in the gut (low pH) - Quick absorption into the systemic circulation - Physicochemical properties of drugs important Administration: Cutaneous Topical - Cream, ointment, gel, lotion, powder etc - Used for skin disorders - Local effect with minimal systemic exposure Ocular - Eye drops most common - Eye ointment (more viscous) - Technique is extremely important Administration: Intraocular (intravitreal) injections Ocular - Injection (27-30G) of drug solution into the vitreous cavity - Necessary for drugs with poor topical penetration - Eg, an-VEGF therapy for wet AMD Intraocular implants - Devices transplanted directly into the vitreous body - Contain a drug reservoir with a mechanism allowing controlled release over a period of time (weeks-months) - Non-biodegradable and biodegradable material Absorption PK Parameters - IV Bolus: The fastest road to achieve the highest plasma concentration. - Oral: Longer absorption phase because it takes time for the drug to go through the GI system and get absorbed into the bloodstream. - Subcutaneous injection: Absorbed from the site of injection. Reasonably slow increase in plasma construct. This is exactly what we want for insulin subcutaneous injection in the treatment of diabetes. We don't want insulin to be absorbed too fast to cause a problem called hypoglycemia where the blood sugar level is too low Extended Release Many clinically important drugs have a short half life Formulations - Therefore require frequent administration to maintain a steady state plasma concentration (Css) in the therapeutic range - Example, nicotine (t1/2 ~ 2 hours) and opioids: oxycodone and fentanyl (t1/2 ~3 hours) These drugs are often available in formulations that release the drug slowly over time - Transdermal patches - Slow-release capsules/tablets Opioid Plasma Concentrations in Function of the Administration Route - IR: immediate-release - ER: extended-release Oral: Absorption peaks and wears off, after taking another dose wears off again. So we have to take the drug frequently. The peak plasma concentration is problematic because it's way beyond the minimum toxic concentration causing a lot of side effects. Administration: You need to three injections in a short time frame to maintain the drug Concession. The peak concentrations are problematic as well because it's way above the minimum toxic concentration causing problems. Extended release formulation: Constant plasma concentration with one dose, you can keep the plasma concentration within the therapy range, is steady, lasts much longer. And gives desired efficacy. Methylphenidate The standard drug treatment for ADHD is methylphenidate (MPH), first sold under the Formulation trade name Ritalin Development Ritalin tablets require 2-3 times daily dosing to provide beneficial responses covering periods of academic and behavioural difficulties - Dose 1 ~7am - prior to school so Cp is within therapeutic levels by 9am - Dose 2 ~11am – second dose to maintain Cp for school classes after lunch - ± third dose after school if behavioural issues were experienced at home MPH is a CNS stimulant and has a high abuse potential - 11am dose typically delivered to the school by parent and locked in a safe - Frequent reports of stolen/diverted/sold Ritalin tablets - Most commonly young parents or siblings of child with ADHD  1. Concerta - An extended-release tablet that produces a similar Cp profile to immediate-release tablets taken 3 times daily - Methylphenidate shows 23 peaks over 12 hours after dosing and then uh wears off. - One capsule taken of Concerta gives stable plasma concentration over the 12 hours over. No need to worry about frequency of the doses and unstable plasma concentrations that might cause toxicity. Osmotic release Has 3 stages of drug release oral system in Outer coat of tablet is IR methylphenidate Concerta There are two separate compartments inside the tablet, one releases ~3 hours after dose and the other compartment ~6 hours after dose Push layer of swelling polymers The drug is released from these compartments via a laser drilled hole - Convenient for children with ADHD treatment in children 2. Daytrana The latest methylphenidate formulation development is patch Daytrana, an extended release transdermal patch for ADHD treatment - Patches are applied to the hip first thing in the morning and removed in the late afternoon – once daily - The patch allows for delivery of methylphenidate at a constant rate across the skin into blood vessels and the systemic circulation - Avoid oral dosing-related first pass effect - Drugs to be able to be administered transdermally must meet physicochemical criteria that allow diffusion across the skin (low MW, high lipophilicity) Oral Absorption Majority of enteral drug absorption occurs in the small intestine Rate and Extent Factors that impact absorption include: – Drug physicochemical properties (MW, lipid solubility, pKa, MW etc) – GI motility (how much surface area is the drug exposed to) – GI content (fed vs fasted, fatty meals, chelators, grapefruit/enzyme  inhibitor) – Splanchnic blood flow (increased after eating) Impact of Red line is optimal Unexpected Absorption Rates - We want the drug to come up above the MEC (minimal effective concentration) and stay there for a bit - The blue curve, we're going to get a peak concentration. above the MTC, the minimum toxic concentration = side effects or toxicity. - If the drug absorption is too slow as seen in the green curve, it can potentially not even reach the MEC = drug won't work. Bioavailability (F) F = 1 or 100% if given an IV bolus & Oral Fraction of an orally administered dose that reaches the systemic circulation Bioavailability F is less than 1.0 because of: - incomplete absorption - first pass metabolism Oral Bioavailability Determination  - First, we need to measure the IUC following IV bolus dose, the purple area. - Here it shows a sharp increase of the plasma concentration and a quick elimination phase. - Secondly, we measure the AUC area under the curve following an oral dose. - So it's absorbed slowly and reaches ac max and wears off. - Then we compare the IUC between these two dose routes using F - If the IUC of the oral is very small, the bioavailability is going to be very small as well. - This is a very important parameter in drug development because you need to ensure new drug compounds to have adequate oral bioavailability. For example: anti hypertensive drugs have a low oral F = high degree of first pass metabolism, paracetamol and some antibiotics, they can have a Biovail if over 90% Practice Question Impact of F on Cmax, tmax and AUC Practice Question Oral Availability Calculation Drug X has an oral availability (F) = 0.25 - If a patient was given a 100 mg tablet, how much of that dose would reach the systemic circulation? - Dose x F = Effective dose Practice Question Practice Question  Wk 3 Lecture 2: Pharmacokinetics 2 - Drug Distribution & Metabolism Distribution – Rate limiting step for distribution of most drugs Importance of Rapid equilibration for well perfused tissues Blood Flow ○ lungs, kidneys, liver, heart, brain Slow equilibration for poorly perfused tissues – bone, adipose tissue and large tumours Diseases and old age can alter distribution – eg, decreased cardiac output Distribution of - Given by IV Bolus Dose Thiopental - Pental = highly lipid soluble and can enter the brain rapidly to have a rapid anaesthetic effect. It also can penetrate to a cell membrane and slip into the fat tissue. Blood: A single IV bolus pushes the drug into the blood all at once. It gets the peak blood concentration right at the start and then it's chipping away quickly from the blood and going to tissues. - Within 20 minutes, you'll notice the next tissue turn up with the drug is the tissues that are well perfused liver and skeletal muscle Adipose: Slow distribution however then the drug accumulates, and stays in the fat tissue for much longer after half an hour or two hours and keeps going Distribution of the drug and tissues is dependent on the blood flow. Different perfusion rate Distribution – Albumin is alkaline and a major protein in the bloodstream Protein Binding Binds acidic drugs and therefore traps them in the blood Protein bound drug vs, free/unbound drug Plasma protein binding limits the concentration of the drug in tissues - Only unbound/free drug can cross membranes Tissue protein binding also occurs - Results in accumulation in tissues rather than the blood Impact the efficacy and elimination of drugs Drugs that are bound to proteins in the blood cannot be metabolised and are too big to be filtered at the kidneys Therefore, changes in the level of plasma protein binding can impact: - Efficacy (more/less free drug gets to the site of action) - Elimination (only free drug can be eliminated) - Example: hypoalbuminemia (low albumin concentration) =Liver disease, burns in patients Distribution – Fat Drugs that are lipid soluble will distribute and “store” in body fat, whereas water Reservoir soluble drugs mostly remain in the blood After distribution to the fat reservoir, the lipid soluble drugs often leak out slowly back into the blood Example: some components of cannabis - Able to be detected in blood/saliva days after administration - Post mortem samples can detect use months later Obesity levels can impact the PK of lipid soluble drug Volume of “The volume into which the drug is distributed uniformly Distribution (Vd) to provide the same concentration as measured in the blood plasma” Also called apparent Vd, not an actual physical volume of body fluid A measure of the tendency of a drug to distribute away from the plasma Determined by measuring plasma concentration immediately after administering a known dose D= Dose administered Cp= Plasma concentration Volume of Distribution Determination NB, Vd is sometimes expressed as L/kg body weight Drug with Large Drugs with a large Vd appear to be dissolved in a very large volume (up to 10, 000L) Volume of Indicates that the drug has left the plasma, so a small Cp Distribution Highly permeable into cells Highly lipid soluble, so most likely to be found in body fat Highly bound to tissue compartments Example – digoxin is highly tissue bound Vd = 640 L Small Volume of A small Vd indicated the drug is confined to plasma Distribution High plasma protein binding than tissue proteins Very polar drugs, water soluble or not lipid Drug is too large to cross out of blood Example – warfarin is extensively plasma protein bound Vd = 10 L Steady State Plasma The rate of missing the drug, which means Concentrations the rate of drug going into the body = the rate (Css) of the drug going out of the body because it's not going up, it's not going down. Vd and Loading A loading dose is a high single dose used to rapidly establish a desired Cp Dose - Metabolism A D Metabolism E Metabolism is the process where lipid soluble and non polar compounds are converted into water soluble and polar compounds This process occurs for all xenobiotics (including drugs) Drug Metabolism – Phase I reactions The Phases – Small primary modifications – Oxidation, reduction or hydrolysis reactions – Often introduce a functional group – Generally lead to drug inactivation Phase II reactions – Conjugation reactions – Add hydrophilic moiety – Increased water solubility – Increased MW Phase I Drug The largest contributor to Phase I metabolism is the superfamily of enzymes Metabolism – Cytochrome P450 (CYP450, P450) CYP450 Primarily located in the liver, but also found in kidney, lung and skin Located in the endoplasmic reticulum Heme containing proteins - The heme iron binds oxygen in the active site Enzyme Constitutive Terminology – Synthesis of enzyme at constant rate – Constant basal level of enzyme Inducible – Enzyme number (& activity) increases following exposure to drug – Increases synthesis rate Inhibition – Enzyme activity is decreased – Competitive Inhibition - Blocks enzyme active site  – Suicide Inhibition - Binds and destroys enzymes – Must synthesise more enzymes to restore activity - If the enzyme is induced, the rate of metabolism will be increased. Therefore, the substrate will be metabolised at a higher rate C.I = competitive inhibitor, called perpetrator drug - If the active site of the enzyme is blocked by another compound C.I, then the substrate (victim drug) cannot be metabolised. The CYP450 Superfamily contains a very large number of enzymes Superfamily – Divided into families and subfamilies – Each individual enzyme is called a CYP450 isozyme - CYP3A4 is the most prominent CYP450 isozyme in humans, being responsible for the metabolism of about 50% of existing drugs Human Hepatic Five most important CYP450 enzymes in the liver of humans CYP450 Content Together they are responsible for the metabolism of ~90% of existing drugs CYP3A4 is responsible for the metabolism of ~50% of all drugs CYP450 Basic CYP450 catalysed reaction is monooxygenation Enzyme-Catalyzed – One atom of oxygen is incorporated into a substrate, another Oxidation oxygen is reduced to a water molecule – It requires two electrons from co-factor NADPH Demethylation Example - Caffeine - Caffeine metabolised into theobromine losing a methyl group (occurs when caffeine wears off and we become tired) CYP450 Induction Smoking induces the CYP450 isozyme CYP1A2 by binding to aryl hydrocarbon receptor Example (AhR), which activates transcription of the CYP1A2 gene Since caffeine is a CYP1A2 substrate smokers metabolise caffeine at a higher rate Caffeine consumption in smokers is generally higher - Higher doses or more frequent doses required to produce same level of efficacy Practice question Practice Question  Wk 3 Lecture 3: Pharmacokinetics 3 Metabolism CYP3A4 – Drug CYP3A4 is responsible for the metabolism of a large number of clinically used drugs Interactions - Example substrate: cyclosporin → immuno-suppresive drug used to turn down the Example I immune system - Example inducer: St John’s Wort → herbal antidepressant - Example inhibitor: verapamil → used to treat heart problems Case example: - 62-yr old pt presents for endomyocardial biopsy 20 months after transplant because of rejection suspicions - Patient maintained on immunosuppressive drugs including cyclosporine - 3 weeks prior to biopsy had started taking St John’s Wort (hypericin) for depression - Biopsy results - acute tissue rejection CYP3A - Drug A 72 year old terminal cancer patient is being treated with fentanyl (CYP3A4 substrate) Interaction patches for analgesia Example II - 4 weeks later the pt is started on verapamil for HTN - 2 days later the pt presents to ED with respiratory depression and heavy sedation - Verapamil → uses a calcium channel blocker to decrease blood pressure CYP450 induction Ethanol is a strong inducer of the CYP450 isozyme CYP2E1 – Alcohol and - Ethanol increases the synthesis of CYP2E1 Paracetamol - This increases the enzymatic activity of CYP2E1 - CYP2E1 substrates are metabolised at a higher rate Paracetamol is metabolised by CYP2E1 to a toxic metabolite  CYP2E1 Metabolism Cytosolic Phase I Alcohol Dehydrogenase (ADH) Enzymes - Converts alcohols to aldehydes Aldehyde Dehydrogenase (ALDH) - Converts aldehydes to carboxylic acids Inhibition of cytosolic enzymes can have a clinical advantage Example - disulfiram (AntabuseTM) - Treatment for chronic alcoholism - Inhibits ALDH - Results in acetaldehyde build-up with alcohol ingestion - Intense nausea, headache, flushing, vomiting, vertigo will occur following ingestion of alcohol Disulfiram - Disulfiram → blocks the enzyme so you Mechanism experience hangover symptoms for days Bioactivation - Sometimes, activity (efficacy) of the metabolite is greater than the parent Pharmacology compound When the efficacy lies in the metabolite, the drug is known as a PRODRUG L-DOPA (levodopa) - Patients with Parkinson’s disease require dopamine - Dopamine (DA) cannot cross BBB (blood brain barrier) - L-DOPA crosses BBB, and is then metabolised to DA - Able to get drug to site of action Bioactivation - Sometimes, activity (toxicity) of the metabolite is greater than the parent compound Toxicology - When the toxicity lies in the metabolite, the drug/chemical is said to be BIOACTIVATED Examples include: - Paracetamol – NAPQI by CYP2E1 - Aflatoxin (peanut mould) – Epoxide by CYP450 enzyme For any of the above, if they do not get metabolised, they will not exhibit any toxicity Phase II Drug Increases the MW and water solubility Metabolism → final - Now targeted for excretion stage in drug Products of Phase II reactions are not active and not toxic metabolism → all Three main enzymes involved about modifying - Uridine diphospho (UDP) glucuronsyl transferase (UDP-GT) – the drug by Sulfotransferase (ST) making it heavier - Glutathione S-transferase (GST → NOTE: The T at the end tell us its phase II enzyme) Broad substrate specificity → can take on multiple drugs Glucuronidation Enzyme (UDP-GT) transfers glucuronic acid from the co- factor (UDP-GA) to the substrate to form a glucuronide (final metabolic product of most drugs) Quantitatively the most important Phase II pathway - For drugs and endogenous compounds - The majority of drugs are excreted as glucuronides Large family of UDP-GT enzymes - All have broad substrate specificity - Glucuronides are excreted in urine and bile Drugs can either be directly conjugated or metabolised by CYP450 first and then conjugated Example Glucuronidation - Paracetamol Sulphation by Reactions catalysed by sulfotransferase (ST) Conjugate sulphate from the Sulfotransferase co-factor (ST) - 3’-phosphoadenosine-5’-phosphosulfate (PAPS) Broad range of endogenous (stuff already in the body) and exogenous (stuff you take and add into your body → drugs) substrates Activity level is controlled by the level of co-factor (PAPS) - Increased dietary sulphate can increase PAPS stores - Poor diet (eg anorexia nervosa → not enough sulphate in their diet) can deplete PAPS inhibiting this Phase II pathway Example Sulphation - Paracetamol Glutathione Glutathione-S-transferases (GST) Conjugation by Glutathione is abundant, mostly as reduced form, a GST tripeptide Glutathione (GSH) conjugates to endogenous and exogenous toxins GSH conjugation is catalysed by GST  Targets reactive electrophiles - Protects cellular macromolecules Paracetamol – NAPQI Detoxification Paracetamol Paracetamol metabolism → goes through 3 Metabolism phase II and 1 phase I pathway CYP450 All of the genes coding for CYP450 families 1-3 are polymorphic (big variation Pharmacogenetics across humans) (PGx) - Four main phenotypes have been identified - Poor metabolizers (PM) who lack the functional enzyme - Intermediate metabolizers (IM) who are heterozygous - Extensive metabolizers (EM) who carry two normal alleles → considered the standard dose for all drugs - Ultrarapid metabolizers (UM) who have multiple gene copies For Example,rates of drugs metabolised by CYP2D6 has been found to differ by 1000-fold Can Result In Unintentional Plasma Concentrations - Too high in PM, causing ADR (adverse drug reactions) or even toxicity - Too low in UM, resulting in decreased efficacy Clinical Relevance It is well established that a small percentage of patients with depression do not of polymorphisms respond to antidepressant treatment - Known as treatment-refractory The majority of antidepressants are metabolised by CYP2D6 In a recent study, it was found that the patients who treatment refractory have a 10-fold higher rate of the CYP2D6 UM phenotype In Contrast,patientswhoareCYP2D6PMshowincreased incidences of ADR (because drug is not getting metabolised fast enough → getting stuck in the plasma → increased plasma concentration which leads to increased side effects) - Can result in non-compliance So what can we do PGx is an emerging area. Currently, genotyping for CYP450 enzymes does not about it? routinely occur Drug companies currently screen all new drugs to ensure they are not substrates for polymorphic CYP450 enzymes - If they are, they are removed from development, usually in favour of  another candidate which is not a polymorphic substrate - The FDA have recently included clauses about polymorphic CYP450 enzymes in their guidelines It is expected that genotyping will increase drug efficacy y10-20%, and reduce ADR’s by 10-15% Genotyping Cost Decreasing,some become common practice the future Example question  Wk 4 Lecture 1: Pharmacokinetics 4 - Elimination Majority of Drugs are Renally Excreted Following metabolism, compounds are more water soluble Drugs bound to plasma proteins are unable to be filtered Excretion Rate Amount of drug removed per unit time Proportional to concentration First-order kinetics At steady state plasma concentration (Css), elimination rate = dose rate Drug out = Drug in First Order First order kinetics occur when a constant Kinetics proportion of the drug is eliminated per unit /Elimination time Half-life (t1/2) of a drug is the time it takes for the amount of a drug’s active substance in your body to reduce by half. - It is a constant - ~93% of drug is gone after 4-5 half-lives Half life The time taken for the plasma concentration to fall by half Renal Clearance Renal clearance (CLren) is defined as: the volume of plasma that is cleared of a drug by kidneys per unit time Hence CL represents the volume of plasma cleared of drug per unit time  A low GFR (kidney disease) would result in a lower Vu, therefore reduced clearance Zero Order Some drugs can only be metabolised at a constant rate (constant amount per unit Kinetics (Zero time) Order Elimination) For these drugs elimination rate is constant, regardless of Cp Example: ethanol - NAD+ required as a co-factor for both ADH and ALDH - NAD+ is produced by the liver at a constant limited rate - Ethanol metabolism is saturable - Ethanol is metabolised at a constant ~10 g/hr - Follow a linear elimination phase as the system is saturated Example: Ethanol NAD+ required as a co-factor for both ADH and ALDH NAD+ is produced by the liver at a constant limited rate Ethanol metabolism is saturable Ethanol is metabolised at a constant ~10 g/hr Follow a linear elimination phase as the system is saturated Drug Excretion – Some drugs are excreted without requiring metabolism or with low extent of Fraction excreted metabolism (eg, gentamicin, digoxin) unchanged - Therefore drug action is terminated by excretion The fraction of a drug excreted unchanged in the urine is the pharmacokinetic parameter fe If fe = 1.0 then all the drug is excreted unchanged in the urine - Therefore renal function is very important If fe is < 1.0 then metabolic elimination has a role - Hepatic function is important The Relationship Clearance (total body clearance) is the volume of plasma that is cleared of drug per Between Clearance unit time and Rate of - Clearance is the sum of renal CL, hepatic CL and pulmonary CL Elimination - Normally expressed in mL/min or L/hr Clearance is the proportionality factor that relates the rate of elimination to Cp - Rate of elimination = CL x Cp Half Life is Dependent on Elimination Rate Half-Life is Elimination for most drugs involves filtration through the kidneys Dependent on Vd - Therefore, this is dependent on how much is available in the plasma Volume of distribution influences the elimination rate constant CL can also be Instead of collecting urine samples, CL can be determined by collecting blood determined from samples at known intervals and determining AUC AUC Half-Life and The time to reach steady state plasma Dosing concentration (Css) is dependent on t1/2 Generally the dosing interval is one t1/2  - Compartment Model Single This model assumes the body is one single compartment Compartment Model Two Compartment This model assumes there is a central compartment and a connected peripheral Model compartment Theoretical plasma concentration time curve - Acids and Bases Weak Acid Drugs Weak bases Henderson-Hassel balch equation Aspirin + Stomach vs Small Intestine - pKa and pH can allow us to predict where orally administered drugs are absorbed Ion Trapping – The standard treatment for aspirin Aspirin Overdose overdose is urinary alkalinisation – Keeps the drug in an ionised state in the urine – Increases urinary excretion Example MCQ  Wk 4 Lecture 2: Toxic Effects of Drugs - Terminology Adverse drug reaction (ADR) – aka side effects “Any response to a medicine which is noxious and unintended, and which occurs at doses normally used (for prophylaxis, diagnosis, therapy) in humans” Excludes overdose, drug abuse, medication errors Drug toxicity Adverse effects of a drug that occur when the dose or plasma concentration of the drug has risen above the therapeutic range, either unintentionally (eg medication error) or intentionally (eg drug overdose) Serious adverse A serious adverse drug reaction / effect is any that: drug reaction Is fatal (ADR) Is life-threatening Is permanently or significantly disabling Requires intervention to prevent permanent impairment or damage Requires or prolongs hospitalisation Causes a congenital anomaly Margin of Safety Some drugs have a small margin of safety but are still commonly utilised: warfarin (anticoagulant) lithium (mood stabiliser) Digoxin (inotrope for heart failure) Some drugs have a relatively large margin of safety Penicillin (antibiotic) All drugs can produce adverse drug reactions (ADRs)! Therapeutic Index (TI) Context is Theophylline – drug administered intravenously to children in hospital emergency Important rooms for acute bronchoconstriction Serum concentration and ADEs: - 15 μg/ml produces life-saving bronchodilation - 25 μg/ml can cause mild side-effects - 35 μg/ml can cause potentially serious adverse effects - 42 μg/ml can cause severe adverse effects Margin of safety is very low (