Pharmacology

Name the piece of legislation that covers:

1. Veterinary medicine

2. Veterinarians prescribing human medicine

3. Veterinary use of controlled drugs

1. Agricultural Compounds and Veterinary Medicines Act (ACVMA)

2. Medicines Acts (MA)

3. Misuse of Drugs Act (MDA)

12 Obligations veterinarians have when authorising RVM or PM

1. Choice and use of product is justified (with appropriate training + information to owner)

2. ONLY authorise RVMs after vet consultation (sufficient information)

3. If complaint is laid, actions other vets would take in the same situation is considered (decision must be consistent with peers)

4. Can authorise treatment without seeing animal (must have good knowledge of situation eg. regular farmer + knowledge of what diseases they commonly encounter + confidence that what the farmer describes is accurate + competent to administer themselves)

5. Must be bona fide client (animal is under your care)

6. Confirm client is competent at administering treatment (training)

7. Arranged provision of ongoing care for patient (in case of adverse drug reaction)

8. Documented authorisation (findings from clinical exam and details of RVM authorised) kept for at least 5 years

9. Can authorise for future supply for production animals (vet must have good knowledge of client, farm, and have ongoing recheck) + finite time and amount of drug + client must keep record of how much is used

10. Must provide written authorisation to client to obtain drug elsewhere (you are still responsible)

11. Critically important antibiotics - Alternatives? Drive to reduce

12. Controlled drugs: Record every sale or use + reconcile at least monthly

Authorising - Vet creates documented approval allowing client to (3):

1. Purchase a particular RVM to administer to particular animal in accordance with instructions of vet

2. Hold an RVM for anticipation of its use (eg. intracillin for mastitis)

3. In accordance with instructions of authorising vet

Dispensing vs. prescribing medicine

Dispensing - Preparing vet medicine for owner (eg. packing 2 dosages from commercial packaging to appropriately labelled alternative)

Prescription medicine - Human medicine that can only be sold under prescription

5 Features of a consultation (required for authorisation of RVMs)

1. Interview with client and exam animal

2. Collect + record sufficient information to ensure course of action is appropriate

3. Obtain consent to proposed course of action

4. MUST be responsible for the ONGOING health and welfare of the animal (eg. arranging after care when considering potential for adverse reaction)

5. Determine and provide appropriate level of advice and training of owner

What is “discretionary use” of medicine?

Use of medicine that is NOT listed on the label (eg. prescribing sheep medication for alpacas)

• Must be able to justify discretionary use (reasonable decision by peers)

Describe the cascade for authorising/prescribing medicine

1. Is there an appropriate on-label (RVM authorised by vet with instructions following the label eg. correct species, dose, frequency, route)?

2. Is there an appropriate off-label (RVM used in a manner NOT specified in the conditions on registration for the product eg. incorrect species, different dose rate, frequency etc.)?

3. Is there a human PM (Under the Medicines Act)?

4. Compounding (specifically compounded by/on authority of that vet)

5. Overseas (talk to MPI to seek permission to import)

Registered vs. restricted vs. controlled veterinary medicines

Registered = All drugs must be registered with the ACVMA (ensures the benefits of the treatment will outweigh the adverse effects)

• Contains compulsory information: species, condition to treat, dose rate, route, withholding periods (with sufficient trial data)

Restricted = Registered vet meds which pose some risks and use must be authorised by a vet with an APC (annual practicing certificate)

• CAN be held by owner

• Unrestricted - Anyone can purchase from supermarket (eg. Drenches, flea preparations, supplements, flea collars)

Controlled = Potential for addiction, and abuse (cannot use class A) with controlled drug register

List the 4 pillars of pharmokinetics (+ definitions)

1. Absorption = Movement of drug into circulation

2. Distribution = Movement of drug from circulation to tissues

3. Metabolism = Body response to drug is to remove it by converting it to a water-soluble metabolite which is NOT toxic

4. Excretion = Elimination of water-soluble metabolite via faeces, urine, milk, sweat

Describe the concentration of drugs in circulation overtime for IV vs. other routes of administration (eg. SC, PO, IM, IP)

IV = No absorption as directly into bloodstream → Gradual decreasing concentration (elimination)

Oral/SQ/IM/IP = Requires absorption and transport across the epithelium → Gradual increasing concentration (absorption) and then decreasing concentration (elimination)

List 3 routes of enteral administration (+ descriptions)

1. Oral = Absorbed through SI mucosa → Portal vein → Liver metabolism → Systemic

2. Sublingual = Absorption under tongue → Bypass liver = More effective

3. Rectal

List 7 routes of parenteral administration (+ cautions)

1. Intravenous (IV) = No absorption required

   • Caution: No insoluble salt suspensions or oils (must be completely soluble)

2. Intramuscular (IM)

   • Caution: Avoid semitendinosus → Potential injection in femoral artery and sciatic nerve

   • Use cranial thigh (quadriceps)

3. Subcutaneous (SC)

4. Intraperitoneal (IP) = Rats (IV difficult)

   • Caution: Massive absorption due to high SA and blood supply

5. Inhalational = Rapid absorption across alveolar membranes

6. Transdermal = Across skin

   • Caution: Slow absorption as natural barrier (faster in highly vascular dermis eg. MM)

7. Epidural/intrathecal

3 things required for drug absorption into bloodstream

1. Dissolution of drug

2. Movement of drug out of site of administration

3. Movement of drug into blood vessels = Cessation of absorption

Describe the pathway of absorption for oral drugs

1. Oral pills have coatings to pass through the acidic stomach in order to spare the drug

2. Absorbed across the small intestinal membrane

3. Cranial mesenteric vein → Hepatic portal vein

4. Liver metabolises drug (hence metabolites necessary for action)

Describe how solubility of drugs influences the route of administration and the concentration overtime (eg. 3 forms of penicillin)

IV = Water-soluble ONLY → Rapid onset

IM/SC = Suspensions of insoluble salts/oils → Slow release

Example: Penicillin

1. Sodium penicillin IV = Water-soluble → Rapid absorption (highest Cmax) and rapid elimination

2. Procaine penicillin IM = Insoluble salt solution → Slower absorption (lower Cmax) and remains in body for longer

3. Benzathine penicillin IM = Insoluble → Extremely slow absorption and long duration of action (lowest Cmax → increase dose)

List 4 ways drugs can pass across the phospholipid membrane with examples (+ exception)

1. Passive diffusion = Small, highly lipid-soluble drugs (eg. fentanyl, diazepam, ethyl alcohol)

2. Facilitated diffusion = Large, highly lipid-soluble drugs (eg. corticosterone and drugs that must cross the BBB)

3. Active diffusion = Uncommon (eg. medicinal drugs, iron salts, fluorouracil)

4. Endocytosis = Large molecules which cannot enter by other means → Invagination of the cell membrane to form a vesicle around the drug (eg. oily preparations and cholesterol)

Exception: Penicillin = Water-soluble and requires specific transport molecules (excreted in urine as penicillin)

Drugs must be lipophilic to cross the phospholipid membrane to exert their effects

List 4 factors influencing absorption rate of PO drugs

1. Blood flow (shock → reduced absorption)

2. Contact time (vomiting/diarrhoea → reduced absorption)

3. Food (drug binds food and reduces availability for absorption)

4. Carrier-mediated transport (eg. P-glycoprotein)

List 4 factors influencing absorption rate of IM/SC drugs

1. Blood flow

   • Exercise → increased absorption of IM drug

   • Accidental intrafat injection → decreased absorption as poorer blood supply

   • SC → slow and variable (not as vascular as muscle)

   • Higher temperature → increased absorption rate

2. Inflammation (disrupted membrane → good absorption)

3. pH

4. Formulation (oil, insoluble salts, water-soluble etc.)

Use the Henderson-Hasselbalch logic to explain how drug pKa and environment pH affects the solubility of the drug

• pH = pKa → 50% ionised / 50% unionised

• pH < pKa → more protonated

• pH > pKa → more unprotonated

Weak Acids: HA ⇌ H⁺ + A⁻

• Low pH (acidic) environment = Shift → HA (unionised and lipid-soluble) → Cross cell membranes

• High pH (alkaline) environment = Shift → A⁻ (ionised and water-soluble) → Trapped in compartment

Weak Bases: BH⁺ ⇌ H⁺ + B

• Low pH (acidic) environment = Shifts → BH⁺ (ionised and water-soluble) → Trapped in compartment

• High pH (alkaline) environment = Shifts → B (unionised and lipid-soluble) → Cross cell membranes

EXAMPLE: Describe absorption of aspirin in mouth vs. stomach vs. duodenum

• Aspirin pKa = 4.4

• Oral pH = 7.4

• Stomach pH = 1

• Duodenum pH = 6

Aspirin = Weak acid

1. Mouth pH > Aspirin pKa → More drug unprotonated and IONISED (A-) = Water-soluble and cannot enter through oral mucosa = ION TRAPPING

2. Stomach pH < Aspirin pKa → More drug protonated and UNIONISED (HA) = Lipid-soluble and absorbed through gastric mucosa

3. Duodenum pH > Aspirin pKa → Although more drug unprotonated and ionised, the higher SA of the SI means more aspirin is absorbed here than in the stomach

What is “ion trapping”?

• Use erythromycin as an example when used IMM for mastitis

Ion trapping = Drug is initially lipid-soluble upon administration and is transported across the cell membrane. In the new site, the pH is different and it becomes water-soluble and therefore CANNOT move back

Example: Erythromycin = Weak base

1. Erythromycin given IMM into pH of 6.8 (acidic)

2. pH < pKa → Drug remains protonated (BH+) and ionised (water-soluble) and hence cannot enter plasma

3. Drug remains in milk where it is required

Define bioavailability

Fraction of drug that reaches systemic circulation (IV always 100%)

List 4 factors that distribution of a drug is affected by (+ examples)

1. Blood flow (higher blood flow → more distribution of drug to tissue)

   • Brain > Muscle > fat

2. Capillary permeability (more permeable → more distribution of drug to tissue)

   • BBB = Astrocytes tightly adhere endothelial cells together → Fewer drugs pass

     • No access to ionised drugs (eg. penicillin and aminoglycosides) EXCEPT with meningitis → Compromised BBB

     • Lipid-soluble drugs rapidly equilibrate and redistribute to the brain

3. Drug structure (size eg. peptides and proteins and lipid solubility)

4. Protein binding (higher PB → lower distribution)

   1. Many drugs bound to albumin which keeps them in circulation, preventing distribution to tissues

   2. Example: Thiopentone Na and phenylbutazone = High PB → Lower subsequent doses required as fewer proteins available for binding → More free drug available

Define volume of distribution (Vd) and its use

Definition: The theoretical volume that would be required to contain the total amount of drug in the body at the same concentration as in plasma

• Higher Vd (eg. thiopentone) → higher distribution to all parts of body (extensive tissue binding)

• Lower Vd (eg. aspirin) → stays in plasma for long periods so lower dose required to obtain necessary concentrations

Use: Used to calculate loading dose (higher Vd = higher loading dose)

Features of acidic vs. basic drugs (5)

Acidic drugs tend to: eg. Aspirin

1. Have higher oral bioavailability

2. Have higher hepatic clearance

3. Have higher protein binding

4. Have smaller Vd

5. Get absorbed better in stomach and duodenum

Basic drugs tend to: eg. Morphine

1. Have poorer protein binding

2. Have larger Vd

3. Have better CNS penetration

4. Be less selective

3 Outcomes of biotransformation

1. Prodrug → Active drug (eg. aspirin → salicylic acid)

2. Toxic → Non-toxic

3. Active drug → Inactive metabolite

Describe the 2 phases of drug metabolism (+ drug examples)

1. PHASE 1 = Reactive “handle” attached by cytochrome P450 enzyme (CYP450) via

   1. Oxidation (hydroxylation, dealkylation, deamination) eg. dexmedetomidine

   2. Reduction eg. warfarin

   3. Hydrolysis eg. lignocaine

2. PHASE 2 = Conjugation with POLAR group (water-soluble molecule) via

   1. Glucuronidation

   2. Sulfation

   3. Methylation

   4. Acetylation

What is cytochrome P450? Why is it important?

Protein in hepatocytes which carry out phase 1 of drug metabolism (also in GIT, lungs, kidney and skin)

Importance: Important to know which CYP enzyme metabolises which drug (drug interactions)

List 5 factors influencing the CYP450 system (+ examples)

1. Enzyme Inducers = Drugs that increase synthesis and activity of CYP450 enzymes → Faster metabolism

   • eg. Phenobarbitone, alcohol, St John’s wort, griseofulvin

   • 2nd dose of phenobarbitone remains in the system for LESS time → Increase dose rate to have the same effect

2. Enzyme Reducers = Drugs that reduce effect of CYP450

   • eg. Ketoconazole, tramadol, quinidine, grapefruit juice

3. Abnormal Phenotype = Abnormal CYPs which turn harmless compounds into toxins due to altered metabolism (faster/slower) of drugs

4. Liver Disease = Slower metabolism → Lower dose required

5. Neonatal/Geriatric = Lack of enzymes to metabolise drugs

List 6 compounds which drugs are conjugated with during phase 2 of drug metabolism (+ species differences)

1. Glucuronide (NOT cats) = glucuronidation

2. Sulphate (NOT pigs) = sulfation

3. Acetyl (NOT cats and dogs) = acetylation

4. Methyl = methylation

5. Glycine = methylation

6. Ornithine (ONLY birds)

Define “first pass metabolism”

Liver metabolises the drug FIRST before entering the systemic circulation to the target organ

Describe the mechanism and effect of enterohepatic recycling

Mechanism:

1. Conjugated drug excreted in bile (with glucuronic acid)

   1. Unconjugated drug enters sinusoids to be excreted in the kidney

2. GIT bacteria remove conjugate for energy (sugar)

3. Drug available for resorption as no longer water-soluble

4. Absorbed back into bloodstream for re-use

Effects: Prolonged duration of action (esp. opioids) → Blips in drug concentration

3 Factors that influence renal elimination of drugs

1. GFR

2. Active excretion of water-soluble drugs (organic anion and cation transporters)

   • eg. Penicillin (given transporter competitor eg. TMP to make penicillin remain in body for longer)

3. Passive resorption of lipid-soluble drugs (pH important for ion-trapping)

Define withholding period and times for different species and products

• Ruminants

• Pigs/horses

• Birds

• Camelids

• Rabbits

WHP: Time taken for drug to fall below the maximum residual limits (MRL) in nearly all animals → Defines period which must elapse between last treatment and use of animal as a food product

Ruminants: 91d (meat) and 35d (milk)

Pigs/Horses/Birds/Camelids/Rabbits: 63d (meat)

Birds: 10d (eggs)

List 6 drug targets (+ definitions)

1. Receptors = Protein molecules which bind specific ligands (drugs) and exert a response

2. Ion channels

3. Enzymes = Drugs compete with substrate for enzyme active site

4. Carrier molecule = Transportation of small molecules into and out of the cell

5. DNA (eg. antibiotics and chemotherapy drugs)

6. Non-specific (eg. osmotic diuretic and radioactive iodine)

Describe the structure of the drug molecule required to bind to extracellular vs. intracellular receptors

Extracellular Receptors - Drugs do NOT cross the cellular membrane → Large and water-soluble

Intracellular Receptors - Drugs must cross the cellular membrane to exert effect → Small and lipid-soluble

List 4 superfamilies of receptors

• Onset of action/response

• Intracellular/extracellular receptor

• MoA

• Example receptors

1. Ionotropic Receptors = Ligand-gated ion channels

   • Response: Rapid

   • Extracellular

   • MoA: Ligand binds receptor to open ion channel

   • Examples: Nicotinic ACh, AMPA, GABA receptors

2. Metabotropic Receptors = G protein coupled receptors

   • Response: Slower than ionotropic (s - mins)

   • Extracellular

   • MoA: Ligand binds receptor to change conformation of G-protein → Open channel or activate enzyme

   • Examples: Opioid, adrenergic, muscarinic ACh receptors

3. Tyrosine Kinase Coupled Receptors = Controlling cell growth

   • Response: Minutes to hours

   • Extracellular

   • MoA: Ligand (hormone) binds receptor to activate tyrosine kinase which phosphorylates the tyrosine residue on their substrates intracellularly → Activation of many intracellular signalling pathways

   • Examples: Hormones eg. insulin, thyroid, growth factor receptors

4. Nuclear Receptors = Receptors in nucleus or cytoplasm

   • Response: Hours to days BUT widespread response throughout body (longer duration of action)

   • Intracellular

   • MoA: Interferes with DNA to increase/decrease gene transcription and therefore, protein synthesis

   • Example: Corticosteroid receptors

How do must drugs influence ion channels? Provide an example

Most drugs block ion channels (NOT open) eg. lignocaine local anaesthetic → Blocks Na+ channels to prevent entry into nociceptive neurons

List 3 drugs which bind enzymes to exert their effects

1. Most antibiotics

2. Organophosphates: Block enzyme that breaks down ACh → Accumulation of ACh

3. NSAIDs: Bind cyclooxygenase enzyme necessary for conversion of arachidonic acid to prostaglandins

List 2 drugs which depend on binding to carrier molecules to exert their effects

1. Antidepressants = Block reuptake transporters → Prevents reuptake of serotonin (5-HT) and noradrenaline into presynaptic neuron → Increased neurotransmitter concentration in synaptic cleft → Increased ruing and intensity of post-synaptic effect

2. Ivermectin = Actively pumped out of the BBB by P-glycoprotein carrier molecule (small and lipophilic enough to passively cross the BBB)

Describe 3 ways that receptors can be complex (+ examples)

1. Same transmitter → Multiple receptor subtypes → Different effects

   • Example: Adrenaline/NA

     • α₁: vasoconstriction

     • α₂: ↓ neurotransmitter release (presynaptic)

     • β₁: ↑ heart rate & contractility

     • β₂: bronchodilation, vasodilation

2. Same receptor → Different effects in different tissues

   • Example: β₂ receptors

     • Bronchi → Bronchodilation

     • Uterus → Relaxation

     • Skeletal muscle vasculature → Vasodilation

3. Receptor regulation = Number depends of level of stimulation

   • Up-regulation (chronic antagonist exposure) → Supsensitivity when drug withdrawn

     • eg. Chronic β-blocker use → β-receptor up-regulation → rebound tachycardia if stopped suddenly

   • Down-regulation (chronic agonist exposure) → Tolerance

     • Chronic β-agonist use → β-receptor down-regulation → reduced bronchodilator response

   • Paradoxical pharmacology = Long-term drug exposure produces opposite effects to acute effects due to receptor regulation

What is tachyphylaxis? 3 Mechanisms

Definition: Rapid decrease in receptor response to a large initial dose of drug over a short period of time

• Acute and reversible reduction in drug effect

• Increasing drug dose will NOT change response as ALL receptors are blocked (must wait 1 week to give smaller dose)

Mechanisms: Desensitisation

1. Decreased synthesis of receptors

2. Increased synthesis of enzymes to deactivate existing receptors (eg. specific kinases which phosphorylate receptors)

3. Internalise receptors via endocytosis

What is tolerance? 3 Mechanisms

Definition: Chronic decrease in receptor response to repeated exposure of drug

• Chronic and irreversible reduction in drug effect

• Increasing drug dose WILL change cellular response

Mechanisms: Desensitisation

1. Decreased synthesis of receptors

2. Increased synthesis of enzyme to metabolise drug → Faster elimination of drug from body

3. Internalise receptors via endocytosis

Dose response curve

• Definition

• Emax

• EC50

Definition: Semi-log plot with log (drug concentration) on x-axis and % of maximum response on y-axis

• Phase 1 = Small initial response

• Phase 2 = Exponential response

• Phase 3 = Drug occupies ALL receptors which become saturated (more drug → NO response)

Emax: Maximum response of drug

EC50: Effective concentration 50 = Potency of drug (i.e. concentration drug required to be 50% effective)

• High specificity and drug binding afinity

Which drug is most potent? Which drug is most effective? Why?

Most potent = Lowest EC50 → Left shift on x-axis

• Drug X = Most potent

• Drug Z = Least potent (highest EC50)

Most efficacious = Highest Emax → Drug exerts more response AFTER binding

• Drug X and Y = Highest efficacy (highest Emax)

• Drug Z = Lowest efficacy (lowest Emax)

What is the “therapeutic index (TI)"?

Measure of how safe the drug is = Range of doses which are effective WITHOUT toxic side effects

TI = TD50 ÷ ED50

• TD50 = Toxic dose for 50% of the population

• ED50 = Effective dose for 50% of the population

ALWAYS choose drug with higher TI → Higher margin of safety

Types of Drug-Receptor Interactions

1. Full agonist

2. Partial agonist

3. Inverse agonist

4. Competitive antagonist

5. Non-competitive antagonist

Drug which interacts with a receptor to produce:

1. Full agonist = Maximum response → Achieve Emax

2. Partial agonist = Response LESS than Emax → Low efficacy

   • Acts as antagonist in presence of full agonist

   • X-axis shifts to the right when partial agonist added to full agonist

3. Inverse agonist = Negative/opposite response

4. Competitive antagonist = Irreversible/reversible competition with agonist for SAME binding site

5. Non-competitive antagonist = Binds to another site (allosteric site) to change conformation of agonist receptor to prevent agonist from binding

Pain vs. nociception

Pain = Unpleasant sensory and emotional experience associated with actual/potential tissue damage

Nociception = Transmission of pain signals to cerebral cortex (ONLY sensory component and NO emotional component)

Hyperalgesia vs. allodynia

Hyperalgesia = Increased sensitivity to a stimulus which is normally painful and may persist AFTER pain is treated

• eg. Hurt toe → Extreme pain when touched again

• Mechanism: Excitation of peripheral nerve endings stimulate other receptors to increase response of EP receptors

Allodynia = Pain caused by stimulus that is NON-painful

• eg. Clothing or grooming causes pain

List 5 ways to assess pain

1. Behaviour = Unprovoked assessment of response to normal stimuli)

2. Response to analgesia = Change in behaviour, ANS etc.

3. Autonomic signs = HR and RR

4. Electroencephalogram (EEG)

5. Pain scores = Glasgow composite pain scale

Describe the pain pathway

1. Initiation of pain signal = Excitation at the peripheral nerve endings

   1. Tissue injury damages the cell membrane → Release of arachidonic acid

   2. Arachidonic acid → PGE2 by COX enzymes

   3. PGE2 binds EP receptors which open Na+ and Ca2+ channels

      • Also binds bradykinin receptors and acid-sensing ion channels which sensitise EP receptors

   4. Influx of Na+ and Ca2+ → Membrane depolarisation

   5. Action potential generated

2. Conduction of pain signal = Through A-delta and C fibres which synapse in the dorsal horn of the spinal cord (lamina I and II)

   • A-delta = Fast, sharp and well-localised pain

   • C fibres = Slow, dull and aching pain

   • Lamina II (substantia gelatinosa): Region of the dorsal horn in the spinal cord where modulation of pain signals in the 2nd-order neuron occur through release of excitatory/inhibitory neurotransmitters

     • Excitatory = Substance P and glutamate released by 1st-order neurons to activate 2nd-order neurons

     • Inhibitory = GABA, glycine, endogenous opioid, noradrenaline, serotonin → Inhibit substance P

     • Complex interaction of 1st-order, 2nd-order neurons and descending pathways which synapse gap between 1st- and 2nd-order neurons

3. TWO ascending tracts of spinal cord

   • Spinothalamic Tract = 2nd-order neuron decussates and ascends to the contralateral nuclei in the thalamus → Synapse with 3rd-order neuron → Somatosensory cortex → Localise pain

     • Projections into the periaqueductal grey matter to modulate pain via descending inhibitory pain pathways (opioid analgesia)

   • Spinoreticular Tract = 2nd-order neuron ascends to the contralateral reticular formation of the brainstem → Thalamus → Hypothalamus → Cortex → Emotional component of pain

Describe the gate control theory of pain

Aggressive touch/pressure on a painful area modulates the pain signal in the substantia gelatinosa → Decreased pain sensation

1. Rubbing/pressure → Activation of non-nociceptive fibres (Aβ-fibres = touch-sensitive) which synapse in the substantia gelatinosa (dorsal horn)

2. The Aβ-fibre activity recruits GABAnergic interneurons in the substantia gelatinosa

3. Interneurons release GABA inhibitory neurotransmitter

4. GABA neurotransmitters inhibit the C-fibre nociceptors and modulate the nociceptive pathway (prevent conduction of pain signal to the somatosensory cortex)

2 Types of chronic pain (+ mechanisms)

Long-standing or repeated noxious stimulus (injury, inflammation) →

1. Peripheral sensitisation = Increased pain perception from normally painful stimuli (hyperalgesia) due to:

   1. Up-regulation of Na+ ion channels → Increased sensitivity of nociceptive fibres (C and A-delta fibres) → Lower threshold for activation

   2. Release of neuropeptides (substance P and CGRP = calcitonin gene-related peptide) → Release of PGE2 and bradykinins

2. Central sensitisation = Pain becomes amplified, persistent, and independent of original injury (allodynia) due to:

   1. Neuropeptides and glutamate activate NMDA receptors in the dorsal horn

   2. NMDA (memory) receptor activation → Increased Ca2+ influx

   3. Changes in gene expression and ion channel activity → Long-term increase in neuronal excitability

   4. SAME sensory input, but increased response at the level of the CNS (increased excitability of neurons in the dorsal horn)

List 6 classes of analgesic drugs

1. Opioids

2. NSAIDs

3. Local anaesthetics

4. Alpha-2-agonists

5. NMDA antagonists (ketamine)

6. GABA agonist (gabapentin)

5 Desired effects of NSAIDs

1. Anti-inflammatory #1

2. Analgesic via anti-inflammatory effect (pain = cardinal sign of inflammation)

   • Inferior analgesic effects to opioids

   • Corticosteroids are anti-inflammatory ONLY → Suggests TWO anti-inflammatory mechanisms of action

3. Anti-pyretic

4. Anti-thrombotic = Reduce thromboxane concentration in circulation

5. Anti-endotoxic = Reduce effect of endotoxins (eg. LPS) released by G- bacteria

   • Antibiotics do NOT treat endotoxins (only bacteria itself)

Describe the MoA of NSAIDs

NSAIDs inhibit action of COX enzymes necessary to convert arachidonic acid (from cell membrane damage) into prostaglandins

• Free arachidonic acid takes the lipoxygenase pathway to produce leukotrienes (instead of cyclooxygenase → PGE2)

Function of COX1 vs. COX2 vs. COX3

COX1 = Assists with NORMAL physiological functions

1. Protect gastric mucosa

2. Maintain renal blood flow

3. Mediate production of thromboxane in platelets AND assists with platelet aggregation

COX2 = Inducible enzyme triggered by inflammation and injury

• Produces PGI2 = Potent vasodilator

• ALSO physiologically present in brain and kidneys

COX3 = Present in brain with unknown function

Why is meloxicam superior to aspirin?

Meloxicam has a higher affinity to COX2 enzymes → Superior analgesic and anti-inflammatory properties AND fewer side effects (less inhibition of physiological COX1)

List 7 side effects of NSAIDs (animals)

1. GASTRIC ULCERATION → Melaena, anorexia and haematemesis (coffee-grounds)

2. KIDNEY FAILURE

3. Vomiting/diarrhoea/inappetence (unknown MoA)

4. Increased bleeding time

5. Carprofen (Rimadyl) = Rare idiosyncratic hepatotoxicity (Labradors)

6. Phenylbutazone = Agranulocytosis → Increased risk of infection

7. Dermal reactions

List and describe the MoA of 2 side effects of NSAIDs on HUMANS

1. COXIBS (eg. firocoxib) = Heart failure (humans ONLY)

   • COXIB drugs only target COX2 → Inhibited release of PGI2 (potent vasodilator) → Vasoconstriction of coronary and brain arteries → Myocardial infarct and stroke

2. Asthma (humans ONLY)

   • Leukotrienes produced cause bronchoconstriction in patients with pre-existing lung pathology

Describe the mechanism of gastric ulceration caused by NSAIDs (or stress)

Prostaglandins play an important role in NORMAL physiological processes by binding receptors on gastric glands which protects the gastric mucosa from acidic stomach contents. NSAIDs inhibit PG synthesis →

1. Decreased mucosal blood flow

2. Decreased bicarbonate production

3. Decreased mucus secretion

4. Increased release of H+ from chief cells

ANY NSAID given for 5 - 7 days produces clinical signs associated with gastric ulcers

List 8 ways to reduce NSAID-induced gastric ulceration (when required for long-term therapy eg. OA)

1. Intermittent treatment (give for 5 days then wait)

2. Multimodal analgesia to reduce dose rates required

3. Misoprostol

4. Sucralfate

5. Antacid

6. Omeprazole

7. Atropine

8. Antihistamines (eg. ranitidine and cimetidine)

Describe the normal physiological effect of prostaglandins on the kidney

NSAIDs rarely cause AKI in healthy patients with normal BP

1. Decreased BP detected by JG cells which activates RAAS

2. Angiotensin II released causes generalised vasoconstriction (i.e. efferent arteriole constriction)

3. Efferent arteriole constriction results in increased GFR and stabilisation of pressure

4. PG acts as angiotensin II ANTAGONIST to cause vasodilation and relieve pressure in the efferent arteriole (maintains homeostasis)

Describe the mechanism on AKI due to NSAIDs

1. Hypotensive patient administered NSAIDs → No PG to cause vasodilation

2. → Permanent efferent arteriole constriction

3. Causes ischaemic necrosis and kidney failure (efferent arterioles supply the peritubular capillaries)

Avoid NSAIDs while under GA as it induces kidney failure due to hypotension (use opioids for analgesia instead)

List 5 indications of NSAIDs

1. Muscle damage

2. Mild pain

3. Osteoarthritis (intermittent use during flare-up periods)

4. Post-op pain

5. Colic

7 Contraindications of NSAIDs

1. Pre-existing GI complications (or consider omeprazole to reduce gastric ulceration risks)

2. Avoid COX2 inhibitors with history of myocardial infarction, angina, chronic CHF and strokes (inhibit PGI2)

3. Liver disease/hepatotoxic drugs (NSAIDs metabolised by liver)

4. Kidney disease/nephrotoxic drugs (eg. diuretics, ACE-i, aminoglycosides)

5. Patients with altered haemostasis (thromboxane is COX1-dependent)

6. Patients with low circulating volume (CHF, ascites, diuretics, dehydration, hypotension) → Exacerbate renal damage

7. Glucocorticoids

3 Specific NSAIDs which are contraindicated in different species

1. Aspirin contraindicated in cats and cattle

   1. Cat half-life = 22hr

   2. Cattle half-life = 25 minutes

2. Phenylbutazone contraindicated in cattle (long half-life NOT for production animals)

3. Naproxen and ibuprofen contraindicated in animals (long half-life due to enterohepatic recycling)

List the commonly used NSAIDs in cattle, horses, dogs and cats (+ COX enzyme affinity)

Cattle

1. Flunixin ((non-selective)

2. Ketoprofen (non-selective)

3. Tolfenamic acid (COX2)

Horse

1. Phenylbutazone (COX1 = Right dorsal colitis)

2. Flunixin (non-selective)

3. Ketoprofen (non-selective)

4. Tolfenamic acid (COX2)

Dogs/Cats

1. Meloxicam (COX2)

2. Carprofen (COX2)

3. Deracoxib/firocoxib (highly COX2)

4 Drug interactions with NSAIDs

1. Other drugs inhibiting prostaglandins (eg. EP4 inhibitors and glucocorticoids)

2. other drugs inhibiting renal blood flow (eg. diuretics)

3. Combination with highly protein-bound drugs → Increased risk of adverse reactions in patients with compromised hepatic function

4. Other drugs inhibiting CYP450 (eg. chloramphenicol, cimetidine, imidazole, antifungals)

2 Recommendations or NSAID use

1. Use short-term with lowest dose possible

2. Use in combination with opioids to produce superior analgesia with fewer side effects

What is neuropathic pain?

Maladaptive pain which originates from nerves (NOT tissue injury) eg. IVDD or chronic OA pain

List 6 drugs which help with neuropathic pain

1. Carbamazepine (anti-convulsant benzodiazepine) = Inhibits voltage-gated Na+ channels → Prevents sustained firing of neurons

2. Gabapentin/pregabalin = Inhibits voltage-gated Ca2+ channels → Disrupts NMDA receptors and excitatory systems

3. Methadone = NMDA receptor antagonist

4. Amantadine = NMDA receptor antagonist

5. Ketamine = NMDA receptor antagonist

6. Amitriptyline (TCA) = Anti-depressant for chronic pain to inhibit re-uptake of serotonin and NA → Increased descending inhibitory pathway activity

3 Advantages and 2 disadvantages of multimodal analgesia

Multimodal analgesia = Combination of ≥2 different drug classes

Advantages:

1. Superior analgesia (target different points on the pain pathway)

2. Reduced dose → Fewer side effects

3. Useful for longer term dosing regimes (eg. OA = Fentanyl patch with low dose NSAIDs)

Disadvantage: $$$

What are corticosteroids? What are the 2 classes?

Corticosteroids = Class of steroid hormones produced by the adrenal cortex

Classes:

1. Glucocorticoids = Produced by the zona fasciculata and zona reticularis

   • Cortisol #1 (cortisone in birds/rats)

2. Mineralocorticoids = Produced by the zona glomerulosa

   • Aldosterone #1

List 5 examples of glucocorticoids from least → most potent (+ duration of action)

1. Hydrocortisone = Short-acting (12hr)

2. Prednisone (dogs) → Prednisolone (horses/cats) = Intermediate-acting (12 - 36hr)

3. Triamcinolone (topical/intra-articular) = Intermediate-acting

4. Dexamethasone = Long-acting (48hr)

5. Bethametasone

6. Methylprednisolone acetate (insoluble salt) = Long-acting (3 - 5d)

Describe 5 levels of immune suppression

1. NSAIDs = Suppress immune response through anti-inflammatory effects

2. Low dose corticosteroids = Anti-inflammatory ± mild immunosuppression

3. Immunomodulators = Selective immune modulation (eg. oclacitinib)

4. High dose corticosteroids = Immunosuppression

5. Old anticancer drugs = ZERO immune response

Describe the mechanism of glucocorticoid binding to receptors

Glucocorticoid receptors are within almost EVERY cell → Widespread reaction

• Intracellular receptor → Long onset of action but long duration

Mechanism:

1. Cortisol crosses the cell membrane and binds to a receptor to enter the nucleus

2. Altered gene transcription

3. → Increased synthesis of lipocortin = Phospholipase A2 antagonist responsible for preventing synthesis of arachidonic acid

4. BOTH prostaglandins and leukotrienes are NOT synthesised

Do glucocorticoids have analgesic properties?

No (anti-inflammatory ONLY)

List 5 indications of corticosteroids

1. Reduce inflammation (eg. trauma or IBD)

2. Reduce allergies (eg. atopy)

3. Immunosuppression (eg. autoimmune disorders or immune system cancers)

4. Shock therapy (esp. refractory septic shock) → NOT indicated now

5. Parturition in cows (unethical)

How should glucocorticoids be used?

Lowest dose for shortest period possible (longer therapy → longer tapering-off period to restart the hypothalamic-pituitary-adrenal axis)

1. Induction dose for 1 - 2 days until no clinical signs of inflammation are observed

2. Transition to every other day treatment

3. Skip more and more days of treatment to taper drug off

10 Side effects of corticosteroids (+ MoA)

Corticosteroids are released during fight-flight response

1. Hyperglycaemia via protein catabolism, lipolysis and gluconeogenesis

   • Cortisol = Anti-insulin agent

2. PU/PD via ADH inhibition → UTI

   • Every glucocorticoid has a mineralocorticoid effect at high/long-term doses

   • Increased GFR (PU and generalised vasoconstriction due to lack of PG)

3. Osteoporosis via 2˚ hyperparathyroidism

   • Cortisol = Inhibited absorption of Ca2+ from duodenum and increased elimination from kidneys → Increased PTH → Increased osteoclastic activity

4. Immunosuppression via WBC apoptosis (except neutrophils) and reduced capillary permeability (reduced emigration of neutrophils and macrophages) → Increased risk of infection AND delayed wound healing

5. Euphoria/depression and polyphagia via increased dopamine and serotonin release

6. Thin and fragile skin via reduced fibroblast activity

7. Calcinosis cutis and alopecia due to increased PTH → Ca2+ deposited on skin

8. Muscle wasting via protein catabolism → Temporalis muscle wasting and pot belly

9. Infertility via reduced GnRH release from hypothalamus

   1. Teratogenic

   2. Inhibited ovulation and spermatogenesis

   3. Induce abortion/parturition (promotes foetal lung maturity and surfactant production at the end of gestation)

10. GIT via gastric ulceration (inhibited PG), hepatic lipidosis, pancreatitis

Tacrolimus

• MoA

• 5 Indications

MoA: Binds immunophilin which inhibits calcineurin → NFATc is NOT transported to the nucleus → Prevents production of cytokines (eg. IL, TNF, IFN) → T-cell activation blocked which prevents clonal expansion and a cytotoxic response

Indications: Immunosuppression

1. Atopic dermatitis (eczema)

2. Keratoconjunctivitis sicca

3. Anal furunculosis

4. Adjunct to corticosteroids

5. Transplants

List 11 antibiotics groups (+ which of the 5 antibiotic classes they fit under)

Cell wall synthesis inhibitors

1. Beta-lactams (penicillins and cephalosporins)

2. Bacitracin

Protein synthesis inhibitors

3. Aminoglycosides

4. Tetracyclines

5. Fenicols

6. Macrolides

7. Lincosamides

Nucleic acid synthesis inhibitors

8. Potentiated sulphonamides

9. Fluoroquinolones

10. Nitroimidazoles

Cell membrane destruction

11. Polymyxin B

Others

List 6 mechanisms of antibiotic resistance

1. Inactivation of drug (eg. β-lactamases and plasmid-mediated chloramphenicol acetyl transferase)

2. Increased efflux pumps → Enhanced removal of antibiotic

3. Decreased cell membrane permeability to antibiotic

4. Alter binding site (eg. Methicillin-resistant staphylococcus aureus)

5. Increased target protein (eg. increased PABA production for TMP)

6. Target in protected environment

Describe the structure of G+ vs. G- cell walls

G+: Thick peptidoglycan wall and NO LPS layer

• Alternating chains of N-acetylmuramic acid and N-acetylglucosamine which are joined together by peptide chains

• No protein channels

• Highly porous and easy for penicillin to pass through

G-: Thin peptidoglycan wall

• Lipopolysaccharide (LPS) layer - Outer layer with LPS which are highly toxic to mammalian cells when broken down and released

  • Destruction of G- bacteria → Release of LPS causing endotoxaemia

  • NOT a porous cell wall (contains specific protein channels for nutrients and metabolic products to enter and exit the cell wall)

    • Penicillin must enter through protein channels to exert effect

• Thin peptidoglycan wall

Periplasmic space - Separates the cell wall and the cellular membrane

β-lactams: Structure

• β-lactam ring = Important for antibiotic activity (broken ring → inactivated AB)

• R chain = Determines spectrum of action and resistance

  • Large/complex R chain = Narrow spectrum (large R group cannot fit through G- protein channels) BUT highly resistant to β-lactamases (large R group protects the ring)

  • Small R chain = Broad spectrum (G-/G+) BUT less protection

β-lactams: MoA

1. Drug MUST cross cell wall to enter the periplasmic space

   • G+: Porous cell wall and easy entry

   • G-: NOT porous cell wall (must enter through protein channels)

2. Drug covalently binds to penicillin binding protein in the cell membrane

   • PBP = Produces transpeptidase to assist in bonds between peptidoglycan chains of the cell wall via inter-bridging

3. Inhibited production of bacterial cell wall

4. Cell wall weakens causing the cell membrane to bulge and rupture

What is β-lactamase? How does it differ between G+ and G- bacteria?

Definition: Enzymes which destroy the β-lactam ring of penicillin and prevents it from working

G+: β-lactamases produced OUTSIDE of the bacteria → NOT confined to the periplasmic space

G-: β-lactamases trapped within the periplasmic space → Penicillin must penetrate through the cell wall to be inactivated

Clavulanic acid

• Definition

• Indication

Definition: β-lactamase inhibitor which increases activity of penicillins

• Covalently bind with bacterial β-lactamase to inhibit destruction of the β-lactam ring

Indication: Given in conjunction with broad-spectrum penicillins against G- (not necessary for G+ as β-lactams are NOT confined to the periplasmic space)

List 4 examples of penicillins (+ spectrum of action)

1. Penicillin G (benzylpenicillin) = G+ (large R chain)

   • NOT PO

   • G- bacteria have LPS cell wall which prevents uptake of penicillin (small protein channels)

   • Na+ and K+ salts can be given IV (K+ concentrated resulting in disruption of membrane potentials)

2. Cloxacillin = Staphylococcus

   • Frequent component of IMM preparations in cattle

3. Amoxycillin/ampicillin ± clavulanic acid = G+/G- (small polar amino group)

   • Amoxycillin has -OH group to increase PO absorption

   • Ampicillin IM/SC ONLY (absorption impeded by food)

4. Ticarcillin/piperacillin = Extended spectrum (Pseudomonas)

Penicillins = Spectrum depends on R chains (G+ aerobes → G+ anaerobes → G-)

β-lactams: Pharmacokinetics

Absorption: Natural benzylpenicillin (G) must be given parenterally (ring destroyed by stomach pH)

• Synthetic penicillins eg. ampicillin and amoxycillin can be given PO

Distribution: Poor (hydrophilic)

• Ionised at blood pH (pKa ~2.7) → Restricted to the ECF (cannot cross cell membranes easily)

  • Cannot access BBB, eye or prostate

Metabolism: Minimal

Elimination: Urine (penicillin eliminated unchanged in urine)

• Some synthetic penicillins accumulate in bile → Useful for GI infections

β-lactams: Duration of action

• Na/K salt (IV) lasts 2hr

• Procaine salts (IM) last 12 - 24hr

• Procaine in oil lasts 48hr

  • Common on farm

• Benzathine lasts ≥48hr

Cephalosporins

• Structure

• 6 Examples

• Caution

Structure: TWO R chains (protects β-lactam ring well → β-lactamase resistant)

Examples: Increasing spectrum of action

1. 1st generation = Cephalexin, cefapirin, cephazoline (G+)

2. 3rd generation = Ceftiofur, cefovecin (Convenia injection = 2w duration)

3. 4th generation = Cefquinome

Caution: Avoid 3rd and 4th generation cephalosporins

β-lactams: 4 Adverse effects

1. Allergic reaction (rare in animals)

2. Electrolyte disturbances

3. Procaine reactions (esp. horses)

4. Antibiotic-associated diarrhoea (AAD) through damage of commensal GIT bacteria

   • Avoid in guinea pigs and hamsters

β-lactams: 5 Indications of use

1. Prophylaxis

2. UTI

3. Meningitis (BBB disrupted)

4. Skin infections

5. Respiratory infections

Aminoglycosides: MoA

Irreversibly binds to receptor protein of bacterial 30S ribosomal subunit → Inhibit protein synthesis

• Distort codon arm and forces production of nonsense proteins

Aminoglycosides: Spectrum of action

G- aerobes ONLY

• MUST have O2 transporters to enter the cell membrane

• Useful synergism with β-lactams

Aminoglycosides: 4 Examples

1. Streptomycin

2. Gentamicin

3. Neomycin

4. Amikacin

Aminoglycosides: Pharmacokinetics

Absorption: ALWAYS give parenterally (no GIT absorption)

Distribution: Poor

Metabolism: Post-antibiotic effect = Bacterial growth suppression AFTER drug concentrations falling below MIC in the blood

Elimination: Kidneys (NEVER use in food animals as present in kidneys for months)

Aminoglycosides: 2 Adverse effects

Positively charged molecules attracted to negatively charged cell membranes of the:

1. Proximal convoluted tubules → Nephrotoxic

2. Cochlear of the ear → Ototoxic (kill hair cells)