Pharmaco: NSAIDs Flashcards
Do a quick comparison of NSAIDs, Paracetamol, and Opioids
NSAIDs: block acute inflammatory response at the site of injury
Paracetamol: modulates how the brain interprets pain signals
Opioids: blocks transmission and signaling of pain through the nerve at the spinothalamic relay and the CNS
Recap: 4 components of acute inflammatory response
- Warmth
- Redness
- Swelling
- Pain
- Loss of function (additional component)
Describe the pathways of phospholipase A2 (from cell membrane phospholipids)
Phospholipase A2 => Arachidonic acid (AA)
AA has three pathways
- 15-Lipoxygenase => Lipoxins
- Cyclooxygenase (COX) => Prostanoids
- more selective for acute inflammatory response
- 5-Lipoxygenase => Leukotrienes
- more involved in chronic inflammatory + immune responses
Lipoxins, Prostanoids, and Leukotrienes are known as Eisosanoids, involved in the inflammation process
Difference between steroids and NSAIDs MOA
Steroids block Phospholipase A2, thereby blocking all the subsequent pathways (potent anti-inflammatory response, potentially immunosuppressive when used long-term)
NSAIDs block cyclooxygenase (COX) and therefore the prostaglandins production (selective for acute inflammation)
What are the prostaglandins produced by COX?
PGI2 (prostacyclin)
- inhibit platelet aggregation (antiplatelet)
- vasodilation
- might expect more in closed wounds, infections etc.
PGE2 (classical prostaglandins)
- directly involved in acute inflammation
- increased vascular permeability (swelling, edema)
- pain
- vasodilation (warmth, redness, swelling)
TXA2 (thomboxanes)
- platelet aggregation
- vasoconstriction
- might expect more in open wounds
Explain the analgesic effects of Aspirin (NSAID)
Why is NSAID only sufficient for mild-mod pain?
NSAIDs block the production of prostaglandins, which sensitize the nociceptive fibers to stimulation by other inflammatory mediators (e.g., bradykinin, leukotrienes)
Block of sensitization rather than direct nociceptive activation explains why NSAIDs have an ‘analgesic ceiling’ (i.e. there may still be too much bradykinin, leukotrienes) - still pain signaling by other inflammatory mediators
NSAIDs also have additional analgesic actions in the CNS
Explain the antipyretic effects of Aspirin (NSAID)
In infections/tissue damage/inflammation, there is incr in neutrophils, leading to incr release of cytokines which increases the expression of COX in the hypothalamus of the brain. This leads to increase PGE2 that results in warmth and fever
NSAIDs are antipyretic as they inhibit COX thereby reducing PGE2 production and alleviating the fever
Explain Aspirin as an antiplatelet
Aspirin is an irreversible COX inhibitor, inhibits COX-1 (TXA2) more than COX-2 (PGI2), therefore inhibiting platelet production of TXA2
*Recall pdn of TXA2 in platelets (1-2 weeks), pdn of PGI2 by endothelial cells (few hours)
*antiplatelet dose is low
Therefore, NSAIDs (such as aspirin) are:
- anti-inflammatory (block PGE2, PGI2, TXA2)
- analgesic (block sensitization)
- antipyretic (block PGE2)
- antiplatelet (aspirin is an irreversible COX inhibitor, block TXA2 > PGI2)
Adverse effects of Aspirin are ________
dose-dependent
What are some side effects at low doses (therapeutic range for NSAID)?
SEs are due to COX inhibition (up to 1g)
- Gastric intolerance
- Bleeding
- Hypersensitivity
What are some side effects at high doses?
SEs are due to salicylate toxicity
To progressively higher doses (1g and onwards):
- Tinnitus
- Uricosuric
- Central hyperventilation
- Respiratory alkalosis
- Fever, dehydration
- Metabolic acidosis
- Respiratory acidosis
- Hypoprothrombinemia
- Vasomotor collapse
- Coma
- Renal failure
- Respiratory failure
What is Reye’s syndrome?
What are the symptoms?
Who is at increased risk of it?
Rare, but life-threatening condition
Swelling of the brain (encephalitis) and liver
Symptoms include:
- Severe vomiting with viral symptoms such as fever, aches, pain
- Personality/behavior changes
- Listlessness
- Delirium
- Convulsions
- Loss of consciousness
Increased risk:
- Children with viral infections taking Aspirin
Therefore, aspirin is now contraindicated in children
Effectiveness of Naproxen
- More effective in women (free fraction >40% higher)
- Often use for dysmenorrhea
- Half-life 12-14h (dosed two times daily)
note that most older NSAIDs have shorter half-life and are dosed more frequently; Naproxen is an exception (BD)
Effectiveness of Diclofenac
- in terms of half-life
- Short plasma half-life <2h, therefore low GI risk, low systemic SEs
- Longer half-life in synovial fluid, hence useful in inflammatory joint diseases
- Can be applied topically
- Additional CVD risk if taken systemically
Effectiveness of Indomethacin
- Strongly anti-inflammatory due to additional steroid-like phospholipase A2 inhibition (thus, it is useful in rheumatic joint conditions)
- However, less frequently 1st choice due to reported CNS adverse effects: 15-25% report headache, altered mental status, confusion or depression, also have cases of psychosis and hallucinations
[Adverse effects of NSAIDs due to COX-inhibition]
- Explain the mechanism of GI adverse effects
In normal circumstances,
- Prostaglandins (PGE2) reduces gastric acid secretions, increases mucosal blood flow, increases mucus secretion, and increases bicarbonate secretion
When NSAID is administered,
- PGE2 is inhibited
- Common SEs: dyspepsia, nausea, vomiting, anorexia, abdominal pain
- More serious: Risk of peptic ulcer formation, and hemorrhagic risk, GI bleed/ulcer/perforation
- Risk of peptic ulcer greatly increased if used for >5days
- Coupled with antiplatelet effects (if aspirin) => incr risk of bleeding
Therefore, older NSAIDs may not be appropriate for long-term use in chronic arthritis due to risk of GI ulcer and bleed
[Adverse effects of NSAIDs due to COX-inhibition]
- Risk factors for GI toxicity - GI bleed, perforation, ulceration (IC16)
- > 65y
- History of ulcer
- Use of high dose/chronic NSAID
- Concurrent glucocorticoids/antiplatelets/anticoagulants
High risk:
- 3 or more of the 4 risk factors OR
- history of complicated ulcer (ulcer + bleed)
If high risk, choose coxib, consider adding PPI
Other considerations: Ibuprofen has less GI SEs than Naproxen; use paracetamol if possible
[Adverse effects of NSAIDs due to COX-inhibition]
- When is urgent referral needed for suspected NSAID-induced GI complications? (IC16)
- Fatigue symptoms
- Severe dyspepsia
- Signs of GI bleeding (melena - black tarry stools, coughing out coffee-ground-like substance)
- Unexplained blood loss anemia
- Iron deficiency
[Adverse effects of NSAIDs due to COX-inhibition]
- Explain the mechanism of renal adverse effects
Inhibition of PGE2 and PGI2 alters the renal blood flow dynamics:
PGE2 actions
- Inhibits sodium reabsorption in the thick ascending limb (25% of Na+ reabsorbed)
Inhibition of PGE2 production
- Sodium retention
- Water retention
- Peripheral edema (esp seen in long acting NSAIDs such as Naproxen and Celecoxib)
- Hypertension
PGI2 actions
- PGI2 stimulates secretion of renin and hence aldosterone
- Aldosterone increases Na+ reabsorption and K+ excretion
Inhibition of PGI2 production
- Suppression of renin and aldosterone secretion (therefore prevents reabsorption of Na+ in the distal convoluted tubule, but only 1-2%)
- Hyperkalemia (incr K+ retention)
- Acute renal failure
How do PG and Ang II play a role in kidney function?
PG - vasodilation of afferent arteriole, increase GFR, increase RBF
Ang II - vasoconstriction of efferent arteriole, increase GFR, decrease RBF