K4i / 17B01: Compare and contrast the pharmacology of ibuprofen and paracetamol

17B01: Exam Report

Compare and contrast the pharmacology of ibuprofen and paracetamol.

65% of candidates passed this question.

This was a standard compare and contrast question of common analgesic pharmacology and it was generally well answered. The use of a table ensured all areas were covered including class, indications, pharmaceutics, mode of action, pharmacodynamics, pharmacokinetics and adverse effects. The uncertain nature (and possibilities) of the mechanism of action of paracetamol was alluded to in better responses.

Details of the comparative pharmacokinetics were often lacking. Answers should have included a comment on first-pass effect, the significance of the difference in protein binding and the details of metabolism, particularly paracetamol. Metabolism limited to “hepatic metabolism and renal excretion” gained no marks as better responses were more detailed and clearer about the differences between the two drugs. Knowledge of metabolism at therapeutic doses and the effect of overdose were expected. Better answers included potential interactions with other drugs (e.g. warfarin) and contraindications to the use of these drugs.

K4i / 17B01: Compare and contrast the pharmacology of ibuprofen and paracetamol

Chemical

Ibuprofen

Proprionic acid derivative

Paracetamol

An acetanilide derivative

Comments

An acetanilide derivative

Use

Ibuprofen

  1. Inflammatory MSK disorders
  2. Post op pain 

Paracetamol

  1. Analgesia
  2. Antipyretic

Comments

Can be used synergistically for analgesia

Both are common in the post operative setting

Presentatation

Ibuprofen

Tablets 200 – 600mg

Racemic mixture

R-enantiomer → converted to S-enantiomer → S is the pharmacologically active drug

Paracetamol

  • Tablets
  • Syrup
  • Rectal suppositories

IV → glass vial & argon, drug is unstable in O2 rich environment

Comments

Paracetamol has a more favourable administration profile

Dose

Ibuprofen

20mg/kg/day divided doses

Paracetamol

1g QID

Comments

Both easily dosed

Route

Ibuprofen

PO

Paracetamol

PO/PR/IV

Commets

Onset

Ibuprofen

30 mins

Paracetamol

  • Peak [ ] 1hr 30mcg/mL
  • Therapeutic range 10 – 20mcg/mL

In 4hrs [   ] <10mcg/mL

Comments

Similar onset & offset

DoA

Ibuprofen

2 – 4 hrs

Paracetamol

2 – 4 hrs

Commets

MoA

Ibuprofen

= antiplatelet, analgesia, antipyretic

  • Non-selective COX enzyme inhibition
  • ↓ synthesis of PGs, prostacyclin & thromboxane

Paracetamol

Antipyretic

  • Inhibits central PG synthesis
  • Especially PGE in anterior hypothalamus

Analgesia

  • Inhibits peripheral Subs P actions
  • Potent central PG synthesis
  • Peripherally blocks impulse generation with BK sensitive chemoreceptors

Comments

Both exhibit antipyretic & analgesic properties

PD

Ibuprofen

CNS

  • Analgesia
  • Antipyretic
  • Peripheral ↓ n. sensitisation to pain

CVS

  • Oedema & exac HF
  • Variable antiplatelet activity

Resp – bronchospasm

GI – GI ulceration, bleeding

GU

  • ↓renal BF
  • ↑urea

Papillary necrosis

Paracetamol

CNS

  • Analgesia
  • Antipyretic

Commets

Paracetamol has a more straightforward PD profile with less side effects when dosed correctly

PK

Ibuprofen

A

OBA 80%

D

>99% PPB

VD 0.15L/kg

M

Hepatic → oxidation to inactive metabolites +/- conjugated prior to excretion

E

Metabolites via urine

t ½ 2 hrs

Paracetamol

A

High lipid solubility

Rapid absorption

OBA 60 – 90%

More PR (OBA)

D

pKa 9.5 (weak acid)

<1% PPB

Highly UNIONISED at pH 7.4

Lipid soluble

Penetrates BBB +++

VD 1L/kg

M

HER 0.3

80% to glucuronide & sulphate

10% to NAPGI via CYP450 which is then conjugated with glutathione

E

<5% unchanged in urine

Metabolites actively secreted into renal tubules

t ½ B 2 – 4hrs

Comments

Both rely on liver and kidneys for metabolism & clearance

A/E

Ibuprofen

Fluid retention

Abdo pain

N&V

Vertigo

Dizziness

Dyspepsia

Nephrotoxic

Paracetamol

Irreversible Liver & Kidney toxicity w overdose

Comments

Both need to be used w caution in patients w renal/hepatic impairment

However, unlike paracetamol, the margin of safety w Ibuprofen is high and not associated with serious ADRs in case of overdose

D/I

Ibuprofen

Caution/ do not use with ACEI & ARBs

Risk of increased GI bleeding when used w other NSAIDs

Increased thrombotic risk when given w aspirin due to COX1 competition

Paracetamol

Drug may lead to increased INR in patients taking warfarin (reduced synthesis of Vit K dep clotting factors)

Comments

Brufen does not interact w warfarin but concurrent use of NSAIDs w anticoagulants increases the risk of bleeding, and this risk is further increased by concominant use of paracetamol