Methylnaltrexone

[SH4:p121]

Structure

• Quaternary opioid receptor antagonist
• Highly ionised quaternary methyl group limits transfer across BBB
  --> Methylnaltrexone is only active at peripheral opioid receptors

Effects

• Attenuates morphine-induced changes in gastric emptying
• Decreases incidence of N&V
  * Direct antagonism of morphine at the chemoreceptor trigger zone (located outside BBB)
  * Improved gastric emptying