Methylnaltrexone (MNTX)

Methylnaltrexone is one of the newer agents of peripherally-acting μ-opioid antagonists that act to reverse some of the side effects of opioid drugs such as constipation without affecting analgesia or precipitating withdrawals. Because it contains a permanently charged tetravalent nitrogen atom, it cannot cross the blood–brain barrier, and so has antagonist effects throughout the body, counteracting effects such as itching and constipation, but without affecting opioid effects in the brain such as analgesia. Methylnaltrexone binds to the same receptors as opioid analgesics such as morphine, but it acts as an antagonist, blocking the effects of those analgesics, specifically the constipating effects on the gastrointestinal tract. Furthermore, as methylnaltrexone cannot cross the blood–brain barrier, it does not reverse the pain-killing properties of opioid agonists or cause withdrawal symptoms.