In vitro- forsøg har vist, at methylnaltrexon er en my- opioidreceptorantagonist (inhibitionskonstant [Ki] = 28 nM) med 8- fold mindre effekt på kappa- opioidreceptorer (Ki = 230 nM) og stærkt reduceret affinitet til delta- opioidreceptorer.
In vitro studies have shown methylnaltrexone to be a mu-opioid receptor antagonist (inhibition constant [Ki] = 28 nM), with 8-fold less potency for kappa opioid receptors (Ki = 230 nM) and much reduced affinity for delta opioid receptors.