Opioid-induced
constipation (OIC) is a common adverse effect of
opioid analgesics. Peripherally acting μ opioid receptor antagonists
(PAMORAs) can be applied in the treatment of OIC without compromising
the analgesic effects. NAP, a 6β-N-4-pyridyl-substituted naltrexamine
derivative, was previously identified as a potent and selective MOR
antagonist mainly acting peripherally but with some CNS effects. Herein,
we introduced a highly polar aromatic moiety, for example, a pyrazolyl
or imidazolyl ring to decrease CNS MPO scores in order to reduce passive
BBB permeability. Four compounds 2, 5, 17, and 19, when administered orally, were able
to increase intestinal motility during morphine-induced constipation
in the carmine red dye assays. Among them, compound 19 (p.o.) improved GI tract motility by 75% while orally administered
NAP and methylnaltrexone showed no significant effects at the same
dose. Thus, this compound seemed a promising agent to be further developed
as an oral treatment for OIC.