Endomorphins (EMs) are potent pharmaceuticals
for the treatment
of pain. Herein, we investigated several novel EM analogues with multiple
modifications and oligoarginine conjugation. Our results showed that
analogues 1–6 behaved as potent μ-opioid agonists and
enhanced stability and lipophilicity. Analogues 5 and 6 administered
centrally and peripherally induced significant and prolonged antinociceptive
effects in acute pain. Both analogues also produced long-acting antiallodynic
effects against neuropathic and inflammatory pain. Furthermore, they
showed a reduced acute antinociceptive tolerance. Analogue 6 decreased
the extent of chronic antinociceptive tolerance, and analogue 5 exhibited
no tolerance at the supraspinal level. Particularly, they displayed
nontolerance-forming antinociception at the peripheral level. In addition,
analogues 5 and 6 exhibited reduced or no opioid-like side effects
on gastrointestinal transit, conditioned place preference (CPP), and
motor impairment. The present investigation established that multiple
modifications and oligoarginine-vector conjugation of EMs would be
helpful in developing novel analgesics with fewer side effects.