A BSTRACTTwo recent developments of opioid peptide-based analgesics are reviewed. The fi rst part of the review discusses the dermorphin-derived, cationic-aromatic tetrapeptide H-Dmt-D-Arg-Phe-Lys-NH 2 ([Dmt 1 ]DALDA, where Dmt indicates 2 ′ ,6 ′ -dimethyltyrosine), which showed subnanomolar m receptor binding affi nity, extraordinary m receptor selectivity, and high m agonist potency in vitro. In vivo, [Dmt 1 ]DALDA looked promising as a spinal analgesic because of its extraordinary antinociceptive effect (3000 times more potent than morphine) in the mouse tail-fl ick assay, long duration of action (4 times longer than morphine), and lack of effect on respiration. Unexpectedly, [Dmt 1 ]DALDA also turned out to be a potent and long-acting analgesic in the tail-fl ick test when given subcutaneously (s.c.), indicating that it is capable of crossing the blood-brain barrier. Furthermore, little or no cross-tolerance was observed with s.c. [Dmt 1 ]DALDA in morphine-tolerant mice. The second part of the review concerns the development of mixed µ agonist/ d antagonists that, on the basis of much evidence, are expected to be analgesics with a low propensity to produce tolerance and physical dependence. The prototype pseudopeptide H-Dmt-Tic Y [CH 2 NH]PhePhe-NH 2 (DIPP-NH 2 [ Y ], where Tic indicates 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) showed subnanomolar m and d receptor binding affi nities and the desired m agonist/ d antagonist profi le in vitro. [ Y ] produced a potent analgesic effect after intracerebroventricular administration in the rat tail-fl ick assay, no physical dependence, and less tolerance than morphine. The results obtained with DIPP-NH 2 [ Y ] indicate that mixed m agonist/ d antagonists look promising as analgesic drug candidates, but compounds with this profi le that are systemically active still need to be developed.
INTRODUCTIONThe clinical treatment of severe pain relies heavily upon opioid analgesics, most of which act via m opioid receptors.Morphine and other centrally acting m opioid analgesics produce, in addition to the analgesic effect, several side effects -including inhibition of gastrointestinal motility, respiratory depression, tolerance, and physical dependence -that limit their use in pain treatment. Kappa opioid agonists have also been shown to be potent analgesics; however, it has long been recognized that centrally acting k agonists have limited usefulness in humans because of their psychotomimetic and dysphoric effects. 1 , 2 Delta opioid agonists are known to produce analgesic effects when given intrathecally (i.th) 3 , 4 or intracerebroventricularly (i.c.v.). 4 There is evidence to indicate that they induce less tolerance and physical dependence than m analgesics, no respiratory depression, and few or no adverse gastrointestinal effects. [5][6][7] Among the peptide d opioid agonists, [D-Pen 2 , D-Pen 5 ]enkephalin (DPDPE) produced only a weak, centrally mediated analgesic effect after systemic administration, indicating that it does not cross the blood-brain barrie...