LPK-26, a novel kappa-opioid receptor agonist with potent antinociceptive effects and low dependence potential.

Analgesics such as morphine cause many side effects including addiction, but kappa-opioid receptor agonist can produce antinociception without morphine-like side effects. With the aim of developing new and potent analgesics with lower abuse potential, we studied the antinociceptive and physical dependent properties of a derivate of ICI-199441, an analogue of (-)U50,488H, named (2-(3,4-dichloro)-phenyl)-N-methyl-N-[(1S)-1-(2-isopropyl)-2-(1-(3-pyrrolinyl))ethyl] acetamides (LPK-26). LPK-26 showed a high affinity to kappa-opioid receptor with the Ki value of 0.64 nM and the low affinities to micro-opioid receptor and delta-opioid receptor with the Ki values of 1170 nM and >10,000 nM, respectively. It stimulated [(35)S]GTPgammaS binding to G-proteins with an EC50 value of 0.0094 nM. In vivo, LPK-26 was more potent than (-)U50,488H and morphine in analgesia, with the ED50 values of 0.049 mg/kg and 0.0084 mg/kg in hot plat and acetic acid writhing tests, respectively. Moreover, LPK-26 failed to induce physical dependence, but it could suppress naloxone-precipitated jumping in mice when given simultaneously with morphine. Taken together, our results show that LPK-26 is a novel selective kappa-opioid receptor agonist with highly potent antinociception effects and low physical dependence potential. It may be valuable for the development of analgesic and drug that can be used to reduce morphine-induced physical dependence.

Morphine inhibits doxorubicin-induced reactive oxygen species generation and nuclear factor kappaB transcriptional activation in neuroblastoma SH-SY5Y cells.

Morphine is recommended as a first-line opioid analgesic in the pain management of cancer patients. Accumulating evidence shows that morphine has anti-apoptotic activity, but its impact on the therapeutic applications of antineoplastic drugs is not well known. The present study was undertaken to test the hypothesis that morphine might antagonize the pro-apoptotic activity of DOX (doxorubicin), a commonly used antitumour drug for the treatment of neuroblastoma, in cultured SH-SY5Y cells. In the present study we demonstrated that morphine suppressed DOX-induced inhibition of cell proliferation and programmed cell death in a concentration-dependent, and naloxone as well as pertussis toxin-irreversible, manner. Further studies showed that morphine inhibited ROS (reactive oxygen species) generation, and prevented DOX-mediated caspase-3 activation, cytochrome c release and changes of Bax and Bcl-2 protein expression. The antioxidant NAC (N-acetylcysteine) also showed the same effects as morphine on DOX-induced ROS generation, caspase-3 activation and cytochrome c release and changes in Bax (Bcl-2-associated X protein) and Bcl-2 protein expression. Additionally, morphine was found to suppress DOX-induced NF-kappaB (nuclear factor kappaB) transcriptional activation via a reduction of IkappaBalpha (inhibitor of nuclear factor kappaB) degradation. These present findings support the hypothesis that morphine can inhibit DOX-induced neuroblastoma cell apoptosis by the inhibition of ROS generation and mitochondrial cytochrome c release, as well as by blockade of NF-kappaB transcriptional activation, and suggests that morphine might have an impact on the antitumour efficiency of DOX.