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Structure−Antitussive Activity Relationships of Naltrindole Derivatives. Identification of Novel and Potent Antitussive Agents

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journal contribution
posted on 14.08.2008 by Satoshi Sakami, Masayuki Maeda, Koji Kawai, Takumi Aoki, Kuniaki Kawamura, Hideaki Fujii, Ko Hasebe, Mayumi Nakajima, Takashi Endo, Shinya Ueno, Tsuyoshi Ito, Junzo Kamei, Hiroshi Nagase
We have previously reported antitussive effects of naltrindole (NTI), a typical δ opioid receptor antagonist, in a rat model. The ED50 values of NTI by intraperitoneal and peroral injections were 104 μg/kg and 1840 μg/kg, respectively, comparable to those of codeine. Codeine, one of the most reliable centrally acting antitussive drugs, has μ agonist activity and thus the same side effects as morphine, e.g., constipation, dependency, and respiratory depression. Because NTI is a δ opioid antagonist, its derivatives have potential as highly potent antitussives, free from the μ opioid agonist side effects. We attempted to optimize the NTI derivatives to develop novel antitussive agents. On the basis of the studies of structure−antitussive activity relationships of alkyl substituted NTI derivatives, we designed NTI derivatives with extra ring fused structures. As a clinical candidate, we identified a highly potent new compound, (5R,9R,13S,14S)-17-cyclopropylmethyl-6,7-didehydro-4,5-epoxy-5′,6′-dihydro-3-methoxy-4′H-pyrrolo[3,2,1-ij]quinolino[2′,1′:6,7]morphinan-14-ol (5b) methanesulfonate (TRK-850) which was effective even by oral administration (ED50 6.40 μg/kg).