On the basis of the three-dimensional pharmacophore model of opioid κ agonists, we simplified the structure of nalfurafine (selective κ agonist) to find the essential structural moieties for binding the opioid receptors, especially κ receptor type. As a result, we found that the trans-fused decahydroisoquinoline derivatives without a phenol ring bound the opioid receptor in micromolar order and that both the amide side chain and the nitrogen substituted by the cyclopropylmethyl group were indispensable moieties for eliciting the κ selectivity. The simple decahydroisoquinoline without amide side chain also bound the opioid receptor without receptor type selectivity, suggesting that the message-address concept would be applicable to even these simple derivatives. These findings that the simple decahydroisoquinoline derivatives showed the affinities for the opioid receptors, especially some of the compounds showed κ selectivity, are the first example in the opioid field.Key words opioid; κ receptor; decahydroisoquinoline; nalfurafine; three-dimensional pharmacophore model Three types of opioid receptors (μ, δ, κ) are now well established not only by pharmacological studies but also by molecular biological studies.1) Narcotic addiction is believed to be derived from the μ receptor type, and therefore δ and κ types are promising drug targets for analgesics without addiction. To obtain ideal analgesics without addiction and other side effects derived from the μ receptor, we have synthesized various kinds of naltrexone derivatives and have reported selective ligands for κ 2-9) and δ [10][11][12][13][14] receptors. Quite recently, one of our designed κ selective agonists, nalfurafine hydrochloride 2,3,6,8,9) Fig. 1), was launched in Japan as an antipruritic for patients undergoing dialysis. 6,8,9) Although many arylacetamide derivatives such as U-50,488H 15,16) (Fig. 1) and U-69,593 17) were synthesized and developed as κ agonists, all of these derivatives were eliminated from clinical trials as not only analgesics but also as antipruritics because of their serious side effects like psychotomimetic and aversive reactions. 18,19) In contrast, nalfurafine has neither aversive nor addictive effects. 20) Our interest in the differences in the pharmacological effects between nalfurafine and the arylacetamide derivatives led us to conduct a detailed structure activity relationship investigation of nalfurafine derivatives. From these studies, we developed the hypothesis that in the active conformation of nalfurafine (Fig. 2), the C-ring would assume the boat form, thereby elevating the amide side chain to bind the κ receptor. 4,5,21,22) Based on this hypothesis, we designed and synthesized KNT-63 with an oxabicyclo[2.2.2] octane skeleton ( Fig. 1), and confirmed its high affinity for the κ receptor. 5) We also proposed a new three-dimensional pharmacophore model applicable to some κ agonists with various chemotypes. 21,22) Our new pharmacophore model of κ agonists supported the proposed active conformation of nalfurafine and...