7β-Methyl substituent is a structural locus associated with activity cliff for nepenthone analogues

With the purpose of identifying novel selective κ opioid receptor (KOR) antagonists as potential antidepressants from nepenthone analogues, starting from N-nor-N-cyclopropylmethyl-nepenthone (SLL-020ACP), a highly selective and potent KOR agonist, a series of 7β-methyl-nepenthone analogues was conceived, synthesized and assayed on opioid receptors based on the concept of hybridization. According to the pharmacological results, the functional reversal observed in orvinol analogues by introduction of 7β-methyl substituent could not be reproduced in nepenthone analogues. Alternatively, introduction of 7β-methyl substituent was associated with substantial loss of both subtype selectivity and potency but not efficacy for nepenthone analogues, which was not found in 7β-methyl orvinol analogues. Surprisingly, SLL-603, a 7β-methyl analogue of SLL-020ACP, was identified to be a KOR full agonist. The possible molecular mechanism for the heterogeneity in activity cliff was also investigated. In conclusion, 7β-methyl substituent was a structural locus associated with activity cliff and demonstrated as a pharmacological heterogeneity between nepenthone and orvinol analogues that warrants further investigations.