From agonist to antagonist profile in LP1 analogues through the introduction of different N-substituents in benzomorphan-based ligands

The design and discovery of new analgesics with diminished adverse side effects are a major challenge for medicinal chemists1. An emerging approach is the use of multitarget opioid ligands, e.g. mixed MOR/DOR compounds2. The benzomorphan-based compound LP1, a multitarget opioid ligand with a MOR agonist/DOR antagonist profile, produced centrally-mediated antinociception with a low tolerance-inducing capability3. Moreover, a significant antiallodynic and antihyperalgesic of LP1 in animal models of neuropathic pain and inflammatory pain determined effect was detected4. These results emphasized LP1 as analgesic candidate for chronic pain treatment. The nature of the N-substituent conferred to LP1 affinity and intrinsic activity versus MOR and DOR. In this study, we examined the results obtained by a structure-activity relationships performed by the introduction of p-substituted-phenyl, naphtyl, diphenyl, indolinyl and tetrahydroquinolyl rings in the N-substituent. Thus, nine analogues of LP1 were synthesized and tested for the calcium mobilization in CHO cells stably expressing human MOR or KOR coupled to the Gαqi5 chimeric protein or DOR coupled to the GαqG66Di5 chimeric protein in the fluorometric imaging plate reader FlexStation II. The data showed that the increase of N-substituent size led to a MOR antagonist profile. An antagonist activity at all three opioid receptors was detected for derivatives with 1-Nph and 2-Nph as substituent. Interestingly, the 1-Nph-substuted compound inhibited the effects of Dermorphin, DPDPE and Dynorphin A agonists showing higher potency at MOR (pKB 7.90) than DOR (pKB 6.78) and KOR (pKB 5.86). Moreover, a MOR antagonist profile was detected for derivatives bearing in the N-substituent the diphenyl or the indolinyl rings. The insertion of tetrahydroquinolinyl ring led to a shift of the KOR profile from antagonist to agonist maintaining the MOR antagonist activity. In comparison with LP1, the p-substituted-phenyl compounds showed a lower potency on MOR and DOR and a switched functional profile. Briefly, the bulkier size of the substituent modified the DOR and KOR functional profile from agonist to antagonist with retention of the MOR agonist activity. The p-methoxy derivative did not follow this trend and just the p-F-phenyl analogue shared with LP1 the same functional profile. Collectively, these data suggest that the benzomorphan-N-phenylpropanamide-based structure may represent a tool to achieve a specific opioid functional profile going from a pure agonist to a pure antagonist via mixed agonist/antagonist ligands, simply by increasing or decreasing the aromatic group size.

The design and discovery of new analgesics with diminished adverse side effects are a major challenge for medicinal chemists1. An emerging approach is the use of multitarget opioid ligands, e.g. mixed MOR/DOR compounds2. The benzomorphan-based compound LP1, a multitarget opioid ligand with a MOR agonist/DOR antagonist profile, produced centrally-mediated antinociception with a low tolerance-inducing capability3. Moreover, a significant antiallodynic and antihyperalgesic of LP1 in animal models of neuropathic pain and inflammatory pain determined effect was detected4. These results emphasized LP1 as analgesic candidate for chronic pain treatment. The nature of the N-substituent conferred to LP1 affinity and intrinsic activity versus MOR and DOR. In this study, we examined the results obtained by a structure-activity relationships performed by the introduction of p-substituted-phenyl, naphtyl, diphenyl, indolinyl and tetrahydroquinolyl rings in the N-substituent. Thus, nine analogues of LP1 were synthesized and tested for the calcium mobilization in CHO cells stably expressing human MOR or KOR coupled to the Gqi5 chimeric protein or DOR coupled to the GqG66Di5 chimeric protein in the fluorometric imaging plate reader FlexStation II. The data showed that the increase of N-substituent size led to a MOR antagonist profile. An antagonist activity at all three opioid receptors was detected for derivatives with 1-Nph and 2-Nph as substituent. Interestingly, the 1-Nph-substuted compound inhibited the effects of Dermorphin, DPDPE and Dynorphin A agonists showing higher potency at MOR (pKB 7.90) than DOR (pKB 6.78) and KOR (pKB 5.86). Moreover, a MOR antagonist profile was detected for derivatives bearing in the N-substituent the diphenyl or the indolinyl rings. The insertion of tetrahydroquinolinyl ring led to a shift of the KOR profile from antagonist to agonist maintaining the MOR antagonist activity. In comparison with LP1, the p-substituted-phenyl compounds showed a lower potency on MOR and DOR and a switched functional profile. Briefly, the bulkier size of the substituent modified the DOR and KOR functional profile from agonist to antagonist with retention of the MOR agonist activity. The p-methoxy derivative did not follow this trend and just the p-F-phenyl analogue shared with LP1 the same functional profile. Collectively, these data suggest that the benzomorphan-N-phenylpropanamide-based structure may represent a tool to achieve a specific opioid functional profile going from a pure agonist to a pure antagonist via mixed agonist/antagonist ligands, simply by increasing or decreasing the aromatic group size.