The functional in vitro study of the enantiomers of imidazolines 4-7 highlighted the role played by the nature of the ortho phenyl substituent in determining the preferred α(2C)-AR configuration. Indeed, the (S) enantiomers of 4-6 or (R) enantiomer of 7 behave as eutomers and activate this subtype as full agonists; the corresponding distomers are partial agonists. Because in clinical pain management with opioids α(2C)-AR agonists, devoid of the α(2A)-AR-mediated side effects, may represent an improvement over current therapies with clonidine like drugs, 4 and its enantiomers, showing α(2C)-agonism/α(2A)-antagonism, have been studied in vivo. The data suggest that partial α(2C)-activation is compatible with effective enhancement of morphine analgesia and reduction both of morphine tolerance acquisition and morphine dependence acquisition and expression. On the contrary, full α(2C)-activation appears advantageous in reducing morphine tolerance expression. Interestingly, the biological profile displayed by 4 (allyphenyline) and its eutomer (S)-(+)-4 has been found to be very unusual.

No abstract

The imidazoline nucleus linked in position 2 via an oxyethylene bridge to a phenyl ring carrying an ortho substituent of moderate steric bulk provided alpha(2)-adrenergic (AR) ligands endowed with significant alpha(2C)-agonism/alpha(2A)-antagonism. Similar behavior was displayed by cirazoline (12). For their positive morphine analgesia modulation (due to alpha(2C)-AR stimulation) and sedation overcoming (due to alpha(2A)-AR antagonism), 8 and 11 might be useful as adjuvant agents in the management of pain with morphine.

The goal of the present study was to modulate the receptor interaction properties of known alpha 2-adrenoreceptor (AR) antagonists to obtain novel alpha 2-AR agonists with desirable subtype selectivity. Therefore, a phenyl group or one of its bioisosteres or aliphatic moieties with similar steric hindrance were introduced into the aromatic ring of the antagonist lead basic structure. The functional properties of the novel compounds allowed our previous observations to be confirmed. The high efficacy of 7, 12, and 13 as alpha 2-AR agonists and the significant alpha 2C-AR subtype selective activation displayed by 11 and 15 demonstrated that favorable interactions to induce alpha 2-AR activation were formed between the pendant groups of the ligands and the aromatic cluster present in transmembrane domain 6 of the binding site cavity of the receptors.

The study of two series of 2-aryl-ethylen-imidazolines 3-7 and 8-12 inspired by I2-IBS ligands phenyzoline (1) and diphenyzoline (2), respectively, confirmed the interesting "positive" or "negative" morphine analgesia modulation displayed by their corresponding leads and demonstrated that these effects might be correlated with morphine tolerance and dependence, respectively. By comparative examination of rationally designed compounds, some analogies between binding site cavity of I2-IBS proteins and alpha 2C-adrenoreceptor emerged.