Fruitful Adrenergic α2C-Agonism/α2A-Antagonism Combination to Prevent and Contrast Morphine Tolerance and Dependence

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 alpha(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 alpha(2C)-AR agonists, devoid of the alpha(2A)-AR-mediated side effects, may represent an improvement over current therapies with clonidine like drugs, 4 and its enantiomers, showing alpha(2C)-agonism/alpha(2A)-Aantagonism, have been studied in vivo. The data suggest that partial alpha(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 alpha(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.