Relationship between 4,5-epoxymorphinan structure and in vitro modulation of cell proliferation.

Morphine belongs to the class of compounds known as 4,5-epoxymorphinans, which can alter immune function directly via receptors expressed by immune cells. However, the opioid characteristics of these receptors are not clear. Therefore, the aim of this study was to investigate the in vitro immunomodulatory effects of 24 structurally related 4,5-epoxymorphinans to allow further characterisation of the receptor that mediates the immunomodulation and to ascertain if there is any structure-effect relationship. The immunomodulation of 4,5-epoxymorphinans using isolated mouse splenocytes stimulated with concanavalin A resulted in five types of responses: an inverted bell shaped curve (oxycodone, inhibitory EC(50)=1.6 nM), an inhibitory concentration response curve (buprenorphine, inhibitory EC(50)=12.6 microM), an inverted bell-shaped curve with induction (morphine, induction EC(50)=1.7 microM), an induction concentration response curve (oxymorphone, induction EC(50)=20 nM), and the lack of any response (e.g. noroxycodone). Non-stereoselectivity, naloxone-insensitivity, naloxone-sensitivity and non-classical opioid rank order of effect were all observed. A structure-effect relationship was developed and significant evidence for non-classical opioid receptor function on immune cells was concluded.