Fig. 1.

Experimental considerations for the assessment of opioid pharmacology in vitro and in vivo. In vitro and in vivo characterisations of opioid action are both essential in drug discovery. However, important factors need to be considered to provide a consistent and systematic framework of opioid action. In vitro, compounds are tested for their binding to MOR, and for their ability to activate downstream signalling. These assays need to take into account distinct variables than can affect the measured outcome. In vivo, candidates are usually first assessed for their antinociceptive efficacy, and then for their ability to elicit adverse effects. OIC: Opioid-induced constipation, OIH: Opioid-induced hyperalgesia.