In vitro agonist activities at the MOP receptor of compounds 1–6, oxycodone and 14-OMC. Concentration-response curves in (A) [35S]GTPγS functional assay with membranes from CHO expressing human MOP receptors and (B) calcium mobilization experiments performed with CHO cells co-expressing the human MOP receptor and the Gαqi5 protein. Activity is calculated as percentage of maximal DAMGO stimulation in each assay (225 ± 32% and 217 ± 21% in [35S]GTPγS functional and calcium mobilization assay, respectively, set as 100%). Calculated potencies and efficacies are presented in Table 2. Data are shown as the mean ± SEM (n ≥ 3).