However, 1a was an agonist in both the tail flick (ED50 0.14 mg/kg) and PPQ-induced writhing assays (ED50 0.016 mg/kg) confirming it as a partial agonist in vivo.Of the C20 analogues of buprenorphine (1a), the one carbon homologues 1b and 1c displayed similar efficacy to 1a at MOP receptors (all 20???29% relative to DAMGO), and were of very similar potency (2.6 ??? 10.2 nM).