(a) The chemical structures of the lead compounds oxycodone, tetrapeptide and JWH-018, and the bivalent compounds SD-11 and SD-19. (b) The chemical structure of the MOR–CXCR4 bivalent ligand VZMX001 (with attachment points shown). (c) The chemical structures of the DOR antagonist (Tyr-Tic-OH), MOR antagonist (H-Tyr-Pro-Phe-D1Nal-NH2), and the bivalent compound D24M. (d) The chemical structures of the MOR agonist hydromorphone, the D2-likeR agonist DPAT and antagonist DAP, and the bivalent compound MQ-12d.