Synthesis and characterization of 7α-O-iodoallyl diprenorphine: A new ligand for potential spect imaging of opioid receptors

Diels–Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels–Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

A radioiodinated 7alpha-O-iodoallyl diprenorphine for mapping opioid receptors

The aim of the current research has been to validate an original radioiodinated diprenorphine (DPN) derivative suitable for imaging studies of opioid receptors. [(125)I]7alpha-O-iodoallyl diprenorphine (7alpha-O-IA-DPN) was prepared by radioiododestannylation and in vitro and in vivo opioid receptor binding assays were performed with CDF1 mouse brains. In vitro binding studies showed high affinity (K(i)= 0.4 +/- 0.2 nM) for mouse brain membranes. In vivo studies showed 63% specific binding. Ex vivo autoradiography of brain sections confirmed high uptake and retention of [(125)I]7alpha-O-IA-DPN in regions rich in opioid receptors. This new radioiodinated DPN analogue appears to be a potential radioprobe for in vivo visualization of human cerebral opioid receptors with single photon emission computed tomography (SPECT).

Evaluation of [125I]7 alpha-O-iodoallyl diprenophine as a new potential SPECT opioid receptor imaging agent

A new iodinated diprenorphine analog, [125I]7 alpha-O-iodoallyl diprenorphine ([125I]7 alpha-O-IA-DPN), was prepared by iododestannylation and characterized. As an opioid antagonist, this agent showed very high affinity (Ki = 0.4 +/- 0.2 nM) and 63% of specific binding by in vitro and in vivo binding studies. Inhibition curves indicated that this tracer labeled with the same affinities to three opioid receptors (mu = delta = kappa). The findings demonstrate that this proposed compound appears to be potential radioprobe for future study of opioid receptors by in vivo SPECT.