Solution conformations of the 6α and 6β epimers of oxymorphamine

Opioid ligands addressing unconventional binding sites and more than one opioid receptor subtype

Opioid receptors (ORs) represent one of the most significant groups of G-protein coupled receptor (GPCR) drug targets and also act as prototypical models for GPCR function. In a constant effort to develop drugs with less side effects, and tools to explore the ORs nature and function, various (poly)pharmacological ligand design approaches have been performed. That is, besides classical ligands, a great number of bivalent ligands (i.e. aiming on two distinct OR subtypes), univalent heteromer-selective ligands and bitopic and allosteric ligands have been synthesized for the ORs. The scope of our review is to present the most important of the aforementioned ligands, highlight their properties and exhibit the current state-of-the-art pallet of promising drug candidates or useful molecular tools for the ORs.

Design, synthesis, and characterization of 6beta-naltrexol analogs, and their selectivity for in vitro opioid receptor subtypes

Since the mu opioid receptor (MOR) is known to be involved in the therapeutically relevant pathways leading to the manifestation of pain and addiction, we are currently studying the specific structural characteristics that promote antagonism at the MOR. The opiates 6beta-naltrexol and 6beta-naltrexamide function as neutral antagonists in in vitro and in vivo systems previously exposed to morphine, and are under investigation as improved treatments for narcotic dependence. In this research, we synthesized and characterized carbamate and sulfonate ester derivates of 6beta-naltrexol that do not contain a protic group at C(6), and evaluated these compounds for opioid receptor affinity. In vitro receptor subtype (mu, kappa, and delta opioid receptors) binding data of the carbamate and sulfonate derivatives is reported. All four compounds synthesized exhibited affinity for the MOR better than the standard 6beta-naltrexol HCl. Based on K(i) data, the order of MOR affinity is as follows: 9>13>14>10>6beta-naltrexol HCl. Carbamate 9 and tosylate 13 displayed subnanomolar affinity for the MOR, while 10 was the most mu-selective compound synthesized. In conclusion, our data indicate that the absence of a hydrogen-bond donor on the C(6) oxygen enhances rather than impedes the in vitro affinity of naltrexol derivatives for the MOR. Additionally, data also suggest that increasing the bulk around C(6) may allow control of subtype selectivity within these compound series.

Opioid antagonist activity of naltrexone-derived bivalent ligands: importance of a properly oriented molecular scaffold to guide "address" recognition at kappa opioid receptors

The presence of a molecular scaffold to orient a basic group is important for potent and selective kappa opioid antagonist selectivity. An attempt to determine how the geometry of the scaffold affects this selectivity has led to the synthesis of a bivalent ligand (5) whose linker constrains the N17' basic nitrogen (the "address") to a position that is 6.5 A from N17' in the kappa antagonist norBNI (1) when these molecules are superimposed. The fact that compound 5 was found to be a highly selective and potent mu-selective antagonist supports the idea that the position of N17' in 5 precludes effective ion pairing with the nonconserved residue Glu297 on outer loop 3 of the kappa opioid receptor. The high mu receptor binding affinity and in vitro pharmacological selectivity of 5 coupled with its presumed low central nervous system bioavailability suggest that it may be a useful antagonist for the investigation of peripheral mu opioid receptors.

Naloxone-6 alpha-spirohydantoin: a long-acting opiate antagonist

We have synthesized a novel derivative of naloxone, naloxyl-6 alpha-spirohydantoin. The compound was rigorously purified and its stereochemical structure characterized. In tests in vivo in mice, it was less toxic than the parent compound and it showed weak anticonvulsive activity. It exerted antagonist effects comparable to those of naloxone but its effects were longer lasting, persisting for a week.