For decades the opioid receptors have been an attractive therapeutic target for the treatment of pain. Since the first discovery of enkephalin, approximately a dozen endogenous opioid peptides have been known to produce opioid activity and analgesia, but their therapeutics have been limited mainly due to low blood brain barrier penetration and poor resistance to proteolytic degradation. One versatile approach to overcome these drawbacks is the cyclization of linear peptides to cyclic peptides with constrained topographical structure. Compared to their linear parents, cyclic analogs exhibit better metabolic stability, lower offtarget toxicity, and improved bioavailability. Extensive structure-activity relationship studies have uncovered promising compounds for the treatment of pain as well as further elucidate structural elements required for selective opioid receptor activity. The benefits that come with employing cyclization can be further enhanced through the generation of polycyclic derivatives. Opioid ligands generally have a short peptide chain and thus the realm of polycyclic peptides has yet to be explored. In this review, a brief history of designing ligands for the opioid receptors, including classic linear and cyclic ligands, is discussed along with recent approaches and successes of cyclic peptide ligands for the receptors. Various scaffolds and approaches to improve bioavailability are elaborated and concluded with a discourse towards polycyclic peptides.

Transporters (expressed) at the blood-brain barrier (BBB) can play an essential role in the treatment of brain injury by transporting neuroprotective substance to the central nervous system. The goal of this study was to understand the role of organic anion transporting polypeptide (OATP1; OATP1A2 in humans and oatp1a4 in rodents) in the transport of a potent opioid receptor agonist, biphalin, across the BBB during ischemic stroke. Brain microvascular endothelial cells (BMECs) that were differentiated from human induced pluripotent stem cells (iPSCs) were used in the present study. The effect of oxygen-glucose deprivation (OGD) and reperfusion on the OATP1 expression, uptake, and transport of biphalin was measured in induced pluripotent stem cells differentiated brain microvascular endothelial cells (iPSC–BMECs) in the presence and absence of an OATP1 substrate, estrone-3-sulfate (E3S). Biphalin brain permeability was quantified while using a highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. It was found that iPSC-BMECs expressed OATP1. In vitro studies showed that biphalin BBB uptake and transport decreased in the presence of an OATP1 specific substrate. It was also observed that OGD and reperfusion modulate the expression and function of OATP1 in BMECs. This study strongly demonstrates that OATP1 contributes to the transport of biphalin across the BBB and increased expression of OATP1 during OGD-reperfusion could provide a novel target for improving ischemic brain drug delivery of biphalin or other potential neurotherapeutics that have affinity to this BBB transporter.

Background: Ischemic stroke is one of the leading causes of mortality and morbidity in the world and effective neuroprotectants are yet to be developed. Recent studies have demonstrated excellent neuroprotective effects of a bivalent enkephalin opioid agonist, biphalin in multiple stroke models. Methods: The purpose of this study is to evaluate novel multifunctional enkephalin-fentanyl opioid agonists, LYS436, LYS739 and LYS416 for their neuroprotective potential using in vitro and in vivo ischemic stroke models and to compare the effect to that of biphalin. Results: In general, all non-selective opioid agonists significantly decreased neuronal cell death and levels of reactive oxygen species in primary neurons subjescted to hypoxia-aglycemia/re-oxygenation or NMDA neurotoxicity. Fluorinated enkephalin-fentanyl conjugate, LYS739 showed enhanced neuroprotection in both in vitro models compared to biphalin. Based on further in vitro screening and comparative studies to biphalin, LYS739 was selected as a lead for in vivo experimentation. A mouse middle cerebral artery occlusion (MCAO) stroke model was utilized to study biphalin and the lead analog, LYS739. Both agonists significantly decreased brain infarct and edema ratios compared to saline treated group. Neurological impairment after stroke was statistically significantly improved in terms of neurological score and locomotor activities with LYS739 and biphalin treatment. Importantly, LYS739 and biphalin demonstrated better neuroprotection compared to fentanyl, and this effect was reversed by non-selective opioid antagonist naltrexone. Conclusion: In summary, the results of this study suggest that the multifunctional fluorinated enkephalin analog, LYS739 can be considered as a potential lead for ischemic stroke research and may provide advantages given the multimeric peptide-opiate structure.

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.