Design, syntheses, and pharmacological characterization of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3'-carboxamido)morphinan analogues as opioid receptor ligands

Systematic Structure-Activity Relationship Study of Nalfurafine Analogues toward Development of Potentially Nonaddictive Pain Management Treatments

Despite the availability of numerous pain medications, the current array of Food and Drug Administration-approved options falls short in adequately addressing pain states for numerous patients and consequently worsens the opioid crisis. Thus, it is imperative for basic research to develop novel and nonaddictive pain medications. Toward addressing this clinical goal, nalfurafine (NLF) was chosen as a lead and its structure-activity relationship (SAR) systematically studied through design, syntheses, and in vivo characterization of 24 analogues. Two analogues, 21 and 23, showed longer durations of action than NLF in a warm-water tail immersion assay, produced in vivo effects primarily mediated by KOR and DOR, penetrated the blood-brain barrier, and did not function as reinforcers. Additionally, 21 produced fewer sedative effects than NLF. Taken together, these results aid the understanding of NLF SAR and provide insights for future endeavors in developing novel nonaddictive therapeutics to treat pain.

Blocking potential metabolic sites on NAT to improve its safety profile while retaining the pharmacological profile

The number of opioid-related overdose deaths and individuals that have suffered from opioid use disorders have significantly increased over the last 30 years. FDA approved maintenance therapies to treat opioid use disorder may successfully curb drug craving and prevent relapse but harbor adverse effects that reduce patient compliance. This has created a need for new chemical entities with improved patient experience. Previously our group reported a novel lead compound, NAT, a mu-opioid receptor antagonist that potently antagonized the antinociception of morphine and showed significant blood-brain barrier permeability. However, NAT belongs to thiophene containing compounds which are known structural alerts for potential oxidative metabolism. To overcome this, 15 NAT derivatives with various substituents at the 5'-position of the thiophene ring were designed and their structure-activity relationships were studied. These derivatives were characterized for their binding affinity, selectivity, and functional activity at the mu opioid receptor and assessed for their ability to antagonize the antinociceptive effects of morphine in vivo. Compound 12 showed retention of the basic pharmacological attributes of NAT while improving the withdrawal effects that were experienced in opioid-dependent mice. Further studies will be conducted to fully characterize compound 12 to examine whether it would serve as a new lead for opioid use disorder treatment and management.

Molecular Pharmacology Profiling of Phenylfentanil and Its Analogues to Understand the Putative Involvement of an Adrenergic Mechanism in Fentanyl-Induced Respiratory Depression

While there are approved therapeutics to treat opioid overdoses, the need for treatments to reverse overdoses due to ultrapotent fentanyls remains unmet. This may be due in part to an adrenergic mechanism of fentanyls in addition to their stereotypical mu-opioid receptor (MOR) effects. Herein, we report our efforts to further understanding of the functions these distinct mechanisms impart. Employing the known MOR neutral antagonist phenylfentanil as a lead, 17 analogues were designed based on the concept of isosteric replacement. To probe mechanisms of action, these analogues were pharmacologically evaluated in vitro and in vivo, while in silico modeling studies were also conducted on phenylfentanil. While it did not indicate MOR involvement in vivo, phenylfentanil yielded respiratory minute volumes similar to those caused by fentanyl. Taken together with molecular modeling studies, these results indicated that respiratory effects of fentanyls may also correlate to inhibition of both α1A- and α1B-adrenergic receptors.

IUPHAR review: Recent progress in the development of Mu opioid receptor modulators to treat opioid use disorders

Opioid Use Disorder (OUD) can be described as intense preoccupation with using or obtaining opioids despite the negative consequences associated with their use. As the number of OUD cases in the U.S. increase, so do the number of opioid-related overdose deaths. In 2022, opioid-related overdose became the No. 1 cause of death for individuals in the U.S. between the ages of 25 and 64 years of age. Because of the introduction of highly potent synthetic opioids (e.g. fentanyl) to the illicit drug market, there is an urgent need for therapeutics that successfully reduce the number of overdoses and can help OUD patients maintain sobriety. Most abused opioids stimulate the mu-opioid receptor (MOR) and activation of this receptor can lead to positive (e.g., euphoria) consequences. However, the negative side effects of MOR stimulation can be fatal (e.g., sedation, respiratory depression). Therefore, the MOR is an attractive target for developing medications to treat OUD. Current FDA drugs include MOR agonists that aid in detoxification and relapse prevention, and MOR antagonists that also serve as maintenance therapies or reverse overdose. These medications are limited by their abuse potential, adverse effects, or pharmacological profiles which leaves ample room for research into designing new chemical entities with optimal physiological effects. These includes, orthosteric ligands that target the primary binding site of the MOR, allosteric ligands that positively, negatively, or "silently" modulate receptor function, and lastly, bitopic ligands target both the orthosteric and allosteric sites simultaneously.

Reversal of subtype-selectivity and function by the introduction of a para-benzamidyl substituent to N-cyclopropylmethyl nornepenthone

The discovery and development of novel μ-opioid receptor (MOR) antagonists is a significant area to combat Opioid Use Disorder (OUD). In this work, a series of para-substituted N-cyclopropylmethyl-nornepenthone derivatives were designed and synthesized and pharmacologically assayed. Compound 6a was identified as a selective MOR antagonist both in vitro and in vivo. Its molecular basis was elucidated using molecular docking and MD simulations. A subpocket on the extracellular side of the TM2 domain of MOR, in particular the residue Y^2.64, was proposed to be responsible for the reversal of subtype selectivity and functional reversal of this compound.

Structural Alterations of the "Address" Moiety of NAN Leading to the Discovery of a Novel Opioid Receptor Modulator with Reduced hERG Toxicity

The search for selective opioid ligands with desired pharmacological potency and improved safety profile has always been an area of interest. Our previous effort yielded a potent opioid modulator, NAN, a 6α-N-7'-indolyl-substituted naltrexamine derivative, which exhibited promising pharmacological activities both in vitro and in vivo. However, significant human ether-a-go-go-related gene (hERG) liability limited its further development. Therefore, a systematic structural modification on NAN was conducted in order to alleviate hERG toxicity while preserving pharmacological properties, which led to the discovery of 2'-methylindolyl derivative compound 21. Compared to NAN, compound 21 manifested overall improved pharmacological profiles. Follow-up hERG channel inhibition evaluation revealed a seven-fold decreased potency of compound 21 compared to NAN. Furthermore, several fundamental drug-like property evaluations suggested a reasonable ADME profile of 21. Collectively, compound 21 appeared to be a promising opioid modulator for further development as a novel therapeutic agent toward opioid use disorder treatments.

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 Biological Evaluation of NAP Isosteres: A Switch from Peripheral to Central Nervous System Acting Mu-Opioid Receptor Antagonists

The μ opioid receptor (MOR) has been an intrinsic target to develop treatment of opioid use disorders (OUD). Herein, we report our efforts on developing centrally acting MOR antagonists by structural modifications of 17-cyclopropylmethyl-3,14-dihydroxy-4,5α-epoxy-6β-[(4'-pyridyl) carboxamido] morphinan (NAP), a peripherally acting MOR-selective antagonist. An isosteric replacement concept was applied and incorporated with physiochemical property predictions in the molecular design. Three analogs, namely, 25, 26, and 31, were identified as potent MOR antagonists in vivo with significantly fewer withdrawal symptoms than naloxone observed at similar doses. Furthermore, brain and plasma drug distribution studies supported the outcomes of our design strategy on these compounds. Taken together, our isosteric replacement of pyridine with pyrrole, furan, and thiophene provided insights into the structure-activity relationships of NAP and aided the understanding of physicochemical requirements of potential CNS acting opioids. These efforts resulted in potent, centrally efficacious MOR antagonists that may be pursued as leads to treat OUD.

Exploring naltrexamine derivatives featuring azaindole moiety via nitrogen-walk approach to investigate their in vitro pharmacological profiles at the mu opioid receptor

In the present work, we reported the application of a nitrogen-walk approach on developing a series of novel opioid ligands containing an azaindole moiety at the C6-position of the epoxymorphinan skeleton. In vitro study results showed that introducing a nitrogen atom around the indole moiety not only retained excellent binding affinity, but also led to significant functional switch at the mu opioid receptor (MOR). Further computational investigations provided corroborative evidence and plausible explanations of the results of the in vitro studies. Overall, our current work implemented a series of novel MOR ligands with high binding affinity and considerably low efficacy, which may shed light on rational design of low efficacy MOR ligands for opioid use disorder therapeutics.

Structural Characterization of KOR Inactive and Active States for 3D Pharmacology and Drug Discovery

The structure of the human kappa opioid receptor (KOR) in complex with the long-acting antagonist JDTic was solved crystallographically in 2012 and, along with structures of other opioid receptors, revolutionized our understanding of opioid system function and pharmacology. More recently, active state KOR structure was also determined, giving important insights into activation mechanisms of the receptor. In this review, we will discuss how the understanding of atomistic structures of KOR established a key platform for deciphering details of subtype and functional selectivity of KOR-targeting ligands and for discovery of new chemical probes with potentially beneficial pharmacological profiles.

Verifying the role of 3-hydroxy of 17-cyclopropylmethyl-4,5α-epoxy-3,14β-dihydroxy-6β-[(4'-pyridyl) carboxamido]morphinan derivatives via their binding affinity and selectivity profiles on opioid receptors

In the present study, the role of 3-hydroxy group of a series of epoxymorphinan derivatives in their binding affinity and selectivity profiles toward the opioid receptors (ORs) has been investigated. It was found that the 3-hydroxy group was crucial for the binding affinity of these derivatives for all three ORs due to the fact that all the analogues 1a-e exhibited significantly higher binding affinities compared to their counterpart 3-dehydroxy ones 6a-e. Meanwhile most compounds carrying the 3-hydroxy group possessed similar selectivity profiles for the kappa opioid receptor over the mu opioid receptor as their corresponding 3-dehydroxy derivatives. [³⁵S]-GTPγS functional assay results indicated that the 3-hydroxy group of these epoxymorphinan derivatives was important for maintaining their potency on the ORs with various effects. Further molecular modeling studies helped comprehend the remarkably different binding affinity and functional profiles between compound 1c (NCP) and its 3-dehydroxy analogue 6c.

Computational insights into the molecular mechanisms of differentiated allosteric modulation at the mu opioid receptor by structurally similar bitopic modulators

Targeting the mu opioid receptor (MOR) by applying orthosteric ligands is the most frequently employed method to treat opioid use disorder (OUD). Unfortunately, most of MOR orthosteric ligands produce severe side effects, mainly due to their low selectivity over other opioid receptors. In contrast, some G protein-coupled receptor allosteric modulators have been reported to exhibit high subtype selectivity and can effectively modulate the potency and/or efficacy of orthosteric ligands. Recently, NAQ and its analog NCQ were identified as novel MOR bitopic modulators. Interestingly, NAQ and NCQ were similar in structure but exhibited different efficacy profiles to the MOR. NAQ exhibited an antagonism activity to the MOR while NCQ showed a partial agonism activity to the MOR. In the present study, molecular modeling methods were applied to explore the putative molecular mechanisms of their different functional profiles to the MOR. When NAQ binding with the inactive MOR, the 'address' portion of NAQ interacted with the MOR allosteric site but showed no significant allosteric modulation of the efficacy of the 'message' portion of NAQ. However, when NCQ binding with the inactive and active MOR, the 'address' portion of NCQ seemed to be able to positively modulate the efficacy of the 'message' portion of NCQ at varying levels. Evidentially, the substituents at the 1'- and 4'-positions of the isoquinoline ring of NCQ seemed to play a critical role in the modulatory function of the 'address' portion of NCQ. These findings will be invaluable to develop our next generation of MOR bitopic modulators with high affinity and subtype selectivity to potentially treat OUD.

Application of Bivalent Bioisostere Concept on Design and Discovery of Potent Opioid Receptor Modulators

Here, we described the structural modification of previously identified μ opioid receptor (MOR) antagonist NAN, a 6α-N-7'-indolyl substituted naltrexamine derivative, and its 6β-N-2'-indolyl substituted analogue INTA by adopting the concept of "bivalent bioisostere". Three newly prepared opioid ligands, 25 (NBF), 31, and 38, were identified as potent MOR antagonists both in vitro and in vivo. Moreover, these three compounds significantly antagonized DAMGO-induced intracellular calcium flux and displayed varying degrees of inhibition on cAMP production. Furthermore, NBF produced much less significant withdrawal effects than naloxone in morphine-pelleted mice. Molecular modeling studies revealed that these bivalent bioisosteres may adopt similar binding modes in the MOR and the "address" portions of them may have negative or positive allosteric modulation effects on the function of their "message" portions compared with NAN and INTA. Collectively, our successful application of the "bivalent bioisostere concept" identified a promising lead to develop novel therapeutic agents toward opioid use disorder treatments.

Characterization of 17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN) as a Novel Opioid Receptor Modulator for Opioid Use Disorder Treatment

The opioid crisis is a significant public health issue with more than 115 people dying from opioid overdose per day in the United States. The aim of the present study was to characterize the in vitro and in vivo pharmacological effects of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN), a μ opioid receptor (MOR) ligand that may be a potential candidate for opioid use disorder treatment that produces less withdrawal signs than naltrexone. The efficacy of NAN was compared to varying efficacy ligands at the MOR, and determined at the δ opioid receptor (DOR) and κ opioid receptor (KOR). NAN was identified as a low efficacy partial agonist for G-protein activation at the MOR and DOR, but had relatively high efficacy at the KOR. In contrast to high efficacy MOR agonists, NAN did not induce MOR internalization, downregulation, or desensitization, but it antagonized agonist-induced MOR internalization and stimulation of intracellular Ca2+ release. Opioid withdrawal studies conducted using morphine-pelleted mice demonstrated that NAN precipitated significantly less withdrawal signs than naltrexone at similar doses. Furthermore, NAN failed to produce fentanyl-like discriminative stimulus effects in rats up to doses that produced dose- and time-dependent antagonism of fentanyl. Overall, these results provide converging lines of evidence that NAN functions mainly as a MOR antagonist and support further consideration of NAN as a candidate medication for opioid use disorder treatment.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Effectiveness comparisons of G-protein biased and unbiased mu opioid receptor ligands in warm water tail-withdrawal and drug discrimination in male and female rats

One emerging strategy to address the opioid crisis is the development of mu opioid receptor (MOR) ligands that preferentially signal the G-protein vs. β-arrestin pathway. The present study compared the relative potency and effectiveness of two G-protein biased (GPB)-MOR ligands TRV130 and SR-14968 to five unbiased MOR ligands (NAQ, nalbuphine, buprenorphine, morphine, and methadone) on therapeutic-related (e.g. antinociception) and abuse-related (e.g. discriminative stimulus effects) endpoints. Male and female rats were tested in a warm water tail-withdrawal procedure (50 °C) or trained to discriminate fentanyl (0.04 mg/kg, SC) from saline in a two-lever food-reinforced discrimination procedure. TRV130 and SR-14968 were approximately two-fold more potent to produce fentanyl stimulus effects vs. antinociception. Morphine, fentanyl, and methadone were significantly more potent in the fentanyl discrimination vs. tail withdrawal procedure. In addition, maximum antinociceptive and discriminative stimulus effects of fixed-proportion fentanyl/naltrexone mixtures (1:0.018, 1:0.054, 1:0.18, 1:0.3, and 1:0.54) were used to quantify 1) the relative in vivo efficacy of the two GPB-MOR agonists and five unbiased MOR ligands, and 2) potential species differences in MOR ligand effects between rats and monkeys. The efficacy-effect function generated from the fentanyl/naltrexone mixtures stratified the five unbiased ligands consistent with agonist-stimulated GTPγS binding (NAQ = nalbuphine < buprenorphine < morphine < methadone). However, TRV130 and SR-14968 produced greater antinociception and less fentanyl-like stimulus effects than was predicted. Furthermore, there was a significant positive correlation between rat and monkey antinociceptive effects. Overall, these results demonstrate GPB-MOR agonists produce undesirable abuse-related effects, albeit with slightly better potency and efficacy ratios compared to unbiased agonists. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

In vitro and in vivo functional profile characterization of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3-carboxamido)morphinan (NAQ) as a low efficacy mu opioid receptor modulator

Evidence has shown that downstream signaling by mu opioid receptor (MOR) agonists that recruit β-arrestin2 may lead to the development of tolerance. Also, it has been suggested that opioid receptor desensitization and cyclic AMP overshoot contributes to the development of tolerance and occurrence of withdrawal, respectively. Therefore, studies were conducted with 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3-carboxamido)morphinan (NAQ), a MOR selective partial agonist discovered in our laboratory, to characterize its effect on β-arrestin2 recruitment and precipitation of a cyclic AMP overshoot. DAMGO, a MOR full agonist dose-dependently increased β-arrestin2 association with the MOR, whereas NAQ did not. Moreover, NAQ displayed significant, concentration-dependent antagonism of DAMGO-induced β-arrestin2 recruitment. After prolonged morphine treatment of mMOR-CHO cells, there was a significant overshoot of cAMP upon exposure to naloxone, but not NAQ. Moreover, prolonged incubation of mMOR-CHO cells with NAQ did not result in desensitization nor downregulation of the MOR. In functional studies comparing NAQ with nalbuphine in the cAMP inhibition, Ca2+ flux and [35S]GTPγS binding assays, NAQ did not show agonism in the Ca2+ flux assay but showed partial agonism in the cAMP and [35S]GTPγS assays. Also, NAQ significantly antagonized DAMGO-induced intracellular Ca2+ increase. In conclusion, NAQ is a low efficacy MOR modulator that lacks β-arrestin2 recruitment function and does not induce cellular hallmarks of MOR adaptation and fails to precipitate a cellular manifestation of withdrawal in cells pretreated with morphine. These characteristics are desirable if NAQ is pursued for opioid abuse treatment development.

Application of Receptor Theory to the Design and Use of Fixed-Proportion Mu-Opioid Agonist and Antagonist Mixtures in Rhesus Monkeys

Receptor theory predicts that fixed-proportion mixtures of a competitive, reversible agonist (e.g., fentanyl) and antagonist (e.g., naltrexone) at a common receptor [e.g., mu-opioid receptors (MORs)] will result in antagonist proportion-dependent decreases in apparent efficacy of the agonist/antagonist mixtures and downward shifts in mixture dose-effect functions. The present study tested this hypothesis by evaluating behavioral effects of fixed-proportion fentanyl/naltrexone mixtures in a warm-water tail-withdrawal procedure in rhesus monkeys (n = 4). Fentanyl (0.001-0.056 mg/kg) alone, naltrexone (0.032-1.0 mg/kg, i.m.) alone, and fixed-proportion mixtures of fentanyl/naltrexone (1:0.025, 1:0.074, and 1:0.22) were administered in a cumulative-dosing procedure, and the proportions were based on published fentanyl and naltrexone Kd values at MOR in monkey brain. Fentanyl alone produced dose-dependent antinociception at both 50 and 54°C thermal intensities. Up to the largest dose tested, naltrexone alone did not alter nociception. Consistent with receptor theory predictions, naltrexone produced a proportion-dependent decrease in the effectiveness of fentanyl/naltrexone mixtures to produce antinociception. The maximum effects of fentanyl, naltrexone, and each mixture were also used to generate an efficacy-effect scale for antinociception at each temperature, and this scale was evaluated for its utility in quantifying 1) efficacy requirements for antinociception at 50 and 54°C and 2) relative efficacy of six MOR agonists that vary in their efficacies to produce agonist-stimuated GTPγS binding in vitro (from lowest to highest efficacy: 17-cyclopropylmethyl-3,14β-dihyroxy-4,5α-epoxy-6α-[(3'-isoquinolyl)acetamindo]morphine, nalbuphine, buprenorphine, oxycodone, morphine, and methadone). These results suggest that fixed-proportion agonist/antagonist mixtures may offer a useful strategy to manipulate apparent drug efficacy for basic research or therapeutic purposes.

AgOTf-catalyzed sequential synthesis of 4-isoquinolones via oxidative ring opening of aziridines and aza-Michael addition

An efficient AgOTf-catalyzed sequential reaction involving the oxidative ring-opening of aziridines by DMSO and aza-Michael addition has been developed. A series of 2,3-dihydro-4(1H)-isoquinolones were afforded in moderate to good yields by the formation of one new C[double bond, length as m-dash]O bond and one new C-N bond. The features of this sequential reaction include high bonding efficiency, use of a catalytic amount of catalysts, a broad substrate scope and mild conditions. This methodology provides a good choice for constructing the libraries of 2,3-dihydro-4(1H)-isoquinolones.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) Modulating the Mu Opioid Receptor in a Biased Fashion

Mounting evidence has suggested that G protein-coupled receptors can be stabilized in multiple conformations in response to distinct ligands, which exert discrete functions through selective activation of various downstream signaling events. In accordance with this concept, we report biased signaling of one C6-heterocyclic substituted naltrexamine derivative, namely, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) at the mu opioid receptor (MOR). NAP acted as a low efficacy MOR partial agonist in the G protein-mediated [(35)S]GTPγS binding assay, whereas it did not significantly induce calcium flux or β-arrestin2 recruitment. In contrast, it potently blocked MOR full agonist-induced β-arrestin2 recruitment and translocation. Additionally, NAP dose-dependently antagonized MOR full agonist-induced intracellular calcium flux and β-arrestin2 recruitment. Further results in an isolated organ bath preparation confirmed that NAP reversed the morphine-induced reduction in colon motility. Ligand docking and dynamics simulation studies of NAP at the MOR provided more supporting evidence for biased signaling of NAP at an atomic level. Due to the fact that NAP is MOR selective and preferentially distributed peripherally upon systemic administration while β-arrestin2 is reportedly required for impairment of intestinal motility by morphine, biased antagonism of β-arrestin2 recruitment by NAP further supports its utility as a treatment for opioid-induced constipation.