Synthesis of a New Class of ß-Carbonyl Selenides Functionalized with Ester Groups with Antioxidant and Anticancer Properties-Part II.

A series of phenyl ß-carbonyl selenides with o-ester functionality substituted on the oxygen atom with chiral and achiral alkyl groups was synthesized. All compounds are the first examples of this type of organoselenium derivatives with an ester substituent in the ortho position. The obtained derivatives were tested as antioxidants and anticancer agents to see the influence of an ester functionality on the bioactivity of ß-carbonyl selenides by replacing the o-amide group with an o-ester group. The best results as an antioxidant agent were observed for O-((1R,2S,5R)-(-)-2-isopropyl-5-methylcyclohexyl)-2-((2-oxopropyl)selanyl)benzoate. The most cytotoxic derivative against breast cancer MCF-7 cell lines was O-(methyl)-2-((2-oxopropyl)selanyl)benzoate and against human promyelocytic leukemia HL-60 was O-(2-pentyl)-2-((2-oxopropyl)selanyl)benzoate.

New uracil analog as inhibitor/modulator of ABC transporters or/and NF-κB in taxol-resistant MCF-7/Tx cell line.

PURPOSE: The global increase in breast cancer cases necessitates ongoing exploration of advanced therapies. Taxol (Tx), an initial breast cancer treatment, induces mitotic arrest but faces limitations due to side effects and the development of resistance. Addressing Tx resistance involves understanding the complex molecular mechanisms, including alterations in tubulin dynamics, NF-κB signaling, and overexpression of ABC transporters (ABCB1 and ABCG2), leading to multidrug resistance (MDR). METHODS: Real-time PCR and ELISA kits were used to analyze ABCB1, ABCG2 and NF-κB gene and protein expression levels, respectively. An MDR test assessed the resistance cell phenotype. RESULTS: MCF-7/Tx cells exhibited a 24-fold higher resistance to Tx. Real-time PCR and ELISA analysis revealed the upregulation of ABCB1, ABCG2, and NF-κB. U-359 significantly downregulated both ABCB1 and ABCG2 gene and protein levels. Co-incubation with Tx and U-359 further decreased the mRNA and protein expression of these transporters. The MDR test indicated that U-359 increased MDR dye retention, suggesting its potential as an MDR inhibitor. U-359 and Tx, either individually or combined, modulated NF-κBp65 protein levels. CONCLUSION: The development of a Taxol-resistant MCF-7 cell line provided valuable insights. U-359 demonstrated effectiveness in reducing the expression of ABC transporters and NF-κB, suggesting a potential solution for overcoming multidrug resistance in breast cancer cells. The study recommends a strategy to enhance the sensitivity of cancer cells to chemotherapy by integrating U-359 with traditional drugs.

Historical Perspective and Current Trends in Anticancer Drug Development.

Cancer is considered one of the leading causes of death in the 21st century. The intensive search for new anticancer drugs has been actively pursued by chemists and pharmacologists for decades, focusing either on the isolation of compounds with cytotoxic properties from plants or on screening thousands of synthetic molecules. Compounds that could potentially become candidates for new anticancer drugs must have the ability to inhibit proliferation and/or induce apoptosis in cancer cells without causing too much damage to normal cells. Some anticancer compounds were discovered by accident, others as a result of long-term research. In this review, we have presented a brief history of the development of the most important groups of anticancer drugs, pointing to the fact that they all have many side effects.

Analgesic Peptides: From Natural Diversity to Rational Design.

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

Synthesis of New Chiral ß-Carbonyl Selenides with Antioxidant and Anticancer Activity Evaluation-Part I.

A series of unsymmetrical phenyl ß-carbonyl selenides with o-amido function substituted on the nitrogen atom with chiral alkyl groups was obtained. The compounds form a series of enantiomeric and diastereomeric pairs and present the first examples of this type of chiral Se derivatives. All obtained selenides were further evaluated as antioxidants and anticancer agents to define the influence of the particular stereochemistry of the attached functional groups on the bioactivity of the molecules. The highest H2O2 reduction potential was observed for N-(cis-2-hydroxy-1-indanyl)-2-((2-oxopropyl)selanyl)benzamide, and the best radical scavenging properties for N-(-1-hydroxy-2-butanyl)-2-((2-oxopropyl)selanyl)benzamide. Also, both enantiomers of the N-(1-hydroxy-2-butanyl) selenide expressed the highest cytotoxic potential towards human promyelocytic leukemia HL-60 cell line with similar IC50 values 14.4 ± 0.5 and 16.2 ± 1.1 µM, respectively. On the other hand, breast cancer cell line MCF-7 was most sensitive to N-((R)-(-)-1-hydroxy-2-butanyl)- 2-((2-oxopropyl)selanyl)benzamide (IC50 of 35.7 ± 0.6 µM). The structure-activity dependence of the obtained Se derivatives was discussed, and the most potent compounds were selected.

Synthesis And Anticancer Activity Of New Hybrid 3-Methylidene-2,3-Dihydro-1,8-Naphthyridinones.

Synthesis of molecular hybrids, obtained by combination of two or more pharmacophoric groups of different bioactive substances in order to produce more efficient drugs, is now a frequently used approach in medicinal chemistry. Following this strategy, we synthetized a library of 3-methylidene-1-tosyl-2,3-dihydro-1,8-naphthyridin-4(1H)-ones, combining a 1,8-naphthyridin-4-one motif with an exo-methylidene bond conjugated with a carbonyl group, pharmacophoric units that are present in many natural, biologically active compounds with anticancer potential. We reasoned that such bifunctional conjugates may have enhanced cytotoxic activity. The title compounds were synthesized in a four step reaction sequence. ß-Ketophosphonate, obtained from methyl N-tosylnicotinate and diethyl methylphosphonate, was reacted with various aldehydes giving 3-diethoxyphosphoryl-2,3-dihydro-1,8-naphthyridin-4(1H)-ones as keto-enol tautomers. Later, these compounds were transformed into 3-methylidene-1-tosyl-2,3-dihydro-1,8-naphthyridin-4(1H)-ones applying the Horner-Wadsworth-Emmons methodology. Then, the cytotoxicity of the new compounds was assessed on two cancer cell lines, promyelocytic leukemia HL-60 and breast cancer adenocarcinoma MCF-7, and for comparison, on human umbilical vein endothelial cells HUVEC. The most active and selective analog, 2-ethyl-3-methylidene-1-tosyl-2,3-dihydro-1,8-naphthyridin-4(1H)-one 4 a was chosen for more detailed studies on HL-60 cell line, to determine molecular mechanisms of its anticancer activity. It was shown that 4 a strongly inhibited proliferation and induced apoptosis which could be attributed to its ability to cause DNA damage.

The Influence of Long Carbon Chains on the Antioxidant and Anticancer Properties of N-Substituted Benzisoselenazolones and Corresponding Diselenides.

Organoselenium compounds are well-known for their numerous biocapacities, which result from the uniqueness of the selenium atom and the possibility of constructing heterorganic molecules that can mimic the activity of selenoenzymes, crucial for a multitude of important physiological processes. In this paper, we have synthesized a series of N-substituted benzisoselenazolones and corresponding diphenyl diselenides possessing lipophilic long carbon chains, solely or with additional polar insets: phenyl linkers and ester groups. Evaluation of their antioxidant and cytotoxic activity revealed an increased H2O2-reduction potential of diphenyl diselenides bearing N-octyl, ethyl N-(12-dodecanoate)- and N-(8-octanoate) groups, elevated radical scavenging activity of 2,2'-diselenobis(N-dodecylbenzamide) and a promising cytotoxic potential of N-(4-dodecyl)phenylbenzisoselenazol-3(2H)-one.

Facile synthesis of chiral phenylselenides as novel antioxidants and cytotoxic agents.

Organoselenium compounds are well-known for their unique biological properties, including antioxidant, anticancer and anti-inflammatory. They result from the presence of a particular Se-moiety enclosed in a structure that provides physicochemical features necessary for effective drug-target interactions. Looking for a proper drug design that considers the influence of each structural element has to be conducted. In this paper, we have synthesized a series of chiral phenylselenides, possessing an additional N-substituted amide moiety, and evaluated their antioxidant and anticancer potential. The presented derivatives, as a group of enantiomeric and diastereomeric pairs, enabled a thorough investigation of the 3D structure-activity dependence in correlation with the presence of the phenylselanyl group as the potential pharmacophore. The N-indanyl derivatives possessing a cis- and trans-2-hydroxy group were selected as the most promising antioxidants and anticancer agents.

Anticancer Properties of 3-Dietoxyphosphorylfuroquinoline-4,9-dione.

Herein, the antitumor activity of a novel synthetic analog with 5,8-quinolinedione scaffold, diethyl (2-(2-chlorophenyl)-4,9-dioxo-4,9-dihydrofuro [3,2-g]quinolin-3-yl)phosphonate (AJ-418) was investigated on two breast cancer cell lines. This analog was selected from a small library of synthetic quinolinediones on the basis of its strong antiproliferative activity against MCF-7 and MDA-MB-231 cells and 4-5-fold lower cytotoxicity towards healthy MCF-10A cells. The morphology of MCF-7 and MDA-MB-231 cancer cells treated with AJ-418 changed drastically, while non-tumorigenic MCF-10A cells remained unaffected. In MCF-7 cells, after 24 h incubation, the increased number of apoptotic cells coincided with a decrease in proliferation and cell viability. The 24 h treatment of MDA-MB-231 cells with the tested compound reduced their cell viability and proliferation rate; however, a significant pro-apoptotic effect was visible only after longer incubation times (48 h and 72 h). Then, the maximum tolerated dose (MTD) of compound AJ-418 in C3H mice after subcutaneous administration was determined to be 160 mg/kg, showing that this analog was well tolerated and can be further evaluated to assess its potential therapeutic effect in tumor-bearing mice.

Trifluoromethylated 4,5-Dihydro-1,2,4-triazin-6(1H)-ones via (3+3)-Annulation of Nitrile Imines with α-Amino Esters.

The synthesis of two series of monocyclic and bicyclic trifluoromethylated 4,5-dihydro-1,2,4-triazin-6(1H)-one derivatives based on (3+3)-annulation of methyl esters derived from natural α-amino acids with in situ generated trifluoroacetonitrile imines has been described. The devised protocol is characterized by a wide scope, easily accessible substrates, remarkable functional group tolerance, and high chemical yield. In reactions with chiral starting materials, no racemization at the stereogenic centers was observed and the respective enantiomerically pure products were obtained. Selected functional group interconversions carried out under catalytic hydrogenation and mild PTC oxidation conditions were also demonstrated.

Synthesis and Anticancer Properties of New 3-Methylidene-1-sulfonyl-2,3-dihydroquinolin-4(1H)-ones.

Quinolinones have been known for a long time as broad-spectrum synthetic antibiotics. More recently, the anticancer potential of this group of compounds has been investigated. Following this direction, we obtained a small library of 3-methylidene-1-sulfonyl-2,3-dihydroquinolin-4(1H)-ones with various substituents at positions 1, 2, 6, and 7 of the quinolinone ring system. The cytotoxic activity of the synthesized analogs was tested in the MTT assay on two cancer cell lines in order to determine the structure-activity relationship. All compounds produced high cytotoxic effects in MCF-7, and even higher in HL-60 cells. 2-Ethyl-3-methylidene-1-phenylsulfonyl-2,3-dihydroquinolin-4(1H)-one, which was over 5-fold more cytotoxic for HL-60 than for normal HUVEC cells, was selected for further tests. This analog was shown to inhibit proliferation and induce DNA damage and apoptosis in HL-60 cells.

Stereoselective Synthesis and Anticancer Activity of 2,6-Disubstituted trans-3-Methylidenetetrahydropyran-4-ones.

In this report, we present efficient and stereoselective syntheses of 2,6-disubstituted trans-3-methylidenetetrahydropyran-4-ones and 2-(4-methoxyphenyl)-5-methylidenetetrahydropyran-4-one that significantly broaden the spectrum of the available methylidenetetrahydropyran-4-ones with various substitution patterns. Target compounds were obtained using Horner-Wadsworth-Emmons methodology for the introduction of methylidene group onto the pyranone ring. 3-Diethoxyphosphoryltetrahydropyran-4-ones, which were key intermediates in this synthesis, were prepared by fully or highly stereoselective addition of Gilman or Grignard reagents to 3-diethoxyphosphoryldihydropyran-4-ones. Addition occurred preferentially by axial attack of the Michael donors on the dihydropyranone ring. Relative configurations and conformations of the obtained adducts were assigned using a detailed analysis of the NMR spectra. The obtained methylidenepyran-4-ones were evaluated for cytotoxic activity against two cancer cell lines (HL-60 and MCF-7). 2,6-Disubstituted 3-methylidenetetrahydropyran-4-ones with isopropyl and phenyl substituents in position 2 were more cytotoxic than analogs with n-butyl substituent. Two of the most cytotoxic analogs were then selected for further investigation on the HL-60 cell line. Both analogs induced morphological changes characteristic of apoptosis in cancer cells, significantly inhibited proliferation and induced apoptotic cell death. Both compounds also generated DNA damage, and one of the analogs arrested the cell cycle of HL-60 cells in the G2/M phase. In addition, both analogs were able to inhibit the activity of topoisomerase IIα. Based on these findings, the investigated analogs may be further optimized for the development of new and effective topoisomerase II inhibitors.

Synthesis and Cytotoxic Activity of Lepidilines A-D: Comparison with Some 4,5-Diphenyl Analogues and Related Imidazole-2-thiones.

A straightforward access to 2-unsubstituted imidazole N-oxides with subsequent deoxygenation by treatment with Raney-nickel followed by N-benzylation opens up a convenient route to lepidilines A and C. Both imidazolium salts were used to generate in situ the corresponding imidazol-2-ylidenes, which smoothly reacted with elemental sulfur, yielding imidazole-2-thiones. These reactions were performed either under classical conditions in pyridine solutions or mechanochemically using solid Cs2CO3 as a base. The structure of lepidiline C was unambiguously confirmed by X-ray analysis of its hexafluorophosphate. An analogous protocol toward lepidilines B and D and their 4,5-diphenyl analogues is less efficient due to observed instability of the key precursors, i.e., the respective 2-methylimidazole N-oxides. Comparison of cytotoxic activity against HL-60 and MCF-7 cell lines of all lepidilines, as well as their selected structural analogues (e.g., 4,5-diphenyl derivatives and PF6 salts), revealed slightly more potent activity of the 2-methylated series, irrespectively of the type of counterion present in the imidazolium salt. Remarkably, the well-known 1,3-diadamantylimidazolium bromide (the "Arduengo salt"), known as the precursor of the first, shelf-stable NHC representative, and its adamantyloxy analogue displayed the most significant cytotoxic activity in the studied series.

The Gliopeptide ODN, a Ligand for the Benzodiazepine Site of GABAA Receptors, Boosts Functional Recovery after Stroke.

Following stroke, the survival of neurons and their ability to reestablish connections is critical to functional recovery. This is strongly influenced by the balance between neuronal excitation and inhibition. In the acute phase of experimental stroke, lethal hyperexcitability can be attenuated by positive allosteric modulation of GABAA receptors (GABAARs). Conversely, in the late phase, negative allosteric modulation of GABAAR can correct the suboptimal excitability and improves both sensory and motor recovery. Here, we hypothesized that octadecaneuropeptide (ODN), an endogenous allosteric modulator of the GABAAR synthesized by astrocytes, influences the outcome of ischemic brain tissue and subsequent functional recovery. We show that ODN boosts the excitability of cortical neurons, which makes it deleterious in the acute phase of stroke. However, if delivered after day 3, ODN is safe and improves motor recovery over the following month in two different paradigms of experimental stroke in mice. Furthermore, we bring evidence that, during the subacute period after stroke, the repairing cortex can be treated with ODN by means of a single hydrogel deposit into the stroke cavity.SIGNIFICANCE STATEMENT Stroke remains a devastating clinical challenge because there is no efficient therapy to either minimize neuronal death with neuroprotective drugs or to enhance spontaneous recovery with neurorepair drugs. Around the brain damage, the peri-infarct cortex can be viewed as a reservoir of plasticity. However, the potential of wiring new circuits in these areas is restrained by a chronic excess of GABAergic inhibition. Here we show that an astrocyte-derived peptide, can be used as a delayed treatment, to safely correct cortical excitability and facilitate sensorimotor recovery after stroke.

Thapsigargin-From Traditional Medicine to Anticancer Drug.

A sesquiterpene lactone, thapsigargin, is a phytochemical found in the roots and fruits of Mediterranean plants from Thapsia L. species that have been used for centuries in folk medicine to treat rheumatic pain, lung diseases, and female infertility. More recently thapsigargin was found to be a potent cytotoxin that induces apoptosis by inhibiting the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) pump, which is necessary for cellular viability. This biological activity encouraged studies on the use of thapsigargin as a novel antineoplastic agent, which were, however, hampered due to high toxicity of this compound to normal cells. In this review, we summarized the recent knowledge on the biological activity and molecular mechanisms of thapsigargin action and advances in the synthesis of less-toxic thapsigargin derivatives that are being developed as novel anticancer drugs.

Synthesis, Selected Transformations, and Biological Activity of Alkoxy Analogues of Lepidilines A and C.

Condensation of diacetyl monooxime with formaldimines derived from alkoxyamines in glacial acetic acid at room temperature leads to corresponding 2-unsubstituted imidazole N-oxides bearing an alkoxy substituent at the N(1) atom of the imidazole ring. Subsequent O-benzylation afforded, depending on the type of alkylating agent, either symmetric or nonsymmetric alkoxyimidazolium salts considered as structural analogues of naturally occurring imidazole alkaloids, lepidilines A and C. Some of the obtained salts were tested as precursors of nucleophilic heterocyclic carbenes (NHCs), which in situ reacted with elemental sulfur to give the corresponding N-alkoxyimidazole-2-thiones. The cytotoxic activity of selected 4,5-dimethylimidazolium salts bearing either two benzyloxy or benzyloxy and 1-adamantyloxy groups at N(1) and N(3) atoms was evaluated against HL-60 and MCF-7 cell lines using the MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay. Notably, in two cases of alkoxyimidazolium salts, no effect of the counterion exchange (Br- → PF6-) on the biological activity was observed.

A New Hybrid δ-Lactone Induces Apoptosis and Potentiates Anticancer Activity of Taxol in HL-60 Human Leukemia Cells.

In the search for new drug candidates, researchers turn to natural substances isolated from plants which may be either used directly or may serve as a source for chemical modifications. An interesting strategy in the design of novel anticancer agents is based on the conjugation of two or more biologically active structural motifs into one hybrid compound. In this study, we investigated the anticancer potential of 4-benzyl-5,7-dimethoxy-4-methyl-3-methylidene-3,4-dihydro-2H-chroman-2-one (DL-247), a new hybrid molecule combining a chroman-2-one skeleton with an exo-methylidene bond conjugated with a carbonyl group, in human myeloid leukemia HL-60 cell line. The cytotoxicity of the new compound was tested using MTT assay. The effect of DL-247 on cell proliferation and apoptosis induction were studied by flow cytometry, fluorometric assay and ELISA analysis. DL-247 displayed high cytotoxic activity (IC50 = 1.15 µM, after 24 h incubation), significantly inhibited cell proliferation and induced apoptosis by both, the intrinsic and extrinsic pathways. A combination of DL-247 with taxol exhibited a strong synergistic effect on DNA damage generation, apoptosis induction and inhibition of cell growth.

Molecular mechanisms of apoptosis induced by a novel synthetic quinolinone derivative in HL-60 human leukemia cells.

The mortality rates for acute myeloid leukemia are very high, necessitating the search for novel chemotherapeutic candidates. Herein, we investigated the anticancer potential of a new synthetic compound, 2-ethyl-3-methyliden-1-tosyl-2,3-dihydroquinolin-4-(1H)-one (AJ-374) against myeloid leukemia HL-60 cell line. This analog was selected from the small library of synthetic dihydroquinolinones on the basis of its strong antiproliferative activity against HL-60 cells and 30-fold lower cytotoxicity towards healthy HUVEC cells. AJ-374 promoted the arrest of the cells in the subG0/G1 phase of the cell cycle in the first 24 h. Treatment of HL-60 cells with AJ-374 caused an increase in annexin-V positive cells, activation of caspase-8, -9 and -3, dissipation of the mitochondrial membrane potential and enhancement of FAS protein level. Apoptosis induction triggered by this quinolinone was blocked by the pre-treatment of the cells with caspase-8, -9 and -3 inhibitors. The obtained results indicated that AJ-374-induced apoptosis was executed by both, the extrinsic and intrinsic pathways. The cytotoxic activity of AJ-374 was also associated with down-regulation of the mitogen-activated protein kinase (MAPK) pathway and was independent of reactive oxygen species generation. Taken together, these results suggest that AJ-374 exerts a potent anticancer effect on leukemia cells, with a wide safety margin, which makes this analog an attractive drug candidate for further testing.

Involvement of α-methylene-γ- and δ-lactones in the suppression of multidrug resistance in MCF-7 cells.

BACKGROUND: The development of multidrug resistance to chemotherapy remains a challenge in the treatment of cancer and is a major factor causing failure of many forms of chemotherapy. The ATP binding cassette (ABC) family of proteins are efflux pumps that transport various potentially dangerous substances out of the cells. Several of the ABC transporters are related to chemoresistance, as the rapidly dividing malignant cells use them to protect themselves from medical interventions. Inhibitors of ABC transporters have the potential to enhance the efficacy of anticancer drugs. Two new synthetic compounds, AD-06 and AD-013, were tested as possible multidrug resistance inhibitors in MCF-7 cells. METHODS: The cytotoxicity of new compounds was tested in MCF-7 and MCF-10A cell lines using the MTT method. Gene expression was measured by real-time PCR and changes in the protein levels were evaluated by flow cytometry and ELISA. A method based on the use of a fluorescent dye, being a marker of the ABC transporter activity, was used for screening the tested compounds as potential multidrug resistance inhibitors. RESULTS: AD-06 and AD-013 down-regulated NF-κB mRNA levels and decreased the population of cells with activated NF-κB. Both compounds were found to be strong ABCB1 and ABCG2 transporter inhibitors. They showed synergistic effects when incubated with taxol or oxaliplatin. CONCLUSIONS: α-Methylene-γ- and -δ-lactones AD-06 and AD-013 are promising lead structures for further development as multidrug resistance inhibitors.

In vitro and in vivo activity of cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH2, a mu opioid receptor agonist biased toward ß-arrestin.

Morphine and related drugs, which are the most effective analgesics for the relief of severe pain, act through activating opioid receptors. The endogenous ligands of these receptors are opioid peptides which cannot be used as antinociceptive agents due to their low bioactivity and stability in biological fluids. The major goal of opioid research is to understand the mechanism of action of opioid receptor agonists in order to improve therapeutic utility of opioids. Analgesic effects of morphine are mediated mostly through activation of the mu opioid receptor. However, in the search for safer and more effective drug candidates, analogs with mixed opioid receptor profile gained a lot of interest. Recently, the concept of biased agonists able to differentially activate GPCR downstream pathways, became a new approach in the design of novel drug candidates. It is hypothesized that compounds promoting G-protein signaling may produce analgesia while ß-arrestin recruitment may be responsible for opioid side effects. In this report we showed that replacement of the tyrosine residue in the mu-selective ligand Tyr-c[d-Lys-Phe-Asp]NH2 with 2',6'-dimethyltyrosine (Dmt) produced a cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH2 with mu/delta opioid receptor agonist profile. This analog showed improved antinociception in the hot-plate test, probably due to the simultaneous activation of mu and delta receptors but also significantly inhibited the gastrointestinal transit. Using the bioluminescence resonance energy transfer (BRET) assay it was shown that this analog was a mu receptor agonist biased toward ß-arrestin. ß-Arrestin-dependent signaling is most likely responsible for the observed inhibition of gastrointestinal motility exerted by the novel cyclopeptide.