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Background: In the last decade there has been a progressive increase in the use of new psychoactive substances (NPSs) that are not yet under international control. In particular, novel synthetic opioids (NSOs) have reappeared on the recreational drug market in the last few years. As a result, the use of NSOs has increased rapidly. This poses an emerging and demanding challenge to public health. Aim: To raise awareness among clinicians and other professionals about NPSs, especially NSOs, to summarize current knowledge about pharmacological properties, forms of NSO on the market, pattern of use, effects and consequences of use. Methods: An electronic search was carried out on the Medline/PubMed and Google Scholar databases to find selected search terms. Results: Some NPSs are already controlled, while others can be legally sold directly on the drug market (mainly via internet, less so by drug dealers) or be used as precursors for the synthesis of other designer drugs that mimic the psychoactive effects of controlled substances. Potential side-effects of NSOs include miosis, sedation, respiratory depression, hypothermia, inhibition of gastrointestinal propulsion, death (from opioid overdose). Conclusions: The severity of the opioid crisis has intensified with the introduction of highly potent NSOs on the drug market. As long as addicts are dying from overdose or similar causes, there is something more constructive to do than waiting for addicts to overdose on heroin at a place located near a remedy, as if to say, within reach of naloxone.

2-Benzylbenzimidazole ‘nitazene’ opioids are presenting a growing threat to public health. Although various nitazenes were previously studied, systematic comparisons of the effects of different structural modifications to the 2-benzylbenzimidazole core structure on μ-opioid receptor (MOR) activity are limited. Here, we assessed in vitro structure–activity relationships of 9 previously uncharacterized nitazenes alongside known structural analogues. Specifically, we focused on MOR activation by ‘ring’ substituted analogues (i.e., N-pyrrolidino and N-piperidinyl modifications), ‘desnitazene’ analogues (lacking the 5-nitro group), and N-desethyl analogues. The results from two in vitro MOR activation assays (β-arrestin 2 recruitment and inhibition of cAMP accumulation) showed that ‘ring’ modifications overall yield highly active drugs. With the exception of 4′-OH analogues (which are metabolites), N-pyrrolidino substitutions were generally more favorable for MOR activation than N-piperidine substitutions. Furthermore, removal of the 5-nitro group on the benzimidazole ring consistently caused a pronounced decrease in potency. The N-desethyl modifications showed important MOR activity, and generally resulted in a slightly lowered potency than comparator nitazenes. Intriguingly, N-desethyl isotonitazene was the exception and was consistently more potent than isotonitazene. Complementing the in vitro findings and demonstrating the high harm potential associated with many of these compounds, we describe 85 forensic cases from North America and the United Kingdom involving etodesnitazene, N-desethyl etonitazene, N-desethyl isotonitazene, N-pyrrolidino metonitazene, and N-pyrrolidino protonitazene. The low-to-sub ng/mL blood concentrations observed in most cases underscore the drugs’ high potencies. Taken together, by bridging pharmacology and case data, this study may aid to increase awareness and guide legislative and public health efforts.

Novel synthetic opioids continue to emerge on recreational drug markets worldwide. In response to legislative bans on fentanyl analogues, non-fentanyl structural templates, such as 2-benzylbenzimidazoles (‘nitazenes’), are being exploited to create new μ-opioid receptor (MOR) agonists. Here, we pharmacologically characterize an emerging cyclic analogue of etonitazene, called N- pyrrolidino etonitazene (etonitazepyne), using in vitro and in vivo methods. A series of analytically confirmed fatalities is described to complement preclinical findings. Radioligand binding assays in rat brain tissue revealed that N- pyrrolidino etonitazene has high affinity for MOR (Ki = 4.09 nM) over δ-opioid (Ki = 959 nM) and κ-opioid (Ki = 980 nM) receptors. In a MOR-β-arrestin2 activation assay, N- pyrrolidino etonitazene displayed high potency (EC 50  = 0.348 nM), similar to etonitazene (EC 50  = 0.360 nM), and largely exceeding the potencies of fentanyl (EC 50  = 14.9 nM) and morphine (EC 50  = 290 nM). When administered s.c. to male Sprague Dawley rats, N- pyrrolidino etonitazene induced opioid-like antinociceptive, cataleptic, and thermic effects. Its potency in the hot plate test (ED 50  = 0.0017 mg/kg) was tenfold and 2,000-fold greater than fentanyl (ED 50  = 0.0209 mg/kg) and morphine (ED 50  = 3.940 mg/kg), respectively. Twenty-one overdose fatalities associated with N- pyrrolidino etonitazene were found to contain low blood concentrations of the drug (median = 2.2 ng/mL), commonly in the context of polysubstance use. N- Pyrrolidino etonitazene was reported as a cause of death in at least two cases, demonstrating toxicity in humans. We demonstrate that N- pyrrolidino etonitazene is an extremely potent MOR agonist that is likely to present high risk to users. Continued vigilance is required to identify and characterize emergent 2-benzylbenzimidazoles, and other non-fentanyl opioids, as they appear in the marketplace.

Following isotonitazene scheduling in 2019, the availability of alternative 2-benzylbenzimidazole opioids (nitazenes) on the global drug market increased, resulting in many fatalities worldwide. Nitazenes are potent µ-opioid receptor agonists with strong narcotic/analgesic effects, and their concentrations in biological matrices are low, making the detection of metabolite biomarkers of consumption crucial to document use in clinical and forensic settings. However, there is little to no data on the metabolism of the most recently available nitazenes, especially desnitro-analogues. The aim of the research was to assess isotonitazene, metonitazene, etodesnitazene, and metodesnitazene human metabolism and identify specific metabolite biomarkers of consumption. The four analogues were incubated with 10-donor-pooled human hepatocytes, and the incubates were analyzed by liquid chromatography-high-resolution tandem mass spectrometry and data mining with Compound Discoverer (Thermo Scientific); the analysis was supported by in silico metabolite predictions with GLORYx open-access software. Metabolites were identified in postmortem blood and/or urine samples from two metonitazene-positive and three etodesnitazene-positive cases following the same workflow, with and without glucuronide hydrolysis in urine, to confirm in vitro results. Twelve, nine, twenty-two, and ten metabolites were identified for isotonitazene, metonitazene, etodesnitazene, and metodesnitazene, respectively. The main transformations were N-deethylation at the N,N-diethylethanamine side chain, O-dealkylation, and further O-glucuronidation. In vitro and autopsy results were consistent, demonstrating the efficacy of the 10-donor-pooled human hepatocyte model to predict human metabolism. We suggest the parent and the corresponding O-dealkyl- and N-deethyl-O-dealkyl metabolites as biomarkers of exposure in urine after glucuronide hydrolysis, and the corresponding N-deethyl metabolite as additional biomarker in blood.

Opioid misuse and its rising rates of morbidity and associated mortality is an increasing area of concern worldwide. The licit/illicit consumption of opioids ranging from plant-based substances and pharmaceutical drugs (particularly analgesia) to the new synthetic opioids, has brought opioid use disorder (OUD) back to the public health concerns, including not only prevention but also availability of evidence-based treatments. Agonist opioids have demonstrated by long high efficacy and effectiveness for OUD treatment. Although methadone has been the more prescribed drug in most of the countries where opioid agonist treatment is available, other agonist opioids can be prescribed. We will present a start of the art of other agonist opioids available for the treatment of OUD, emphasizing in the differences among them, in line with of personalizing treatment in addiction. We will focus on morphine slow release, buprenorphine (with or without naloxone, sublingual or long-lasting) and diacetylmorphine.

New synthetic opioids (NSO) constitute one of the fastest-growing group of New Psychoactive Substances, which emerged on the illicit drug marker in the second half of 2000's. The most popular and the largest NSO subgroup are high potency fentanyl and its analogs. Subsequent to core-structure scheduling of fentanyl-related substances many opioids with different chemical structures are now emerging on the illicit drug market, rendering the landscape highly complex and dynamic. PubMed, Scopus and Google Scholar were searched for appropriate articles up to December 2022. Moreover, a search for reports was conducted on Institutional websites to identify documentation published by World Health Organization, United Nations Office on Drugs and Crime, United States Drug Enforcement Administration, and European Monitoring Centre for Drugs and Drug Addiction. Only articles or reports written in English were selected. Non-fentanyl derived synthetic opioids, i.e., 2-benzylbenzimidazoles (nitazenes), brorphine, U-compounds, AH-7921, MT-45 and related compounds are characterized, describing them in terms of available forms, pharmacology, metabolism as well as their toxic effects. Sample procedures and analytical techniques available for detection and quantification of these compounds in biological matrices are also presented. Finally, as overdoses involving highly potent NSO may be difficult to reverse, the effectiveness of naloxone as a rescue agent in NSO overdose is discussed. Current review presents key information on non-fentanyl derived NSO. Access to upto-date data on substances of abuse is of great importance for clinicians, public health authorities and professionals performing analyses of biological samples. •New synthetic opioids (NSO) are the fast growing group of novel psychoactive substances.•The most popular and largest NSO subgroup were high potency fentanyl and its analogs.•Many non-fentanyl NSO are now emerging on the illicit drug market.•Non-fentanyl NSO are responsible for hundreds of deaths and intoxications.•Analytical methods have to be updated to detect NSO, which are abused in low doses.

N -Piperidinyl etonitazene (‘etonitazepipne’) represents a recent addition to the rapidly expanding class of 2-benzylbenzimidazole ‘nitazene’ opioids. Following its first identification in an online-sourced powder and in biological samples from a patient seeking help for detoxification, this report details its in-depth chemical analysis and pharmacological characterization. Analysis of the powder via different techniques (LC-HRMS, GC–MS, UHPLC-DAD, FT-IR) led to the unequivocal identification of N -piperidinyl etonitazene. Furthermore, we report the first activity-based detection and analytical identification of N -piperidinyl etonitazene in authentic samples. LC-HRMS analysis revealed concentrations of 1.21 ng/mL in serum and 0.51 ng/mL in urine, whereas molecular networking enabled the tentative identification of various (potentially active) urinary metabolites. In addition, we determined that the extent of opioid activity present in the patient’s serum was equivalent to the in vitro opioid activity exerted by 2.5–10 ng/mL fentanyl or 10–25 ng/mL hydromorphone in serum. Radioligand binding assays in rat brain tissue revealed that the drug binds with high affinity ( K i  = 14.3 nM) to the µ-opioid receptor (MOR). Using a MOR-β-arrestin2 activation assay, we found that N -piperidinyl etonitazene is highly potent (EC 50  = 2.49 nM) and efficacious ( E max  = 183% versus hydromorphone) in vitro. Pharmacodynamic evaluation in male Sprague Dawley rats showed that N -piperidinyl etonitazene induces opioid-like antinociceptive, cataleptic, and thermic effects, its potency in the hot plate assay (ED 50  = 0.0205 mg/kg) being comparable to that of fentanyl (ED 50  = 0.0209 mg/kg), and > 190 times higher than that of morphine (ED 50  = 3.940 mg/kg). Taken together, our findings indicate that N- piperidinyl etonitazene is a potent opioid with the potential to cause harm in users.

New synthetic opioids (NSOs) are one of the fastest growing groups of new psychoactive substances. Amid this dynamic landscape, insight into the pharmacology of NSOs is important to estimate the harm potential of newly emerging drugs. In this work, we determined the µ-opioid receptor (MOR) affinity and activation potential of seven poorly characterized non-fentanyl NSOs ( N -ethyl-U-47700, 3,4-difluoro-U-47700, U-47931E/bromadoline, 2,4-difluoro-U-48800, U-62066/spiradoline, 2F-viminol, ketobemidone) and a panel of nine reference opioids. MOR affinity was determined via [ 3 H]-DAMGO binding in rat brain tissue homogenates, and was found to correlate well with different functional parameters. MOR activation potential was studied at different levels of receptor signaling using three distinct assays (NanoBiT ® MOR-β-arrestin2/mini-G αi and AequoScreen ® ). The most active compounds were ketobemidone (EC 50 32.8–528 nM; E max 105–271%, relative to hydromorphone) and N -ethyl-U-47700 (EC 50 241–767 nM; E max 139–247%). The same opioids showed the strongest MOR affinity. As most of the other NSOs only weakly activated MOR in the three assays (EC 50 values in the high nM–µM range), they likely do not pose a high overdose risk. 2F-viminol (EC 50 2.2–4.5 µM; E max 21.2–61.5%) and U-47931E/bromadoline (EC 50 0.55–2.9 µM; E max 52.8–85.9%) were partial agonists compared to hydromorphone, and maximum receptor activation was not reached for 2,4-difluoro-U-48800 (EC 50  > 22 µM). We further highlight the importance of considering specific assay characteristics upon interpretation of potencies, efficacies and biased agonism. As absolute values may greatly differ between assays with varying experimental set-ups, a comparison of functional parameters to those of well-characterized reference agonists is considered the most informative.

Background 2-Benzylbenzimidazole ‘nitazene’ opioids pose a growing threat to public health. Nitazene analogues are increasingly found mixed with or (mis)sold as heroin and in falsified (non-)opioid medications, posing a great risk of intoxication in users (un)knowingly exposed to these potent opioids. Lateral flow immunoassay nitazene test strips (NTS; BTNX Rapid Response™) became commercially available in Q1 2024, with the aim to enable rapid detection of nitazene analogues in drug samples. As only limited independent data is available on the performance of these strips, this lab-based study aimed at evaluating their potential for drug checking applications. Methods Following dilution of drug standards in water, the NTS readouts were analyzed independently by two individuals and by ImageJ. The limit of detection for isotonitazene was determined using two manufacturing lots of NTS. Cross-reactivity with 32 other nitazene analogues was evaluated. Six sourced drug samples were tested to explore the ability of NTS to detect the presence of a nitazene analogue in authentic samples. Results The limits of detection for isotonitazene were 2000 or 3000 ng/mL, depending on the lot. Twenty-four of the 33 tested nitazene analogues cross-reacted with the NTS at concentrations ≤ 9000 ng/mL. Structural analysis indicated that either substitution or removal of the 5-nitro group, or lengthening the linker between the two aromatic rings, generally hampered detection. All six authentic drug samples consistently tested positive, with no observed false negatives. Conclusions This study provides a better understanding of the potential of NTS for drug checking purposes. Our findings indicate that NTS can theoretically alert to the presence of most nitazene analogues that have emerged on recreational drug markets. However, ‘desnitazenes’ (lacking the 5-nitro group) may yield false negative results due to low cross-reactivity. Although factors like specificity, lot-to-lot variability, nitazene analogue content in drug samples, solubility, and different testing conditions should be considered, our study results indicate that, at least under the conditions evaluated here (using reference standards and sourced powders), NTS are capable of detecting the presence of a wide range of nitazene analogues. Hence, NTS may alert users of the presence of nitazene analogues in drug samples.

The emergence of structurally diverse new synthetic opioids (NSOs) has caused the opioid crisis to spiral to new depths. Little information is available about the pharmacology of most novel opioids when they first emerge. Here, using a β-arrestin 2 recruitment assay, we investigated the in vitro μ-opioid receptor (MOR) activation potential of dipyanone, desmethylmoramide, and acetoxymethylketobemidone (O-AMKD) — recent NSOs that are structurally related to the prescription opioids methadone and ketobemidone. Our findings indicate that dipyanone (EC 50 =39.9 nM; E max =155% vs. hydromorphone) is about equally active as methadone (EC 50 =50.3 nM; E max =152%), whereas desmethylmoramide (EC 50 =1335 nM; E max =126%) is considerably less active. A close structural analogue of ketobemidone (EC 50 =134 nM; E max =156%) and methylketobemidone (EC 50 =335 nM; E max =117%), O-AMKD showed a lower potency (EC 50 =1262 nM) and efficacy ( E max =109%). Evaluation of the opioid substitution product buprenorphine and its metabolite norbuprenorphine confirmed the increased in vitro efficacy of the latter. In addition to in vitro characterization, this report details the first identification and full chemical analysis of dipyanone in a seized powder, as well as a postmortem toxicology case from the USA involving the drug. Dipyanone was quantified in blood (370 ng/mL), in which it was detected alongside other NSOs (e.g., 2-methyl AP-237) and novel benzodiazepines (e.g., flualprazolam). While dipyanone is currently not commonly encountered in forensic samples worldwide, its emergence is worrisome and representative of the dynamic NSO market. Graphical Abstract