Analysis of U-47700, a Novel Synthetic Opioid, in Human Urine by LC-MS-MS and LC-QToF

Review of analytical methods for screening and quantification of fentanyl analogs and novel synthetic opioids in biological specimens

Fentanyl, fentanyl analogs, and other novel synthetic opioids (NSO), including nitazene analogs, prevail in forensic toxicology casework. Analytical methods for identifying these drugs in biological specimens need to be robust, sensitive, and specific. Isomers, new analogs, and slight differences in structural modifications necessitate the use of high-resolution mass spectrometry (HRMS), especially as a non-targeted screening method designed to detect newly emerging drugs. Traditional forensic toxicology workflows, such as immunoassay and gas chromatography mass spectrometry (GC-MS), are generally not sensitive enough for detection of NSOs due to observed low (sub-μg/L) concentrations. For this review, the authors tabulated, reviewed, and summarized analytical methods from 2010-2022 for screening and quantification of fentanyl analogs and other NSOs in biological specimens using a variety of different instruments and sample preparation approaches. Limits of detection or quantification for 105 methods were included and compared to published standards and guidelines for suggested scope and sensitivity in forensic toxicology casework. Methods were summarized by instrument for screening and quantitative methods for fentanyl analogs and for nitazenes and other NSO. Toxicological testing for fentanyl analogs and NSOs is increasingly and most commonly being conducted using a variety of liquid chromatography mass spectrometry (LC-MS)-based techniques. Most of the recent analytical methods reviewed exhibited limits of detection well below 1 μg/L to detect low concentrations of increasingly potent drugs. In addition, it was observed that most newly developed methods are now using smaller sample volumes which is achievable due to the sensitivity increase gained by new technology and new instrumentation.

Metabolic Evaluation of Synthetic Opioids on the Example of U-47700 with the Use of In Vitro and In Vivo Methods for Forensic Toxicology Application

Legal highs present a great threat to health, especially in groups of people experimenting with psychoactive substances. The lack of available knowledge on the biotransformation of these substances necessitates symptomatic treatment in the event of intoxication, which, unfortunately, may be ineffective. Opioids, including heroin analogues, such as U-47700, constitute a special group of designer drugs. In this study, a multi-directional approach to trace the biotransformation of U-47700 in living organisms was used. For this purpose, the in silico assessment (ADMET Predictor) was used first and then followed by an in vitro study using human liver microsomes and the S9 fraction. The biotransformation was then followed in an animal model (Wistar rats). Tissues such as blood, brain and liver were collected for analysis. The study was performed using liquid chromatography with tandem mass spectrometry (LC-MS/MS). The obtained results were compared to those obtained from the analysis of autopsy materials (cases analysed in the Toxicology Laboratory of the Department of Forensic Medicine, Jagiellonian University Medical College in Krakow).

Trace Residue Identification, Characterization and Longitudinal Monitoring of the Novel Synthetic Opioid β-U10, from Discarded Drug Paraphernalia

Empirical data regarding dynamic alterations in illicit drug supply markets in response to the COVID-19 pandemic, including the potential for introduction of novel drug substances and/or increased poly-drug combination use at the 'street' level, i.e., directly proximal to the point of consumption, is currently lacking. Here, a high-throughput strategy employing ambient ionization-mass spectrometry is described for the trace residue identification, characterization and longitudinal monitoring of illicit drug substances found within >6,600 discarded drug paraphernalia (DDP) samples collected during a pilot study of an early warning system for illicit drug use in Melbourne, Australia from August 2020-February 2021, while significant COVID-19 lockdown conditions were imposed. The utility of this approach is demonstrated for the de novo identification and structural characterization of β-U10, a previously unreported naphthamide analogue within the 'U-series' of synthetic opioid drugs, including differentiation from its α-U10 isomer without need for sample preparation or chromatographic separation prior to analysis. Notably, β-U10 was observed with 23 other drug substances, most commonly in temporally distinct clusters with heroin, etizolam and diphenhydramine, and in a total of 182 different poly-drug combinations. Longitudinal monitoring of the number and weekly 'average signal intensity' (ASI) values of identified substances, developed here as a semi-quantitative proxy indicator of changes in availability, relative purity and compositions of street level drug samples, revealed that increases in the number of identifications and ASI for β-U10 and etizolam coincided with a 50% decrease in the number of positive detections and an order of magnitude decrease in the ASI for heroin.

Toxicokinetics of U-47700, tramadol, and their main metabolites in pigs following intravenous administration: is a multiple species allometric scaling approach useful for the extrapolation of toxicokinetic parameters to humans?

New synthetic opioids (NSOs) pose a public health concern since their emergence on the illicit drug market and are gaining increasing importance in forensic toxicology. Like many other new psychoactive substances, NSOs are consumed without any preclinical safety data or any knowledge on toxicokinetic (TK) data. Due to ethical reasons, controlled human TK studies cannot be performed for the assessment of these relevant data. As an alternative animal experimental approach, six pigs per drug received a single intravenous dose of 100 µg/kg body weight (BW) of U-47700 or 1000 µg/kg BW of tramadol to evaluate whether this species is suitable to assess the TK of NSOs. The drugs were determined in serum and whole blood using a fully validated method based on solid-phase extraction and LC–MS/MS. The concentration–time profiles and a population (pop) TK analysis revealed that a three-compartment model best described the TK data of both opioids. Central volumes of distribution were 0.94 L/kg for U-47700 and 1.25 L/kg for tramadol and central (metabolic) clearances were estimated at 1.57 L/h/kg and 1.85 L/h/kg for U-47700 and tramadol, respectively. The final popTK model parameters for pigs were upscaled via allometric scaling techniques. In comparison to published human data, concentration–time profiles for tramadol could successfully be predicted with single species allometric scaling. Furthermore, possible profiles for U-47700 in humans were simulated. The findings of this study indicate that unlike a multiple species scaling approach, pigs in conjunction with TK modeling are a suitable tool for the assessment of TK data of NSOs and the prediction of human TK data.

Perimortem Distribution of U-47700, Tramadol and their Main Metabolites in pigs Following Intravenous Administration

In spite of a decreasing number of new releases, New Synthetic Opioids (NSO) are gaining increasing importance in postmortem (PM) forensic toxicology. For the interpretation of analytical results, toxicokinetic (TK) data, e.g. on tissue distribution, are helpful. Concerning NSO, such data are usually not available due to the lack of controlled human studies. Hence, a controlled TK study using pigs was carried out and the tissue distribution of U-47700 and tramadol as reference was examined. Twelve pigs received an intravenous dose of 100 µg/kg body weight (BW) U-47700 or 1000 µg/kg BW tramadol, respectively. Eight hours after administration, the animals were put to death with T61. Relevant organs, body fluids and tissues were sampled. After homogenization and solid-phase extraction, quantification was performed applying standard addition and liquid chromatography-tandem mass spectrometry. At the time of death, the two parent compounds were determined in all analyzed specimens. Regarding U-47700, concentrations were highest in duodenum content, bile fluid and adipose tissue (AT). Concerning tramadol, next to bile fluid and duodenum content, highest concentrations were determined in the lung. Regarding the metabolites, N-desmethyl-U-47700 and O-desmethyltramadol (ODT) were detected in all analyzed specimens except for AT (ODT). Higher metabolite concentrations were found in specimens involved in metabolism. N-desmethyl-U-47700 showed much higher concentrations in routinely analyzed organs (lung, liver, kidney) than U-47700. To conclude, besides the routinely analyzed specimens in PM toxicology, AT, bile fluid and duodenum content could serve as alternative matrices for blood, urine or standard specimens such as kidney or liver. In case of U-47700, quantification of the main metabolite N-desmethyl-U-47700 is highly recommendable.

Fatal intoxication with U-47700 in combination with other NPS (N-ethylhexedrone, adinazolam, 4-CIC, 4-CMC) confirmed by identification and quantification in autopsy specimens and evidences

Purpose

We present a case of fatal intoxication with U-47700 in combination with other NPS (N-ethylhexedrone, adinazolam, 4-chloro-N-isopropylcathinone (4-CIC), 4-chloromethcathinone (4-CMC) and sertraline) confirmed by identification and quantification in biological materials and evidences found at the scene in 2017 in Poland.

Methods

Blood and urine samples were extracted with ethyl acetate from alkaline medium (pH 9); powders/crystals were diluted with methanol. The analysis was carried out using ultra-high-performance liquid chromatography–tandem mass spectrometry. Validation criteria were evaluated for blood and urine at the concentrations of 10 and 100 ng/mL.

Results

The validation parameters of the method were within acceptable ranges. In the presented case, the determined concentrations of drugs were as follows, in blood: U-47700, 1470 ng/mL; N-ethylhexedrone, 58 ng/mL; adinazolam, 18 ng/mL; 4-CIC, 8.0 ng/mL; 4-CMC, 1.7 ng/mL; in urine: U-47700, 3940 ng/mL; N-ethylhexedrone, 147 ng/mL; adinazolam, 82 ng/mL; 4-CIC, 130 ng/mL; 4-CMC, 417 ng/mL. Sertraline (blood, 89 ng/mL; urine, 32 ng/mL) was also determined in both materials. The same substances were also found in 5 powders/crystals: U-47700 (12% by weight), N-ethylhexedrone (54%), adinazolam (14%), 4-CIC (23%), 4-CMC (26%). After 775 days of storage, biological samples at + 4 °C, the most stable substance was sertraline and the less, synthetic cathinones, especially 4-CIC and 4-CMC.

Conclusions

The described case of fatal intoxication with NPS presented postmortem concentrations of U-47700, 4-CMC, N-ethylhexedrone, adinazolam and 4-CIC for the first time in the literature. The paper also showed stability study of these substances stored at + 4 °C for 775 days.

Spectral trends in GC-EI-MS data obtained from the SWGDRUG mass spectral library and literature: A resource for the identification of unknown compounds

Rapid identification of new or emerging psychoactive substances remains a critical challenge in forensic drug chemistry laboratories. Current analytical protocols are well-designed for confirmation of known substances yet struggle when new compounds are encountered. Many laboratories initially attempt to classify new compounds using gas chromatography-electron ionization-mass spectrometry (GC-EI-MS). Though there is a large body of research focused on the analysis of illicit substances with GC-EI-MS, there is little high-level discussion of mass spectral trends for different classes of drugs. This manuscript compiles literature information and performs simple exploratory analyses on evaluated GC-EI-MS data to investigate mass spectral trends for illicit substance classes. Additionally, this work offers other important aspects: brief discussions of how each class of drugs is used; illustrations of EI mass spectra with proposed structures of commonly observed ions; and summaries of mass spectral trends that can help an analyst classify new illicit compounds.

U-47700 and Its Analogs: Non-Fentanyl Synthetic Opioids Impacting the Recreational Drug Market

The recreational use of opioid drugs is a global threat to public health and safety. In particular, an epidemic of opioid overdose fatalities is being driven by illicitly manufactured fentanyl, while novel synthetic opioids (NSOs) are appearing on recreational drug markets as standalone products, adulterants in heroin, or ingredients in counterfeit drug preparations. Trans-3,4-dichloro-N-[2-(dimethylamino)cyclohexyl]-N-methylbenzamide (U-47700) is a prime example of a non-fentanyl NSO that is associated with numerous intoxications and fatalities. Here, we review the medicinal chemistry, preclinical pharmacology, clandestine availability, methods for detection, and forensic toxicology of U-47700 and its analogs. An up-to-date summary of the human cases involving U-47700 intoxication and death are described. The evidence demonstrates that U-47700 is a potent μ-opioid receptor agonist, which poses a serious risk for overdosing and death. However, most analogs of U-47700 appear to be less potent and have been detected infrequently in forensic specimens. U-47700 represents a classic example of how chemical entities from the medicinal chemistry or patent literature can be diverted for use in recreational drug markets. Lessons learned from the experiences with U-47700 can inform scientists, clinicians, and policymakers who are involved with responding to the spread and impact of NSOs.

Classics in Chemical Neuroscience: U-47700

U-47700, 3,4-dichloro-N-((1R,2R)-2-(dimethylamino)cyclohexyl)-N-methyl benzamide, is a novel synthetic opioid (NSO), discovered by the Upjohn company in the late 1970s. With potent in vivo activity, ∼10-times greater than that of morphine, U-47700 has become a drug of widespread abuse due to its ease of synthesis and, until recently, lack of robust detection methods by law enforcement. U-47700 has been found in counterfeit oxycodone tablets and is a key ingredient in "gray death." Due to its emergence worldwide in the past 5 years, it is now a Schedule I drug in the United States and similarly designated around the world; moreover, at autopsy, U-47700 was found to have contributed to the death of the pop artist Prince. This Review will capture the >40 year history of U-47700 and go in-depth regarding the synthesis, medicinal chemistry, in vitro/in vivo pharmacology, drug metabolism (from postmortem overdose cases), and societal impact of this DARK Classic in chemical neuroscience.

Determination of 37 fentanyl analogues and novel synthetic opioids in hair by UHPLC-MS/MS and its application to authentic cases

The recent emergence of new fentanyl analogues and synthetic opioids on the drug market poses a global public health threat. However, these compounds cannot typically be identified using existing analytical methods. In this study, we aimed to develop and validate a rapid and sensitive method based on ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) for the simultaneous determination of 37 fentanyl analogues and novel synthetic opioids in hair samples. Hair samples (20 mg) were extracted by cryogenic grinding in an extraction medium of methanol, acetonitrile, and 2 mmol/L ammonium acetate (pH 5.3). Following centrifugation of the samples, the analytes were separated using a WATERS Acquity UPLC HSS T₃ column. The limits of detection (LODs) and limits of quantification (LOQs) ranged from 0.5 to 2.5 pg/mg and from 2 to 5 pg/mg, respectively. The intraday and interday precisions were within 13.32% at LOQ, low, medium, and high levels. The accuracies were within the range of 85.63–116.1%. The extraction recoveries were in the range of 89.42–119.68%, and the matrix effects were within the range of 44.81–119.77%. Furthermore, the method was successfully applied to the detection and quantification of fentanyl and sufentanil in hair samples from two authentic cases. Thus, this method has great potential for detecting fentanyl analogues and novel synthetic opioids in forensic work.

Identification and structure characterization of five synthetic opioids: 3,4-methylenedioxy-U-47700, o-methyl-acetylfentanyl, 2-thiophenefentanyl, benzoylfentanyl and benzoylbenzylfentanyl

Purpose

The number of new synthetic opioids, especially fentanyl analogs, on the illicit market is increasing each year and becoming a disturbing phenomenon. Due to the very high potency, ease of addiction and high risk of overdose with the fatal effect, they pose a serious threat to public health. The ever-changing drug market, lack of reference standards and low doses make this class of new psychoactive substances (NPS) a continual analytical challenge in terms of detection and identification. Structure elucidations of five synthetic opioids are described in this paper.

Methods

Unequivocal identifications were performed using liquid chromatography–high resolution tandem mass spectrometry with quadrupole time-of-flight analyzer (LC–QTOF-MS/MS) and nuclear magnetic resonance (NMR) spectroscopy with prior chromatographic purification of the sample in each case.

Results

Identified NPS included one non-fentanyl opioid belonging to “U-compounds group”—3,4-methylenedioxy-U-47700 and four fentanyl analogs: o-methyl-acetylfentanyl, benzoylfentanyl, 2-thiophenefentanyl and benzoylbenzylfentanyl.

Conclusions

Complementary LC–QTOF-MS/MS and NMR techniques enabled the unambiguous identification of new synthetic opioids. To the best of our knowledge, this is the first comprehensive characterization of these five NPS with complete data which are published to facilitate their detection and identification by forensic and clinical laboratories.

Studies on the in vitro and in vivo metabolism of the synthetic opioids U-51754, U-47931E, and methoxyacetylfentanyl using hyphenated high-resolution mass spectrometry

New Synthetic Opioids (NSOs) are one class of New Psychoactive Substances (NPS) enjoying increasing popularity in Europe. Data on their toxicological or metabolic properties have not yet been published for most of them. In this context, the metabolic fate of three NSOs, namely, trans-3,4-dichloro-N-[2-(dimethylamino)cyclohexyl]-N-methyl-benzenacetamide (U-51754), trans-4-bromo-N-[2-(dimethylamino)cyclohexyl]-N-methyl-benzamide (U-47931E), and 2-methoxy-N-phenyl-N-[1-(2-phenylethyl)piperidin-4-yl] acetamide (methoxyacetylfentanyl), was elucidated by liquid chromatography high-resolution mass spectrometry after pooled human S9 fraction (phS9) incubations and in rat urine after oral administration. The following major reactions were observed: demethylation of the amine moiety for U-51754 and U-47931E, N-hydroxylation of the hexyl ring, and combinations thereof. N-dealkylation, O-demethylation, and hydroxylation at the alkyl part for methoxyacetylfentanyl. Except for U-47931E, parent compounds could only be found in trace amounts in rat urine. Therefore, urinary markers should preferably be metabolites, namely, the N-demethyl-hydroxy and the hydroxy metabolite for U-51754, the N-demethylated metabolite for U-47931E, and the N-dealkylated metabolite as well as the O-demethylated one for methoxyacetylfentanyl. In general, metabolite formation was comparable in vitro and in vivo, but fewer metabolites, particularly those after multiple reaction steps and phase II conjugates, were found in phS9. These results were consistent with those of comparable compounds obtained from human liver microsomes, human hepatocytes, and/or human case studies.

Novel synthetic opioids - toxicological aspects and analysis

Over the past few years, there has been an emerging number of new psychoactive drugs. These drugs are frequently mentioned as "legal highs", "herbal highs", "bath salts" and "research chemicals". They are mostly sold and advertised on online forums and on the dark web. The emerging new psychoactive substances are designed to mimic the effects of psychoactive groups, which are often abused drugs. Novel synthetic opioids are a new trend in this context and represent an alarming threat to public health. Given the wide number of fatalities related to these compounds reported within the last few years, it is an important task to accurately identify these compounds in biologic matrices in order to administer an effective treatment and reverse the respiratory depression caused by opioid related substances. Clinicians dealing with fentanyl intoxication cases should consider that it could, in fact, be a fentanyl analogue. For this reason, it is a helpful recommendation to include synthetic opioids in the routine toxicological screening procedures, including analysis in alternative matrices, if available, to investigate poly-drug use and possible tolerance to opioids. To address this public health problem, better international collaboration, effective legislation, effective investigation, control of suspicious "research chemicals" online forums and continuous community alertness are required. This article aims to review diverse reported fatalities associated with new synthetic opioids describing them in terms of pharmacology, metabolism, posology, available forms, as well as their toxic effects, highlighting the sample procedures and analytical techniques available for their detection and quantification in biological matrices.

A validated workflow for drug detection in oral fluid by non-targeted liquid chromatography-tandem mass spectrometry

Oral fluid is recognized as an important specimen for drug testing. Common applications are monitoring in substance abuse treatment programs, therapeutic drug monitoring, pain management, workplace drug testing, clinical toxicology, and driving under the influence of drugs (DRUID). In this study, we demonstrate that non-targeted LC-MS/MS with subsequent compound identification by tandem mass spectral library search is a valuable tool for comprehensive detection and confirmation of drugs in oral fluid samples. The workflow developed involves solid-phase extraction and chromatographic separation on reversed phase materials. Mass spectrometric detection is accomplished on a quadrupole-quadrupole-time-of-flight instrument operated with data-dependent acquisition control. The workflow was optimized for 500 μl of neat oral fluid collected with the Greiner Bio-One saliva collection system. The fitness of the developed method was tested and proven by analyzing blank and spiked samples as well as 59 authentic patient samples. We could demonstrate that compounds with logP values in the range 0.5-5.5 are efficiently detected at low nanograms per milliliter concentrations. The true positive and true negative rates of automated library search were equal or close to 100%. The beauty of the non-targeted LC-MS/MS approach is the ability to detect compounds hardly included in routinely applied targeted assays, and this was demonstrated by detecting the synthetic opioid U-47700 in two patient samples. Graphical abstract ᅟ.

Non-Medical Use of Novel Synthetic Opioids: A New Challenge to Public Health

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.

The search for the "next" euphoric non-fentanil novel synthetic opioids on the illicit drugs market: current status and horizon scanning

PURPOSE

A detailed review on the chemistry and pharmacology of non-fentanil novel synthetic opioid receptor agonists, particularly N-substituted benzamides and acetamides (known colloquially as U-drugs) and 4-aminocyclohexanols, developed at the Upjohn Company in the 1970s and 1980s is presented.

METHOD

Peer-reviewed literature, patents, professional literature, data from international early warning systems and drug user fora discussion threads have been used to track their emergence as substances of abuse.

RESULTS

In terms of impact on drug markets, prevalence and harm, the most significant compound of this class to date has been U-47700 (trans-3,4-dichloro-N-[2-(dimethylamino)cyclohexyl]-N-methylbenzamide), reported by users to give short-lasting euphoric effects and a desire to re-dose. Since U-47700 was internationally controlled in 2017, a range of related compounds with similar chemical structures, adapted from the original patented compounds, have appeared on the illicit drugs market. Interest in a structurally unrelated opioid developed by the Upjohn Company and now known as BDPC/bromadol appears to be increasing and should be closely monitored.

CONCLUSIONS

International early warning systems are an essential part of tracking emerging psychoactive substances and allow responsive action to be taken to facilitate the gathering of relevant data for detailed risk assessments. Pre-emptive research on the most likely compounds to emerge next, so providing drug metabolism and pharmacokinetic data to ensure that new substances are detected early in toxicological samples is recommended. As these compounds are chiral compounds and stereochemistry has a large effect on their potency, it is recommended that detection methods consider the determination of configuration.

Case report: relevance of metabolite identification to detect new synthetic opioid intoxications illustrated by U-47700

Today, new psychoactive substances (NPS) producers increasingly appear to be targeting new synthetic opioids (NSOs), and the recent emergence of NSOs is causing considerable concern in North America and in Europe. For toxicologists, NSO detection in a forensic context presents three additional difficulties to the general NPS analytical detection challenge: (i) high frequency of new products, (ii) low concentrations (in μg/L range and under) in biological samples related to their high opioid potency, and (iii) extensive metabolism. In this context, the present work aims to highlight the relevance of NSO metabolite detection in potential intoxication cases. Illustration is given with U-47700, an emerging NSO, (i) that was identified in a powder recently collected in France and in a fatality case, (ii) whose metabolites were in vitro produced using human liver microsomes and their mass spectra (MS) added in our MS/MS and HRMS libraries, and (iii) for which metabolism data were compared to those of the literature: U-47700 was identified in the powder and at 3040 μg/L in peripheral blood in the fatality case. In addition, high amounts of several U-47700 metabolites, especially N-desmethyl-U-47700, were observed in urine. Even if metabolite formation may largely depend on the enzymatic activity as well as on the length of the survival time, confrontation of these results to data found in the literature strongly suggests that this metabolite is regularly a better blood and (mainly) urine biomarker of U-47700 intake than U-47700 itself. Indeed, in this fatality and in other previous reports, N-desmethyl-U-47700 produced the main observed chromatographic signal (i) systematically in vitro and (ii) commonly in vivo, especially in urines. N,N-Didesmethyl-U-47700 is also sometimes a better biomarker of U-47700 intake than U-47700 itself. Accordingly, we suggest adding N-desmethyl-U-47700 (and N,N-didesmethyl-U-47700) in mass spectrum databases used for toxicological screening in order to reduce the risk of false-negative results in intoxication cases involving U-47700.

Drug Interactions With New Synthetic Opioids

Fentanyl, fentanyl analogs, and other new synthetic opioids (NSO) have burst onto the illegal drug market as new psychoactive substances (NPS). They are often sold as heroin to unsuspecting users and produce euphoria through their agonist action on μ- opioid receptors. Their high consumption, often combined with other substances, has led to multiple intoxications during recent years. In some countries, such as the United States, the consumption of opioids, whether for medical or recreational purposes, has become epidemic and is considered a public health problem. Fentanyl analogs are more potent than fentanyl which in turn is 50 times more potent than morphine. Furthermore, some fentanyl analogs have longer duration of action and therefore interactions with other substances and medicines can be more serious. This review is focused on the potentially most frequent interactions of opioid NPS taking into account the drugs present in the reported cases of poly-intoxication, including other illegal drugs of abuse and medication. Substances involved are mainly antidepressants, antihistamines, antipsychotics, benzodiazepines, analgesics, anesthetics, psychostimulants, other opioids, alcohol, and illegal drugs of abuse. The interactions can be produced due to pharmacokinetic and pharmacodynamic mechanisms. Naloxone can be used as an antidote, although required doses might be higher than for traditional opioid intoxications. It is crucial that doctors who habitually prescribe opioids, which are often misused by patients and NPS users, be aware of designer opioids' potentially life-threatening drug-drug interactions in order to prevent new cases of intoxication.

Novel Synthetic Opioids: The Pathologist's Point of View

Background: New Psychoactive Substances (NPS) constitute a broad range of hundreds of natural and synthetic drugs, including synthetic opioids, synthetic cannabinoids, synthetic cathinones, and other NPS classes, which were not controlled from 1961 to 1971 by the United Nations drug control conventions. Among these, synthetic opioids represent a major threat to public health. Methods: A literature search was carried out using public databases (such as PubMed, Google Scholar, and Scopus) to survey fentanyl-, fentanyl analogs-, and other synthetic opioid-related deaths. Keywords including “fentanyl”, “fentanyl analogs”, “death”, “overdose”, “intoxication”, “synthetic opioids”, “Novel Psychoactive Substances”, “MT-45”, “AH-7921”, and “U-47700” were used for the inquiry. Results: From our literature examination, we inferred the frequent implication of fentanyls and synthetic opioids in side effects, which primarily affected the central nervous system and the cardiovascular and pulmonary systems. The data showed a great variety of substances and lethal concentrations. Multidrug-related deaths appeared very common, in most reported cases. Conclusions: The investigation of the contribution of novel synthetic opioid intoxication to death should be based on a multidisciplinary approach aimed at framing each case and directing the investigation towards targeted toxicological analyses.

Pharmacotoxicology of Non-fentanyl Derived New Synthetic Opioids

A class of opioid agonists not structurally related to fentanyl, derived from research publications of pharmaceutical companies or patents within the United States and abroad are contributing to the current opioid epidemic. Novel synthetic opioids (NSOs) created to circumvent drug control laws such as U-47700, U-49900, AH-7921, or MT-45 have no recognized therapeutic use, are clandestinely manufactured and sold on conventional or dark web. We herein provide a review of the pharmacological properties available on most of these substances trying to provide a better knowledge on these compounds, particularly with respect to toxicity and dangerous adverse effects in users. Indeed, these NSOs share not only a great potency of action and receptor affinity with respect to natural or synthetic opiates (e.g., morphine, heroin, and methadone) but also a non-negligible toxicity leading to intoxications and fatalities, posing a serious harm to public health and society.

Synthetic Opioids

Opioid abuse has been a global menace for centuries, but the proliferation of synthetic opioids and their use within this current decade have created epidemic-level harms in some countries. According to the United Nations Office on Drugs and Crime, almost 12 million years were estimated loss of "healthy" life resulting in premature death and disability attributable to global opioid abuse just in 2015. Law enforcement and regulatory authorities have been particularly challenged abating the spread of synthetic opioids because soon after controlling the currently recognized abused opioids, their structures are tweaked, and new entities replace them. Drug racketeers have most often exploited the fentanyl phenylpiperidine structure in this regard, but non-fentanyl-like and classical morphinan-like structures have been pirated as well. A growing number of anecdotal reports identify respiratory depression induced by these newer synthetic opioids to be especially refractive to reversal by antagonists, with consequently high levels of lethality. This review examines three of these synthetic opioids representing three chemical classes (U-47700, MT-45, and acetylfentanyl) that have emerged to be of such menace that they have been brought under international control in recent years and addresses factors that could make synthetic opioids especially untreatable by opioid antagonists.

U-47700: An Emerging Threat

Illicit opioid use continues to be an ever-growing problem in the United States. The rise of synthetic opioids is an emerging threat that is beginning to draw attention over the past few years. Herein, we present an overview of the rise of a synthetic opioid known as U-47700. We describe U-47700's history, legal status, ease of obtainment, consequences of its use, and a proposal to increase the awareness of this synthetic opioid.

An Expanding World of Novel Psychoactive Substances: Opioids

The abuse of novel psychoactive substances (NPS) has been increasing dramatically worldwide since late 2000s. By the end of 2015, more than 560 NPS had been reported to the European Monitoring Centre for Drugs and Drug Addiction. Although the most popular compounds are synthetic cannabinoids and psychostimulatory derivatives of cathinone (so-called β-keto-amphetamines), novel synthetic opioids have recently emerged on the recreational drug market. They include fentanyl (a potent narcotic analgesic) and its analogs (e.g., acetylfentanyl, acryloylfentanyl, carfentanil, α-methylfentanyl, 3-methylfentanyl, furanylfentanyl, 4-fluorobutyrylfentanyl, 4-methoxybutyrylfentanyl, 4-chloroisobutyrylfentanyl, 4-fluoroisobutyrylfentanyl, tetrahydrofuranylfentanyl, cyclopentylfentanyl, and ocfentanil) and compounds with different chemical structures, such as AH-7921, MT-45, and U-47700. This survey provides an overview of the pharmacological properties, pattern of use, and desired and unwanted effects of the above-listed novel opioids. Special emphasis is given to cases of non-fatal and lethal intoxication involving these compounds.