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New fentanyl analogs have recently emerged as new psychoactive substances and have caused numerous fatalities worldwide. To determine if the new analogs follow the same metabolic pathways elucidated for fentanyl and known fentanyl analogs, we performed in vitro and in vivo metabolite identification studies for acetylfentanyl, acrylfentanyl, 4-fluoro-isobutyrylfentanyl, and furanylfentanyl. All compounds were incubated at 10 μM with pooled human hepatocytes for up to 5 h. For each compound, four or five authentic human urine samples from autopsy cases with and without enzymatic hydrolysis were analyzed. Data acquisition was performed in data-dependent acquisition mode during liquid chromatography high-resolution mass spectrometry analyses. Data was analyzed (1) manually based on predicted biotransformations and (2) with MetaSense software using data-driven search algorithms. Acetylfentanyl, acrylfentanyl, and 4-fluoro-isobutyrylfentanyl were predominantly metabolized by N-dealkylation, cleaving off the phenethyl moiety, monohydroxylation at the ethyl linker and piperidine ring, as well as hydroxylation/methoxylation at the phenyl ring. In contrast, furanylfentanyl’s major metabolites were generated by amide hydrolysis and dihydrodiol formation, while the nor-metabolite was minor or not detected in case samples at all. In general, in vitro results matched the in vivo findings well, showing identical biotransformations in each system. Phase II conjugation was observed, particularly for acetylfentanyl. Based on our results, we suggest the following specific and abundant metabolites as analytical targets in urine: a hydroxymethoxy and monohydroxylated metabolite for acetylfentanyl, a monohydroxy and dihydroxy metabolite for acrylfentanyl, two monohydroxy metabolites and a hydroxymethoxy metabolite for 4-fluoro-isobutyrylfentanyl, and a dihydrodiol metabolite and the amide hydrolysis metabolite for furanylfentanyl.

According to the CDC, approximately 87,000 drug overdose deaths occurred in the United States from October 2023 to September 2024, and the estimated number of overdose deaths in the US attributed to fentanyl and fentanyl analogs was around 54,000 in 2024. While fentanyl and its analogs are abused via numerous administration routes, the number of seized counterfeit tablets containing these drugs is significantly high. The DEA seized over 60 million fentanyl-laced counterfeit pills in 2024, equivalent to more than 380 million lethal doses of fentanyl. Portable techniques can be especially useful for law enforcement. Raman spectroscopy, especially surface-enhanced Raman spectroscopy (SERS), is one such technique which can be useful in both field and laboratory applications due to its sensitivity and selectivity. This study demonstrates SERS for presumptive detection of fentanyl and fentanyl analogs in counterfeit tablets using two portable Raman instruments equipped with 785 nm and 1064 nm lasers, combined with commercial Ag nanoparticles and synthesized Au/Ag nanostars. The analysis of 14 counterfeit tablets revealed that SERS correctly identified fentanyl or a fentanyl analog in all samples, achieving 100 % sensitivity. However, the 1064 nm laser configuration demonstrated slightly superior performance, yielding clearer and more consistent spectral features compared to the 785 nm laser. These findings suggest that SERS, particularly with a 1064 nm laser, is a promising tool for presumptive field detection of fentanyl and its analogs in counterfeit tablets. [Display omitted] •Portable SERS enables rapid presumptive detection of fentanyl in counterfeit tablets.•Compares different nanoparticles and excitation lasers for reliable field detection.•Supports fast threat assessment, sample screening, and prioritization for analysis.•Provides on-site detection of fentanyl analogs for practical forensic applications.

Recently, a number of fentanyl analogs have been implicated in overdose deaths in Europe and in the US. So far, little is known of the molecular behavior of the structurally related subgroup; the alicyclic fentanyls. In this study, reference standards of cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and 2,2,3,3-tetramethylcyclopropyl fentanyl (TMCPF) at a final concentration of 5 µM were incubated with cryopreserved human hepatocytes (1 × 106 cells/mL) for 0, 1, 3 and 5 h. The metabolites formed were identified by liquid chromatography–quadrupole time-of-flight mass spectrometry analysis. The most abundant biotransformation found was N-dealkylation (formation of normetabolites) and oxidation of the alicyclic rings. As ring size increased, the significance of N-dealkylation decreased in favor of alicyclic ring oxidation. An example of this was cyclopropyl fentanyl, with a three-carbon ring, whose normetabolite covered 82% of the total metabolic peak area and no oxidation of the alicyclic ring was observed. In contrast, TMCPF, with a seven-carbon ring structure, rendered as much as 85% of its metabolites oxidized on the alicyclic ring. Other biotransformations found included oxidation of the piperidine ethyl moiety and/or the phenethyl substructure, glucuronidation as well as amide hydrolysis to form metabolites identical to despropionyl fentanyl. Taken together, this study provides a base for understanding the metabolism of a number of structurally related fentanyl analogs formed upon intake.

Nasal spray formulations of naloxone, a mu-opioid receptor (MOR) antagonist, are currently used for the treatment of opioid overdose. They may have additional therapeutic utility also in the absence of opioid agonist drugs, but the onset and duration of action at brain MORs have been inadequately characterized to allow such projections. This study provides initial characterization of brain MOR availability at high temporal resolution following intranasal (IN) naloxone administration to healthy volunteers in the absence of a competing opioid agonist. Fourteen participants were scanned twice using positron emission tomography (PET) and [11C]carfentanil, a selective MOR agonist radioligand. Concentrations of naloxone in plasma and MOR availability (relative to placebo) were monitored from 0 to 60 min and at 300–360 min post naloxone. Naloxone plasma concentrations peaked at ~20 min post naloxone, associated with slightly delayed development of brain MOR occupancy (half of peak occupancy reached at ~10 min). Estimated peak occupancies were 67 and 85% following 2 and 4 mg IN doses, respectively. The estimated half-life of occupancy disappearance was ~100 min. The rapid onset of brain MOR occupancy by IN naloxone, evidenced by the rapid onset of its action in opioid overdose victims, was directly documented in humans for the first time. The employed high temporal-resolution PET method establishes a model that can be used to predict brain MOR occupancy from plasma naloxone concentrations. IN naloxone may have therapeutic utility in various addictions where brain opioid receptors are implicated, such as gambling disorder and alcohol use disorder.

RationaleThe potent synthetic opioid fentanyl, and its analogs, continue to drive opioid-related overdoses. Although the pharmacology of fentanyl is well characterized, there is little information about the reinforcing effects of clandestine fentanyl analogs (FAs).ObjectivesHere, we compared the effects of fentanyl and the FAs acetylfentanyl, butyrylfentanyl, and cyclopropylfentanyl on drug self-administration in male and female rats. These FAs feature chemical modifications at the carbonyl moiety of the fentanyl scaffold.MethodsSprague-Dawley rats fitted with intravenous jugular catheters were placed in chambers containing two nose poke holes. Active nose poke responses resulted in drug delivery (0.2 mL) over 2 s on a fixed-ratio 1 schedule, followed by a 20 s timeout. Acquisition doses were 0.01 mg/kg/inj for fentanyl and cyclopropylfentanyl, and 0.03 mg/kg/inj for acetylfentanyl and butyrylfentanyl. After 10 days of acquisition, dose-effect testing was carried out, followed by 10 days of saline extinction.ResultsSelf-administration of fentanyl and FAs was acquired by both male and female rats, with no sex differences in acquisition rate. Fentanyl and FAs showed partial inverted-U dose-effect functions; cyclopropylfentanyl and fentanyl had similar potency, while acetylfentanyl and butyrylfentanyl were less potent. Maximal response rates were similar across drugs, with fentanyl and cyclopropylfentanyl showing maximum responding at 0.001 mg/kg/inj, acetylfentanyl at 0.01 mg/kg/inj, and butyrylfentanyl at 0.003 mg/kg/inj. No sex differences were detected for drug potency, efficacy, or rates of extinction.ConclusionsOur work provides new evidence that FAs display significant abuse liability in male and female rats, which suggests the potential for compulsive use in humans.

Transcranial Direct Current Stimulation (tDCS) is a method of non-invasive brain stimulation that has been frequently used in experimental and clinical pain studies. However, the molecular mechanisms underlying tDCS-mediated pain control, and most important its placebo component, are not completely established. In this pilot study, we investigated in vivo the involvement of the endogenous μ-opioid system in the global tDCS-analgesia experience. Nine healthy volunteers went through positron emission tomography (PET) scans with [11C]carfentanil, a selective μ-opioid receptor (MOR) radiotracer, to measure the central MOR activity during tDCS in vivo (non-displaceable binding potential, BPND)--one of the main analgesic mechanisms in the brain. Placebo and real anodal primary motor cortex (M1/2mA) tDCS were delivered sequentially for 20 minutes each during the PET scan. The initial placebo tDCS phase induced a decrease in MOR BPND in the periaqueductal gray matter (PAG), precuneus, and thalamus, indicating activation of endogenous μ-opioid neurotransmission, even before the active tDCS. The subsequent real tDCS also induced MOR activation in the PAG and precuneus, which were positively correlated to the changes observed with placebo tDCS. Nonetheless, real tDCS had an additional MOR activation in the left prefrontal cortex. Although significant changes in the MOR BPND occurred with both placebo and real tDCS, significant analgesic effects, measured by improvements in the heat and cold pain thresholds, were only observed after real tDCS, not the placebo tDCS. This study gives preliminary evidence that the analgesic effects reported with M1-tDCS, can be in part related to the recruitment of the same endogenous MOR mechanisms induced by placebo, and that such effects can be purposely optimized by real tDCS.

Availability of fentanyl is at a record high with 3138 kg of fentanyl and related substances being seized in 2019. Fentanyl’s high toxicity makes a lethal dose for most mere milligrams. With such a high potency and a consistent rise of abuse, the chances of injury or death of frontline workers increase with every interaction. Development of a non-contact detection method for fentanyl would decrease the chances of a workplace mishap. To aid in the development of a non-contact detection method, target analytes in the vapor profile of fentanyl need to be identified. In order to achieve this goal, semi-quantitative headspace analysis of fentanyl analogs and confiscated fentanyl exhibits was accomplished using solid-phase microextraction and gas chromatography coupled with mass spectrometry (SPME-GC-MS). The vapor signatures of these samples were compared to a previously reported reference-grade fentanyl vapor signature to determine the target analyte(s) for fentanyl detection in the vapor phase. A total of 20 fentalogs and confiscated exhibits, with masses ranging from 2 to 19 mg, were sampled. N-Phenylpropanamide(NPPA) or N-phenethyl-4-piperidone(NPP) was identified as target analytes in 75% of these samples. This is a crucial component for the development of a non-contact detection method for fentanyl.

BackgroundIllicitly manufactured fentanyl and its analogs are a major driving force behind the ongoing opioid crisis. Cyclopropylfentanyl is a fentanyl analog associated with many overdose deaths, but limited knowledge is available about its pharmacology. In the present study, we developed a bioanalytical method for the determination of cyclopropylfentanyl and its main metabolite cyclopropylnorfentanyl and evaluated pharmacokinetic-pharmacodynamic relationships in rats.MethodAn ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method was developed and validated for determination of cyclopropylfentanyl and cyclopropylnorfentanyl in rat plasma. Male Sprague–Dawley rats fitted with jugular catheters and temperature transponders received cyclopropylfentanyl (30, 100, and 300 μg/kg) or saline subcutaneously. Blood specimens were withdrawn over an 8-h time period, along with measurements of pharmacodynamic endpoints.ResultsThe analytical method was validated, and both analytes exhibited a low limit of quantification (15 pg/mL). Cyclopropylfentanyl caused dose-related increases in hot plate latency (ED50 = 48 µg/kg) and catalepsy (ED50 = 87 µg/kg) and produced long-lasting hypothermia at the highest dose. Plasma cyclopropylfentanyl rose rapidly in a dose-related fashion, reaching maximal concentration (Cmax) after 15–28 min, whereas metabolite Cmax occurred later at 45–90 min. Cyclopropylfentanyl Cmax values were similar to concentrations measured in non-fatal intoxications in humans; however, differences in parent drug: metabolite ratio indicated possible interspecies variance in metabolism.ConclusionOur study shows that cyclopropylfentanyl produces typical opioid-like effects in male rats. Cyclopropylfentanyl displays much greater analgesic potency when compared to morphine, suggesting that cyclopropylfentanyl poses increased overdose risk for unsuspecting users.

•Acrylfentanyl caused 40 fatal intoxications in Sweden between April and October 2016.•Non-fatal acrylfentanyl intoxications occurred in lower number.•Acrylfentanyl and other new fentanyl-analogs keep appearing on the drug market. The European Nordic Countries are the most exposed to opioid-related deaths. Between April and October 2016, a series of forty lethal intoxications occurred in Sweden, in which the presence of the synthetic opioid acrylfentanyl was determined to be the main – or a contributing – cause of death. In the reported cases, the blood concentration of acrylfentanyl – mostly detected in combination with other drugs – ranged from 0.01ng/g to 5ng/g; victims were predominantly males (34 males and 6 females), and their age varied between 18 and 53 years. We further describe five cases, representative of the different drug administration route (nasal spray, tablets) and intentions (accidental or voluntary intoxication). Moreover, we address nine cases of non-lethal intoxication, in single (8 cases) or polydrug scenario (1 case). We discuss the present characteristics of the Swedish drug market for fentanyl-analogs in general and acrylfentanyl in particular, reporting a structural difficulty to effectively counteracting the appearance of unscheduled substances due to the constant turnover of new molecules on the recreational drug market.

•To develop and validate an UPLC-MS/MS method for the analysis of 16 fentanyl analogues in hair.•To determine of the prevalence of fentanyl analogue drugs in authentic hair samples from fentanyl users.•First time where concentrations of tetrahydrofuranylfentanyl, and valerylfentanyl in hair are reported.•Determination of poly drug history with segmented hair. Seizures of synthetic opioids have increased since 2012, with a 45 % increase in synthetic opioid related deaths between 2016 and 2017 in US. Recently, concerns have arisen around these substances and their illicit use also in several European countries. Our aim was to develop and validate an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the analysis of 16 synthetic opioids in segmented hair, including fentanyl, norfentanyl, acetylfentanyl, U-47700, AH-7921, acrylfentanyl, crotonylfentanyl, butyrylfentanyl, methoxacetylfentanyl, U-49900, valeryfentanyl, 4-fluoro-iso-butyrylfentanyl, ocfentanyl, furanylfentanyl, tetrahydrofuranylfentanyl, and alfetanyl. Sample preparation involved washing the hair in dichloromethane, water and methanol, and extraction in methanol, followed by solid phase extraction clean-up. This method was validated for linearity, limit of quantification (LLOQ), precision and bias, selectivity, stability, matrix effects, extraction efficiency of the clean up procedure, and carryover. LLOQs ranged from 0.15−1pg/mg, and the calibration ranged from the LLOQ up to 500pg/mg. Intra and inter-day precision were evaluated at low and high concentrations, with spiked QCs, during 8 days and the results were satisfactory with RSD<15 % for all the compounds except for norfentanyl (22 %) and alfentanyl (19 %). Two external certified QCs containing fentanyl at 11 and 105pg/mg were also analysed within each batch and the RSD and bias were lower than 16 % and 10 %, respectively. Matrix effects compensated by internal standard fentanyl-d5 (MEIS), were between 77–115 % (RSD<10 %) and extraction efficiency of the clean-up procedure was between 66–93 % (RSD<21 %). Processed sample stability and carryover were acceptable for all of the compounds. The method was applied to 17 authentic hair samples (body or head hair) from US fentanyl analogue users. When head hair was available, the hair strands were analysed in 1cm/segment. Concentrations ranges were as follows: fentanyl (n=16) 2->ULOQ (500) pg/mg, norfentanyl (n=14) 1−38pg/mg, acetylfentanyl (n=7) 0.6->ULOQ (250) pg/mg, furanylfentanyl (n=5) 2−123pg/mg, tetrahydrofuranylfentanyl (n=1) 0.5−63pg/mg and valerylfentanyl (n=1) 2.1->ULOQ (50) pg/mg, along the hair strands. To our knowledge, this is the first time where concentrations of tetrahydrofuranylfentanyl, and valerylfentanyl in hair are reported. The same samples were also analysed for the determination of other drugs of abuse using our routine method (also in 1cm/segment for head hair when available). The results demonstrated poly-drug use in these fentanyl-analogue users population (mean drugs: n=5): amphetamine and/or methamphetamine (n=10), buprenorphine (n=5), cocaine (n=8), methadone (n=8), 6-MAM (n=17), meperidine (n=1), oxycodone (n=11), tramadol (n=3). Evaluation of the concentrations of these drugs, together with the fentanyl analogues is discussed in the present paper. Two authentic samples from two Belgian post-mortem cases, were also analysed showing fentanyl use and in one case polydrug use. The results demonstrated multi-analyte quantitative methods, including fentanyl analogues, are becoming useful in forensic laboratories involved in hair analysis, and in particular when polydrug use is suspected.