Fentanyl Analogs Research Articles

High-Throughput Screening of Fentanyl Analogs

High-Throughput Screening of Fentanyl Analogs

A Fentanyl Hapten‐Displaying Lipid Nanoparticle Vaccine that Non‐Covalently Encapsulates a TLR7/8 Agonist and T‐helper Epitope Induces Protective Anti‐Fentanyl Immunity

Opioid use disorder ‐ particularly involving fentanyl ‐ has precipitated a public health crisis characterized by a significant increase in addiction and overdose‐related deaths. Fentanyl‐specific immunotherapy, which aims at inducing fentanyl‐specific antibodies capable of binding fentanyl molecules in the bloodstream, preventing their entry in the central nervous system, is therefore gaining momentum. Conventional opioid designs rely on the covalent conjugation of fentanyl analogues to immunogenic carrier proteins that hold the inherent capacity of mounting immunodominant responses. Here, we present an alternative fentanyl vaccine design that utilizes a non‐covalent assembly of lipid nanoparticles (LNPs) to deliver fentanyl haptens in conjunction with a CD4+ T‐helper peptide epitope and an imidazoquinoline TLR7/8 agonist. Our results demonstrate that a single intramuscular administration of the LNP‐based nanovaccine elicits fentanyl‐specific antibodies, significantly mitigating the effects of opioid overdose in preclinical mouse models. Furthermore, we analyzed the immunobiological behavior of the vaccine in vivo in mouse models, providing evidence that covalent attachment of a fentanyl hapten to a carrier proteins or peptide epitope is not necessary for inducing an effective immune response. However, co‐delivery ‐ specifically, the physical assembly of all immune cues into an LNP ‐ remains essential for inducing hapten‐specific immunity.

A Fentanyl Hapten-Displaying Lipid Nanoparticle Vaccine that Non-Covalently Encapsulates a TLR7/8 Agonist and T-helper Epitope Induces Protective Anti-Fentanyl Immunity.

Opioid use disorder - particularly involving fentanyl - has precipitated a public health crisis characterized by a significant increase in addiction and overdose-related deaths. Fentanyl-specific immunotherapy, which aims at inducingfentanyl-specificantibodies capable of binding fentanyl molecules in the bloodstream, preventing their entry in the central nervous system, is therefore gaining momentum.Conventional opioid designsrely on the covalent conjugation of fentanyl analogues to immunogenic carrier proteins that hold the inherent capacity of mounting immunodominant responses. Here, we present an alternative fentanyl vaccine design that utilizes a non-covalent assembly of lipid nanoparticles (LNPs) to deliver fentanyl haptens in conjunction with a CD4+T-helper peptide epitope and an imidazoquinoline TLR7/8 agonist. Our results demonstrate that a single intramuscular administration of the LNP-based nanovaccine elicits fentanyl-specific antibodies, significantly mitigating the effects of opioid overdose in preclinical mouse models. Furthermore, we analyzed the immunobiological behavior of the vaccine in vivo in mouse models, providing evidence that covalent attachment of a fentanyl hapten to a carrier proteins or peptide epitope is not necessary for inducing an effective immune response. However, co-delivery - specifically, the physical assembly of all immune cues into an LNP - remains essential for inducing hapten-specific immunity.

Determination of Synthetic Opioids Belonging to the Fentanyl Class in Silt Using Electron Ionization Gas Chromatography-Mass Spectrometry (GC-MS).

An extraction protocol from silt sediment of fentanyl and three analogs: acetylfentanyl, thiofentanyl and acetylthiofentanyl, spiked at two concentrations each and separately (at ∼1 and ∼10 µg/g), is described. In addition, the identity of the fentanyls preliminarily identified by electron ionization gas chromatography-mass spectrometry (EI-GC-MS) analysis, can be corroborated by reacting each opioid in the silt’s extract with 2,2,2-trichloroethoxycarbonyl chloride (Troc-Cl). Reaction between Troc-Cl and each opioid generates two unique products that can be used to retrospectively identify the original opioid therefore serving as a corroborating tool for known opioids as well as new, unknown fentanyl analogs.

Slow dissociation kinetics of fentanyls and nitazenes correlates with reduced sensitivity to naloxone reversal at the μ-opioid receptor.

Fentanyls and nitazenes are μ-opioid receptor agonists responsible for a large number of opioid overdose deaths. Here, we determined the potency, dissociation kinetics and antagonism by naloxone at the μ receptor of several fentanyl and nitazene analogues, compared to morphine and DAMGO. In vitro assays of G protein activation and signalling and arrestin recruitment were performed. AtT20 cells expressing μ receptors were loaded with a membrane potential dye and changes in fluorescence used to determine agonist potency, dissociation kinetics and susceptibility to antagonism by naloxone. BRET experiments were undertaken in HEK293T cells expressing μ receptors to assess Gi protein activation and β-arrestin 2 recruitment. The apparent rate of agonist dissociation from the μ receptor varied: morphine, DAMGO, alfentanil and fentanyl dissociated rapidly, whereas isotonitazene, etonitazene, ohmefentanyl and carfentanil dissociated slowly. Slowly dissociating agonists were more resistant to antagonism by naloxone. For carfentanil, the slow apparent rate of dissociation was not because of G protein receptor kinase-mediated arrestin recruitment as its apparent rate of dissociation was not increased by inhibition of G protein-coupled receptor kinases (GRKs) with Compound 101. The in vitro relative potencies of fentanyls and nitazenes compared to morphine were much lower than that previously observed in in vivo experiments. With fentanyls and nitazenes that slowly dissociate from the μ receptor, antagonism by naloxone is pseudo-competitive. In overdoses involving fentanyls and nitazenes, higher doses of naloxone may be required for reversal than those normally used to reverse heroin overdose.

In vivo pharmacokinetic, pharmacodynamic and brain concentration comparison of fentanyl and para-fluorofentanyl in rats.

In 2022, para-fluorofentanyl (pFF) rose to the 6th most reported drug and the most reported fentanyl analog in the United States according to the Drug Enforcement Administration (DEA). pFF differs from fentanyl by the addition of a single fluorine group. To date, pFF has not been extensively evaluated in vivo and is frequently seen in combination with fentanyl. In the present study, the pharmacodynamic (PD) and pharmacokinetic (PK) properties and brain region-specific concentrations of pFF were evaluated in male Sprague-Dawley rats and compared to fentanyl. A 300μg/kg subcutaneous dose of fentanyl or pFF was administered to assess PD and PK parameters as well as brain region concentrations. PD parameters were evaluated via a tail flick test to evaluate analgesia and core body temperature to measure hypothermia, a surrogate marker of overall opioid toxicity. Fentanyl and pFF were found to be equally active at the tested dose in terms of tail flick response with both compounds producing an analgesic response that lasted up to 240min post-drug treatment. pFF induced a significantly greater hypothermic effect compared to fentanyl with a maximum temperature decrease of -5.6 ℃. Plasma PK parameters (T1/2, AUC, etc.) did not differ between fentanyl and pFF. However, pFF concentrations in the medulla, hippocampus, frontal cortex and striatum were more than two times the fentanyl concentrations. The increase in brain concentrations and greater hypothermic effect suggests that pFF is potentially more dangerous than fentanyl.

Structure-Based Engineering of Monoclonal Antibodies for Improved Binding to Counteract the Effects of Fentanyl and Carfentanil.

The opioid overdose epidemic is a growing and evolving public health crisis fueled by the widespread presence of fentanyl and fentanyl analogues (F/FAs) in both street mixtures and counterfeit pills. To expand current treatment options, drug-targeting monoclonal antibodies (mAbs) offer a viable therapeutic for both pre- and postexposure clinical scenarios. This study reports the isolation, in vitro characterization, and in vivo efficacy of two murine mAb families targeting fentanyl, carfentanil, or both. Because humanization of the mAbs by CDR grafting negatively impacted affinity for both fentanyl and carfentanil, crystal structures of mAbs in complex with fentanyl or carfentanil were analyzed to identify key residues involved in ligand binding in murine versus humanized structures, and site-directed mutagenesis was used to verify their functional importance. The structural analysis identified a framework residue, Tyr36, present in the murine germline sequence of two mAbs, which was critical for binding to fentanyl and carfentanil. These studies emphasize the importance of structural considerations in mAb engineering to optimize mAbs targeting small molecules including opioids and other drugs of public health interest.

In vitro and in vivo study of butyrylfentanyl and 4-fluorobutyrylfentanyl in female and male mice: Role of the CRF1 receptor in cardiorespiratory impairment.

Fentanyl analogues have been implicated in many cases of intoxication and death with overdose worldwide. The aim of this study is to investigate the pharmaco-toxicology of two fentanyl analogues: butyrylfentanyl (BUF) and 4-fluorobutyrylfentanyl (4F-BUF). In vitro, we measured agonist opioid receptor efficacy, potency, and selectivity and ability to promote interaction of the μ receptor with G protein and β-arrestin 2. In vivo, we evaluated thermal antinociception, stimulated motor activity and cardiorespiratory changes in female and male CD-1 mice injected with BUF or 4F-BUF (0.1-6 mg·kg-1). Opioid receptor specificity was investigated using naloxone (6 mg·kg-1). We investigated the possible role of stress in increasing cardiorespiratory toxicity using the corticotropin-releasing factor 1 (CRF1) antagonist antalarmin (10 mg·kg-1). Agonists displayed the following rank of potency at μ receptors: fentanyl > 4F-BUF > BUF. Fentanyl and BUF behaved as partial agonists for the β-arrestin 2 pathway, whereas 4F-BUF did not promote β-arrestin 2 recruitment. In vivo, we revealed sex differences in motor and cardiorespiratory impairments but not antinociception induced by BUF and 4F-BUF. Antalarmin alone was effective in blocking respiratory impairment induced by BUF in both sexes but not 4F-BUF. The combination of naloxone and antalarmin significantly enhanced naloxone reversal of the cardiorespiratory impairments induced by BUF and 4F-BUF in mice. In this study, we have uncovered a novel mechanism by which synthetic opioids induce respiratory depression, shedding new light on the role of CRF1 receptors in cardiorespiratory impairments by μ agonists.

B-268 Detection of xylazine in fentanyl-positive urine drug screen specimens using a quantitative LCMS/MS method

Abstract Background Fentanyl, a synthetic opioid used clinically for severe pain management, sedation, and as a perioperative analgesic, has been at the forefront of fatal drug-involved overdoses for almost a decade. Xylazine, an α-2 adrenergic receptor agonist, is an increasingly common adulterant in illicitly manufactured fentanyl products and analogs. Given its class, xylazine is non-responsive to naloxone, an opioid receptor antagonist used to reverse opioid overdose. Importantly, the U.S. Drug Enforcement Administration reported xylazine in 23% of seized fentanyl powders and 7% of fentanyl pills; additionally, a recent study by Kariisa et. al. identified an increase of xylazine presence in fatal fentanyl-involved overdoses (2.9% in January 2019 to 10.9% in June 2022) across twenty states.1 Collectively, these trends indicate a need for increased surveillance of xylazine in overdose-presenting patients to inform appropriate clinical responses. In this study, we developed a sensitive LC-MS/MS method for xylazine quantitation and used it to determine xylazine presence in our fentanyl positive patients. Methods An LC-MS/MS method for quantitation of xylazine from urine samples was developed using a Waters Xevo TQ-XS and reversed-phase CORTECS C8 2.7 µm column. MRMs and optimal MS tuning parameters were generated for xylazine and xylazine-d6 via IntelliStart, with transition ions of 221.18→90.04 (Quan) and 221.18→136.31 (Qual) selected for xylazine quantitation against a 227.23→89.98 xylazine-d6 transition as an internal standard. An LC method was developed using water (2 mM Ammonium Acetate; 0.1% Formic Acid v/v) and methanol (2 mM Ammonium Acetate; 0.1% Formic Acid v/v) gradient for a 4-minute runtime. Calibrators were prepared by addition of the pure drug to water, while QCs were prepared using healthy volunteer urine that was confirmed negative in a routine urine drug screen (UDS). Calibrators were value-assigned using a reference laboratory. A simple processing method consisting of the addition of 200 µL methanol containing 25 ng/mL xylazine-d6 to 50 µL of standards, QCs, or urine was used for sample preparation. Remnant fentanyl-positive patient samples (determined by ARK Fentanyl II Immunoassay - Abbott Architect C16000 analyzer) submitted for routine UDS evaluation were collected for xylazine analysis. Results Using custom xylazine standards, the method yielded a linear range from 2 ng/mL to 500 ng/mL with an average R2 of 0.9995 across runs (n = 12). The LLoQ for this assay was 2 ng/mL with a CV of 18.1%. The intra-assay CVs were < 5.0% and inter-assay CVs were < 15.0%. Of preliminary patient samples tested (n = 25), xylazine was detected in 32% of fentanyl-positive samples, ranging in concentration from 2.8 ng/mL to 192.8 ng/mL with an average of 64.8 ng/mL. Conclusions Here we report a simple LC-MS/MS method for quantitation of xylazine in remnant fentanyl-positive UDS specimens by LC-MS/MS. Preliminary testing revealed a xylazine positivity rate of 32% in fentanyl-positive patients. Uncomplicated sample work-up makes this method applicable to xylazine surveillance in all UDS-positive patient samples in high-throughput clinical laboratories. Reference: 1. Kariisa M, et. al. “Illicitly Manufactured Fentanyl-Involved Overdose Deaths with Detected Xylazine.” MMWR Morb Mortal Wkly Rep 2023;72:721-727.

Lethal fentanyl analog shows increase across U.S.

Lethal fentanyl analog shows increase across U.S.

Identification of fentanyl analogs and potential biomarkers in urine using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and liquid chromatography-quadrupole/time of flight mass spectrometry (LC-Q/TOF-MS)

Novel synthetic opioids are a class of drugs abused for their potent analgesic effect and are responsible for many fatal intoxications, particularly within the United States. A targeted assay was developed and validated using LC-MS/MS, capable of identifying nineteen fentalogs. Solid phase extraction was used to isolate analytes of interest from urine. Limits of detection ranged from 0.05 to 0.1 ng/mL and the limit of quantitation was 0.5 ng/mL. Extraction efficiencies using the optimized procedure were 77–88 % for all targeted species. Bias, precision, matrix effects and interferences were within acceptable thresholds for all analytes. The validated assay was used to identify analytes of interest from thirty-seven individuals that had used fentanyl and related substances. In addition to quantitative analyses, a non-targeted liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q/TOF-MS) assay was also used to identify additional substances and potential biomarkers. Additional N-oxide and N-dealkylated species were identified using this approach, and the potential for biomarker use is presented, given the stability of some analytes within this class.

Bio‐inspired 3D Sub‐Microstructures Coupling of Au Nanoparticles for SERS Detection of Trace Fentanyl

Abstract3D surface‐enhanced Raman scattering (SERS) platforms with multi‐dimensional hotspots and better tolerance to laser misfocus show great potential for the rapid detection of trace fentanyl. However, the complex fabrication process of 3D fine structures limits the wide‐ranging application of the SERS technique. Herein, a 3D SERS‐active platform is fabricated utilizing a scalable and cost‐effective strategy that involves the electroless deposition of dense Au nanoparticles on natural butterfly wing scales (b‐wings@Au). The resultant b‐wings@Au proves to be sensitive and reliable, with a lower limit of detection of 4× 10−12 m and an enhancement factor (EFexp) of 1.74 × 108 for fentanyl. Furthermore, it exhibits excellent reproducibility, as evidenced by a relative standard deviation of less than 5%, and also demonstrates outstanding stability. The obtained high EFexp is due to the combined effects of the electromagnetic mechanism, involving the enhanced electromagnetic field generated by 3D periodic hotspots, and the chemical mechanism, involving the charge transfer from fentanyl to Au surface. Additionally, the as‐prepared 3D substrate realizes the trace detection of nine fentanyl analogs. This study provides new ideas for the controllable construction of sensitive SERS platforms for drug detection, ultimately advancing the fields of 3D substrates in booming SERS‐based technologies.

Evaluating fentanyl test strips as a harm reduction strategy in rural and urban counties: study protocol for a randomized controlled trial

BackgroundOpioid-related fatalities are a leading cause of death in Ohio and nationally, with an increasing number of overdoses attributable to fentanyl. Rapid fentanyl test strips can identify fentanyl and some fentanyl analogs in urine samples and are increasingly being used to check illicit drugs for fentanyl before they are used. Fentanyl test strips are a promising harm reduction strategy; however, little is known about the real-world acceptability and impact of fentanyl test strip use. This study investigates fentanyl test strip distribution and education as a harm reduction strategy to prevent overdoses among people who use drugs.MethodsThe research team will recruit 2400 individuals ≥ 18 years with self-reported use of illicit drugs or drugs purchased on the street within the past 6 months. Recruitment will occur at opioid overdose education and naloxone distribution programs in 16 urban and 12 rural Ohio counties. Participating sites will be randomized at the county level to the intervention or non-intervention study arm. A brief fentanyl test strip educational intervention and fentanyl test strips will be provided to participants recruited from sites in the intervention arm. These participants will be eligible to receive additional fentanyl test strips for 2 years post-enrollment. Participants recruited from sites in the non-intervention arm will not receive fentanyl test strip education or fentanyl test strips. All participants will be followed for 2 years post-enrollment using biweekly, quarterly, and 6-month surveys. Primary outcomes include (1) identification of perceived barriers and facilitating factors associated with incorporating fentanyl test strip education and distribution into opioid overdose education and naloxone distribution programs; (2) differences in knowledge and self-efficacy regarding how to test drugs for fentanyl and strategies for reducing overdose risk between the intervention and non-intervention groups; and (3) differences in non-fatal and fatal overdose rates between the intervention and non-intervention groups.DiscussionFindings from this cluster randomized controlled trial will contribute valuable information about the feasibility, acceptability, and impact of integrating fentanyl test strip drug checking in rural and urban communities in Ohio and help guide future overdose prevention interventions.Trial registrationClinicalTrials.gov NCT05463341. Registered on July 19, 2022. https://clinicaltrials.gov/study/NCT05463341

Abstract P231: Self-administration of Remifentanil Induces Endothelial Dysfunction in Thoracic Aorta and Impaired Cavernosal Reactivity of Wistar Rats

Aims: Recent studies have implicated the use of prescription opioids as a risk factor for cardiovascular diseases (CVD), with associations including myocardial infarction, coronary artery disease and vascular deterioration. Further, opioid-dependent patients show increased vascular aging and arterial stiffness when compared to opioid-naïve controls. Our hypothesis was that rats dependent on remifentanil (a fentanyl analog) after a 12-day operant self-administration (SA) study lead to vascular dysfunction in thoracic aorta. Because erectile dysfunction (ED) is associated with arterial stiffness and is positively correlated with its severity, we hypothesized that opioid SA impaired corpus cavernosum (CC) reactivity as well. Methods: Male Wistar rats (12 weeks-old) were implanted with intravenous catheters and learned to lever press (FR1) for remifentanil (1µg/kg/infusion). A separate group of rats received passive yoked infusions of saline administered in synchrony with remifentanil-treated animals to control volumetric vascular effects. Following administration, animals were euthanized, and aorta and CC collected to evaluate function using both pin and strip myographs. Concentration-response curves to phenylephrine (Phe; 10 -11 -10 -4 M), sodium nitroprusside (SNP) and acetylcholine (Ach or SNP; 10 -10 -10 -4 M) were performed to test contractility and endothelium-independent and -dependent relaxation respectively. Results: Remifentanil-treated rats showed rapidly acquired SA behavior, exhibiting robust and selective drug-paired lever pressing when compared to the non-drug paired lever. In contrast, yoked saline-treated controls rarely exhibited lever pressing. Endothelium-dependent relaxation in the aorta was diminished in the remifentanil group (Rmax: Control: 68.7±2.7 vs. Remi: 47.8±3.7%; p<0.05). There were no observed alterations in endothelium-independent relaxation or vascular contraction. However, CC contraction was slightly shifted to the right in remifentanil treated rats (pEC 50 : Control: 6.04±0.09 vs. Remi: 5.66±0.12; p<0.05). No changes were observed in the CC relaxant response. These findings indicate that 12 days of remifentanil administration results in endothelial dysfunction in large arteries and impaired cavernosal reactivity. Long-term exposure may lead to aortic stiffness exacerbating ED severity and increases in pulse-wave velocity in opioid-dependent patients contributing to the onset of hypertension.

The fentanyl made me feel like I needed more methadone”: changes in the role and use of medication for opioid use disorder (MOUD) due to fentanyl

BackgroundFentanyl and fentanyl analogues have disrupted the illicit drug supply through contamination of other substances (i.e., methamphetamine and cocaine) and replacement of heroin in illicit markets. Increasingly, they are contributing to opioid-overdose related deaths. The rapid and growing presence of fentanyl has led to gaps in research on the impact of this illicit market change on people who use drugs (PWUD). We sought to examine how the changing opioid market and growing fentanyl availability influences the role and use of medication for opioid use disorder (MOUD).MethodsSemi-structured qualitative interviews were conducted with a community recruited sample of PWUD (N = 22) in Los Angeles, California between September 2021 and April 2022. Interviews examined opioid use history, current opioid use behaviors and consumption patterns, and MOUD experiences and perceptions. Thematic analysis was used to systematically code and analyze textual interview data.ResultsThe following themes related to fentanyl use and MOUD emerged: (1) Use of deviated MOUD to address fentanyl contamination, (2) Changing perception of the effectiveness of MOUD on fentanyl, and (3) Regulatory limitations of MOUD for fentanyl use disorder.ConclusionsPWUD described several repertoires for adjusting to changes in the illicit market of opioids. Clinicians treating PWUD should ask about recent fentanyl use prior to starting MOUD to account for increased tolerance to opioids. Harm reduction strategies such as naloxone kits, safe supply, and supervised consumption facilities can all prevent overdose deaths due to fentanyl.

Monitoring illicit pentobarbital availability in the United States: A National Drug Early Warning System briefing

BackgroundPentobarbital is a Schedule II/III short-acting barbiturate with limited medical use in humans. Veterinary professionals use pentobarbital to euthanize dogs, cats, and other companion animals. Pentobarbital is also utilized in capital punishment and small amounts are illegally shipped or diverted to assist in suicides. However, five kilograms of pentobarbital smuggled in from Mexico was recently seized by an organized crime drug enforcement task force (along with fentanyl, heroin, and cocaine), which may suggest a shift in illicit supply. We investigated potential indicators of illicit pentobarbital use or availability in the US to help determine whether this drug is becoming an emerging public health concern. MethodsThe National Drug Early Warning System requested information on pentobarbital from its sentinel surveillance sites and collaborators and conducted a search of current literature. ResultsIn early 2024, multiple batches of counterfeit pills (e.g., pressed as “M30s” to represent oxycodone) confiscated near the Southwest border tested positive for pentobarbital plus combinations of fentanyl, fentanyl analogs, and xylazine. Other indicators suggest pentobarbital is being smuggled in powder form and possibly sold as another drug such as heroin. One national drug analysis program detected pentobarbital in 217 drug submissions from 2020 to 2023, and there were at least 12 fatal exposures linked to use from 2020 to 2022. ConclusionContinued monitoring of illicit use and availability is needed as pentobarbital may continue to appear on the illicit market. Unknown exposure can occur if the drug is mixed into counterfeit pills or sold in powder form represented to be another drug.

Isobutyryl-carfentanyl has strong acute toxicity and analgesic effects with high addiction potential.

Isobutyryl-carfentanyl is the most recently discovered fentanyl analogue with a chemical structure that is similar to that of carfentanyl. Its analogue, carfentanyl, is regarded as one of the most lethal drugs in the world, with a potency of 10,000 times that of morphine. Therefore, isobutyryl-carfentanyl may possess a comparably high potency and its harmful effects cannot be ignored. This study was designed to assess the analgesic effect of isobutyryl-carfentanyl and the potential risks associated with its misuse. In this study, we assessed the acute toxicity of isobutyryl-carfentanyl by up-and-down-procedure, the analgesic efficacy by hot-plate test, the abuse potential by conditioned place preference (CPP), drug self-administration, and drug discrimination tests, and compared it with fentanyl and carfentanyl. The estimated median lethal dose (LD50) of isobutyryl-carfentanyl administered were 175mg/kg (intragastric administration, IG), 15.84mg/kg (intraperitoneal injection, IP), 15.84mg/kg (subcutaneous injection, SC), and 1.6mg/kg (intravenous injection, IV), respectively. The 50% maximal analgesic effect (ED50) of isobutyryl-carfentanyl was determined to be 0.00319mg/kg, with an analgesic potency 14 times that of fentanyl and 0.82 times that of carfentanyl. Isobutyryl-carfentanyl exhibited a significant positional preference at a minimum dose of 0.1mg/kg, while fentanyl exhibited a significant positional preference at a minimum dose of 0.3mg/kg. In the heroin (0.05mg/kg/infusion) self-administration substitution experiment, isobutyryl-carfentanyl showed significant self-administration behaviour at doses of 0.0005-0.001mg/kg/infusion, with the maximum number of infusions observed at a dose of 0.001mg/kg. In the heroin (1mg/kg) drug discrimination experiment, fentanyl (0.005-0.02mg/kg), carfentanyl (0.0005-0.002mg/kg), and isobutyryl-carfentanyl (0.001-0.005mg/kg) were tested in the dose-effect curves. The results showed that all three drugs exhibit dose-dependent increase in the number of drug-associated nose pokes responses and reduction in the rate of nose pokes. The subjective effect potency of isobutyryl-carfentanyl was found to be 4.4 times that of fentanyl and 0.5 times that of carfentanyl. In summary, isobutyryl-carfentanyl has high acute toxicity and analgesic effect, with strong psychological dependence approximately 5 times that of fentanyl and 0.5 times that of carfentanyl, and has extremely high abuse potency.

Fluorofentanyl and Novel Synthetic Opioids in Accidental Overdose Deaths.

One of the quickest-growing subclasses of novel psychoactive substances is novel synthetic opioids (NSO), which are categorized as fentanyl analogs (fentalogs) or non-fentanyl opioids that bind to the mu-opioid receptor. Increased detections of NSO have been observed in the United States. However, limited information on their prevalence outside of the East Coast is available. This study details the prevalence of NSO, specifically fluorofentanyl, in the biological and drug paraphernalia specimens of accidental overdose deaths in San Francisco in 2022. A recently developed and validated LC-MS-MS method was utilized for the analysis of over 250 NSO. Out of the 649 accidental overdose deaths in 2022, 617 cases were available for blood analysis, with at least one NSO detected in 48 cases (7.8%). Fentalogs were detected in all 48 cases, with fluorofentanyl being detected in 40 cases. In postmortem femoral blood, estimated concentrations of fluorofentanyl ranged from 0.1 to 8.9 ng/mL, and 0.05 to 85 ng/mL in urine. Polysubstance use with NSO was seen with fentanyl (89.6%), methamphetamine (70.8%), cocaine (33.3%), and heroin (18.8%). NSO, mainly, fluorofentanyl were observed in matched drug paraphernalia. This report documents the migration of fluorofentanyl to the West Coast, specifically California.

Heroin or fentanyl: Prevalence of confirmed fentanyl in ED patients with suspected heroin overdose.

United States drug overdose deaths are being driven by the increasing prevalence of fentanyl, but whether patients are knowingly using fentanyl is unclear. We examined the analytical confirmation of fentanyl in emergency department (ED) patients with documented heroin overdose. We hypothesized that the proportion of fentanyl and fentanyl analogs would be higher than that of confirmed heroin. This is a subgroup analysis from a prospective multicenter consecutive cohort of ED patients age 18+ with opioid overdose presenting to 10 US sites within the Toxicology Investigators Consortium from 2020 to 2021. Toxicology analysis was performed using liquid chromatography quadrupole time-of-flight mass spectrometry. De-identified toxicology results were paired with the clinical database. The primary outcome was the proportion of patients with fentanyl analytes detected in their serum. Of 1006 patients screened, 406 were eligible, and of 168 patients who reported that they had taken heroin or had a documented heroin overdose, 88% (n=147) were in fact found to have fentanyl and/or a fentanyl analog present on serum analysis (p<0.0001). In contrast, only 46 of the 168 patients with reported or documented heroin overdose (27%) were found to have heroin biomarkers present. The prevalence of confirmed fentanyl in ED patients with suspected heroin overdose was extremely high, while the prevalence of heroin was very low. There was a high degree of mismatch between the opioids believed to be the overdose agent versus the actual opioids identified on serum toxicology. Clinicians in the United States should presume that fentanyl is involved in all illicit opioid overdoses and should counsel patients on harm reduction measures.

Reconsidering the usefulness of long-term high-dose buprenorphine.

Buprenorphine has been successfully used for decades in the treatment of opioid use disorder, yet there are complexities to its use that warrant attention to maximize its utility. While the package insert of the combination product buprenorphine\naloxone continues to recommend a maximum dose of 16 mg daily for maintenance, the emergence of fentanyl and synthetic analogs in the current drug supply may be limiting the effectiveness of this standard dose. Many practitioners have embraced and appropriately implemented novel practices to mitigate the sequelae of our current crisis. It has become common clinical practice to stabilize patients with 24 - 32 mg of buprenorphine daily at treatment initiation. Many of these patients, however, are maintained on these high doses (>16 mg/d) indefinitely, even after prolonged stability. Although this may be a necessary strategy in the short term, there is little evidence to support its safety and efficacy, and these high doses may be exposing patients to more complications and side effects than standard doses. Commonly known side effects of buprenorphine that are likely dose-related include hyperhidrosis, sedation, decreased libido, constipation, and hypogonadism. There are also complications related to the active metabolite of buprenorphine (norbuprenorphine) which is a full agonist at the mu opioid receptor and does not have a ceiling on respiratory suppression. Such side effects can lead to medical morbidity as well as decreased medication adherence, and we, therefore, recommend that after a period of stabilization, practitioners consider a trial of decreasing the dose of buprenorphine toward standard dose recommendations. Some patients' path of recovery may never reach this stabilization phase (i.e., several months of adherence to medications, opioid abstinence, and other clinical indicators of stability). Side effects of buprenorphine may not have much salience when patients are struggling for survival and safety, but for those who are fortunate enough to advance in their recovery, the side effects become more problematic and can limit quality of life and adherence.