Nalmefene reverses carfentanil-induced loss of righting reflex and respiratory depression in rats

Nalmefene Hydrochloride: Potential Implications for Treating Alcohol and Opioid Use Disorder

Nalmefene hydrochloride was first discovered as an opioid antagonist derivative of naltrexone in 1975. It is among the most potent opioid antagonists currently on the market and is differentiated from naloxone and naltrexone by its partial agonist activity at the kappa-opioid receptor which may benefit in the treatment of alcohol use disorder. Oral nalmefene has been approved in the European Union for treatment of alcohol use disorder since 2013. As of 2023, nalmefene is available in the United States as an intranasal spray for reversal of opioid overdose but is not approved for alcohol or opioid use disorder as a maintenance treatment. The substantially longer half-life of nalmefene and 5-fold higher binding affinity to opioid receptors makes it a superior agent over naloxone in the reversal of high potency synthetic opioids like fentanyl and the emerging nitazenes. Nalmefene presents with a comparable side effect profile to other opioid antagonists and should be considered for further development as a maintenance treatment for opioid and other substance use disorders.

Mechanisms of Neurorespiratory Toxicity Induced by Fentanyl Analogs—Lessons from Animal Studies

In 2020, fentanyl and its analogs contributed to ~65% of drug-attributed fatalities in the USA, with a threatening increasing trend during the last ten years. These synthetic opioids used as potent analgesics in human and veterinary medicine have been diverted to recreational aims, illegally produced and sold. Like all opioids, central nervous system depression resulting from overdose or misuse of fentanyl analogs is characterized clinically by the onset of consciousness impairment, pinpoint miosis and bradypnea. However, contrasting with what observed with most opioids, thoracic rigidity may occur rapidly with fentanyl analogs, contributing to increasing the risk of death in the absence of immediate life support. Various mechanisms have been proposed to explain this particularity associated with fentanyl analogs, including the activation of noradrenergic and glutamatergic coerulospinal neurons and dopaminergic basal ganglia neurons. Due to the high affinities to the mu-opioid receptor, the need for more elevated naloxone doses than usually required in morphine overdose to reverse the neurorespiratory depression induced by fentanyl analogs has been questioned. This review on the neurorespiratory toxicity of fentanyl and analogs highlights the need for specific research focused on these agents to better understand the involved mechanisms of toxicity and develop dedicated strategies to limit the resulting fatalities.

The efficacy of nalmefene on anesthetic recovery of patients: a study protocol for a multicenter randomized controlled trial

INTRODUCTION

So far, the recovery quality after general anesthesia is still unsatisfied. Nalmefene is a drug to treat opioid overdose and reverse opioid actions. We aim to investigate the efficacy of nalmefene on optimizing the recovery quality of patients after general anesthesia.

METHODS

It is a prospective, placebo-controlled, two-arm parallel groups, multicentre, double-blind, randomized (PPPMDR) clinical trial. The participants (n = 520) will be randomly assigned into two groups. Each patient will receive either: a single dose of nalmefene 0.25 µg/kg in the intervention group, or the same volume of 0.9% NaCl solution in the control group at the end of the surgery. The primary outcome will be the time interval between the end of anaesthesia and recovery endpoints achieved (Aldrete recovery score ≥ 9) in post-anesthesia care unit (PACU). The other variables are the time interval from the end of operation to extubation; Richmond Agitation Sedation Scale (RASS) score at extubation; the time at Montreal Cognitive Assessment Scale (MoCA) orientation score ≥ 5; visual analog scale (VAS) score and adverse effects including postoperative nausea and vomiting (PONV), and pruritus in PACU and 24 h postoperatively.

ANALYSIS

This trial aims to study whether small dose of nalmefene can shorten the time from the end of surgery to Aldrete score ≥ 9 and improve opioid-induced side effects.This trial focuses on providing the reliable clinical evidence for satisfactory quality of recovery.

ETHICS AND DISSEMINATION

This clinical trial has been approved and supported by the ethics committee of the Renji Hospital, Shanghai Jiaotong University, School of Medicine (KY2020-150); Shanghai Tongren Hospital (2021-030-01);The First Affiliated Hospital of Guangxi Medical University (2021-032); and The First Affiliated Hospital of Zhengzhou University(2021-KY-0495-003). Analysis of the study results will be submitted to a peer-reviewed journal for publication.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT04713358, Registered on September 23, 2021.

Pharmacotherapy of Opioid Use Disorder-Update and Current Challenges

The incidence of opioid use disorder (OUD) and overdose deaths is rising yearly within the United States. Many cases are associated with illicitly manufactured fentanyl use. In addition to offering patients medications for OUD (methadone, buprenorphine, and naltrexone), the approach to this epidemic should involve increasing provider awareness and education about substance use disorders, expanding urine toxicology screens to test for fentanyl, and using low-threshold treatment approaches.

Hypoxia driven opioid targeted automated device for overdose rescue

Opioid use disorder has been designated a worsening epidemic with over 100,000 deaths due to opioid overdoses recorded in 2021 alone. Unintentional deaths due to opioid overdoses have continued to rise inexorably. While opioid overdose antidotes such as naloxone, and nalmefene are available, these must be administered within a critical time window to be effective. Unfortunately, opioid-overdoses may occur in the absence of antidote, or may be unwitnessed, and the rapid onset of cognitive impairment and unconsciousness, which frequently accompany an overdose may render self-administration of an antidote impossible. Thus, many lives are lost because: (1) an opioid overdose is not anticipated (i.e., monitored/detected), and (2) antidote is either not present, and/or not administered within the critical frame of effectiveness. Currently lacking is a non-invasive means of automatically detecting, reporting, and treating such overdoses. To address this problem, we have designed a wearable, on-demand system that comprises a safe, compact, non-invasive device which can monitor, and effectively deliver an antidote without human intervention, and report the opioid overdose event. A novel feature of our device is a needle-stow chamber that stores needles in a sterile state and inserts needles into tissue only when drug delivery is needed. The system uses a microcontroller which continuously monitors respiratory status as assessed by reflex pulse oximetry. When the oximeter detects the wearer’s percentage of hemoglobin saturated with oxygen to be less than or equal to 90%, which is an indication of impending respiratory failure in otherwise healthy individuals, the microcontroller initiates a sequence of events that simultaneously results in the subcutaneous administration of opioid antidote, nalmefene, and transmission of a GPS-trackable 911 alert. The device is compact (4 × 3 × 3 cm), adhesively attaches to the skin, and can be conveniently worn on the arm. Furthermore, this device permits a centralized remotely accessible system for effective institutional, large-scale intervention. Most importantly, this device has the potential for saving lives that are currently being lost to an alarmingly increasing epidemic.

Treatment of overdose in the synthetic opioid era

Overdose deaths are often viewed as the leading edge of the opioid epidemic which has gripped the United States over the past two decades (Skolnick, 2018a). This emphasis is perhaps unsurprising because opioid overdose is both the number-one cause of death for individuals between 25 and 64 years old (Dezfulian et al., 2021) and a significant contributor to the decline in average lifespan (Dowell et al., 2017). Exacerbated by the COVID 19 pandemic, it was estimated there were 93,400 drug overdose deaths in the United States during the 12 months ending December 2020, with more than 69,000 (that is, >74%) of these fatalities attributed to opioid overdose (Ahmad et al., 2021). However, the focus on mortality statistics (Ahmad et al., 2021; Shover et al., 2020) tends to obscure the broader medical impact of nonfatal opioid overdose. Analyses of multiple databases indicate that for each opioid-induced fatality, there are between 6.4 and 8.4 non-fatal overdoses, exacting a significant burden on both the individual and society. Over the past 7-8 years, there has been an alarming increase in the misuse of synthetic opioids ("synthetics"), primarily fentanyl and related piperidine-based analogs. Within the past 2-3 years, a structurally unrelated class of high potency synthetics, benzimidazoles exemplified by etonitazene and isotonitazene ("iso"), have also appeared in illicit drug markets (Thompson, 2020; Ujvary et al. 2021). In 2020, it was estimated that over 80% of fatal opioid overdoses in the United States now involve synthetics (Ahmad et al., 2021). The unique physicochemical and pharmacological properties of synthetics described in this review are responsible for both the morbidity and mortality associated with their misuse as well as their widespread availability. This dramatic increase in the misuse of synthetics is often referred to as the "3rd wave" (Pardo et al., 2019; Volkow and Blanco, 2020) of the opioid epidemic. Among the consequences resulting from misuse of these potent opioids is the need for higher doses of the competitive antagonist, naloxone, to reverse an overdose. The development of more effective reversal agents such as those described in this review is an essential component of a tripartite strategy (Volkow and Collins, 2017) to reduce the biopsychosocial impact of opioid misuse in the "synthetic era".

Cyclopropyl Scaffold: A Generalist for Marketed Drugs

In recent decades, much attention has been given to cyclopropyl scaffolds, which commonly exist in natural products and synthetic organic molecules. Clinical drug molecules with cyclopropyl rings are an area of focus in therapeutic research due to their interesting chemical properties and unique pharmacology activity. These molecular drugs against different targets are applicable in some therapeutic treatment fields including cancer, infection, respiratory disorder, cardiovascular and cerebrovascular diseases, dysphrenia, nervous system disorders, endocrine and metabolic disorders, skin disease, digestive disorders, urogenital diseases, otolaryngological and dental diseases, and eye diseases. This review is a guide for pharmacologists who are in search of valid preclinical/clinical drug compounds where the progress, from 1961 to the present day, of approved marketed drugs containing cyclopropyl scaffold is examined.

Discriminative stimulus effects of carfentanil in rats discriminating fentanyl: Differential antagonism by naltrexone

BACKGROUND

A significant number of deaths caused by opioids involve fentanyl and/or one of its very potent analogs (e.g., carfentanil). Some clinical reports suggest larger doses of opioid receptor antagonists may be required to reverse the effects of carfentanil compared with other opioid receptor agonists, although this has not been examined extensively in vivo. The current study compared the discriminative stimulus effects of fentanyl, carfentanil, and heroin, and their antagonism by naltrexone.

METHODS

Eight male Sprague-Dawley rats were trained to discriminate 10.0 μg/kg fentanyl from saline while responding under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for the opioid receptor agonists fentanyl (1.0-32.0 μg/kg), carfentanil (0.1-3.2 μg/kg), and heroin (10.0-320.0 μg/kg), then redetermined following a 15-minute pretreatment with 0.1 mg/kg naltrexone.

RESULTS

Fentanyl, carfentanil, and heroin dose-dependently increased responding on the fentanyl-associated lever and decreased the rate of lever pressing. Carfentanil and heroin were approximately 10-fold more and less potent, respectively, than fentanyl at eliciting >80 % responding on the fentanyl-associated lever. Pretreatment with 0.1 mg/kg naltrexone resulted in a significant rightward shift in the fentanyl and heroin but not carfentanil dose-effect curves.

CONCLUSIONS

Differences in the effectiveness of naltrexone to attenuate the discriminative stimulus effects of carfentanil, compared with fentanyl and heroin, suggest that there may be differences in how carfentanil exerts its discriminative stimulus effects compared with other opioids. Further evaluation is needed of potential pharmacological and behavioral differences between carfentanil and other opioids, particularly in the context of toxicity.

Carfentanil - from an animal anesthetic to a deadly illicit drug

The use of novel synthetic opioids as recreational drugs has become a public health concern as they are implicated in numerous fatal intoxications across the world. Synthetic opioids have played a major role in the United States opioid crisis and may contribute to a similar opioid epidemic in Europe. The most prominent group of designer opioids consists of fentanyl and its analogues. At present, carfentanil is the most dangerous fentanyl derivative. It was recently detected as an adulterant to other illicit drugs and counterfeit pharmaceuticals, contributing to life-threatening hospital admissions and fatalities. Toxic exposure to carfentanil typically occurs through injection, insufflation or inhalation. Carfentanil produces similar pharmacotoxicological effects to other opioids. However, due to its extraordinary potency, reversing carfentanil-induced severe and recurring respiratory depression requires administration of multiple or higher than standard doses of naloxone. Toxicological reports indicate that carfentanil use is strongly connected to polydrug use. Detection of carfentanil requires specific and sensitive analytical methods that are not commonly available in hospitals. Since abuse of carfentanil is an emerging problem, particularly in the United States, there is an urgent need to develop new techniques for rapid determination of intoxication evoked by this drug as well as new treatment regimens for effective overdose maintenance. This review presents current knowledge on pharmacological activity of carfentanil, prevalence and patterns of use, and analytical methods of its detection. Special emphasis is given to carfentanil-related non-fatal and lethal overdose cases.

GluN2A-selective positive allosteric modulator-nalmefene-flumazenil reverses ketamine-fentanyl-dexmedetomidine-induced anesthesia and analgesia in rats

Anesthetics are used to produce hypnosis and analgesic effects during surgery, but anesthesia for a long time after the operation is not conducive to the recovery of animals or patients. Therefore, finding appropriate treatments to counter the effects of anesthetics could enhance postoperative recovery. In the current study, we discovered the novel role of a GluN2A-selective positive allosteric modulator (PAM) in ketamine-induced anesthesia and investigated the effects of the PAM combined with nalmefene and flumazenil (PNF) in reversing the actions of an anesthetic combination (ketamine-fentanyl-dexmedetomidine, KFD). PAM treatment dose-dependently decreased the duration of the ketamine-induced loss of righting reflex (LORR). Compared with those in the KFD group, the duration of LORR and the analgesic effect of the KFD + PNF group were obviously decreased. Meanwhile, successive administration of PNF and KFD had no adverse effects on the cardiovascular and respiratory systems. Both the KFD group and the KFD + PNF group showed no changes in hepatic and renal function or cognitive function in rats. Moreover, the recovery of motor coordination of the KFD + PNF group was faster than that of the KFD group. In summary, our results suggest the potential application of the PNF combination as an antagonistic treatment strategy for anesthesia.

Carfentanil toxicity in the African green monkey: Therapeutic efficacy of naloxone

Carfentanil is an ultra-potent opioid with an analgesic potency 10,000 times that of morphine but has received little scientific investigation. Three experiments were conducted to evaluate the toxicity of carfentanil and the efficacy of naloxone in adult male African green monkeys. The first experiment determined the ED₅₀ (found to be 0.71 μg/kg) of subcutaneous carfentanil for inducing bradypnea and/or loss of posture. Experiment 2 attempted to establish the ED₅₀ of naloxone for rapidly reversing the bradypnea/loss of posture induced by carfentanil (1.15 μg/kg). Experiment 3 evaluated the effects of carfentanil (0.575 μg/kg) alone, the safety of naloxone (71-2841 μg/kg), and the efficacy of naloxone (71-710 μg/kg) administration at two time points following carfentanil (1.15 μg/kg) on operant choice reaction time. Collectively, these experiments characterized the temporal progression of carfentanil-induced toxic signs, determined the range of naloxone doses that restored respiratory and gross behavioral function, and determined the time course and range of naloxone doses that partially or completely reversed the effects of carfentanil on operant choice reaction time performance in African green monkeys. These results have practical relevance for the selection of opioid antagonists, initial doses, and expected functional outcomes following treatment of synthetic opioid overdose in a variety of operational/emergency response contexts.

The rising crisis of illicit fentanyl use, overdose, and potential therapeutic strategies

Fentanyl is a powerful opioid anesthetic and analgesic, the use of which has caused an increasing public health threat in the United States and elsewhere. Fentanyl was initially approved and used for the treatment of moderate to severe pain, especially cancer pain. However, recent years have seen a growing concern that fentanyl and its analogs are widely synthesized in laboratories and adulterated with illicit supplies of heroin, cocaine, methamphetamine, and counterfeit pills, contributing to the exponential growth in the number of drug-related overdose deaths. This review summarizes the recent epidemic and evolution of illicit fentanyl use, its pharmacological mechanisms and side effects, and the potential clinical management and prevention of fentanyl-related overdoses. Because social, economic, and health problems that are related to the use of fentanyl and its analogs are growing, there is an urgent need to implement large-scale safe and effective harm reduction strategies to prevent fentanyl-related overdoses.

Monoclonal Antibodies for Combating Synthetic Opioid Intoxication

Opioid abuse in the United States has been declared a national crisis and is exacerbated by an inexpensive, readily available, and illicit supply of synthetic opioids. Specifically, fentanyl and related analogues such as carfentanil pose a significant danger to opioid users due to their high potency and rapid acting depression of respiration. In recent years these synthetic opioids have become the number one cause of drug-related deaths. In our research efforts to combat the public health threat posed by synthetic opioids, we have developed monoclonal antibodies (mAbs) against the fentanyl class of drugs. The mAbs were generated in hybridomas derived from mice vaccinated with a fentanyl conjugate vaccine. Guided by a surface plasmon resonance (SPR) binding assay, we selected six hybridomas that produced mAbs with 10^-11 M binding affinity for fentanyl, yet broad cross-reactivity with related fentanyl analogues. In mouse antinociception models, our lead mAb (6A4) could blunt the effects of both fentanyl and carfentanil in a dose-responsive manner. Additionally, mice pretreated with 6A4 displayed enhanced survival when subjected to fentanyl above LD₅₀ doses. Pharmacokinetic analysis revealed that the antibody sequesters large amounts of these drugs in the blood, thus reducing drug biodistribution to the brain and other tissue. Lastly, the 6A4 mAb could effectively reverse fentanyl/carfentanil-induced antinociception comparable to the opioid antagonist naloxone, the standard of care drug for treating opioid overdose. While naloxone is known for its short half-life, we found the half-life of 6A4 to be approximately 6 days in mice, thus monoclonal antibodies could theoretically be useful in preventing renarcotization events in which opioid intoxication recurs following quick metabolism of naloxone. Our results as a whole demonstrate that monoclonal antibodies could be a desirable treatment modality for synthetic opioid overdose and possibly opioid use disorder.

Evidence for nonlinear accumulation of the ultrapotent fentanyl analog, carfentanil, after systemic administration to male rats

The current opioid overdose crisis is being exacerbated by illicitly manufactured fentanyl and its analogs. Carfentanil is a fentanyl analog that is 10,000-times more potent than morphine, but limited information is available about its pharmacology. The present study had two aims: 1) to validate a method for quantifying carfentanil and its metabolite norcarfentanil in small-volume samples, and 2) to use the method for examining pharmacodynamic-pharmacokinetic relationships in rats. The analytical method involved liquid-liquid extraction of plasma samples followed by quantitation of carfentanil and norcarfentanil using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The method was validated following SWGTOX guidelines, and both analytes displayed limits of detection and quantification at 7.5 and 15 pg/mL, respectively. Male Sprague-Dawley rats fitted with jugular catheters and temperature transponders received subcutaneous carfentanil (1, 3 and 10 μg/kg) or saline. Repeated blood specimens were obtained over 8 h, along with pharmacodynamic measures including core temperature and catalepsy scores. Carfentanil produced dose-related hypothermia and catalepsy that lasted up to 8 h. Carfentanil C_max occurred at 15 min whereas metabolite C_max was at 1-2 h. Concentrations of both analytes increased in a dose-related fashion, but area-under-the-curve values were much greater than predicted after 10 μg/kg. Plasma half-life for carfentanil increased at higher doses. Our findings reveal that carfentanil produces marked hypothermia and catalepsy, which is accompanied by nonlinear accumulation of the drug at high doses. We hypothesize that impaired clearance of carfentanil in humans could contribute to life-threatening effects of this ultrapotent opioid agonist. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Fighting Fire with Fire: Development of Intranasal Nalmefene to Treat Synthetic Opioid Overdose

The dramatic rise in overdose deaths linked to synthetic opioids (e.g., fentanyl, carfentanil) may require more potent, longer-duration opiate antagonists than naloxone. Both the high affinity of nalmefene at μ opiate receptors and its long half-life led us to examine the feasibility of developing an intranasal (IN) formulation as a rescue medication that could be especially useful in treating synthetic opioid overdose. In this study, the pharmacokinetic properties of IN nalmefene were compared with an intramuscular (i.m.) injection in a cohort of healthy volunteers. Nalmefene was absorbed slowly following IN administration, with a median time to reach C_max (T_max) of 2 hours. Addition of the absorption enhancer dodecyl maltoside (Intravail, Neurelis, Inc., Encinitas, CA) reduced T_max to 0.25 hour and increased C_max by ∼2.2-fold. The pharmacokinetic properties of IN nalmefene (3 mg) formulated with dodecyl maltoside has characteristics consistent with an effective rescue medication: its onset of action is comparable to an i.m. injection of nalmefene (1.5 mg) previously approved to treat opioid overdose. Furthermore, the C_max following IN administration was ∼3-fold higher than following i.m. dosing, comparable to previously reported plasma concentrations of nalmefene observed 5 minutes following a 1-mg i.v. dose. The high affinity, very rapid onset, and long half-life (>7 hours) of IN nalmefene present distinct advantages as a rescue medication, particularly against longer-lived synthetic opioids.

Pharmacological Research as a Key Component in Mitigating the Opioid Overdose Crisis

The United States is experiencing an epidemic of opioid overdose deaths. Many of the recent fatalities are associated with illicitly manufactured fentanyl, which is being added to heroin and counterfeit pain pills. The crisis is further exacerbated by the emergence of an increasing number of novel synthetic opioids (NSOs), including various fentanyl analogs and non-fentanyl compounds that display potent agonist actions at the μ-opioid receptor. Importantly, most users are unaware of their exposure to fentanyl and NSOs. Stemming the tide of opioid-related fatalities will require a coordinated multidisciplinary response from policy makers, law enforcement personnel, first responders, treatment providers, family members, and scientists. To this end, basic research in pharmacology can contribute significantly to mitigating the crisis through efforts to characterize the biological effects of NSOs, discover more effective antidotes for overdose rescue, and develop safer medications for treating addiction and alleviating pain.

On the front lines of the opioid epidemic: Rescue by naloxone

Naloxone is a specific, high affinity opioid antagonist that has been used to treat suspected or confirmed overdose for more than 40 years. Naloxone use was initially confined to an emergency room setting, but the dramatic rise in opioid overdose events over the past two decades has, with increasing frequency, shifted naloxone use to first responders including police, emergency medical technicians, and the friends and family of overdose victims. The opioids responsible for overdose events have also evolved, from prescription opioids to heroin and most recently, very high potency synthetic opioids such as fentanyl. In 2016, synthetic opioids were linked to more overdose fatalities than either prescription opioids or heroin. In this review, I will discuss the evolution and use of naloxone products by first responders and the development of additional rescue medications in response to the unprecedented dangers posed by synthetic opioids.

Fentanyls continue to replace heroin in the drug arena: the cases of ocfentanil and carfentanil

Purpose

Ocfentanil and carfentanil are two potent synthetic opioids that are analogues of fentanyl and are actively involved in the recent fentanyl crisis. The aim of this review is to provide all the available information on these two fentanyl analogues.

Methods

All reviewed information was gathered through a detailed search of PubMed and the World Wide Web using relevant keywords.

Results

Like most of the members of the family of fentanyls, they are either sold as heroin to unsuspecting users or used extensively to lace heroin street samples. Despite the fact that ocfentanil was studied clinically in the early 1990s, it did not manage to find its place in clinical practice. On the other hand, carfentanil is mainly used today as an anesthetic agent in large animals. Ocfentanil and carfentanil are used and abused extensively, mainly in Europe and in the United States. As a result, they are the cause of some verified intoxication cases and deaths worldwide. This review provides information concerning chemistry, synthesis, prevalence, pharmacology, and toxicology, as well as the current legal status of these two fentanyl analogues. Analytical methods developed for the determination of ocfentanil and carfentanil in biological specimens and seized materials, as well as related intoxication and lethal cases are also presented.

Conclusions

Ocfentanil and carfentanil are undeniably very dangerous opioid drugs and a very serious matter of concern for public safety. The authorities should take the appropriate actions to avoid the expansion of this threat by taking proper and prompt measures.