The role of gonadal hormones in regulating opioid antinociception.

Opioids are the most prescribed drugs for the alleviation of pain. Both clinical and preclinical studies have reported strong evidence for sex-related divergence regarding opioid analgesia. There is an increasing amount of evidence indicating that gonadal hormones regulate the analgesic efficacy of opioids. This review presents an overview of the importance of gonadal steroids in modulating opioid analgesic responsiveness and focuses on elaborating what is currently known regarding the underlyingmechanism. We sought to identify the link between gonadal hormones and the effect of oipiod antinociception.

Gonadal hormones contribute to the sexual dimorphism of opioid antinociception.Generally, oestradiol is a negative modulator of opioid analgesia via both non-genomic and genomic effects.Testosterone facilitates opioid analgesia mainly through the transcriptional activities of androgen receptors.Under normal physiological conditions, progestin and oestrogen exist in parallel and have a combined effect. However, progestin alone could promote opioid analgesia by increasing the expression of opioid receptors.

Neuro-protective effect of pre-treatment with Sorghum bicolor and vitamin C on tramadol induced brain oxidative stress and anxiety-like behaviour in male albino rats.

The study focused on the neuroprotective role of Sorghum bicolor and vitamin C in the amelioration of oxidative stress and anxiety-like behavoiur induced by tramadol in male albino rats. The study design involved 7 groups and a control group with 5 male albino rats in each group. Tramadol (40 mg/kg) treatment was administered for 21 days. Tramadol 40mg/kg was administered in all groups. Pretreatment with varying doses of Sorghum bicolor and Vitamin C was done in three of the groups. Behavioral assessment of anxiety and locomotors actions of the groups were compared using Elevated Plus Maze (EPM) and Open Field Test (OFT). In conclusion, Sorghum bicolor and Vitamin C tramadol ameliorated oxidative stress and anxiety-like behaviour induced by tramadol. Pretreatment with Sorghum bicolor or vitamin C (100mg) can also reduced anxiogenic responses in male albino rats that are induced by chronic tramadol use.

Transcriptional signatures of fentanyl use in the mouse ventral tegmental area.

Synthetic opioids such as fentanyl contribute to the vast majority of opioid-related overdose deaths, but fentanyl use remains broadly understudied. Like other substances with misuse potential, opioids cause lasting molecular adaptations to brain reward circuits, including neurons in the ventral tegmental area (VTA). The VTA contains numerous cell types that play diverse roles in opioid use and relapse; however, it is unknown how fentanyl experience alters the transcriptional landscape in specific subtypes. Here, we performed single nuclei RNA sequencing to study transcriptional programs in fentanyl-experienced mice. Male and female C57/BL6 mice self-administered intravenous fentanyl (1.5 µg/kg/infusion) or saline for 10 days. After 24 h abstinence, VTA nuclei were isolated and prepared for sequencing on the 10× platform. We identified different patterns of gene expression across cell types. In dopamine neurons, we found enrichment of genes involved in growth hormone signalling. In dopamine-glutamate-GABA combinatorial neurons, and some GABA neurons, we found enrichment of genes involved in Pi3k-Akt signalling. In glutamate neurons, we found enrichment of genes involved in cholinergic signalling. We identified transcriptional regulators for the differentially expressed genes in each neuron cluster, including downregulated transcriptional repressor Bcl6, and upregulated transcription factor Tcf4. We also compared the fentanyl-induced gene expression changes identified in mouse VTA with a published rat dataset in bulk VTA, and found overlap in genes related to GABAergic signalling and extracellular matrix interaction. Together, we provide a comprehensive picture of how fentanyl self-administration alters the transcriptional landscape of the mouse VTA that serves as the foundation for future mechanistic studies.

Abnormal Morphology and Synaptogenic Signaling in Astrocytes Following Prenatal Opioid Exposure.

In recent decades, there has been a dramatic rise in the rates of children being born after in utero exposure to drugs of abuse, particularly opioids. Opioids have been shown to have detrimental effects on neurons and glia in the central nervous system (CNS), but the impact of prenatal opioid exposure (POE) on still-developing synaptic circuitry is largely unknown. Astrocytes exert a powerful influence on synaptic development, secreting factors to either promote or inhibit synapse formation and neuronal maturation in the developing CNS. Here, we investigated the effects of the partial µ-opioid receptor agonist buprenorphine on astrocyte synaptogenic signaling and morphological development in cortical cell culture. Acute buprenorphine treatment had no effect on the excitatory synapse number in astrocyte-free neuron cultures. In conditions where neurons shared culture media with astrocytes, buprenorphine attenuated the synaptogenic capabilities of astrocyte-secreted factors. Neurons cultured from drug-naïve mice showed no change in synapses when treated with factors secreted by astrocytes from POE mice. However, this same treatment was synaptogenic when applied to neurons from POE mice, indicating a complex neuroadaptive response in the event of impaired astrocyte signaling. In addition to promoting morphological and connectivity changes in neurons, POE exerted a strong influence on astrocyte development, disrupting their structural maturation and promoting the accumulation of lipid droplets (LDs), suggestive of a maladaptive stress response in the developing CNS.

Mechanism-based organization of neural networks to emulate systems biology and pharmacology models.

Deep learning neural networks are often described as black boxes, as it is difficult to trace model outputs back to model inputs due to a lack of clarity over the internal mechanisms. This is even true for those neural networks designed to emulate mechanistic models, which simply learn a mapping between the inputs and outputs of mechanistic models, ignoring the underlying processes. Using a mechanistic model studying the pharmacological interaction between opioids and naloxone as a proof-of-concept example, we demonstrated that by reorganizing the neural networks' layers to mimic the structure of the mechanistic model, it is possible to achieve better training rates and prediction accuracy relative to the previously proposed black-box neural networks, while maintaining the interpretability of the mechanistic simulations. Our framework can be used to emulate mechanistic models in a large parameter space and offers an example on the utility of increasing the interpretability of deep learning networks.

Multitarget µ-Opioid Receptor Agonists─Neuropeptide FF Receptor Antagonists Induce Potent Antinociception with Reduced Adverse Side Effects.

The design of bifunctional compounds is a promising approach toward the development of strong analgesics with reduced side effects. We here report the optimization of the previously published lead peptide KGFF09, which contains opioid receptor agonist and neuropeptide FF receptor antagonist pharmacophores and is shown to induce potent antinociception and reduced side effects. We evaluated the novel hybrid peptides for their in vitro activity at MOP, NPFFR1, and NPFFR2 and selected four of them (DP08/14/32/50) for assessment of their acute antinociceptive activity in mice. We further selected DP32 and DP50 and observed that their antinociceptive activity is mostly peripherally mediated; they produced no respiratory depression, no hyperalgesia, significantly less tolerance, and strongly attenuated withdrawal syndrome, as compared to morphine and the recently FDA-approved TRV130. Overall, these data suggest that MOP agonist/NPFF receptor antagonist hybrids might represent an interesting strategy to develop novel analgesics with reduced side effects.

Development of fentanyl-specific monoclonal antibody (mAb) to antagonize the pharmacological effects of fentanyl.

Fentanyl, a critical component of opioid analgesics, poses a severe threat to public health, exacerbating the drug problem due to its potential fatality. Herein, we present two novel haptens designed with different attachment sites conjugated to keyhole limpet hemocyanin (KLH), aiming to develop an efficacious vaccine against fentanyl. KLH-Fent-1 demonstrated superior performance over KLH-Fent-2 in antibody titer, blood-brain distribution, and antinociceptive tests. Consequently, we immunized mice with KLH-Fent-1 to generate fentanyl-specific monoclonal antibodies (mAbs) using the hybridoma technique to compensate for the defects of active immunization in the treatment of opioid overdose and addiction. The mAb produced by hybridoma 9D5 exhibited the ability to recognize fentanyl and its analogs with a binding affinity of 10-10 M. Subsequently, we developed a human IgG1 chimeric mAb to improve the degree of humanization. Pre-treatment with murine and chimeric mAb significantly reduced the analgesic effect of fentanyl and altered its blood-brain biodistribution in vivo. Furthermore, in a mouse model of fentanyl-induced respiratory depression, the chimeric mAb effectively reversed respiratory depression promptly and maintained a certain level during the week. The development of high-affinity chimeric mAb gives support to combat the challenges of fentanyl misuse and its detrimental consequences. In conclusion, mAb passive immunization represents a viable strategy for addressing fentanyl addiction and overdose.

The role of morphine- and fentanyl-induced impairment of intestinal epithelial antibacterial activity in dysbiosis and its impact on the microbiota-gut-brain axis.

Recent evidence suggests that chronic exposure to opioid analgesics such as morphine disrupts the intestinal epithelial layer and causes intestinal dysbiosis. Depleting gut bacteria can preclude the development of tolerance to opioid-induced antinociception, suggesting an important role of the gut-brain axis in mediating opioid effects. The mechanism underlying opioid-induced dysbiosis, however, remains unclear. Host-produced antimicrobial peptides (AMPs) are critical for the integrity of the intestinal epithelial barrier as they prevent the pathogenesis of the enteric microbiota. Here, we report that chronic morphine or fentanyl exposure reduces the antimicrobial activity in the ileum, resulting in changes in the composition of bacteria. Fecal samples from morphine-treated mice had increased levels of Akkermansia muciniphila with a shift in the abundance ratio of Firmicutes and Bacteroidetes. Fecal microbial transplant (FMT) from morphine-naïve mice or oral supplementation with butyrate restored (a) the antimicrobial activity, (b) the expression of the antimicrobial peptide, Reg3γ, (c) prevented the increase in intestinal permeability and (d) prevented the development of antinociceptive tolerance in morphine-dependent mice. Improved epithelial barrier function with FMT or butyrate prevented the enrichment of the mucin-degrading A. muciniphila in morphine-dependent mice. These data implicate impairment of the antimicrobial activity of the intestinal epithelium as a mechanism by which opioids disrupt the microbiota-gut-brain axis.

Co-administration of naloxone and dexmedetomidine to simultaneously reverse acute effects of fentanyl and methamphetamine in rats.

BACKGROUND: The incidence of combination methamphetamine (METH)-opioid overdose has substantially increased in recent years. While agitation is uncommon after the naloxone (NLX) reversal of opioids, it is a major clinical concern in acute METH intoxication and can be physiologically antagonized by opioid-induced sedation. This study aimed to perform initial preclinical analysis of the safety and efficacy of dexmedetomidine (DEXMED) co-administered with NLX to attenuate METH-induced locomotor activity, as a rat model of agitation, after the reversal of fentanyl (FENT)-induced sedation. METHODS: Male Sprague Dawley rats were administered subcutaneous (SC) 0.1mg/kg FENT ± 1mg/kg METH. Fifteen min later, SC 0.1mg/kg NLX ± an increasing (0, 0.032, 0.056, and 0.1mg/kg) DEXMED dose was administered prior to the measurement of locomotor activity. After a washout period, the FENT ± METH and NLX ± DEXMED administration with the highest dose of DEXMED was administered for measurement of blood oxygen saturation and heart rate. RESULTS: After the NLX reversal of FENT-induced sedation, adjunct DEXMED substantially and significantly reduced METH-induced locomotor activity (p<0.05) at all doses tested. While the addition of DEXMED did not significantly reduce blood oxygenation in METH treated rats, it did so in the absence of METH. Also, DEXMED significantly reduced heart rate compared to non-DEXMED treated groups and resulted in further significant reductions in the animals not exposed to METH (p<0.05). CONCLUSIONS: These data provide preclinical evidence that DEXMED may be a safe and effective chemical restraint for METH-induced agitation after NLX opioid reversal.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation.

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Fentanyl Analogs Exert Antinociceptive Effects via Sodium Channel Blockade in Mice.

The formalin test is one approach to studying acute pain in rodents. Similar to formalin, injection with glutamate and veratrine can also produce a nociceptive response. This study investigated whether opioid-related compounds could suppress glutamate- and veratrine-induced nociceptive responses in mice at the same dose. The administration of morphine (3 mg/kg), hydromorphone (0.4 mg/kg), or fentanyl (0.03 mg/kg) suppressed glutamate-induced nociceptive response, but not veratrine-induced nociceptive response at the same doses. However, high doses of morphine (10 mg/kg), hydromorphone (2 mg/kg), or fentanyl (0.1 mg/kg) produced a significant reduction in the veratrine-induced nociceptive response. These results indicate that high doses are required when using morphine, hydromorphone, or fentanyl for sodium channel-related neuropathic pain, such as ectopic activity. As a result, concerns have arisen about overdose and abuse if the dose of opioids is steadily increased to relieve pain. In contrast, trimebutine (100 mg/kg) and fentanyl analog isobutyrylfentanyl (iBF; 0.1 mg/kg) suppressed both glutamate- and veratrine-induced nociceptive response. Furthermore, nor-isobutyrylfentanyl (nor-iBF; 1 mg/kg), which is a metabolite of iBF, suppressed veratrine-induced nociceptive response. Besides, the optimal antinociceptive dose of iBF, unlike fentanyl, only slightly increased locomotor activity and did not slow gastrointestinal transit. Cancer pain is a complex condition driven by inflammatory, neuropathic, and cancer-specific mechanisms. Thus, iBF may have the potential to be a superior analgesic than fentanyl.

ß-caryophyllene inhibits heroin self-administration, but does not alter opioid-induced antinociception in rodents.

A growing body of research indicates that ß-caryophyllene (BCP), a constituent present in a large number of plants, possesses significant therapeutic properties against CNS disorders, including alcohol and psychostimulant use disorders. However, it is unknown whether BCP has similar therapeutic potential for opioid use disorders. In this study, we found that systemic administration of BCP dose-dependently reduced heroin self-administration in rats under an FR2 schedule of reinforcement and partially blocked heroin-enhanced brain stimulation reward in DAT-cre mice, maintained by optical stimulation of midbrain dopamine neurons at high frequencies. Acute administration of BCP failed to block heroin conditioned place preference (CPP) in male mice, but attenuated heroin-induced CPP in females. Furthermore, repeated dosing with BCP for 5 days facilitated the extinction of CPP in female but not male mice. In the hot plate assay, pretreatment with the same doses of BCP failed to enhance or prolong opioid antinociception. Lastly, in a substitution test, BCP replacement for heroin failed to maintain intravenous BCP self-administration, suggesting that BCP itself has no reinforcing properties. These findings suggest that BCP may have certain therapeutic effects against opioid use disorders with fewer unwanted side-effects by itself.

Analgesic Peptides: From Natural Diversity to Rational Design.

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide.

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

Genetic background and sex influence somatosensory sensitivity and oxycodone analgesia in the Hybrid Rat Diversity Panel.

Opioid use disorder (OUD) is an ongoing public health concern in the United States, and relatively little work has addressed how genetic background contributes to OUD. Understanding the genetic contributions to oxycodone-induced analgesia could provide insight into the early stages of OUD development. Here, we present findings from a behavioral phenotyping protocol using several inbred strains from the Hybrid Rat Diversity Panel. Our behavioral protocol included a modified "up-down" von Frey procedure to measure inherent strain differences in the sensitivity to a mechanical stimulus on the hindpaw. We also performed the tail immersion assay, which measures the latency to display tail withdrawal in response to a hot water bath. Initial withdrawal thresholds were taken in drug-naïve animals to record baseline thermal sensitivity across the strains. Oxycodone-induced analgesia was measured after administration of oxycodone over the course of 2 h. Both mechanical and thermal sensitivity are shaped by genetic factors and display moderate heritability (h2 = 0.23-0.40). All strains displayed oxycodone-induced analgesia that peaked at 15-30 min and returned to baseline by 2 h. There were significant differences between the strains in the magnitude and duration of their analgesic response to oxycodone, although the heritability estimates were quite modest (h2 = 0.10-0.15). These data demonstrate that genetic background confers differences in mechanical sensitivity, thermal sensitivity, and oxycodone-induced analgesia.

The Role of Opioid Receptor Antagonists in Regulation of Blood Pressure and T-Cell Activation in Mice Selected for High Analgesia Induced by Swim Stress.

Opioid peptides and their G protein-coupled receptors are important regulators within the cardiovascular system, implicated in the modulation of both heart and vascular functions. It is known that naloxone-an opioid antagonist-may exert a hypertensive effect. Recent experimental and clinical evidence supports the important role of inflammatory mechanisms in hypertension. Since opioids may play a role in the regulation of both blood pressure and immune response, we studied these two processes in our model. We aimed to evaluate the effect of selective and non-selective opioid receptor antagonists on blood pressure and T-cell activation in a mouse model of high swim stress-induced analgesia. Blood pressure was measured before and during the infusion of opioid receptor antagonists using a non-invasive tail-cuff measurement system. To assess the activation of T-cells, flow cytometry was used. We discovered that the non-selective antagonism of the opioid system by naloxone caused a significant elevation of blood pressure. The selective antagonism of µ and κ but not δ opioid receptors significantly increased systolic blood pressure. Subsequently, a brief characterization of T-cell subsets was performed. We found that the blockade of µ and δ receptors is associated with the increased expression of CD69 on CD4 T-cells. Moreover, we observed an increase in the central memory CD4 and central memory CD8 T-cell populations after the δ opioid receptor blockade. The antagonism of the µ opioid receptor increased the CD8 effector and central memory T-cell populations.

Paradoxical potentiation of the exercise pressor reflex by endomorphin 2 in the presence of naloxone.

When contracting muscles are freely perfused, the acid-sensing ion channel 3 (ASIC3) on group IV afferents plays a minor role in evoking the exercise pressor reflex. We recently showed in isolated dorsal root ganglion neurons innervating the gastrocnemius muscles that two mu opioid receptor agonists, namely endomorphin 2 and oxycodone, potentiated the sustained inward ASIC3 current evoked by acidic solutions. This in vitro finding prompted us to determine whether endomorphin 2 and oxycodone, when infused into the arterial supply of freely perfused contracting hindlimb muscles, potentiated the exercise pressor reflex. We found that infusion of endomorphin 2 and naloxone in decerebrated rats potentiated the pressor responses to contraction of the triceps surae muscles. The endomorphin 2-induced potentiation of the pressor responses to contraction was prevented by infusion of APETx2, an ASIC3 antagonist. Specifically, the peak pressor response to contraction averaged 19.3 ± 5.6 mmHg for control (n = 10), 27.2 ± 8.1 mmHg after naloxone and endomorphin 2 infusion (n = 10), and 20 ± 8 mmHg after APETx2 and endomorphin 2 infusion (n = 10). Infusion of endomorphin 2 and naloxone did not potentiate the pressor responses to contraction in ASIC3 knockout rats (n = 6). Partly similar findings were observed when oxycodone was substituted for endomorphin 2. Oxycodone infusion significantly increased the exercise pressor reflex over its control level, but subsequent APETx2 infusion failed to restore the increase to its control level (n = 9). The peak pressor response averaged 23.1 ± 8.6 mmHg for control (n = 9), 33.2 ± 11 mmHg after naloxone and oxycodone were infused (n = 9), and 27 ± 8.6 mmHg after APETx2 and oxycodone were infused (n = 9). Our data suggest that after opioid receptor blockade, ASIC3 stimulation by the endogenous mu opioid, endomorphin 2, potentiated the exercise pressor reflex.NEW & NOTEWORTHY This paper provides the first in vivo evidence that endomorphin 2, an endogenous opioid peptide, can paradoxically increase the magnitude of the exercise pressor reflex by an ASIC3-dependent mechanism even when the contracting muscles are freely perfused.

Pharmacologic Characterization of Substituted Nitazenes at µ, κ, and Δ Opioid Receptors Suggests High Potential for Toxicity.

The benzimidazole opioids (substituted nitazenes) are highly potent µ opiod receptor (MOR) agonists with heroin- or fentanyl-like effects. These compounds have caused hospitalizations and fatal overdoses. We characterized the in vitro pharmacology and structure-activity relationships of 19 nitazenes with substitutions at three positions of the benzimidazole core. Affinities were assessed using agonist radioligand binding assays at human µ, κ, and Δ opioid receptors (MOR, KOR, and DOR, respectively) heterologously expressed in CHO cells. Notably, for MOR binding, nine substituted nitazenes had significantly higher affinities than fentanyl including N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, and N-desethyl isotonitazene; 13 had subnanomolar affinities. Only metodesnitazene and flunitazene had significantly lower affinities than fentanyl. Affinities for the substituted nitazenes at KOR and DOR relative to MOR were 46- to 2580-fold and 180- to 1280-fold lower, respectively. Functional activities were assessed using [35S]GTPγS binding assays. Four nitazenes had subnanomolar potencies at MOR: N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, N-pyrrilidino protonitazene and N-desethyl isotonitazene. Ten substituted nitazenes had significantly higher potencies than fentanyl. All tested nitazenes were full MOR agonists. Potencies at KOR and DOR relative to MOR were 7.3- to 7920-fold and 24- to 9400-fold lower, respectively. Thus, many of these compounds are high affinity/high potency MOR agonists with elevated potential to elicit toxicity and overdose at low doses. SIGNIFICANCE STATEMENT: Substituted nitazenes are a growing public health threat. Although the 19 nitazenes tested vary in their opioid receptor pharmacology, a number are very high affinity, high potency, and high efficacy compounds- higher than fentanyl. Their pharmacology suggests high potential for harm.

Clinical Pharmacokinetics and Pharmacodynamics of Naloxone.

Naloxone is a World Health Organization (WHO)-listed essential medicine and is the first choice for treating the respiratory depression of opioids, also by lay-people witnessing an opioid overdose. Naloxone acts by competitive displacement of opioid agonists at the µ-opioid receptor (MOR). Its effect depends on pharmacological characteristics of the opioid agonist, such as dissociation rate from the MOR receptor and constitution of the victim. Aim of treatment is a balancing act between restoration of respiration (not consciousness) and avoidance of withdrawal, achieved by titration to response after initial doses of 0.4-2 mg. Naloxone is rapidly eliminated [half-life (t1/2) 60-120 min] due to high clearance. Metabolites are inactive. Major routes for administration are intravenous, intramuscular, and intranasal, the latter primarily for take-home naloxone. Nasal bioavailability is about 50%. Nasal uptake [mean time to maximum concentration (Tmax) 15-30 min] is likely slower than intramuscular, as reversal of respiration lag behind intramuscular naloxone in overdose victims. The intraindividual, interindividual and between-study variability in pharmacokinetics in volunteers are large. Variability in the target population is unknown. The duration of action of 1 mg intravenous (IV) is 2 h, possibly longer by intramuscular and intranasal administration. Initial parenteral doses of 0.4-0.8 mg are usually sufficient to restore breathing after heroin overdose. Fentanyl overdoses likely require higher doses of naloxone. Controlled clinical trials are feasible in opioid overdose but are absent in cohorts with synthetic opioids. Modeling studies provide valuable insight in pharmacotherapy but cannot replace clinical trials. Laypeople should always have access to at least two dose kits for their interim intervention.