Reports of gabapentin misuse among people with opioid use disorder have increased, as has the presence of gabapentin in opioid overdose deaths. Despite these trends and known interactions between these drugs in other settings (e.g. gabapentin as an 'opioid-sparing' agent), few studies have evaluated the impact of gabapentin on reinforcing effects of opioids. Intravenous self-administration in rats was used to determine whether gabapentin modifies the reinforcing effects of heroin or cocaine. Rats self-administered drugs under a progressive ratio schedule of reinforcement after saline or gabapentin pretreatment. Pretreatment with 32 mg/kg gabapentin increased the number of infusions earned on the ascending limb of the heroin dose-response curve but had no effect on cocaine self-administration. While the magnitude of effect was modest, these data suggest that gabapentin can potentiate the reinforcing effects of opioids and lays the groundwork for future studies characterizing interactions between gabapentinoids and opioids in the context of reinforcement.

Naloxone and nalmefene are μ opioid receptor (MOR) antagonist medications for reversing opioid overdose and naltrexone is a MOR antagonist medication for treating opioid use disorder (preventing relapse). MOR agonist medications for treating opioid use disorder include methadone and buprenorphine. The magnitude of effect of buprenorphine can be less than the effect of higher efficacy MOR agonists with some effects of buprenorphine (eg, ventilatory depression) resistant to reversal by naloxone, compared with reversal of the effects of other MOR agonists. Drug discrimination was used to compare the ability of naloxone, naltrexone, nalmefene, and methocinnamox (MCAM) to prevent and reverse the effects of buprenorphine and the ultrapotent fentanyl analog carfentanil in male and female rats discriminating fentanyl from saline. Naloxone, naltrexone, nalmefene, and MCAM (0.01-0.1 mg/kg) were equipotent at preventing the discriminative stimulus effects of 0.01 mg/kg buprenorphine and 0.0001 mg/kg carfentanil. Compared with their potencies to prevent discriminative stimulus effects, naloxone, naltrexone, and nalmefene were ≥92-fold less potent in reversing the discriminative stimulus effects of buprenorphine and ≥31-fold less potent in reversing the discriminative stimulus effects of carfentanil. In contrast, MCAM was equipotent in preventing and reversing the discriminative stimulus effects of buprenorphine and carfentanil. There was no significant difference in the onset of action of naloxone, naltrexone, nalmefene, or MCAM. The greater potency of MCAM, compared with naloxone, naltrexone, and nalmefene for reversing the effects of buprenorphine and carfentanil might translate to MCAM reversing adverse effects of a wide range of MOR agonists in patients. SIGNIFICANCE STATEMENT: Buprenorphine, a μ opioid receptor (MOR) agonist used to treat opioid use disorder, can produce adverse effects that are reportedly more difficult to reverse with naloxone, compared with reversal of the effects of other MOR agonists. This study provides evidence for the potential utility of the MOR antagonist methocinnamox to reverse the adverse effects of buprenorphine by demonstrating the relatively greater potency of methocinnamox, compared with currently available MOR antagonists, to reverse the discriminative stimulus effects of buprenorphine and carfentanil.

Approximately 70 % of fatal drug overdoses in the United States are attributed to fentanyl and fentanyl analogs. Current medications for reversing overdose and treating opioid use disorder might not be as effective against fentanyl and fentanyl analogs compared with other opioids, possibly due to their lipophilicity and high potency at the mu-opioid receptor (MOR). Hence, fentanyl and fentanyl analog-targeting monoclonal antibodies (mAb) could be an alternative treatment. The humanized (h) mAb hHY6-F9 has high relative affinity for fentanyl and decreases intravenous (i.v.) fentanyl self-administration in monkeys. hHY6-F9 has lower affinity for fentanyl analogs, including carfentanil; however, the effects of hHY6-F9 on fentanyl analogs in vivo have not been characterized. This study examined the effects of hHY6-F9 on i.v. carfentanil self-administration. hHY6-F9 was administered to two male rhesus monkeys self-administering carfentanil, heroin, cocaine, or fentanyl during twice daily sessions. Based on prior in vitro and in vivo findings, hHY6-F9 was hypothesized to attenuate fentanyl but not carfentanil, heroin, or cocaine self-administration. However, hHY6-F9 significantly decreased carfentanil self-administration for up to 5 weeks while having little or no effect on heroin, cocaine, or fentanyl self-administration. A cell-based pharmacological assay of carfentanil-induced MOR activation supported the carfentanil self-administration findings, showing that murine HY6-F9 reduced the effects of carfentanil. The ability of hHY6-F9 to attenuate the effects of an ultra-potent fentanyl analog could be advantageous for treating opioid use disorder or overdose given the unpredictability of the unregulated opioid supply.

Fentanyl and its analogs continue to drive the overdose crisis in the United States. It is unclear whether there are unique properties of fentanyls that increase the risk of opioid overdose compared with other opioid receptor agonists (e.g., heroin). This study compared the ventilatory depressant effects of the opioid receptor agonists heroin, fentanyl, and carfentanil, and reversal of those effects by opioid receptor antagonists. This study used whole-body plethysmography in rats to determine the ventilatory depressant effects of heroin (178-1780 μg/kg), fentanyl (5.6-56 μg/kg), and carfentanil (0.56-5.6 μg/kg) when administered alone and in mixtures, to compare the profile of effects across drugs and determine whether there are significant interactions between the drugs when co-administered. The potencies of the opioid receptor antagonists naloxone and diprenorphine to reverse opioid-induced ventilatory depression were compared across opioid agonists. All three agonists reduced minute volume with carfentanil being ∼50- and 100-fold more potent than fentanyl and heroin, respectively. The profile of effects on ventilation did not differ across agonists and no significant drug-drug interactions were detected. Naloxone and diprenorphine dose-dependently reversed the ventilatory depressant effects of each opioid agonist. Both antagonists were less potent at reversing the effects of carfentanil compared with reversing a functionally equivalent dose of heroin. These findings suggest that fentanyl might not produce unique effects on breathing that increase risk of overdose. That naloxone and diprenorphine were less potent to reverse the effects of carfentanil compared with an equivalent dose of heroin is consistent with existing literature and emphasizes the need for continued evaluation of potential differences in pharmacological properties across opioid receptor agonists.

During the opioid epidemic there has been a significant increase in the number of prescriptions for gabapentinoids (gabapentin and pregabalin), which block the α2δ-subunit of voltage-gated calcium channels (VGCCs). In rats, gabapentinoids enhance the potency of the mu opioid receptor (MOR) agonists fentanyl and heroin and decrease the potency of the stimulant drugs cocaine and d-methamphetamine to elicit discriminative stimulus effects. Moreover, (±)-BAY-K-8644, a dihydropyridine-type agonist at the α1-subunit of VGCCs (α1-VGCCs), prevents the antiallodynic effects of gabapentin in rats. The mechanism(s) of interaction between gabapentinoids and MOR agonists and between gabapentinoids and stimulant drugs is/are unclear. This study tested the following hypotheses: (1) the dihydropyridine-type α1-VGCC blocker nimodipine increases the potency of MOR agonists to elicit discriminative stimulus effects; and (2) (±)-BAY-K-8644 attenuates the ability of gabapentinoids and nimodipine to increase the potency of MOR agonists to elicit discriminative stimulus effects. In rats trained to discriminate fentanyl (0.0032 mg/kg) or cocaine (3.2 mg/kg) from saline, neither (±)-BAY-K-8644 nor nimodipine elicited significant fentanyl- or cocaine-appropriate responding. (±)-BAY-K-8644 did not significantly alter discrimination dose-effect functions of MOR agonists or stimulant drugs whereas nimodipine dose-dependently shifted the MOR agonist discrimination dose-effect functions to the left and the stimulant drug discrimination dose-effect functions to the right. (±)-BAY-K-8644 dose-dependently attenuated the ability of nimodipine and gabapentinoids to increase the potency of MOR agonists and decrease the potency of stimulant drugs to elicit discriminative stimulus effects. These results suggest that gabapentinoids alter the potency of MOR agonists and stimulant drugs to elicit discriminative stimulus effects via blockade of α1-VGCCs. SIGNIFICANCE STATEMENT: Prescriptions for gabapentinoids (gabapentin and pregabalin) increased significantly during the opioid epidemic. Using (±)-BAY-K-8644, a dihydropyridine-type agonist at the α1-subunit of the voltage-gated calcium channel (α1-VGCC), this study reports that in male and female rats, gabapentinoids increase the potency of fentanyl and heroin and decrease the potency of cocaine and d-methamphetamine to elicit discriminative stimulus effects via blockade of dihydropyridine-binding sites on α1-VGCC. These results suggest that actions of drugs at α1-VGCCs contribute to the opioid epidemic and opioid/stimulant co-use.

During the opioid epidemic there has been a significant increase in the number of prescriptions for gabapentinoids (gabapentin and pregabalin), which block the a2d-subunit of voltage-gated calcium channels (VGCCs). In rats, gabapentinoids enhance the potency of the mu opioid receptor (MOR) agonists fentanyl and heroin and decrease the potency of the stimulant drugs cocaine and d-methamphetamine to elicit discriminative stimulus effects. Moreover, (+/-)-BAY-K-8644, a dihydropyridine-type agonist at the a1-subunit of VGCCs (a1-VGCCs), prevents the antiallodynic effects of gabapentin in rats. The mechanism(s) of interaction between gabapentinoids and MOR agonists and between gabapentinoids and stimulant drugs is/are unclear. This study tested the following hypotheses: (1) the dihydropyridine-type a1-VGCC blocker nimodipine increases the potency of MOR agonists to elicit discriminative stimulus effects; and (2) (+/-)-BAYK-8644 attenuates the ability of gabapentinoids and nimodipine to increase the potency of MOR agonists to elicit discriminative stimulus effects. In rats trained to discriminate fentanyl (0.0032 mg/kg) or cocaine (3.2 mg/kg) from saline, neither (+/-)-BAY-K-8644 nor nimodipine elicited significant fentanyl-or cocaine-appropriate responding. (+/-)-BAY-K-8644 did not significantly alter discrimination dose-effect functions of MOR agonists or stimulant drugs whereas nimodipine dose-dependently shifted the MOR agonist discrimination dose-effect functions to the left and the stimulant drug discrimination dose-effect functions to the right. (+/-)-BAY-K-8644 dose-dependently attenuated the ability of nimodipine and gabapentinoids to increase the potency of MOR agonists and decrease the potency of stimulant drugs to elicit discriminative stimulus effects. These results suggest that gabapentinoids alter the potency of MOR agonists and stimulant drugs to elicit discriminative stimulus effects via blockade of a1-VGCCs. Significance Statement: Prescriptions for gabapentinoids (gabapentin and pregabalin) increased significantly during the opioid epidemic. Using (+/-)-BAY-K-8644, a dihydropyridine-type agonist at the a1-subunit of the voltage-gated calcium channel (a1-VGCC), this study reports that in male and female rats, gabapentinoids increase the potency of fentanyl and heroin and decrease the potency of cocaine and d-methamphetamine to elicit discriminative stimulus effects via blockade of dihydropyridine-binding sites on a1-VGCC. These results suggest that actions of drugs at a1-VGCCs contribute to the opioid epidemic and opioid/stimulant co-use. (c) 2025 American Society for Pharmacology and Experimental Therapeutics. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Medications for opioid use disorder (OUD) and overdose have been available for decades, yet nearly 70

The mu-opioid receptor agonists methadone and buprenorphine are effective for treating opioid use disorder (OUD); however, both drugs are diverted and misused and withdrawal signs can emerge when treatment is tapered or discontinued. Mu-opioid and cannabinoid type 1 receptors are colocalized in several brain regions, and cannabinoids have been proposed as potential treatments for opioid withdrawal. This study tested Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), alone and in mixtures, for treating the behavioral and physiological signs of morphine withdrawal in 3 male rhesus monkeys. The α2-adrenergic receptor agonist lofexidine was also tested. Monkeys received escalating doses of morphine up to 3.2 mg/kg twice daily. After at least 2 weeks of morphine treatment, saline was substituted for morphine for 2 days. Behavioral and physiological signs of opioid withdrawal, including blood pressure, heart rate, body temperature, and activity were measured before and after administration of THC (0.32-1.0 mg/kg), CBD (10-17.8 mg/kg), mixtures of THC (0.32 mg/kg) and CBD (10-17.8 mg/kg), lofexidine (0.032-0.32 mg/kg), or vehicle. Discontinuing morphine treatment markedly increased unusual tongue movements, a characteristic behavioral sign of opioid withdrawal in monkeys, and all physiological signs. The largest THC dose (1.0 mg/kg) decreased unusual tongue movements and heart rate, and the largest lofexidine dose (0.32 mg/kg) decreased unusual tongue movements, blood pressure, heart rate, and activity. CBD alone or with THC had no significant effect. These data demonstrate that THC attenuates some signs of opioid withdrawal; however, THC was not more effective than the currently available medication lofexidine. SIGNIFICANCE STATEMENT: Δ9-Tetrahydrocannabinol (THC) alone, but not cannabidiol either alone or in combination with THC, decreased some behavioral and physiological signs of opioid withdrawal in monkeys. However, those effects of THC were not greater than the effects of the currently available nonopioid medication lofexidine.

The number of drug overdoses and deaths has increased significantly over the past decade and co-use of opioids and stimulants is associated with greater likelihood of overdose and decreased likelihood of accessing treatment, compared with use of opioids alone. Potential adverse effects of opioid/stimulant mixtures, particularly methamphetamine, are not well characterized. Two structurally different drugs with agonist properties at µ-opioid receptors (MOR), fentanyl and heroin, and d-methamphetamine, alone and in mixtures, were assessed for their effects on ventilation in rats breathing normal air. Whole-body phethysmography chambers were equipped with a tower and swivel allowing infusions to indwelling intravenous catheters. After a 45-minute habituation period, saline, fentanyl, heroin, or d-methamphetamine, alone and in mixtures, was administered. Five minutes later, the opioid receptor antagonist naloxone or vehicle was injected. Fentanyl (0.0032–0.1 mg/kg) and heroin (0.32–3.2 mg/kg) decreased ventilation [frequency (f) and tidal volume (VT)] in a dose-related manner whereas d-methamphetamine (0.1–3.2 mg/kg) increased f to >400% of control and decreased VT to <60% of control, overall increasing minute volume (product of f and VT) to >240% of control. When combined, d-methamphetamine (0.1–3.2 mg/kg) attenuated the ventilatory depressant effects of fentanyl (0.1 mg/kg) and heroin (3.2 mg/kg). d-Methamphetamine did not alter the potency of naloxone to reverse the ventilatory depressant effects of fentanyl or heroin. These studies demonstrate that d-methamphetamine can attenuate the ventilatory depressant effects of moderate doses of opioid receptor agonists while not altering the potency of naloxone to reverse opioid hypoventilation. SIGNIFICANCE STATEMENT Co-use of opioids and stimulants is associated with greater likelihood of overdose and decreased likelihood of accessing treatment, compared with use of opioids alone. Potential adverse effects of opioid/stimulant mixtures are not well characterized. This study reports that 1) d-methamphetamine attenuates the ventilatory depressant effects of moderate doses of two structurally different opioid receptor agonists, fentanyl and heroin, and 2) d-methamphetamine does not alter potency or effectiveness of naloxone to reverse the ventilatory depressant effects of these opioid receptor agonists.

The number of opioid overdose deaths has increased significantly over the past decade. The life-threatening effect of opioids is hypoventilation, which can be reversed by the μ-opioid receptor (MOR) antagonist naloxone; however, because of the very short duration of action of naloxone, reemergence of MOR agonist-induced hypoventilation can occur, requiring additional doses of naloxone. The MOR antagonist methocinnamox (MCAM) antagonizes hypoventilation by the nonmorphinan fentanyl and the morphinan heroin in laboratory animals with an unusually long duration of action. Whole-body plethysmography was used to compare the potency and effectiveness of MCAM and naloxone for preventing and reversing hypoventilation by fentanyl, heroin, and the ultrapotent and longer-acting fentanyl analogs carfentanil and 3-methylfentanyl in male rats breathing normal air. Sessions comprised a 45-minute habituation period followed by intravenous administration of saline or an acute dose of MOR agonist. The rank order of potency to decrease ventilation was 3-methylfentanyl > carfentanil > fentanyl > heroin. MCAM (0.0001-0.1 mg/kg) and naloxone (0.0001-0.01 mg/kg) dose dependently reversed hypoventilation by 3-methylfentanyl (0.01 mg/kg), carfentanil (0.01 mg/kg), fentanyl (0.1 mg/kg), or heroin (3.2 mg/kg). For prevention studies, MCAM, naloxone, or vehicle was administered intravenously 22, 46, or 70 hours prior to a MOR agonist. When administered 22 hours earlier, MCAM (0.1-1.0 mg/kg) but not naloxone (1.0 mg/kg) prevented hypoventilation by each MOR agonist. This study demonstrates the effectiveness of MCAM at reversing and preventing hypoventilation by MOR agonists including ultrapotent fentanyl analogs that have a long duration of action. SIGNIFICANCE STATEMENT: The number of opioid overdose deaths increased over the past decade despite the availability of antagonists that can prevent and reverse the effects of opioids. This study demonstrates the effectiveness and long duration of action of the μ-opioid receptor (MOR) antagonist methocinnamox (MCAM) for reversing and preventing hypoventilation by MOR agonists including ultrapotent fentanyl analogs. These results provide support for the notion that MCAM has the potential to positively impact the ongoing opioid crisis by reversing and preventing opioid overdose.

Abstract ID 90672Poster Board 101Nearly 70% of drug overdose deaths in 2022 involved fentanyl or fentanyl analogs. Due to a significant increase in the presence of fentanyls in the opioid and non-opioid drug supply (e.g., stimulants, illicitly manufactured prescription pain pills), both opioid and non-opioid preferring drug users are at increased risk of potentially fatal overdose. The mu-opioid receptor antagonists naloxone and nalmefene are used to reverse overdose. Vivitrol®, a long-acting formulation of the opioid antagonist naltrexone, can prevent overdose; however, nonselectively inhibiting the actions of opioid agonists precludes the use of agonist maintenance therapies and some pain medications. In contrast, highly selective monoclonal antibodies (mAbs) are an alternative for preventing fentanyl-induced overdose. Fentanyl-targeting mAbs are designed to bind and sequester fentanyl in the serum and other organs. Their high degree of selectivity could permit co-administration of other opioid agonist medications. In rats, a promising fentanyl-targeting mAb reversed fentanyl-induced hypoventilation, bradycardia, and antinociception as effectively as naloxone. To extend these findings, the present study characterized the utility of the mAb to reverse and prevent fentanyl-induced hypoventilation in rhesus monkeys. Baseline ventilation was measured over a 30-min habituation period and 45-min test session using head plethysmography. In an initial study, the smallest dose of fentanyl that reliably decreased minute volume (VE) to <70% of the within-session baseline was determined in four monkeys (1 male, 3 females). In subsequent sessions, naloxone, mAb, or vehicle (saline or phosphate-buffered saline) was administered after fentanyl. The mAb was as effective as naloxone in restoring VE to baseline. To examine the magnitude and duration of protection, a cumulative dosing procedure was used to determine a within-session fentanyl dose-effect curve in five monkeys (2 males, 3 females). The dose of fentanyl increased in 0.25 or 0.5 log units up to a dose that decreased VE to <70% of the within-session baseline (maximum, 3 doses/session). Once the dose-effect curve was stable between sessions, the mAb was administered. Post-treatment, larger doses of fentanyl were studied, and the dose sequence was adjusted as sensitivity to fentanyl recovered. The time-course of protection was examined by determining the dose-effect curve 24 hours post-treatment, and then weekly, until the fentanyl ED70 value returned to the pre-treatment baseline. In four monkeys, there was a 3.5 (± 0.64 SD)-fold rightward shift in the fentanyl dose-effect curve 24 hours post-treatment. A 13.6-fold rightward shift was observed in the fifth monkey, who was the most sensitive to fentanyl prior to treatment (ED70 = 0.91 μg/kg). The post-treatment ED70 values slowly decreased over time; taking approximately four weeks to return to baseline in one monkey, and three weeks in the others. These findings support the potential clinical utility of the mAb as a reversal agent and long-acting preventative medication for fentanyl-induced hypoventilation.These studies are supported by USPHS grants U01DA051658 (MP), T32DA031115 (CPF), and the Welch Foundation (AQ-0039 [CPF]).

Cannabis is a pharmacologically complex plant consisting of hundreds of potentially active compounds. One class of compounds present in cannabis that has received little attention are terpenes. Traditionally thought to impart aroma and flavor to cannabis, it has become increasingly recognized that terpenes might exert therapeutic effects themselves. Several recent reports have also indicated terpenes might behave as cannabinoid type 1 (CB1) receptor agonists. This study aimed to investigate whether several terpenes present in cannabis produce discriminative stimulus effects similar to or enhance the effects of Δ9-tetrahydrocannabinol (THC). Subsequent experiments explored other potential cannabimimetic effects of these terpenes. Rats were trained to discriminate THC from vehicle while responding under a fixed-ratio 10 schedule of food presentation. Substitution testing was performed with the CB receptor agonist JWH-018 and the terpenes linalool, limonene, γ-terpinene and α-humulene alone. Terpenes were also studied in combination with THC. Finally, THC and terpenes were tested in the tetrad assay to screen for CB1-receptor agonist-like effects. THC and JWH-018 dose-dependently produced responding on the THC-paired lever. When administered alone, none of the terpenes produced responding predominantly on the THC-paired lever. When administered in combination with THC, none of the terpenes enhanced the potency of THC, and in the case of α-humulene, decreased the potency of THC to produce responding on the THC-paired lever. While THC produced effects in all four tetrad components, none of the terpenes produced effects in all four components. Therefore, the terpenes examined in this report do not have effects consistent with CB1 receptor agonist properties in the brain.

Medications are urgently needed to treat symptoms of drug withdrawal and mitigate dysphoria and psychiatric comorbidities that drive opioid abuse and relapse. ITI-333 is a novel molecule in development for treatment of substance use disorders, psychiatric comorbidities, and pain. Characterize the preclinical profile of ITI-333 using pharmacological, behavioral, and physiological assays. Cell-based assays were used to measure receptor binding and intrinsic efficacy of ITI-333; animal models were employed to assess effects on opioid reinstatement, precipitated oxycodone withdrawal, and drug abuse liability. In vitro, ITI-333 is a potent 5-HT2A receptor antagonist (Ki = 8 nM) and a biased, partial agonist at μ-opioid (MOP) receptors (Ki = 11 nM; lacking β-arrestin agonism) with lesser antagonist activity at adrenergic α1A (Ki = 28 nM) and dopamine D1 (Ki = 50 nM) receptors. In vivo, ITI-333 blocks 5-HT2A receptor-mediated head twitch and MOP receptor-mediated effects on motor hyperactivity in mice. ITI-333 alone is a naloxone-sensitive analgesic (mice) which suppresses somatic signs of naloxone-precipitated oxycodone withdrawal (mice) and heroin cue-induced reinstatement responding without apparent tolerance or physical dependence after chronic dosing (rats). ITI-333 did not acutely impair gastrointestinal or pulmonary function (rats) and was not intravenously self-administered by heroin-maintained rats or rhesus monkeys. ITI-333 acts as a potent 5-HT2A receptor antagonist, as well a biased MOP receptor partial agonist with low intrinsic efficacy. ITI-333 mitigates opioid withdrawal/reinstatement, supporting its potential utility as a treatment for OUD.