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. Author manuscript; available in PMC: 2025 Jul 14.
Published in final edited form as: Neuropharmacology. 2023 Nov 8;243:109777. doi: 10.1016/j.neuropharm.2023.109777

Daily methocinnamox treatment dose-dependently attenuates fentanyl self-administration in rhesus monkeys

David R Maguire a,b, Charles P France a,b,c
PMCID: PMC12257550  NIHMSID: NIHMS2093581  PMID: 37944894

Abstract

Opioid use disorder and opioid overdose continue to be significant public health challenges despite the availability of effective treatments. Methocinnamox (MCAM) is a novel, long-acting mu opioid receptor antagonist that might be an effective treatment for opioid use disorder (i.e., preventing relapse) and overdose. In nonhuman primates, MCAM selectively blocks the positive reinforcing effects of opioid receptor agonists, including heroin, fentanyl, and its ultra-potent analogs (e.g., carfentanil) with a single administration of MCAM being effective for up to two weeks. Because treatment of opioid use disorder would involve repeated administration of a medication, MCAM was studied in rhesus monkeys (3 males and 2 females) responding under a fixed-ratio self-administration procedure for a range of doses of fentanyl (0.000032 to 0.1 mg/kg/infusion). The fentanyl self-administration dose-effect curve was determined before and during treatment with progressively increasing daily doses of MCAM (0.001 to 0.1 mg/kg) given subcutaneously 1 hour before the session. MCAM dose-dependently shifted the fentanyl dose-effect curve rightward and then, at larger doses, downward. The largest treatment dose of MCAM (0.1 mg/kg/day) shifted the curve more than 120-fold rightward with monkeys receiving doses three times larger than the lethal dose of fentanyl with no adverse effect or observable change in behavior. This study demonstrates that MCAM reliably and dose-dependently decreases fentanyl self-administration and prevents opioid overdose, with no evidence of any adverse effects over a broad dose range, further supporting the potential therapeutic utility of this novel antagonist.

Keywords: fentanyl, self-administration, methocinnamox, daily treatment, opioid use disorder, nonhuman primates

1. Introduction

Drug overdose-related deaths are increasing worldwide and in the United States exceeded 100,000 for the first time in 2021, continuing an alarming trend that has emerged over the past two decades and driven lately by fentanyl and related analogs that are cheap, potent, easy to synthesize, widely available, and often used as adulterants to other opioid and non-opioid drugs. (Spencer et al., 2022). High rates of opioid misuse and overdose continue despite the availability of several approved medications, including methadone, buprenorphine, naltrexone, and naloxone; nalmefene was recently approved in the United States for treating opioid overdose (Food and Drug Administration, 2023). Many factors contribute to recent trends including limited access to evidence-based treatments, low treatment compliance, as well as limitations of currently approved medications (e.g., Humphreys et al., 2022; Madras et al. 2020; Volkow and Blanco, 2023). Therefore, in addition to increasing access to and utilization of existing medications for preventing relapse and overdose, there is an urgent need for additional safe, effective treatments.

Methocinnamox (MCAM) is a long-acting mu opioid receptor antagonist that potently blocks effects of drugs acting at mu opioid receptors, including effects contributing to abuse and overdose (e.g., Broadbear et al. 2000; Houyshar et al. 2000; Peckham et al. 2005; see France et al., 2021 and Maguire and France 2023 for recent reviews). MCAM reverses and prevents the ventilatory-depressant effects of opioids such as heroin and fentanyl in rats and nonhuman primates, with a single injection providing protection against opioid-induced ventilatory depression for up to several weeks (Gerak et al., 2019; Jimenez et al. 2021). Moreover, MCAM blocks self-administration of opioids over a broad range of doses and dosing conditions (Gerak and France, 2023; Maguire et al. 2019; Maguire et al., 2020; Maguire and France, 2022; Maguire and France 2023), and its effects appear to be very selective in so far as it has no effect on responding for cocaine or food, and no effect on heart rate, blood pressure, body temperature, overall activity, cognition, or memory (Gerak and France, 2023; Maguire et al., 2019; Maguire et al., 2020; Maguire and France, 2022; Minervini et al., 2020). In addition to its long duration of action following acute administration, MCAM remains effective when given repeatedly over the course of many months (Gerak and France, 2023; Maguire et al., 2020; Maguire and France, 2022), which is essential for potential treatments for substance use disorders which are chronic, relapsing disorders.

Collectively, studies to date strongly support the consideration of MCAM for preventing relapse in individuals with opioid use disorder. However, the consequences of repeated treatment with MCAM have not been fully characterized. Two recent studies examined the effects of daily treatment with a relatively small (0.032 mg/kg) dose of MCAM on opioid self-administration in nonhuman primates. In one study (Maguire and France 2022), monkeys responded under a single-response, fixed-ratio (FR) schedule for a range of doses of fentanyl. Daily MCAM treatment shifted the fentanyl dose-effect curve rightward approximately 20-fold during treatment and responding recovered quickly (within a few days) following discontinuation of treatment. In another study (Gerak and France 2023), monkeys responded under a concurrent FR schedule in which pressing one lever delivered an intravenous infusion and pressing the other lever delivered a sucrose pellet (i.e., food-drug choice procedure). Again, daily MCAM treatment reliably shifted the choice dose-effect curves for fentanyl, carfentanil, 3-methylfentanyl, and heroin rightward but did not alter responding for food or the non-opioid drugs cocaine or methamphetamine. These studies demonstrated remarkable consistency of effects of a single treatment dose of MCAM across studies and selectivity of MCAM for reducing opioid-maintained behavior as responding for non-opioid drugs (cocaine and methamphetamine) and for food was not altered.

The current experiment builds on previous studies by assessing the magnitude of antagonism produced by MCAM across a wide (100-fold) range of daily treatment doses in rhesus monkeys responding under a single-response fentanyl self-administration procedure. Daily MCAM treatment dose progressively increased across conditions, and fentanyl self-administration dose-effect curves were re-determined with each MCAM dose to characterize its antagonist properties. Based on its unusually long duration of action and complex pharmacodynamic properties (e.g., the potential for noncompetitive binding to mu opioid receptors; see Zamora et al. 2021), this study tested the hypothesis that MCAM would shift the fentanyl dose-effect curve rightward and, at larger doses, downward, consistent with a noncompetitive interaction with the mu opioid receptor and potentially insurmountable antagonism.

2. Materials and Methods

2.1. Subjects.

Five rhesus monkeys (3 males and 2 females) were housed individually in stainless steel cages in a colony room maintained under 14/10-hr light/dark cycle. Chow (High Protein Monkey Diet; Harlan Teklad, Madison, WI, USA), fresh fruit, peanuts, and other treats were provided daily in amounts that maintained healthy sex- and age-appropriate weights, and water was continuously available in the home cage; monkeys were weighed daily prior to each session. Monkeys had most recently participated in studies investigating effects of MCAM on drug self-administration (e.g., Maguire et al. 2019; Maguire et al. 2020; Maguire and France 2022). Experiments were conducted in a facility fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International and in accordance with guidelines set forth by the Guide for the Care and Use of Laboratory Animals (8th edition; 2011) as well as ARRIVE 2.0. Protocols were approved by the University of Texas Health Science Center at San Antonio Institutional Animal Care and Use Committee.

2.2. Surgery.

Monkeys were anesthetized with 10 mg/kg ketamine given intramuscularly, intubated, and then maintained on 2 l/min oxygen and isoflurane anesthesia. A 5-fr polyurethane catheter (Access Technologies, Skokie, IL, USA) was inserted in a vein (e.g., femoral or jugular), tunneled subcutaneously (s.c.) to the back, and connected to a stainless steel s.c. access port (Access Technologies). Penicillin B&G (40,000 IU/kg) and meloxicam (0.1-0.2 mg/kg) were given postoperatively, and monkeys were given 3 to 5 days to recover before beginning studies.

2.3. Apparatus.

During daily sessions, monkeys were seated comfortably in a chair and positioned in a ventilated, sound-attenuating operant conditioning chamber facing a custom-made, stainless-steel instrument panel with two horizontally aligned levers; above each lever was a translucent disk that could be trans-illuminated red or green. Drug was delivered i.v. through the s.c. access port connected via a Huber point needle and catheter extension set to a syringe mounted in a pump (Med Associates, Inc., Georgia, VT, USA) located outside of the chamber. Chambers had an exhaust fan and white noise that masked extraneous sounds. Experiments were conducted and data were collected using Med PC IV software and associated interfacing equipment (Med Associates, Inc.).

2.4. Drugs.

Fentanyl hydrochloride and naltrexone hydrochloride were generously provided by the National Institute on Drug Abuse Drug Supply Program (Rockville, MD, USA) and dissolved in sterile saline. MCAM was purchased (Syncom, Groningen, NL) and dissolved in 10% w/v 2-hydroxypropyl-β-cyclodextrin (β-cyclodextrin; Accela ChemBio Co., Ltd., San Diego, CA, USA). Fentanyl was injected i.v. during self-administration sessions in a volume of 1 ml per 10 kg of body weight; naltrexone and MCAM were injected s.c. prior to self-administration sessions in volumes of approximately 0.32 ml per 10 kg of body weight. Doses of all drugs are expressed as the salt. Saline and vehicle (10% β-cyclodextrin in saline) injections were administered in volumes that matched the volume of drug solution. All solutions injected i.v. were first passed through a 0.22 μm syringe filter.

2.5. Procedure.

Sessions were conducted once daily, seven days per week and started at 0800 hr. After monkeys were positioned in the operant chamber, the port was flushed with 5 ml of sterile saline and connected to a syringe containing the solution to be delivered that session, either saline (0.1 ml/kg) or fentanyl (0.000032-0.1 mg/kg/infusion). The pump was activated for 15 sec to fill the port and internalized portion of the catheter. One minute later, the red light was illuminated for 5 sec, and a unit dose of drug (or equivalent volume of saline) available for self-administration that day was delivered response independently. Immediately following the first injection, the green light above the active lever (right for four monkeys and left for the other monkey) was illuminated, signaling the beginning of a response period. In the presence of the green light, 30 consecutive responses on the active lever turned off the green light, turned on the red light for 5 sec, delivered an infusion, and initiated a 180-sec timeout during which no lights were illuminated and responding had no programmed consequence. After the timeout, the green light was illuminated once again, signaling the next response period. Responding on the inactive lever reset the response requirement on the active lever but had no other programmed consequence. Sessions ended after 90 min, at which point the port and catheter were flushed and locked with heparinized saline to promote patency.

2.6. Experimental Design.

For 3 of 5 monkeys, this study began approximately 3 weeks following the conclusion of the previous study (Maguire and France 2022); the other 2 monkeys had not participated in an experiment for at least 2 months. For all monkeys, a dose of fentanyl that maintained at or near the peak number of infusions obtained (0.00032 mg/kg/infusion for 4 monkeys and 0.001 mg/kg/infusion for 1 monkey) was available within and across sessions until the number of infusions obtained in each of 3 consecutive sessions did not vary by more than 20% of the mean of those sessions (i.e., stability criterion). At that point, a naltrexone pretreatment dose-effect curve was determined as described previously (Maguire et al. 2020; Maguire and France 2022). Monkeys received a s.c. injection in the back 15 min prior to each session. Saline was given prior to baseline sessions, and different doses of naltrexone (0.0032-0.032 mg/kg) were substituted for saline, with intervening baseline sessions being conducted until 1 session occurred in which the number of infusions obtained was at least 80% of the 3-session baseline established prior to any pretreatments were given. In total, 7 to 9 sessions, across monkeys, were required to determine the naltrexone dose-effect curve.

After determining a naltrexone pre-treatment dose-effect curve, saline was substituted for fentanyl until 8 or fewer infusions were obtained each session for 3 consecutive sessions (i.e., extinction criterion), at which point a baseline fentanyl self-administration dose-effect curve was determined, as described previously (e.g., Maguire and France 2022). Each unit dose of fentanyl (0.000032 to 0.001 mg/kg/infusion) remained available within and across sessions until either the extinction or stability criterion was satisfied, or 7 sessions occurred without meeting either of the other two criteria. Fentanyl doses were tested in ascending order up to a dose that maintained a at or near the peak number of infusions obtained for each individual monkey. The naltrexone pretreatment dose-effect curve, taken together with the baseline fentanyl self-administration dose-effect curve, confirmed sensitivity to drugs acting at mu opioid receptors was consistent with previous studies prior to beginning daily MCAM treatment.

Following determination of the baseline fentanyl self-administration dose-effect curve, the maintenance dose of fentanyl remained available each session and monkeys began receiving a s.c. injection daily 60 min prior to the session. Initially, the injection was 0.5 ml of the MCAM vehicle (10% β-cyclodextrin in saline). Once responding was stable, the vehicle injection was replaced by 0.001 mg/kg MCAM for at least 7 sessions. For one monkey, this daily dose of MCAM modestly decreased the number of infusions obtained and so the unit dose of fentanyl was increased one half-log unit before increasing the daily MCAM dose. For the remaining monkeys, daily injection 0.001 mg/kg MCAM did not alter infusions obtained. From that point, the daily treatment dose of MCAM increased across phases. For each treatment dose, the maintenance dose of fentanyl (as determined under the previous daily MCAM treatment condition) was initially available for at least 7 sessions and until either the extinction or stability criteria was satisfied, at which point, the unit dose of fentanyl was varied as described above to re-determine the ascending limb of the fentanyl dose-effect curve. The fentanyl dose-effect curve was determined once with daily MCAM doses of 0.0032 and 0.01 and twice with 0.032 and 0.1 mg/kg. Re-determination of the fentanyl dose-effect curve with the larger treatment doses evaluated the stability of antagonist effects of MCAM. MCAM doses were studied in ascending order; fentanyl unit doses (up to 0.1 mg/kg/infusion) generally were tested in ascending order. Table 1 indicates the number of daily treatments received with each dose of MCAM for individual monkeys. To ensure the safety and well-being of the monkeys, the treatment dose of MCAM was increased systematically across conditions, resulting in gradual and progressive increases in the unit doses of fentanyl that could be studied; therefore, treatment doses were not randomized.

Table 1.

Number of daily treatments with each dose of MCAM for individual monkeys

Monkey
MCAM (mg/kg) AC GU PE PR TA
0.001 10 7 7 7 7
0.00321 31 14 10 16 18
0.01 23 23 20 17 25
0.032 47 36 46 36 36
0.1 55 43 39 39 56
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1.

Fentanyl self-administration dose-effect curves were determined once with treatment doses of 0.0032 and 0.01 mg/kg/infusion and twice with 0.032 and 0.1 mg/kg/infusion.

2.7. Data and Statistical Analysis.

The number of infusions obtained each session served as the primary dependent measure. Fentanyl dose-effect curves for individual monkeys were constructed for each test by averaging the number of infusions obtained across each of the last three sessions with each unit dose of fentanyl. The potency of naltrexone to decrease responding for the initial maintenance dose of fentanyl was quantified by fitting the naltrexone dose-effect curve with a straight line and estimating the dose required to produce a 50% reduction in the number of infusions obtained. Effects of MCAM on sensitivity to fentanyl were quantified by evaluating changes in the fentanyl dose-effect curve across treatment doses of MCAM (Maguire and France 2022). First, the average number of infusions obtained with each unit dose of fentanyl was normalized to percent maximum possible effect (%MPE) using the following equation: %MPE = (test-saline) / (maximum-saline) * 100, where test is the average number of infusions obtained with each unit dose, saline is the number of infusions obtained when saline was available for self-administration at the beginning of the study, and maximum is the maximum number of infusions obtained at any dose of fentanyl determined at the beginning of the study. Then, a straight line was fit to the ascending limb of the dose-effect curve using linear regression and the logarithm (base 10) of fentanyl unit dose. The slope and intercept of the fit line were used to estimate the dose producing 50% of the maximal number of infusions obtained at the beginning of the study (ED50). Shifts in the fentanyl dose-effect curve across daily treatment doses of MCAM were quantified by calculating a potency ratio; ED50 values during daily MCAM treatment were divided by the ED50 before treatment. Changes in slope were quantified by calculating a slope ratio; the slopes of the dose-effect curves during daily MCAM were divided by the slope before treatment. Effects on ED50 and slope were considered significant if the 95% confidence interval surrounding the mean for the group did not contain 1. Rightward and downward shifts in the fentanyl dose-effect curve are reflected by an increase in the potency ratio and a decrease in the slope ratio, respectively. Curve-fitting and statistical calculations were completed using Microsoft Excel (Redmond, WA, USA).

3. Results

The initial maintenance dose of fentanyl (0.00032 mg/kg/infusion for four monkeys and 0.001 mg/kg/infusion for the fifth monkey) maintained, on average (± 95% confidence limits), 23.0 (18.8-27.2) infusions per session. Naltrexone dose-dependently decreased infusions in all monkeys (Table 2) with an average ED50 of 0.007 (0.005-0.009) mg/kg. Before daily MCAM treatment, fentanyl dose-dependently increased the number of infusions obtained with an average ED50 of 0.0001 (0.00007-0.0002) mg/kg/infusion (unfilled squares, Figure 1). Daily treatment with MCAM dose-dependently shifted the fentanyl self-administration dose-effect curve rightward and then downward (filled symbols, Figure 1). Re-determination of the fentanyl dose-effect curve with each of the two largest daily doses of MCAM yielded shifts very similar to the initial determination (half-filled symbols, Figure 1). Effects observed in the group averaged data (top left panel, Figure 1) were generally representative of all five individual monkeys, though there was some variance in effect of the largest dose of MCAM across monkeys, particularly in the maximum number of infusions obtained (e.g., compare monkeys PR and TA).

Table 2.

Number of fentanyl infusions1 obtained during baseline sessions and following pretreatment with different dose of naltrexone for individual monkeys.

Monkey
Naltrexone (mg/kg)2 AC GU PE PR TA
Baseline3 17.5 21 23.5 26.3 27.5
0.0032 13 23 15 24 28
0.01 3 6 6 13 5
0.032 5
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1.

The unit dose of fentanyl was 0.00032 mg/kg/infusion for monkeys AC, GU, PE, and PR and 0.001 mg/kg/infusion for monkey TA.

2.

Naltrexone was administered s.c. 15 min prior to the session.

3.

Average number of infusions obtained during sessions immediately preceding a naltrexone pretreatment test.

Figure 1.

Figure 1.

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Effects of daily MCAM treatment on fentanyl self-administration. The number of infusions obtained is plotted for saline (data above “S”) and increasing unit doses of fentanyl (mg/kg/infusion). For group data (top left panel), data points indicate the group mean (n=5) for unit doses of fentanyl for which at least 2 monkeys were tested. The remaining panels indicate data for individual monkeys, and each data point shows the mean of the last three sessions under each condition. Unfilled squares indicate the fentanyl dose-effect curve immediately prior to the beginning of MCAM treatment. Filled and half-filled symbols show data from different daily treatment doses; half-filled symbols show data from the second determination of the fentanyl dose-effect curve with the two largest treatment doses of MCAM. For group data, symbols indicate the mean and standard error of the mean for the group; only data from doses in which at least 2 monkeys were tested are shown. For individual monkey data, symbols indicate the mean (and standard error of the mean), for the last 3 sessions of each condition.

Figure 2 shows quantification of fentanyl dose-effect curves across daily treatment doses of MCAM. Fentanyl ED50 increased as a function of MCAM dose and potency ratios (test ED50 divided by baseline ED50) were significantly greater than 1 for doses of 0.0032 mg/kg and larger (0.001 mg/kg was without effect on the maximal dose of fentanyl, and no ED50 was determined). MCAM also dose-dependently decreased the slope of the fentanyl dose-effect curve, on average, with doses of 0.032 mg/kg (first determination) and 0.1 mg/kg (both determinations) significantly decreasing slope ratios (test slope divided by baseline slope).

Figure 2.

Figure 2.

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Quantification of changes in the fentanyl dose-effect curve across different daily treatment doses of MCAM. Potency (ED50) and slope were estimated using linear regression and the ascending limb of the fentanyl dose-effect curve. Each data point indicates data for an individual monkey, and the horizontal line indicates the group mean. Upper panels show the raw values, and lower panels show the same data expressed as potency and slope ratios (left and right panels, respectively) in which the test value was divided by the baseline value for individual monkeys. The asterisks in the bottom panels indicate that the 95% confidence interval for the group did not include 1.

4. Discussion

This study characterized effects of range of daily treatment doses of MCAM on fentanyl self-administration, assessed the magnitude of antagonism by quantifying changes in the fentanyl dose-effect curve, and tested the hypothesis that MCAM would shift the fentanyl dose-effect curve rightward and, at larger doses, downward, consistent with noncompetitive binding to mu opioid receptors. Results of this study showed that MCAM did the following: 1) dose-dependently shifted the fentanyl self-administration dose-effect curve rightward and, at larger doses in 4 of 5 monkeys, downward; 2) remained effective with repeated daily treatment; and 3) blocked severe adverse (e.g., respiratory depressant) effects of fentanyl in addition to its positive reinforcing effects. Together these results provide additional support for considering the use of MCAM as a treatment for preventing relapse to opioid taking.

Prior to initiation of daily MCAM treatment, fentanyl dose-dependently increased the number of infusions obtained, and pretreatment with naltrexone dose-dependently decreased responding for fentanyl. The potency of fentanyl to maintain responding and of naltrexone to decrease opioid-maintained responding was comparable to previous studies conducted in this laboratory and others (e.g., Broadbear et al., 2004; Carey et al., 2023; Gerak and France 2023; Maguire 2023; Maguire et al. 2020; Maguire and France 2022; Negus et al., 2008), confirming sensitivity of responding to drugs acting at mu opioid receptors before starting daily MCAM dosing.

Consistent with previous studies (Gerak and France, 2023; Maguire et al., 2019; Maguire et al., 2020; Maguire and France, 2022), MCAM attenuated fentanyl self-administration, indicated here by rightward shifts in the fentanyl dose-effect curve. The effects of MCAM were very reliable as demonstrated by across- and within-subject replication in this study as well as replication across studies. Shifts in the fentanyl dose-effect curve were orderly and dose-related, with a dose as small as 0.0032 mg/kg MCAM producing a small, but significant, shift of approximately 2-fold and the largest dose tested (0.1 mg/kg) shifting the curve over 120-fold, on average. Moreover, the larger treatment doses of MCAM (0.032 and 0.1 mg/kg) decreased the slope of the dose-effect curve.

All five monkeys showed qualitatively similar patterns of interaction between MCAM and fentanyl, and effects of the two largest doses of MCAM very closely replicated within each subject. MCAM given acutely has been shown to dose-dependently shift the dose-effect curve for the mu opioid receptor agonist remifentanil rightward and downward in monkeys working under a food-drug choice procedure (Maguire et al. 2019); however, in that study, the range of doses of remifentanil was fixed and the magnitude of shifts in the dose-effect curve could not be quantified. In two more recent studies, daily treatment with a single dose of MCAM (0.032 mg/kg) shifted the fentanyl self-administration dose-effect curve between 20- and 40-fold rightward (Gerak and France 2023; Maguire and France 2022). In the current study, the same dose of MCAM, given via the same route of administration and at the same frequency, shifted the fentanyl dose-effect curve 22- and 28-fold across the two determinations which is within the range observed previously. Taken together these data confirm consistency in antagonist effects of MCAM across subjects, including males and females, and across different experimental arrangements (e.g., food-drug choice versus single-response self-administration procedures).

The current study employed a single-response self-administration procedure, and decreased self-administration can be subject to multiple interpretations including generalized response rate-depressant effects. Although response rate-depressant effects of MCAM cannot be ruled out directly in the current study, a major contribution of such effects seems unlikely. First, in some cases, antagonist effects of MCAM were surmounted, even in the context of very large shifts in the fentanyl dose-effect curve, indicating that shifts were not due primarily to rate-depressant effects. Second, prior studies have shown that daily treatment with 0.032 mg/kg MCAM, a dose producing substantial antagonism of fentanyl self-administration in the current study, selectively reduces responding maintained by opioids but not for other reinforcers. For example, the same dose of MCAM, administered by the same route, frequency, and pretreatment time in monkeys responding under the same schedule of reinforcement, decreased responding for fentanyl but not for cocaine (Maguire and France, 2022). In another study with monkeys responding under a food versus drug choice procedure (Gerak and France, 2023), daily treatment with 0.032 mg/kg MCAM decreased choice of fentanyl (as well as other opioids, such as carfentanil and heroin) and increased choice of food without significantly reducing the total number of trials completed. That is, daily MCAM prompted a reallocation of behavior from one option (drug) to another (food) without evidence of depression of overall response output. Moreover, acute and intermittent (every 12 days) treatment with dose of MCAM 3-fold larger than the largest treatment dose used in the current study (0.32 mg/kg) also selectively decreased responding for fentanyl and heroin but not cocaine (Maguire et al. 2019; Maguire et al. 2020). Finally, acute treatment with a dose of MCAM 30-fold larger than the largest treatment dose in the current study (3.2 mg/kg) did not decrease responding in monkeys working for food under a similar schedule of reinforcement (Maguire et al., 2019); daily treatment with a much smaller dose would be unlikely to produce sustained response rate-depressant effects. Taken together, these data suggest effects of MCAM on opioid self-administration in the current study, as in other studies, were primarily due to blockade of opioid receptors rather than a generalized depression of operant responding.

In addition to selectively attenuating the positive reinforcing effects of opioids, which likely contribute to their abuse, MCAM blocks other adverse effects of opioids. For example, MCAM reverses and prevents the ventilatory-depressant effects of opioids such as heroin and fentanyl in rats and nonhuman primates, with a single injection providing substantial protection against opioid-induced ventilatory depression for up to several weeks (Gerak et al., 2019; Jimenez et al. 2021). Although not an explicit goal of the current study, results showed that daily treatment also blocks severe adverse effects of very large doses of fentanyl. As the fentanyl dose-effect curve shifted progressively rightward with increasing treatment doses of MCAM, monkeys that continued to respond for infusions of fentanyl reached daily intake levels of fentanyl that exceed those known to produce severe adverse effects in otherwise untreated individuals (Figure 3). For example, bolus doses of 0.032 mg/kg of fentanyl and larger markedly decrease ventilation in rhesus monkeys (e.g., Nussmeier et al. 1991; Ko et al. 2002). In the current study, monkeys treated daily with 0.1 mg/kg MCAM received nearly 50 times the amount of fentanyl that would be expected to produce marked ventilatory depression and many times what would very likely be lethal without any evidence of ventilatory depression or other signs of distress, demonstrating significant protection against severe adverse effects.

Figure 3.

Figure 3.

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Daily fentanyl intake before and during daily treatment with 0.1 mg/kg/day MCAM. Intake (mg/kg/session) is plotted as a function of the unit dose of fentanyl available for self-administration (mg/kg/infusion). Data points indicate the group mean. Squares show intake before daily MCAM treatment, and circles show intake during daily treatment with 0.1 mg/kg MCAM (first fentanyl dose-effect curve determination). The horizontal dashed line indicated the minimal dose of fentanyl estimated to produce marked ventilatory depression in otherwise untreated rhesus monkeys.

Although the magnitude of shifts in the fentanyl dose-effect curve grew larger with increasing treatment doses of MCAM, changes in the magnitude did not appear to be proportional to the increase in dose as might be expected with a competitive antagonist such as naltrexone (e.g., Bertalmio and Woods, 1989). For example, increasing MCAM dose from 0.0032 to 0.01 mg/kg (3-fold) resulted in an average increase in the potency ratio of approximately 2-fold on average, while further increasing the treatment dose from 0.01 to 0.032 and then from 0.032 to 0.1 (also 3-fold steps) increased the magnitude of shift 5- and 6-fold on average, respectively, exceeding the 3-fold shifts that would be expected with a simple, competitive interaction. Greater-than-expected rightward shifts and flattening of the dose-effect curve are hallmarks of a noncompetitive interaction of the antagonist with the receptor (e.g., Adams et al., 1990; Walker et al. 1998; Zimmerman et al. 1987), which has previously been observed following treatment with a structurally related compound, clocinnomox, in nonhuman primates self-administering opioids (Zernig et al., 1997).

These results, taken together with data showing that MCAM remains effective even with very low concentrations circulating in plasma (e.g., Maguire et al., 2020), suggest that antagonist effects of MCAM might be due to pharmacodynamic and not pharmacokinetic factors. Indeed, in vitro studies have shown that MCAM insurmountably blocks effects of fentanyl on cyclic adenosine monophosphate production in human embryonic kidney cells expressing the human mu opioid receptors (Zamora et al. 2021), consistent with long-term and noncompetitive binding of MCAM to mu opioid receptors. At present it is unclear what role, if any, noncompetitive binding of MCAM plays in its effects in vivo. If noncompetitive binding is involved, then the mechanism(s) by which MCAM blocks effects of opioids differs from antagonists currently approved to treat opioid use disorder and overdose (i.e., naltrexone, naloxone, and nalmefene) that bind opioid receptors in a competitive, reversible manner. Therefore, one critical difference might be potential insurmountability of antagonist effects in so far as the blockade produced by MCAM, with sufficiently large treatment doses, would be less likely to be overcome by taking more opioid. Whether an antagonist such as MCAM will provide insurmountable protection depends on several factors including the number of receptors occupied by the antagonist, the intrinsic efficacy of the agonist, the efficacy requirement of the mu opioid receptor agonist-mediated effect (e.g., Adams et al. 1990). For example, it is plausible that opioid-mediated effects with relatively low efficacy requirement, such as positive reinforcement, would be expected to require a higher proportion of receptors to be occupied by an antagonist to demonstrate insurmountability compared to effects with a larger efficacy requirement such as antinociception or ventilatory depression (e.g., Zernig et al. 1997; Bergman et al., 2020; see Townsend, 2020 for a discussion). In the current study, differences in efficacy requirement might account, at least in part, for the surmountability of the blockade of reinforcing effects but not more severe adverse effects with daily treatment of a relatively large dose of MCAM. However, the relationship between efficacy requirement of opioid-mediated effects and the effectiveness of MCAM requires additional investigation.

Given severity of the opioid crisis, many novel approaches for treating opioid abuse and preventing overdose are under investigation (e.g., France et al., 2021). Notable among them are drug-targeted therapeutics (e.g., vaccines) that blunt the activity of opioids by binding drug molecules themselves and interfering with their capacity to enter the brain and/or bind to opioid receptors (e.g., Banks et al., 2018; Pravetoni and Comer, 2019). Like MCAM and approved opioid receptor antagonists, these treatments selectively attenuate opioid-mediated abuse- and overdose-related effects, including self-administration (e.g., Townsend et al. 2021). Drug-targeted approaches have the advantage of selectivity in that they can be designed to attenuate effects of drugs with particular chemical structures (e.g., fentanyl and related analogs) without altering effects of drugs with different chemical structures that might have legitimate medical utility (e.g., oxycodone for treating severe pain). However, one drawback to this approach is that drugs with sufficiently different chemical structures could have much lower (or no) binding affinity for a drug-targeting agent, which can be problematic with quickly evolving illicit drug markets and introduction of compounds with different chemical properties (e.g., ‘nitazenes’; see Glatfelter et al., 2023); in such a case, receptor-targeting treatments would likely have an advantage. Whether treatments targeting opioid receptors or the opioid molecules themselves represent the optimal approach for treating opioid abuse and preventing overdose deaths remains unclear.

5. Conclusions

In conclusion, the opioid crisis continues despite the availability of safe, effective, and evidence-based treatments. Although effective in many patients, currently approved medications for treating opioid used disorder and preventing relapse to opioid use have limitations impacting their clinical utility which include abuse potential, diversion, and risk of overdose for mu opioid receptor agonists such as methadone and buprenorphine and short duration of action and surmountable blockade for mu opioid receptor antagonists such as naltrexone. MCAM is a long-acting mu opioid receptor antagonist that reliably blocks abuse- and overdose-related effects of opioids, including highly potent and efficacious agonists such as fentanyl. The current study demonstrates that MCAM dose-dependently, and possibly insurmountably, blocks fentanyl self-administration, remains effective with repeated treatment, and blocks other toxic effects, supporting its potential utility as an additional tool to help curtail the opioid crisis.

Highlights.

  • Methocinnamox (MCAM) is a long-acting antagonist that blocks effects of opioids

  • This study assessed the magnitude of antagonism across treatment doses of MCAM

  • MCAM dose-dependently decreased fentanyl self-administration

  • Results support the potential of MCAM as a novel treatment for relapse and overdose

Acknowledgements:

The authors gratefully acknowledge S Bosworth, M Deande, S Hopper, J Juarez, A Nelson, X Tijerina, J Tovar, and S Womack for excellent technical assistance. David R. Maguire has no conflict of interest to declare. Charles P. France is coholder of U.S. patents for MCAM. The authors confirm that studies described herein were conducted in accordance with local and federal guidelines regarding the use of animals in research.

Funding:

This work was supported by the United States National Institute on Drug Abuse of the National Institutes of Health [grant number R01DA048417]; and by the Welch Foundation [Grant AQ-0039]. The content of this article is solely the responsibility of the authors and does not represent the official views of the National Institutes of Health.

Nonstandard abbreviations:

MCAM

methocinnamox

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