Medications Development for Treatment of Opioid Use Disorder

An economon model of drug addiction

The term "economon" (i:'ka.nə.muhn; plural: economa) is introduced here to describe an economic unit composed of two participants engaged in mutually reinforcing operant behavior. Economa are basic building blocks of transactional behavior that aggregate in social networks called economies. In a drug-addiction economon, operant behavior by one participant (the "supplier") provides an addictive drug as a reinforcer to the second participant (a "Person with Substance Use Disorder; PwSUD"). Reciprocal operant behavior by the PwSUD usually provides money as a reinforcer to the supplier. After defining the features of the drug-addiction economon, this article discusses its implications for (1) prevalence and virulence of drug addiction, (2) opportunities for drug-addiction research in general, (3) the "brain-disease model of addiction" in particular, and (4) factors that mitigate harm or promote risk of drug addiction. The economon model is intended to provide a novel perspective on the uniquely human disorder of drug addiction.

An automated group-housed oral fentanyl self-administration method in mice

RATIONALE AND OBJECTIVES

Social factors play a critical role in human drug addiction, and humans often consume drugs together with their peers. In contrast, in traditional animal models of addiction, rodents consume or self-administer the drug in their homecage or operant self-administration chambers while isolated from their peers. Here, we describe HOMECAGE ("Home-cage Observation and Measurement for Experimental Control and Analysis in a Group-housed Environment"), a translationally relevant method for studying oral opioid self-administration in mice. This setting reduces experimental confounds introduced by social isolation or interaction with the experimenter.

METHODS

We have developed HOMECAGE, a method in which mice are group-housed and individually monitored for their consumption of a drug vs. a reference liquid.

RESULTS

Mice in HOMECAGE preserve naturalistic aspects of behavior, including social interactions and circadian activity. The mice showed a preference for fentanyl and escalated their fentanyl intake over time. Mice preferred to consume fentanyl in bouts during the dark cycle. Mice entrained to the reinforcement schedule of the task, optimizing their pokes to obtain fentanyl rewards, and maintained responding for fentanyl under a progressive ratio schedule. HOMECAGE also enabled the detection of cage-specific and individual-specific behavior patterns and allowed the identification of differences in fentanyl consumption between co-housed control and experimental mice.

CONCLUSIONS

HOMECAGE serves as a valuable procedure for translationally relevant studies on oral opioid intake under conditions that more closely mimic the human condition. The method enables naturalistic investigation of factors contributing to opioid addiction-related behaviors and can be used to identify novel treatments.

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Sensitivity of a fentanyl-vs.-social interaction choice procedure to environmental and pharmacological manipulations

Recent studies have shown that social interaction can serve as an alternative reinforcer to opioid self-administration under a choice context in rats. However, additional parametric studies are needed to evaluate the sensitivity of opioid-vs.-social interaction procedures relative to more established opioid-vs.-food procedures. The current study evaluated the sensitivity of a novel fentanyl-vs.-social interaction choice procedure to environmental and pharmacological manipulations previously shown to affect fentanyl-vs.-food choice. Male and female rats (responder rats; n = 6/sex) were trained to respond in a discrete-trial choice procedure for either 30-s access to a same-sex "partner" rat or an intravenous fentanyl infusion. Once trained, the effects of fentanyl unit dose (0, 0.32-10 μg/kg/inf), partner rat presence, opioid-dependence status, chronic naltrexone administration (0.032, 0.1 mg/kg/h), and response requirement for fentanyl self-administration (fixed ratio 1-320) were determined across weeks. The fentanyl-vs.-social interaction choice procedure was sensitive to the unit dose of fentanyl, chronic naltrexone treatment, and fentanyl response requirement. However, the magnitude of these effects on fentanyl choice was smaller than those reported in published fentanyl-vs.-food choice studies. Furthermore, fentanyl-vs.-social interaction choice was not sensitive to removal of the partner rat or opioid-dependence status. Minimal sex differences were detected. These results suggest that this fentanyl-vs.-social interaction choice procedure is less sensitive to environmental and pharmacological interventions than previously established opioid-vs.-food choice procedures. The observed discrepancy in sensitivity between the procedures suggests that social interaction may have qualitatively different reinforcing properties compared to more commonly assessed alternative reinforcers such as food (preclinical) or money (human laboratory).

Effect of TRV130 and methadone on fentanyl-vs.-food choice and somatic withdrawal signs in opioid-dependent and post-opioid-dependent rats

The high efficacy mu-opioid receptor (MOR) agonist methadone is an effective opioid use disorder (OUD) medication used exclusively in opioid-dependent patients. However, methadone has undesirable effects that limit its clinical efficacy. Intermediate efficacy MOR agonists may treat OUD with fewer undesirable effects. We compared the effects of methadone with the intermediate efficacy MOR agonist TRV130 (oliceridine) on fentanyl-vs.-food choice and somatic withdrawal signs in opioid-dependent and post-opioid-dependent rats. Male rats (n = 20) were trained under a fentanyl-vs.-food choice procedure. Rats were then provided extended fentanyl (3.2 µg/kg/infusion) access (6 p.m.-6 a.m.) for 10 days to produce opioid dependence/withdrawal. Rats were treated with vehicle (n = 7), TRV130 (3.2 mg/kg; n = 8), or methadone (3.2 mg/kg; n = 5) three times per day after each extended-access session (8:30 a.m., 11 a.m., 1:30 p.m.). Withdrawal sign scoring (1:55 p.m.) and choice tests (2-4 p.m.) were conducted daily. Vehicle, TRV130, and methadone effects on fentanyl choice were redetermined in post-opioid-dependent rats. Vehicle-, TRV130-, and methadone-treated rats had similar fentanyl intakes during extended access. Vehicle-treated rats exhibited increased withdrawal signs and decreased bodyweights. Both methadone and TRV130 decreased these withdrawal signs. TRV130 was less effective than methadone to decrease fentanyl choice and increase food choice in opioid-dependent rats. Neither methadone nor TRV130 decreased fentanyl choice in post-opioid-dependent rats. Results suggest that higher MOR activation is required to reduce fentanyl choice than withdrawal signs in fentanyl-dependent rats. Additionally, given that TRV130 did not precipitate withdrawal in opioid-dependent rats, intermediate efficacy MOR agonists like TRV130 may facilitate the transition of patients with OUD from methadone to lower efficacy treatments like buprenorphine.

Lack of effect of the nociceptin opioid peptide agonist Ro 64-6198 on pain-depressed behavior and heroin choice in rats

RATIONALE AND OBJECTIVE

One objective of the National Institutes of Health Helping to End Addiction Long-term (HEAL) initiative is to accelerate research on safer and more effective medications for both pain and opioid use disorder. Ligands that activate the nociceptin opioid peptide receptor (NOP) constitute one class of candidate drugs for both applications. The present preclinical study determined the effectiveness of the NOP agonist Ro 64-6198 to produce antinociception in a pain-depressed behavior procedure and attenuate opioid self-administration in a heroin-vs-food choice procedure.

METHODS

In Experiment 1, Adult Sprague-Dawley rats were equipped with microelectrodes and trained to respond for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. The potency, time course, and receptor mechanism of effects produced by R0 64-6198 alone (0.32-3.2 mg/kg) on ICSS were examined, followed by evaluation of 0.32-1.0 mg/kg Ro 64-6198 effectiveness to block lactic acid-induced depression of ICSS. In Experiment 2, rats self-administered heroin under a heroin-vs-food choice procedure during a regimen of repeated, daily intraperitoneal administration of vehicle or Ro 64-6198 (1-3.2 mg/kg/day).

RESULTS

Ro 64-6198 produced dose- and time-dependent ICSS depression that was blocked by the selective NOP antagonist SB612111 but not by naltrexone. Ro 64-6198 failed to block acid-induced depression of ICSS. Repeated Ro 64-6198 pretreatment also failed to attenuate heroin-vs-food choice up to doses that significantly decreased operant behavior.

CONCLUSIONS

These results do not support the utility of Ro 64-6198 as a stand-alone medication for either acute pain or opioid use disorder.

A drug-vs-food "choice" self-administration procedure in rats to investigate pharmacological and environmental mechanisms of substance use disorders

BACKGROUND

Preclinical drug self-administration procedures are commonly used to investigate expression, mechanisms, and treatment of substance use disorders.

NEW METHOD

The aims were to back-translate an intravenous drug-vs-food choice procedure primarily utilized in monkeys to male and female rats and to develop a surgical method for sustained intravenous catheter patency suitable for long-term drug-choice studies.

RESULTS

The surgical protocol resulted in a median intravenous jugular catheter patency in male and female rats of 126 days (range: 25-365 days). Drug-vs-food choice was established with opioids (fentanyl and heroin), psychostimulants (cocaine, methamphetamine, and amphetamine), and an opioid/psychostimulant mixture (fentanyl + methamphetamine). The average time from catheter implantation to stable choice behavior across all drugs was 27 sessions (range: 16-44 sessions). Choice behavior stabilized more quickly for cocaine and fentanyl than for other drugs. Manipulations of both environmental variables (e.g., response requirement or food reinforcer magnitude) and pharmacological variables (e.g., extended access drug self-administration or continuous buprenorphine treatment via osmotic pump) significantly shifted opioid-vs-food choice consistent with previous monkey studies.

COMPARISON WITH EXISTING METHODS

Duration of intravenous catheter patency in rats was suitable for long-term, within-subject drug choice studies. Effects of environmental and pharmacological manipulations in rats confirmed and extended previous results from monkeys.

CONCLUSIONS

The concordance of behavioral results between rats and monkeys using the present drug-vs-food choice procedure supports its utility to improve our basic understanding of the expression and mechanisms of substance use disorders towards to development of more effective therapeutics.

Improving Translational Research Outcomes for Opioid Use Disorder Treatments

Purpose of Review

Pharmacotherapies are the most effective means of reducing the harms associated with opioid use disorder (OUD). Translational research seeking to develop novel medications to treat OUD has been challenging due to the complex etiology of addiction. Preclinical outcome measures are often behavioral, and it is difficult, if not impossible, to fully mirror the various emotional and cognitive processes that motivate opioid use in humans. The goal of the current narrative review was to summarize the translational progression of three potential medications for OUD, which had varying levels of success.

Recent Findings

Memantine, lorcaserin, and lofexidine all showed promise in preclinical studies; however, only lofexidine was able to consistently replicate these findings in human subjects, and receive FDA approval. It was the authors' objective to use this review to identify areas of needed improvement in translational research for OUD.

Summary

Preclinical studies vary significantly in their ability to forecast effectiveness in clinical trials. Among the various preclinical models, suppression of opioid self-administration appears to have the best predictive validity. As they model a mostly physiological phenomenon, preclinical assessments of opioid withdrawal also appear to have high predictive validity. In our review of the literature, the authors noted numerous examples of clinical trials that were underpowered, lack precision, and proper outcomes. Better-validated preclinical targets and improved design of proof-of-concept human studies should allow investigators to more efficiently develop and test medications for OUD.

A synthetic opioid vaccine attenuates fentanyl-vs-food choice in male and female rhesus monkeys

AIM

Opioid-targeted vaccines are under consideration as candidate Opioid Use Disorder medications. We recently reported that a fentanyl-targeted vaccine produced a robust and long-lasting attenuation of fentanyl-vs-food choice in rats. In the current study, we evaluated an optimized fentanyl-targeted vaccine in rhesus monkeys to determine whether vaccine effectiveness to attenuate fentanyl choice translated to a species with greater phylogenetic similarity to humans.

METHODS

Adult male (2) and female (3) rhesus monkeys were trained to respond under a concurrent schedule of food (1 g pellets) and intravenous fentanyl (0, 0.032-1 μg/kg/injection) reinforcement during daily 2 h sessions. Fentanyl choice dose-effect functions were determined daily and 7-day buprenorphine treatments (0.0032-0.032 mg/kg/h IV; n = 4-5) were determined for comparison to vaccine effects. Subsequently, a fentanyl-CRM₁₉₇ conjugate vaccine was administered at week 0, 3, 8, 15 over a 29-week experimental period during which fentanyl choice dose-effect functions continued to be determined daily.

RESULTS

Buprenorphine significantly decreased fentanyl choice and reciprocally increased food choice. Vaccination eliminated fentanyl choice and increased food choice in four-of-the-five monkeys. A transient and less robust vaccine effect was observed in the fifth monkey. Fentanyl-specific antibody concentrations peaked after the third vaccination to approximately 50 μg/mL while anti-fentanyl antibody affinity increased to a sustained low nanomolar level.

CONCLUSION

These results translate fentanyl vaccine effectiveness from rats to rhesus monkeys to decrease fentanyl-vs-food choice, albeit with greater individual differences observed in monkeys. These results support the potential and further clinical evaluation of this fentanyl-targeted vaccine as a candidate Opioid Use Disorder medication.

Evaluation of a Dual Fentanyl/Heroin Vaccine on the Antinociceptive and Reinforcing Effects of a Fentanyl/Heroin Mixture in Male and Female Rats

Opioid-targeted vaccines represent an emerging treatment strategy for opioid use disorder. To determine whether concurrent vaccination against two commonly abused opioids (fentanyl and heroin) would confer broader spectrum opioid coverage, the current study evaluated dual fentanyl/heroin conjugate vaccine effectiveness using a warm water tail-withdrawal and a fentanyl/heroin-vs-food choice procedure in male and female rats across a 105-day observation period. Vaccine administration generated titers of high-affinity antibodies to both fentanyl and heroin sufficient to decrease the antinociceptive potency of fentanyl (25-fold), heroin (4.6-fold), and a 1:27 fentanyl/heroin mixture (7.5-fold). Vaccination did not alter the antinociceptive potency of the structurally dissimilar opioid agonist methadone. For comparison, continuous treatment with a naltrexone dose (0.032 mg/kg/h) shown previously to produce clinically relevant plasma-naltrexone levels decreased the antinociceptive potency of fentanyl, heroin, and the 1:27 fentanyl/heroin mixture by approximately 20-fold. Naltrexone treatment also shifted the potency of 1:27 fentanyl/heroin mixture in a drug-vs-food choice self-administration procedure 4.3-fold. In contrast, vaccination did not attenuate 1:27 fentanyl/heroin mixture self-administration in the drug-vs-food choice procedure. These data demonstrate that a vaccine can simultaneously attenuate the thermal antinociceptive effects of two structurally dissimilar opioids. However, the vaccine did not attenuate fentanyl/heroin mixture self-administration, suggesting a greater magnitude of vaccine responsiveness is required to decrease opioid reinforcement relative to antinociception.