Human behavioral pharmacology, past, present, and future: symposium presented at the 50th annual meeting of the Behavioral Pharmacology Society

Opioid vaccine clinical testing: lessons learned

PURPOSE OF REVIEW

Opioid use disorder (OUD) presents a serious public health concern, with dramatic increases in opioid-overdose mortality in recent years and a small percentage of those with OUD accessing or remaining engaged with available treatments. Efforts are currently underway to identify vaccines targeting opioids, which could provide a novel and complimentary approach. The current review provides an overview of existing literature, practical considerations for designing and conducting clinical trials with vaccines for opioids, and future directions.

RECENT FINDINGS

This review covers the following themes: clinical trial design and selection of endpoints, timepoint selection, practical considerations and lessons learned from the first (ongoing) trial of a vaccine targeting opioids, and future directions.

SUMMARY

Efforts to develop and test vaccines targeting OUD are based on a foundation of preclinical work and close collaboration between preclinical and clinical researchers. Efforts to learn from shortcomings of prior clinical trials of vaccines for other substances are essential in designing and testing effective vaccines for OUD. Design and implementation of clinical trials for a vaccine for OUD requires careful balance of participant safety and strategies for retention and efforts to gather viable data to inform future work.

Combining noninvasive brain stimulation with behavioral pharmacology methods to study mechanisms of substance use disorder

Psychotropic drugs and transcranial magnetic stimulation (TMS) are effective for treating certain psychiatric conditions. Drugs and TMS have also been used as tools to explore the relationship between brain function and behavior in humans. Combining centrally acting drugs and TMS has proven useful for characterizing the neural basis of movement. This combined intervention approach also holds promise for improving our understanding of the mechanisms underlying disordered behavior associated with psychiatric conditions, including addiction, though challenges exist. For example, altered neocortical function has been implicated in substance use disorder, but the relationship between acute neuromodulation of neocortex with TMS and direct effects on addiction-related behaviors is not well established. We propose that the combination of human behavioral pharmacology methods with TMS can be leveraged to help establish these links. This perspective article describes an ongoing study that combines the administration of delta-9-tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, with neuroimaging-guided TMS in individuals with problematic cannabis use. The study examines the impact of the left dorsolateral prefrontal cortex (DLPFC) stimulation on cognitive outcomes impacted by THC intoxication, including the subjective response to THC and the impairing effects of THC on behavioral performance. A framework for integrating TMS with human behavioral pharmacology methods, along with key details of the study design, are presented. We also discuss challenges, alternatives, and future directions.

The effects of acute oral naltrexone pretreatment on the abuse potential of intranasal methamphetamine, and the relationship between reward/punishment sensitivity and methamphetamine's effects

One potential medication for treating methamphetamine use disorder is the opioid antagonist naltrexone (NLTX). Despite encouraging preclinical findings, the results of clinical studies have been mixed. The primary aim of the current trial was to examine the effects of acute NLTX pretreatment on the subjective and reinforcing effects of intranasal methamphetamine. Nonmedical psychostimulant users completed outpatient testing sessions in which they received oral placebo (0 mg) or NLTX (50 mg) before intranasal methamphetamine (30 mg/70 kg). Primary outcome measures were peak positive subjective effects (e.g. drug 'Liking') assessed on a visual analog scale (0-100), and methamphetamine self-administration using an operant self-administration task. Participants also completed a probabilistic categorization task to assess reward and punishment learning sensitivity. Complete data were available from 13 male and 1 transgender (male-to-female) participant (age: 33.4 ± 7.6 years). Intranasal methamphetamine significantly increased subjective ratings of drug 'Liking', 'Good Effect' and 'High' from baseline (P's < 0.01), but did not significantly vary as a function of placebo or NLTX pretreatment. Similarly, methamphetamine self-administration did not vary between the placebo and NLTX pretreatment conditions. This sample did not demonstrate a significant 'bias' in learning from positive and negative outcomes (i.e. reward and punishment sensitivity), and reward/punishment sensitivity was not correlated with the effects of methamphetamine or the effects of NLTX on methamphetamine. The current study argues against the use of NLTX as a stand-alone medication for treating methamphetamine use disorder.

A Brief Introduction to Human Behavioral Pharmacology: Methods, Design Considerations and Ethics

Human behavioral pharmacology methods have been used to rigorously evaluate the effects of a range of centrally acting drugs in humans under controlled conditions for decades. Methods like drug self-administration and drug discrimination have been adapted from nonhuman laboratory animal models. Because humans have the capacity to communicate verbally, self-report methods are also commonly used to understand drug effects. This perspective article provides an overview of these traditional human behavioral pharmacology methods and introduces some novel methodologies that have more recently been adapted for use in the field. Design (e.g., using placebo controls, testing multiple doses) and ethical (e.g., avoiding enrollment of individuals seeking treatment, determining capacity to consent) considerations that must be addressed when conducting these types of studies are also described.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Positron emission tomography (PET) imaging of nicotine-induced dopamine release in squirrel monkeys using [18F]Fallypride

BACKGROUND

Nicotine, the principal psychoactive tobacco constituent, is thought to produce its reinforcing effects via actions within the mesolimbic dopamine (DA) system. The objective of the current study was to examine the effect of nicotine on DA D₂/D₃ receptor availability in the nonhuman primate brain with the use of the radioligand [¹⁸F]fallypride and positron emission tomography (PET).

METHODS

Ten adult male squirrel monkeys were used in the current study. Each subject underwent two PET scans, one with an injection (IV) of saline and subsequently one with an injection of nicotine (0.032mg/kg). The DA D₂/D₃ antagonist, [¹⁸F]fallypride, was delivered IV at the beginning of each scan, and nicotine or saline was delivered at 45min into the scan. Regions of interest (ROI) were drawn on specific brain regions and these were used to quantify standard uptake values (SUVs). The SUV is defined as the average concentration of radioactivity in the ROI x body weight/injected dose. Using the cerebellum as a reference region, SUV ratios (SUV_ROI/SUV_cerebellum) were calculated to compare saline and nicotine effects in each ROI.

RESULTS

Two-way repeated ANOVA revealed a significant decrease of SUV ratios in both striatal and extrastriatal regions following an injection of nicotine during the PET scans.

CONCLUSIONS

Like other drugs of abuse, these results indicate that nicotine administration may produce DA release, as suggested by the decrease in [¹⁸F]fallypride signal in striatal regions. These findings from a nonhuman primate model provide further evidence that the mesolimbic DA system is affected by the use of products that contain nicotine.

Pharmacogenetics of stimulant abuse liability: association of CDH13 variant with amphetamine response in a racially-heterogeneous sample of healthy young adults

RATIONALE

A previous genome-wide association study (GWAS) in a predominately Caucasian sample of healthy young adults linked greater amphetamine-induced rewarding effects with the rs3784943 G allele of the cadherin 13 (CDH13; i.e., a cell adhesion molecule implicated in neuronal connectivity) gene. This association has not been subsequently examined, nor has it been studied in Asian populations, which may have greater frequencies of the risk allele.

OBJECTIVE

The objective of this study was to examine the association of rs3784943 with amphetamine response in a racially heterogeneous sample (37 % Asian) of healthy young adults.

METHODS

Participants (N = 84; 18-35 years old) genotyped for rs3784943 completed counterbalanced sessions involving 20 mg oral d-amphetamine or placebo administration. At both sessions, cardiovascular and subjective drug effects measures were collected.

RESULTS

In the combined racially heterogeneous sample, amphetamine (vs. placebo) effects were significantly greater on "Feel Drug" ratings (p < 0.05) and marginally greater on "Feel High" ratings and systolic blood pressure (p < 0.10) in G/A + G/G genotypes than A/A genotypes. The G allele was more common among Asian than other racial groups. Among the subsample of Asian participants (N = 31), drug effects were significantly greater on Feel Drug (p < 0.05) and marginally greater on Feel High and heart rate (p < 0.10) among Asians with G/A + G/G (vs. A/A) genotypes.

CONCLUSIONS

In concert with a previous GWAS result, this candidate gene study provides convergent evidence implicating CDH13 rs3784943 variant in d-amphetamine's drug effect profile and suggests generalization to Asian populations. CDH13 and genes coding for other cell adhesion molecules may be worthy of study in the biology of psychostimulant abuse liability.

Emotional traits predict individual differences in amphetamine-induced positive mood in healthy volunteers

BACKGROUND

Previous research on emotional correlates of individual differences in subjective responses to D-amphetamine has focused on relatively broad personality traits. Yet, emotional functioning is best characterized by several narrow subcomponents, each of which may contribute uniquely to amphetamine response. Here, we examine several specific subdomains of emotional functioning in relation to acute amphetamine response.

METHOD

At a baseline session, healthy stimulant-naive volunteers (N = 97) completed measures of several subdomains of baseline trait emotional functioning and then completed two counterbalanced experimental sessions during which they received a single oral dose of 20 mg D-amphetamine or placebo. Acute subjective drug response measures were completed at repeated intervals before and after drug administration. Data from subjective measures that were significantly modulated by amphetamine were reduced using principal component analysis (amphetamine or placebo) into three higher-order factors of "positive mood," "arousal," and "drug high." Amphetamine did not significantly alter any "negative" subjective states. Separate multiple regression analyses were conducted regressing these three drug factors on baseline trait emotional functioning scales.

RESULTS

The combined set of trait emotional functioning indicators accounted for approximately 22 % of the variance in acute amphetamine-induced positive mood changes. Greater anticipatory pleasure and greater anxious distress each uniquely predicted greater amphetamine-induced positive mood. Trait emotional functioning did not significantly predict amphetamine-induced changes in arousal or drug high.

DISCUSSION

Emotional traits appear to moderate drug-induced positive mood but not other dimensions of amphetamine effects. Different facets of emotional functioning may differentially modulate amphetamine's subjective effect profile.

The pharmacogenetics of alcohol use disorder

BACKGROUND

Annually, the use and abuse of alcohol contributes to millions of deaths and billions of dollars in societal costs. To determine the impact of genetic variation on the susceptibility to the disorder and its response to treatment, studies have been conducted to assess the contribution of a variety of candidate genetic variants. These variants, which we review here, were chosen based upon their observed or hypothesized functional relevance to alcohol use disorder (AUD) risk or to the mechanism by which medications used to treat the disorder exert their effects.

METHODS

This qualitative review examines studies in which candidate polymorphisms were tested as moderator variables to identify pharmacogenetic effects on either the subjective response to alcohol or the outcomes of pharmacotherapy.

RESULTS

Although findings from these studies provide evidence of a number of clinically relevant pharmacogenetic effects, the literature is limited and there are conflicting findings that require resolution.

CONCLUSIONS

Pharmacogenetic studies of AUD treatment that use greater methodological rigor and better statistical controls, such as corrections for multiple testing, may help to resolve inconsistent findings. These procedures could also lead to the discovery of more robust and clinically meaningful moderator effects. As the field evolves through methodological standardization and the use of larger study samples, pharmacogenetic research has the potential to inform clinical care by enhancing therapeutic effects and personalizing treatments. These efforts may also provide insights into the mechanisms by which medications reduce heavy drinking or promote abstinence in patients with an AUD.

Opioid-like effects of the neurokinin 1 antagonist aprepitant in patients maintained on and briefly withdrawn from methadone

BACKGROUND

Although opioid substitution therapy is an effective clinical tool used to manage opioid abuse and dependence, concerns regarding the current FDA-approved medications have lead to a search for efficacious, non-opioid medications. Preclinical data indicate that neurokinin 1 (NK1) receptor activity may modulate opioid effects and withdrawal. This investigation sought to examine the ability of the NK1 antagonist aprepitant to alter the effects of methadone as well as withdrawal symptoms induced by brief methadone discontinuation.

METHODS

This blinded, placebo-controlled, within-subjects study consisted of placebo and aprepitant conditions. Experimental assessments occurred on the first three days (days 1-3: placebo or aprepitant + methadone) and again on days 8-10 (aprepitant or placebo + methadone). Fifteen methadone-maintained patients completed the investigation. Outcome measures were the assessments of opioid withdrawal, as well as subjective measures of opioid-like effects.

RESULTS

Statistical trends indicated that aprepitant may reduce opioid withdrawal symptoms. When an active dose of aprepitant was administered an hour before methadone, participants reported less desire to use methadone. However, ratings of methadone "Liking" also appeared to increase.

CONCLUSIONS

These data tentatively suggest that aprepitant has some ability to alleviate withdrawal following methadone abstinence, but also appears to increase subjective indicators of methadone's abuse liability. Since few of the differences between aprepitant and placebo reached statistical significance, these data should only be viewed as preliminary. Findings from other studies indicate that higher doses of aprepitant may be more clinically effective. Further clinical investigations are needed in order to determine whether aprepitant is useful for alleviating opioid withdrawal.

A review of human drug self-administration procedures

Drug self-administration procedures in laboratory settings allow us to closely model drug-taking behavior in real-world settings. This review provides an overview of many of the common self-administration methods used in human laboratory research. Typically, self-administration studies provide a quantifiable measure of the reinforcing effect of a drug, which is believed to be predictive of its potential for abuse. Several adaptations of the self-administration paradigm exist, the simplest of which allows participants free access to the drug under investigation. Free-access procedures allow investigators to observe patterns of drug self-administration and drug effects in a controlled setting. Allowing participants to choose between two simultaneously available reinforcers (choice procedures) is another well-established method of assessing the reinforcing effects of a drug. Offering a choice between two reinforcers (e.g. two different doses of the same drug, two different drugs, or drug and nondrug reinforcers) provides researchers with a point of comparison (e.g. between a drug of known abuse potential and a novel drug). When combined with other endpoints, such as subjective effects ratings, physiological responses, and cognitive performance, human self-administration paradigms have contributed significantly to our understanding of the factors that contribute to, maintain, and alter drug-taking behavior including: craving, positive subjective effects, toxicity, drug interactions and abstinence. This area of research has also begun to incorporate other techniques such as imaging and genetics to further understand the multifaceted nature of substance abuse. The present paper summarizes the different self-administration techniques that are commonly used today and the application of other procedures that may complement interpretation of the drug self-administration findings.

Crowdsourcing black market prices for prescription opioids

Background

Prescription opioid diversion and abuse are major public health issues in the United States and internationally. Street prices of diverted prescription opioids can provide an indicator of drug availability, demand, and abuse potential, but these data can be difficult to collect. Crowdsourcing is a rapid and cost-effective way to gather information about sales transactions. We sought to determine whether crowdsourcing can provide accurate measurements of the street price of diverted prescription opioid medications.

Objective

To assess the possibility of crowdsourcing black market drug price data by cross-validation with law enforcement officer reports.

Methods

Using a crowdsourcing research website (StreetRx), we solicited data about the price that site visitors paid for diverted prescription opioid analgesics during the first half of 2012. These results were compared with a survey of law enforcement officers in the Researched Abuse, Diversion, and Addiction-Related Surveillance (RADARS) System, and actual transaction prices on a “dark Internet” marketplace (Silk Road). Geometric means and 95% confidence intervals were calculated for comparing prices per milligram of drug in US dollars. In a secondary analysis, we compared prices per milligram of morphine equivalent using standard equianalgesic dosing conversions.

Results

A total of 954 price reports were obtained from crowdsourcing, 737 from law enforcement, and 147 from the online marketplace. Correlations between the 3 data sources were highly linear, with Spearman rho of 0.93 (P<.001) between crowdsourced and law enforcement, and 0.98 (P<.001) between crowdsourced and online marketplace. On StreetRx, the mean prices per milligram were US$3.29 hydromorphone, US$2.13 buprenorphine, US$1.57 oxymorphone, US$0.97 oxycodone, US$0.96 methadone, US$0.81 hydrocodone, US$0.52 morphine, and US$0.05 tramadol. The only significant difference between data sources was morphine, with a Drug Diversion price of US$0.67/mg (95% CI 0.59-0.75) and a Silk Road price of US$0.42/mg (95% CI 0.37-0.48). Street prices generally followed clinical equianalgesic potency.

Conclusions

Crowdsourced data provide a valid estimate of the street price of diverted prescription opioids. The (ostensibly free) black market was able to accurately predict the relative pharmacologic potency of opioid molecules.

Personality and the acute subjective effects of d-amphetamine in humans

There is evidence that subjective responses to psychoactive drugs are related to personality traits. Here, we extend previous findings by examining personality measures in relation to acute responses to d-amphetamine (AMPH) in a large sample of healthy volunteers. Healthy adults (n=286) completed the Multidimensional Personality Questionnaire Brief Form (MPQ-BF) and participated in four sessions during which they received oral AMPH (0, 5, 10, 20 mg), under double-blind conditions. Subjective responses to the drug were measured using the Profile of Mood States, Addiction Research Center Inventory, and Drug Effects Questionnaire. Drug responses were reduced via principal components analysis to three higher-order factors ('Euphoria', 'Arousal', 'Dysphoria'). Participants were rank ordered on selected MPQ-BF scales; the top and bottom third on each trait were compared on the drug response factors. High trait physical fearlessness was significantly associated with greater amphetamine-related Arousal, and high trait reward sensitivity was significantly associated with greater Euphoria. In addition, high trait impulsivity was significantly associated with greater Arousal and Euphoria. These results provide further evidence that individual differences in the subjective effects of AMPH are partially explained by differences in personality, and are consistent with the idea that both personality and responses to stimulants depend upon shared neurochemical systems.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.