Controversies in translational research: drug self-administration

Self-administration of cocaine, cannabis and heroin in the human laboratory: benefits and pitfalls

The objective of this review is to describe self-administration procedures for modeling addiction to cocaine, cannabis and heroin in the human laboratory, the benefits and pitfalls of the approach, and the methodological issues unique to each drug. In addition, the predictive validity of the model for testing treatment medications will be addressed. The results show that all three drugs of abuse are reliably and robustly self-administered by non-treatment-seeking research volunteers. In terms of pharmacotherapies, cocaine use is extraordinarily difficult to disrupt either in the laboratory or in the clinic. A range of medications has been shown to significantly decrease cocaine's subjective effects and craving without decreasing either cocaine self-administration or cocaine abuse by patients. These negative data combined with recent positive findings with modafinil suggest that self-administration procedures are an important intermediary step between pre-clinical and clinical studies. In terms of cannabis, a recent study suggests that medications that improve sleep and mood during cannabis withdrawal decrease the resumption of marijuana self-administration in abstinent volunteers. Clinical data on patients seeking treatment for their marijuana use are needed to validate these laboratory findings. Finally, in contrast to cannabis or cocaine dependence, there are three efficacious Food and Drug Administration-approved medications to treat opioid dependence, all of which decrease both heroin self-administration and subjective effects in the human laboratory. In summary, self-administration procedures provide meaningful behavioral data in a small number of individuals. These studies contribute to our understanding of the variables maintaining cocaine, marijuana and heroin intake, and are important in guiding the development of more effective drug treatment programs.

Pharmacotherapy for cannabis dependence: how close are we?

Cannabis is the most widely used illicit drug in the world. Treatment admissions for cannabis use disorders have risen considerably in recent years, and the identification of medications that can be used to improve treatment outcomes among this population is a priority for researchers and clinicians. To date, several medications have been investigated for indications of clinically desirable effects among cannabis users (e.g. reduced withdrawal, attenuation of subjective or reinforcing effects, reduced relapse). Medications studied have included those: (i) known to be effective in the treatment of other drug use disorders; (ii) known to alleviate symptoms of cannabis withdrawal (e.g. dysphoric mood, irritability); or (iii) that directly affect endogenous cannabinoid receptor function. Results from controlled laboratory studies and small open-label clinical studies indicate that buspirone, dronabinol, fluoxetine, lithium and lofexidine may have therapeutic benefit for those seeking treatment for cannabis-related problems. However, controlled clinical trials have not been conducted and are needed to both confirm the potential clinical efficacy of these medications and to validate the laboratory models being used to study candidate medications. Although the recent increase in research towards the development of pharmacotherapy for cannabis use disorders has yielded promising leads, well controlled clinical trials are needed to support broad clinical use of these medications to treat cannabis use disorders.

The safety, tolerability, and subject-rated effects of acute intranasal cocaine administration during aripiprazole maintenance II: increased aripipirazole dose and maintenance period

OBJECTIVES

This experiment determined the safety and tolerability of intranasal cocaine during aripiprazole maintenance.

METHODS

Six cocaine-dependent human subjects were maintained on aripiprazole (15 mg) and placebo for 10 days in counterbalanced order prior to assessing the physiological and subject-rated effects of intranasal cocaine.

RESULTS

Intranasal cocaine produced prototypical stimulant-like effects (e.g., increased blood pressure and heart rate, increased subject ratings of Like Drug and Stimulated), and aripiprazole enhanced these effects on several measures.

CONCLUSIONS

Aripiprazole (15 mg/day) is safe and tolerable when combined with cocaine; however, the usefulness of aripiprazole as a treatment for cocaine-use disorders remains to be determined.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Evaluation of subjective effects of aripiprazole and methamphetamine in methamphetamine-dependent volunteers

A variety of neuropharmacological strategies are being pursued in the search for an effective treatment for methamphetamine (Meth) addiction. In this study, we investigated the safety and potential efficacy of aripiprazole, an antipsychotic agent acting on both dopamine and serotonin systems. We conducted a double-blind in-patient clinical pharmacology study to assess potential interactions between intravenous (i.v.) Meth (15 mg and 30 mg) and oral aripiprazole (15 mg). In addition, the effects of aripiprazole treatment on abstinence-related craving and cue-induced craving were evaluated. Participants included non-treatment-seeking, Meth-dependent patients (n=16), aged 18-45 yr, currently using Meth. Following baseline Meth dosing (15 mg and 30 mg), participants received 2 wk treatment with aripiprazole (n=8) or placebo (n=8). Participants then completed cue exposure sessions using neutral and Meth-related cues. Meth dosing (15 mg and 30 mg) was then repeated. Aripiprazole treatment had no effect on cue-induced Meth craving, or on daily baseline craving assessed over the course of medication treatment, although aripiprazole treatment was associated with increased craving independent of Meth dosing. Aripiprazole treatment was associated with significantly higher ratings on Addiction Research Center Inventory (ARCI) subscales reflecting euphoria and amphetamine-like effects following Meth dosing. Aripiprazole treatment was also associated with significant reductions in ratings of 'bad effects' and reductions on the ARCI subscale for sedation effects following Meth dosing. Aripiprazole treatment reduced the increase in systolic blood pressure following Meth dosing, but had no other effects on cardiovascular responses to Meth. Aripiprazole treatment did not alter the pharmacokinetics of Meth. These findings indicate that aripiprazole treatment appears to be safe in volunteers with Meth dependence, although the finding that aripiprazole increased some of the rewarding and stimulatory effects produced by acute Meth suggests that 15 mg aripiprazole is unlikely to be efficacious for the treatment of Meth dependence. Further research with lower doses of aripiprazole, possibly using study designs aimed at evaluating efficacy for relapse prevention, are needed before ruling out aripiprazole as a treatment for Meth dependence.

Reinforcing effects of stimulants in humans: sensitivity of progressive-ratio schedules

Drug self-administration methodologies have been developed for use in humans to model naturalistic stimulant drug-taking behaviors. These methodologies use a number of schedules of reinforcement, including progressive-ratio schedules. As the name implies, in a progressive-ratio schedule, the response requirement for each subsequent delivery of drug increases, and the primary outcome variable is often the break point (i.e., the last ratio completed to receive a drug delivery). These schedules have been used in a number of human laboratory studies evaluating the reinforcing effects of stimulants. The results of these studies have demonstrated that progressive-ratio schedules are sensitive to manipulation of a pharmacological variable, dose, and to nonpharmacological variables contributing to stimulant drug effects. In addition, findings with progressive-ratio schedules are largely concordant with clinical findings, suggesting that drug self-administration under these schedules has predictive validity in terms of drug abuse and dependence. Future research is necessary, however, to understand better how pharmacological factors like route of administration, onset of effects, and pretreatment influence the reinforcing effects of stimulants under progressive-ratio schedules.

A placebo-controlled trial of modafinil for nicotine dependence

BACKGROUND

Nicotine deprivation symptoms, including fatigue and attentional deficits, predict relapse following smoking cessation. Modafinil (Provigil), a wakefulness medication shown to have efficacy for the treatment of cocaine addiction, was tested as a novel therapy for nicotine dependence in a double-blind placebo-controlled trial.

METHODS

One hundred and fifty-seven treatment-seeking smokers received brief smoking cessation counseling and were randomized to: (1) 8 weeks of modafinil (200mg/day), or (2) 8 weeks of placebo. The primary outcome was biochemically verified 7-day point prevalence abstinence at the end of treatment (EOT). Secondary outcomes included cigarette smoking rate and post-quit nicotine deprivation symptoms (e.g., negative affect, withdrawal).

RESULTS

In this interim study analysis, EOT quit rates did not differ between treatment arms (42% for placebo vs. 34% for modafinil; OR=0.67 [0.34-1.31], p=0.24). Further, from the target quit date to EOT, the daily smoking rate was 44% higher among non-abstainers in the modafinil arm, compared to non-abstainers in the placebo arm (IRR=1.44, CI95=1.09-1.89, p<0.01). Modafinil-treated participants also reported greater increases in negative affect and withdrawal symptoms, vs. participants randomized to placebo (ps<0.05).

CONCLUSIONS

These data do not support the use of modafinil for the treatment of nicotine dependence and, as a consequence, this trial was discontinued. Cigarette smoking should be considered when modafinil is prescribed, particularly among those with psychiatric conditions that have high comorbidity with nicotine dependence.

Effects of the monoamine uptake inhibitors RTI-112 and RTI-113 on cocaine- and food-maintained responding in rhesus monkeys

Cocaine blocks uptake of the monoamines dopamine, serotonin and norepinephrine, and monoamine uptake inhibitors constitute one class of drugs under consideration as candidate "agonist" medications for the treatment of cocaine abuse and dependence. The pharmacological selectivity of monoamine uptake inhibitors to block uptake of dopamine, serotonin and norepinephrine is one factor that may influence the efficacy and/or safety of these compounds as drug abuse treatment medications. To address this issue, the present study compared the effects of 7-day treatment with a non-selective monoamine uptake inhibitor (RTI-112) and a dopamine-selective uptake inhibitor (RTI-113) on cocaine- and food-maintained responding in rhesus monkeys. Monkeys (N=3) were trained to respond for cocaine injections (0.01 mg/kg/inj) and food pellets under a second-order schedule [FR2(VR16:S)] during alternating daily components of cocaine and food availability. Both RTI-112 (0.0032-0.01 mg/kg/hr) and RTI-113 (0.01-0.056 mg/kg/h) produced dose-dependent, sustained and nearly complete elimination of cocaine self-administration. However, for both drugs, the potency to reduce cocaine self-administration was similar to the potency to reduce food-maintained responding. These findings do not support the hypothesis that pharmacological selectivity to block dopamine uptake is associated with behavioral selectivity to decrease cocaine- vs. food-maintained responding in rhesus monkeys.

Challenges for translational psychopharmacology research--some basic principles

We introduce below several principles that recur in the discussion of translating preclinical findings to clinical applications, and conversely, developing animal models of human disorders: 1. The translation of preclinical data to clinical concerns is more successful when the scope of experimental models is restricted to a core symptom of a psychiatric disorder. 2. Preclinical experimental models gain in clinical relevance if they incorporate conditions that induce maladaptive behavioral or physiological changes that have some correspondence with species-normative behavioral adaptations. 3. Preclinical data are more readily translated to the clinical situation when they are based on converging evidence from several experimental procedures, each capturing cardinal features of the disorder. 4. The more closely a model approximates significant clinical symptoms, the more likely it is to generate data that will yield clinical benefits. 5. The choice of environmental, genetic, and/or physiological manipulations that induce a cardinal symptom or cluster of behavioral symptoms reveals the theoretical approach used to construct the model. 6. Preclinical experimental preparations that are validated by predicting treatment success with a prototypic agent are only able to detect alternative treatments that are based on the same mechanism as the existing treatment that was used to validate the screen. 7. The degree to which an experimental model fulfills the criteria of high construct validity relative to face or predictive validity depends on the purpose of the model. 8. Psychological processes pertinent to affect and cognition can only be studied in preclinical models if they are defined in behavioral and neural terms.

The role of human drug self-administration procedures in the development of medications

The purpose of this review is to illustrate the utility and value of employing human self-administration procedures in medication development, including abuse liability assessments of novel medications and evaluation of potential pharmacotherapies for substance use disorders. Traditionally, human abuse liability testing has relied primarily on subjective reports describing drug action by use of questionnaires; similarly, drug interactions between putative treatment agents and the drugs of abuse have relied on these measures. Subjective reports are highly valued because they provide qualitative and quantitative information about the characteristics of central and peripheral pharmacodynamic effects as well as safety and tolerability. However, self-administration procedures directly examine the behavior of interest-that is, drug taking. The present paper (1) reviews the most commonly used human self-administration procedures, (2) discusses the concordance of subjective reports and self-administration within the context of medications development for substance use disorders, focusing primarily on illustrative examples from development efforts with opioid and cocaine dependence, and (3) explores the utility of applying self-administration procedures to assess the abuse liability of novel compounds, including "abuse-deterrent" formulations (ADFs). The review will focus on opioid and cocaine dependence because a rich database from both clinical laboratory and clinical trial research exists for these two drug classes. The data reviewed suggest that drug-induced changes in self-administration and subjective effects are not always concordant. Therefore, assessment of self-administration in combination with subjective effects provides a more comprehensive picture that may have improved predictive validity for translating to the clinical setting.