Known Drugs Of Abuse Research Articles

Investigating the potential of mirtazapine to induce drug-seeking behavior in free-choice drinking mouse model

Addiction to various drugs and chemicals is a significant public health concern worldwide. Addiction to prescription medications has increased due to the psychoactive effects of these medications, their availability, low price, and the lack of legal consequences for abusers. One of such prescription medication is mirtazapine (MIRT). MIRT is an antidepressant that has recently been reported to be abused and could induce withdrawal symptoms in different case studies. No previous study has investigated its abuse potential in animal models of drug addiction. Here, we conducted a free-choice drinking paradigm to investigate voluntary drinking of MIRT at two different concentrations. Male BALB/c mice were given unlimited access to two water bottles for five days before being divided into three groups: the first group had free access to two water bottles. The second group (MIRT10) and the third group (MIRT20) was allowed unlimited choice to one bottle of water and one bottle of MIRT at concentrations of 0.03 and 0.06 mg/mL, respectively. The average daily MIRT intake in the MIRT20 group was significantly higher on all tested days than that in the MIRT10 group. Moreover, mice in the MIRT20 group preferred to self-administer MIRT over water, indicating that MIRT can induce drug-seeking behavior. To further investigate the addictive potential of MIRT and its possible deterioration of memory and recognition, as reported with several known drugs of abuse, animals underwent a novel object recognition test. Mice in the MIRT20 group demonstrated significant deterioration in memory and recognition, indicating its effects on different brain regions involved in recognition, similar to other known drugs of abuse. The forced swimming test and tail suspension test were used to test MIRT-induced withdrawal symptoms after forced abstinence. After eight days of abstinence, mice in the MIRT20 group demonstrated significant depression-like symptoms in both the TST and FST, manifested by a significant increase in immobility time. MIRT was shown to induce drug-seeking behavior, deteriorate recognition, and cause withdrawal symptoms. This might confirm that MIRT has the potential to induce drug dependence and further studies are warranted to explore the neurobiological basis of MIRT-induced drug-seeking behavior.

Δ9-tetrahydrocannabinol: Drug discrimination abuse liability testing in female Lister Hooded rats: Trials, tribulations and triumphs

IntroductionThe assessment of the abuse potential of CNS-active drugs is a regulatory requirement. Drug discrimination is one of the nonclinical tests that contribute to this assessment by providing information on a drug's potential to induce a discriminative stimulus comparable to that of a known drug of abuse. AimThe objective was to validate drug discrimination in the rat for the purpose of supporting regulatory submissions for novel drugs with potential cannabinoid-like activity. MethodsTen female Lister hooded rats were trained to discriminate no-drug from Δ9-THC (1.5 mg/kg, IP) under a FR10 schedule of reinforcement. Once trained, a Δ9-THC dose-response curve was obtained using doses of 0.25, 0.75, 1.5, and 3 mg/kg, IP. This was followed by evaluation of amphetamine (0.3 mg/kg, SC); morphine (3 mg/kg, IP); midazolam (2.5 mg/kg, PO); and the synthetic cannabinoids WIN55,212–2 (0.75 to 2 mg/kg, IP), CP-47,497 (0.5 to 2 mg/kg, IP), and JWH-018 (1 mg/kg, IP) for their discriminative stimulus similarity to Δ9-THC. ResultsPharmacological specificity was demonstrated by achieving the anticipated dose-response curve for Δ9-THC, and a vehicle-like response for the non-cannabinoid drugs. Although full generalisation was obtained for JWH-018, in contrast to published literature, WIN55,212–2 and CP-47,497 failed to generalise to Δ9-THC. DiscussionBased on the literature review performed in light of the results obtained, contrasting and unpredictable behavioural responses produced by cannabinoids in animals and humans raises the question of the reliability and relevance of including drug discrimination and self-administration studies within an abuse potential assessment for novel cannabinoid-like drugs.

Numerical Analysis of Time-Dependent Inhibition by MDMA.

Methylenedioxymethamphetamine (MDMA) is a known drug of abuse and schedule 1 narcotic under the Controlled Substances Act. Previous pharmacokinetic work on MDMA used classic linearization techniques to conclude irreversible mechanism-based inhibition of CYP2D6. The current work challenges this outcome by assessing the possibility of two alternative reversible kinetic inhibition mechanisms known as the quasi-irreversible (QI) model and equilibrium model (EM). In addition, progress curve experiments were used to investigate the residual metabolism of MDMA by liver microsomes and CYP2D6 baculosomes over incubation periods up to 30 minutes. These experiments revealed activity in a terminal linear phase at the fractional rates with respect to initial turnover of 0.0354 ± 0.0089 in human liver microsomes and 0.0114 ± 0.0025 in baculosomes. Numerical model fits to percentage of remaining activity (PRA) data were consistent with progress curve modeling results, wherein an irreversible inhibition pathway was found unnecessary for good fit scoring. Both QI and EM kinetic mechanisms fit the PRA data well, although in CYP2D6 baculosomes the inclusion of an irreversible inactivation pathway did not allow for convergence to a reasonable fit. The kinetic complexity accessible to numerical modeling has been used to determine that MDMA is not an irreversible inactivator of CYP2D6, and instead follows what can be generally referred to as slowly reversible inhibition. SIGNIFICANCE STATEMENT: The work herein describes the usage of computational models to delineate between irreversible and slowly reversible time-dependent inhibition. Such models are used in the paper to analyze MDMA and classify it as a reversible time-dependent inhibitor.

High-throughput and Comprehensive Drug Surveillance Using Multisegment Injection-capillary Electrophoresis Mass Spectrometry.

New analytical methods are urgently needed to enable high-throughput, yet comprehensive drug screening, given an alarming opioid and prescription drug crisis in public health. Conventional urine drug testing based on a two-tier immunoassay screen followed by a gas chromatography-tandem mass spectrometry (GC-MS/MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) method are expensive and prone to bias while being limited to targeted panels of known drugs of abuse (DoA). Herein, we outline an improved method for drug surveillance that allows for the resolution and detection of an expanded panel of DoA and their metabolites when using multisegment injection-capillary electrophoresis-mass spectrometry (MSI-CE-MS). Multiplexed separations of ten urine samples with a quality control by CE (< 3 min/sample) in conjunction with full-scan data acquisition using a time-of-flight mass spectrometer (TOF-MS) under positive ion mode detection allows for the identification and quantification of DoA above recommended cut-off levels. An excellent resolution of drug isomers and isobars, including background interferences, are achieved when using MSI-CE-MS with an electrokinetic spacer between sample segments, where accurate mass/molecular formula together with the comigration of a matching deuterated internal standard and the detection of one or more bio-transformed metabolites facilitate DoA identification over a wider detection window. Additionally, urine samples can be analyzed directly without enzyme deconjugation for the rapid screening without complicated sample workup. MSI-CE-MS enables the surveillance of a broad spectrum of DoA that is required for the treatment monitoring of high-risk patients, including confirming prescribed drug adherence, revealing illicit drug use/substitution, and evaluating optimal dosage regimes as required for new advances in precision medicine.

Human cytochrome P450 kinetic studies on six N-2-methoxybenzyl (NBOMe)-derived new psychoactive substances using the substrate depletion approach

A huge number of new chemical derivatives of known drugs of abuse, so-called new psychoactive substances (NPS), are sold and consumed without prior preclinical and clinical testing. For assessing the elimination behaviors, determination of the kinetic constants Km and Vmax of the cytochrome P450 (CYP) isoforms involved in the hepatic metabolism of NPS could help to predict their contributions to hepatic clearance, drug–drug interactions and polymorphisms. Therefore, the aims of the present study were to determine the Km and Vmax values for CYP isoforms using the substrate depletion approach for the six N-2-methoxybenzyl (NBOMe)-derived NPS 25B-NBOMe, 25C-NBOMe, 25I-NBOMe, 3,4-DMA-NBOMe, 4-EA-NBOMe, and 4-MMA-NBOMe. Furthermore, the contributions of each CYP isozyme to the hepatic net clearance were elucidated using the relative activity factor approach. Several CYPs including CYP1A2, CYP2B6, CYP2C19, CYP2D6, and CYP3A4 were identified to be involved in the metabolism of the investigated compounds. The determined Km values ranged from 0.010 μM (CYP2D6, 4-MMA-NBOMe) to 13 μM (CYP2B6, 4-EA-NBOMe). All NBOMes were good substrates of CYP2C19 and CYP2D6 resulting in very low Km values in the nanomolar range. The main contributors to hepatic net clearance were CYP2D6 for 25B-NBOMe (69%), 25C-NBOMe (83%), 25I-NBOMe (61%), 3,4-DMA-NBOMe (89%) as well as for 4-EA-NBOMe (62%) and CYP2C19 for 4-MMA-NBOMe (64%). As more than one isoform was involved in the particular steps, the risk of harm associated with drug–drug interactions might be considered low. However, in cases where substances with high contributions from polymorphically expressed CYP2C19 and CYP2D6 are encountered, inter-individual variations in metabolism and excretion cannot be excluded.

Molecular mechanisms of serotonergic action of the HIV-1 antiretroviral efavirenz

Efavirenz is highly effective at suppressing HIV-1, and the WHO guidelines list it as a component of the first-line antiretroviral (ARV) therapies for treatment-naïve patients. Though the pharmacological basis is unclear, efavirenz is commonly associated with a risk for neuropsychiatric adverse events (NPAEs) when taken at the prescribed dose. In many patients these NPAEs appear to subside after several weeks of treatment, though long-term studies show that in some patients the NPAEs persist. In a recent study focusing on the abuse potential of efavirenz, its receptor psychopharmacology was reported to include interactions with a number of established molecular targets for known drugs of abuse, and it displayed a prevailing behavioral profile in rodents resembling an LSD-like activity. In this report, we discovered interactions with additional serotonergic targets that may be associated with efavirenz-induced NPAEs. The most robust interactions were with 5-HT3A and 5-HT6 receptors, with more modest interactions noted for the 5-HT2B receptor and monoamine oxidase A. From a molecular mechanistic perspective, efavirenz acts as a 5-HT6 receptor inverse agonist of Gs-signaling, 5-HT2A and 5-HT2C antagonist of Gq-signaling, and a blocker of the 5-HT3A receptor currents. Efavirenz also completely or partially blocks agonist stimulation of the M1 and M3 muscarinic receptors, respectively. Schild analysis suggests that efavirenz competes for the same site on the 5-HT2A receptor as two known hallucinogenic partial agonists (±)-DOI and LSD. Prolonged exposure to efavirenz reduces 5-HT2A receptor density and responsiveness to 5-HT. Other ARVs such as zidovudine, nevirapine and emtricitabine did not share the same complex pharmacological profile as efavirenz, though some of them weakly interact with the 5-HT6 receptor or modestly block GABAA currents.

Abuse liability assessment in preclinical drug development: Predictivity of a translational approach for abuse liability testing using methylphenidate in four standardized preclinical study models

ObjectivesPreclinical abuse liability assessment of novel clinical CNS-active candidates involves several tests, addressing different aspects characteristic for abuse potential, which are considered predictive for substance abuse of these candidates, thus ensuring an appropriate translational approach. To demonstrate how such a strategy could work, a known drug of abuse, methylphenidate was evaluated in a full rodent test battery, comprising four test models, and in accordance with the requirements of the FDA, ICH and EMA guidelines. MethodsMethylphenidate was tested orally at 2.5, 5 or 10mg/kg for its physical dependence potential in a repeated dose non-precipitated withdrawal test, for its drug profiling in a drug discrimination learning procedure (single escalating doses), and for its reinforcing properties in a conditioned place preference test (alternate dosing days) and an intravenous self-administration procedure (0.05 to 1mg/kg/IV infusion during 5 daily 1-h test sessions). The stimulant d-amphetamine served as positive control and was administered subcutaneously at 0.8mg/kg in the first three test models. In the intravenous self-administration procedure rats were habituated to intravenously self-administer d-amphetamine at 0.06mg/kg/IV infusion prior to methylphenidate substitution. ResultsCessation of subchronic dosing up to 10mg/kg methylphenidate led to sustained or even exacerbated effects on locomotion and behavior, body temperature, body weight, food consumption, and alteration of the diurnal rhythm during withdrawal. Clear generalization to d-amphetamine was obtained in the drug discrimination test at 5 and 10mg/kg. Distinct reinforcing properties were present in the conditioned place preference test at 10mg/kg and in the intravenous self-administration study from 0.05mg/kg/IV infusion onwards. The maximum plasma exposure after oral administration of methylphenidate over the dose ranges tested in the present rat studies covered at least 1.9-fold to 18.9-fold the recommended human therapeutic exposure of 10ng/ml, a plasma level that is considered representative of the human efficacious methylphenidate dose. The ratio Cmax Hu/rat calculated from the intravenous self-administration data ranged from 14.9 to 576.5. Consequently the regulatory requirements, stating that preclinical drug abuse liability studies should include high doses that produce plasma levels that are multiples of the therapeutic dose were fulfilled (FDA, EMA, ICH). DiscussionThe presented preclinical models, implemented within a drug development environment, were considered highly predictive to assess the abuse potential of methylphenidate, and in accordance with the regulatory requirements of drug licensing authorities in terms of appropriate methods, dose selection and subsequent plasma exposure.

The HIV Antiretroviral Drug Efavirenz has LSD-Like Properties

Anecdotal reports have surfaced concerning misuse of the HIV antiretroviral medication efavirenz ((4S)-6-chloro-4-(2-cyclopropylethynyl)-4-(trifluoromethyl)-2,4-dihydro-1H-3,1-benzoxazin-2-one) by HIV patients and non-infected teens who crush the pills and smoke the powder for its psychoactive effects. Molecular profiling of the receptor pharmacology of efavirenz pinpointed interactions with multiple established sites of action for other known drugs of abuse including catecholamine and indolamine transporters, and GABAA and 5-HT(2A) receptors. In rodents, interaction with the 5-HT(2A) receptor, a primary site of action of lysergic acid diethylamine (LSD), appears to dominate efavirenz's behavioral profile. Both LSD and efavirenz reduce ambulation in a novel open-field environment. Efavirenz occasions drug-lever responding in rats discriminating LSD from saline, and this effect is abolished by selective blockade of the 5-HT(2A) receptor. Similar to LSD, efavirenz induces head-twitch responses in wild-type, but not in 5-HT(2A)-knockout, mice. Despite having GABAA-potentiating effects (like benzodiazepines and barbiturates), and interactions with dopamine transporter, serotonin transporter, and vesicular monoamine transporter 2 (like cocaine and methamphetamine), efavirenz fails to maintain responding in rats that self-administer cocaine, and it fails to produce a conditioned place preference. Although its molecular pharmacology is multifarious, efavirenz's prevailing behavioral effect in rodents is consistent with LSD-like activity mediated via the 5-HT(2A) receptor. This finding correlates, in part, with the subjective experiences in humans who abuse efavirenz and with specific dose-dependent adverse neuropsychiatric events, such as hallucinations and night terrors, reported by HIV patients taking it as a medication.

Abuse Potential of Non-Nicotine Tobacco Smoke Components: Acetaldehyde, Nornicotine, Cotinine, and Anabasine

This review identified published animal studies evaluating the possible abuse potential of acetaldehyde, nornicotine, cotinine, and anabasine based on five commonly used paradigms. These include their effects on midbrain dopamine (DA) levels, drug discrimination and substitution for known drugs of abuse, place conditioning, self-administration behavior, and somatic withdrawal symptoms. Acetaldehyde had mixed effects on midbrain DA levels and drug discrimination; however, it consistently produced a conditioned place preference and supported self-administration. The single available study on withdrawal found that cessation of acetaldehyde administration resulted in a somatic withdrawal syndrome. Nornicotine increased DA in the midbrain, especially in the nucleus accumbens. Although there are no data on place conditioning, it substituted for nicotine in drug discrimination testing, partially substituted for cocaine and amphetamine, and, though only a single study, supported self-administration. Anabasine increased midbrain DA levels and that it partially substituted for nicotine in drug discrimination testing. Cotinine increased midbrain DA levels and substituted for nicotine. The existing literature suggests that acetaldehyde and nornicotine likely possess abuse potential, with anabasine having possible abuse potential. Although some cotinine data were available, it was insufficient to draw conclusions about possible abuse potential. Further research is needed to determine the role of minor alkaloids on tobacco dependence.

Behavioral and toxicological effects of propofol

There is increasing concern about abuse of propofol, a widely-used surgical anesthetic and sedative that is currently not a controlled substance. The purpose of this study was to establish a rat model of the psychoactive effect of subanesthetic doses of propofol that could be useful for confirming abuse liability and for studying mechanisms of propofol abuse. Sprague-Dawley rats were trained to discriminate propofol (10 mg/kg, intraperitoneally) from vehicle (2% methylcellulose). Carisoprodol (100 mg/kg), chlordiazepoxide (10 mg/kg), and dizocilpine (0.1 mg/kg) were tested for substitution for the discriminative-stimulus effects of propofol (10 mg/kg), whereas pentylenetetrazol (10 mg/kg) was tested for antagonism of the discriminative-stimulus effects. Propofol (10 mg/kg) was tested for substitution in rats trained to discriminate carisoprodol from vehicle. Carisoprodol produced 59% propofol-appropriate responding, chlordiazepoxide produced 65% propofol-appropriate responding, and dizocilpine produced 34% propofol-appropriate responding. Pentylenetetrazol decreased propofol-appropriate responding to 41%. Propofol produced 52% carisoprodol-appropriate responding. Mortality rate during training of propofol (10 mg/kg) was 38%. Postmortem examination revealed cardiovascular abnormalities similar to those observed in propofol-infusion syndrome in humans. The results demonstrate that propofol can be trained as a discriminative stimulus. Its discriminative-stimulus effects were more similar to compounds promoting γ-aminobutyric acid-A receptor activity than to a compound inhibiting N-methyl-d-aspartate receptor activity. As propofol has discriminative-stimulus effects similar to known drugs of abuse, and occasions a high-mortality rate, its potential for continued abuse is of particular concern.

Current approaches and issues in non-clinical evaluation of abuse and dependence

Preclinical assessment of drug abuse and dependence has been the subject of several recent regulatory guidelines. Both the European and US authorities recommend a tiered approach and are generally aligned on the methods which should be used. The first tier simply compares the pharmacology of the novel substance to known drugs of abuse. The second tier aims to identify abuse and dependence liability more directly. The most direct approach to assessing reinforcing properties is the i.v. self-administration procedure. Unfortunately there is no standardized procedure for evaluating substances with differing potencies, reinforcement properties or pharmacokinetics (PK). Indeed, the choice of training substance, species and procedural parameters can radically affect the outcome. Apart from the lower cost of the rat, the primate presents several advantages for self-administration studies (potentially greater similarity to humans in behavioral effects, active doses and PK). Although it does not measure abuse liability directly, drug discrimination is a powerful method for assessing the similarity of a test substance to a known drug of abuse. In this procedure an animal uses the interoceptive effects of the substance as the discriminative stimulus to determine which of two responses to make. For certain classes of substance, such as hallucinogens acting via the 5-HT 2A receptor, discrimination is the only procedure currently able to identify them. Drug dependence is assessed by the occurrence of withdrawal effects on drug discontinuation. Although conceptually simple, many factors (duration and frequency of drug treatment, dose/exposure levels, duration of observation after discontinuation) can complicate interpretation. Telemetry may represent a novel approach which allows continuous observation of somatic and behavioral parameters during drug withdrawal thereby increasing sensitivity. Presently available tools can identify essentially all substances known to cause abuse or dependence with little risk of false positives. It remains unclear how effective these models will be with entirely novel substances. Nonetheless, drug abuse/dependence is an area of safety pharmacology where the predictive value of animal models remains very high.

Drugs of Abuse-Induced Hyperthermia, Blood-Brain Barrier Dysfunction and Neurotoxicity: Neuroprotective Effects of a New Antioxidant Compound H-290/51

The psychostimulants, morphine and methamphetamine are well known drugs of abuse that induce brain pathology and/or neurodegeneration resulting in a huge burden on our society. The possible mechanisms of psychostimulants induced neuropathology and neurodegeneration are still not well known. The drugs of abuse results in profound hyperthermia and widespread alterations in neurochemical metabolism in the central nervous system (CNS). It appears that psychostimulants induced hyperthermia and/or release of neurochemicals influence the blood-brain barrier (BBB) dysfunction leading to brain pathology. The drugs of abuse also induce oxidative stress resulting in generation of free radicals and lipid peroxidation. Thus, further research is needed to understand the basic function of BBB disruption and temperature regulation by psychostimulants and to modify them pharmacologically to attenuate brain dysfunction and neuropathology. This review is focused on the problems of morphine and methamphetamine induced hyperthermia and their effects on breakdown of the BBB function leading to brain damage. Work done in our laboratory suggest that hyperthermia caused by these drugs is responsible for BBB disruption and neurodegeneration. This hypothesis is further supported by our observation that pretreatment with a potent antioxidant compound H-290/51 attenuates the BBB disruption and induces marked neuroprotection following morphine induced withdrawal and methamphetamine induced neurotoxicity. The possible mechanisms and functional significance of these findings are discussed.

Drosophila GABA B receptors are involved in behavioral effects of γ-hydroxybutyric acid (GHB)

γ-hydroxybutyric acid (GHB) can be synthesized in the brain but is also a known drug of abuse. Although putative GHB receptors have been cloned, it has been proposed that, similar to the behavior-impairing effects of ethanol, the in vivo effects of pharmacological GHB may involve metabotropic γ-aminobutyric acid (GABA) GABA B receptors. We developed a fruitfly ( Drosophila melanogater) model to investigate the role of these receptors in the behavioral effects of exogenous GHB. Injecting GHB into male flies produced a dose-dependent motor impairment (measured with a computer-assisted automated system), which was greater in ethanol-sensitive cheapdate mutants than in wild-type flies. These effects of pharmacological concentrations of GHB require the presence and activation of GABA B receptors. The evidence for this was obtained by pharmacological antagonism of GABA B receptors with CGP54626 and by RNA interference (RNAi)-induced knockdown of the GABA B(1) receptor subtype. Both procedures inhibited the behavioral effects of GHB. GHB pretreatment diminished the behavioral response to subsequent GHB injections; i.e., it triggered GHB tolerance, but did not produce ethanol tolerance. On the other hand, ethanol pretreatment produced both ethanol and GHB tolerance. It appears that in spite of many similarities between ethanol and GHB, the primary sites of their action may differ and that recently cloned putative GHB receptors may participate in actions of GHB that are not mediated by GABA B receptors. These receptors do not have a Drosophila orthologue. Whether Drosophila express a different GHB receptor should be explored.

Cannabinoid withdrawal is dependent upon PKA activation in the cerebellum.

Region-specific up-regulation of the cyclic AMP pathway is considered an important molecular mechanism in the origin of the somatic manifestations of the withdrawal syndrome to known drugs of abuse. Nevertheless, the existence of a withdrawal syndrome after prolonged cannabinoid administration has long been a controversial issue. Recent studies, in different species, have shown that withdrawal to prolonged cannabinoid exposure precipitated by the cannabinoid antagonist SR141716A is characterized by physical signs underlying impairment of motor coordination. Interestingly, cannabinoid withdrawal is accompanied by an increase of adenylyl cyclase activity in the cerebellum. Here, we investigate the functional role of the cyclic AMP pathway in the cerebellum in the establishment of cannabinoid withdrawal. We show that after SR141716A precipitation of cannabinoid withdrawal, basal and calcium-calmodulin-stimulated adenylyl cyclase activities as well as active PKA in the cerebellum increase in a transient manner with a temporal profile which matches that of the somatic expression of abstinence. Selectively blocking the up-regulation of the cyclic AMP pathway in the cerebellum, by microinfusing the cyclic AMP blocker Rp-8Br-cAMPS in this region, markedly reduced both PKA activation and the somatic expression of cannabinoid withdrawal. Our results (i) directly link the behavioural manifestations of cannabinoid withdrawal with the up-regulation of the cyclic AMP pathway in the cerebellum, pointing towards common molecular adaptive mechanisms for dependence and withdrawal to most drugs of abuse; (ii) suggest a particular role for the cerebellum as a major neurobiological substrate for cannabinoid withdrawal.

Abuse liability assessment of neuroprotectants.

There has been considerable interest in the potential of N-methyl-D-aspartate (NMDA) receptor antagonists in the treatment of a diverse group of neurological disorders including cerebral ischemia and neurodegeneration. The amino acids L-glutamate and L-aspartate have been shown to possibly mediate excitatory synaptic transmission in the central nervous system (CNS) via selective excitatory amino acid receptors. Competitive and noncompetitive antagonists acting at the NMDA receptors have been shown to possess relevant activity. However, NMDA antagonists can produce a variety of adverse neurobehavioral effects in both animals and humans. These adverse events are particularly pronounced with NMDA antagonists (phencyclidine (PCP), ketamine, and MK-801) that have dissociative anesthetic properties and block NMDA receptor-mediated responses by binding to the cation channel of the NMDA receptor complex. When a new pharmaceutical product demonstrates structural similarity and/or a similar pharmacological profile with a known drug of abuse, the characterization of its abuse potential is needed by the FDA for scientific review. The abuse liability assessment is based upon an evaluation of data on the chemistry, pharmacology (preclinical and clinical), pharmacokinetics, and pharmacodynamic profiles of the drug, and the adverse events/effects reported in clinical trials. The evaluation of the drug's abuse potential is determined relative to pharmacologically similar drugs. This includes determination of the drug's receptor binding efficacy, preclinical pharmacology, reinforcing efficacy, discriminative stimulus effects, dependence-producing potential, pharmacokinetics, and assessment of the clinical efficacy-safety database relative to abuse and clinical abuse liability studies. It has been well established that high-affinity noncompetitive NMDA antagonists have reinforcing efficacy and can serve as discriminative stimuli in operant procedures. In a variety of species in drug discrimination studies, each antagonist is capable of generalizing to the others, and it is believed that these effects may be mediated through the NMDA blockade. The generalization of each substance for another suggests production of common subjective effects in humans.

Drug addiction research: moving toward the 21st century

Advances in science over the last 2 decades have revolutionized our understanding of drug abuse and addiction and continue to significantly impact how we as a Nation approach this major public health problem. We now have strong scientific evidence to show that addiction is a chronic, relapsing disease of the brain that has imbedded behavioral and social context Ž aspects e.g. Leshner, 1997; Nestler and Aghajanian, . 1997; O’Brien, 1997 . Moreover, like other complex brain diseases, addiction’s successful treatment requires what some refer to as ‘whole person treatment’ Ž . Coletti, personal communication attending to all of addiction’s biological, behavioral and social context aspects. We know more about abused drugs and the brain than is known about almost any other aspect of brain function. As discussed throughout this issue, scientists have identified neural circuits that subsume the actions of every known drug of abuse, and they have specified common pathways that are affected by almost all such drugs. Researchers have also identified the primary receptors for every major class of abused drug; identified their genetic code and cloned these receptors; mapped their locations in the brain; and determined the neurotransmitter systems involved. In addition, they have elaborated many of the biochemical cascades within the cell that follow receptor activation by drugs. Moreover, although there appear to be some idiosyncratic effects of each drug taken; there also appears to be some common effects. For example, many studies suggest that independent of a drug’s initial site of action, virtually every drug } be it nicotine, heroin, cocaine, or amphetamine } appears to stimulate increases in dopamine in the nucleus accumbens. This increase in dopamine secretion is believed by many investigators to be a critical element in what continues drug taking behavior. Research has also begun to reveal major differences between the brains of addicted and non-addicted individuals. This is what fundamentally makes addiction a brain disease; it is a condition of changes in brain structure and function. A variety of studies in both humans and other animals have demonstrated that chronic drug use changes the brain in fundamental ways that persist long after the individual has stopped taking the drug. In addition, long-term use of a variety of different drugs has been found to lead to common changes at the cellular and molecular level. We do not yet know which of these changes constitute the switch in state, from voluntary drug use to compulsive, often uncontrollable drug craving, seeking and use, that constitutes addiction. However, we do know that addicted brains are quite different from non-addicted brains and these observable brain changes appear to coincide with the behavioral changes that also occur as addiction develops. It is because of the many recent efforts in clinical neurobiology research that we are able to gain insight into the precise ways in which the brains of human drug abusers are altered. New technologies allow us to observe changes in brain function as individuals move from one state to another or through one phase of abuseraddiction to another. We can now assess in awake and functioning patients many different parameters, such as blood flow, neural activity, receptor and transmitter function, drug pharmacokinetics and pharmacodynamics, as well as some of the neurotoxic effects drugs may have on the human brain. One of the most significant breakthroughs has been the identification of areas of the brain that are specifically involved in the phenomenon of craving. Many clinicians believe that craving is a central factor in many cases of drug relapse, and, therefore, understanding the systems that mediate this phenomenon may provide a foundation for developing much more effective treatments to prevent or reverse the addiction process.

Human drug abuse liability assessment: opioids and analgesics

Methods are described for assessment of the abuse liability of opioids and analgesics. The methods developed by the USPHS Addiction Research Center for assessment of opioid abuse liability have served as the prototype for most currently used abuse liability assessment approaches. The general method is that of comparing the profile of effects of a test drug to the profiles of placebo and of a known drug of abuse. Drugs with profiles of activity similar to the known drug of abuse are considered pharmacologically equivalent and of similar abuse liability. Specific methods, including the Addiction Research Inventory, adjective- and visual analog rating scales, are described, as are methodological developments that have been introduced to improve the sensitivity of the procedures and to adapt them to modern circumstances. It is concluded that the profile-of-effects methodology has proven extremely useful and should remain a basic element in abuse liability assessments.

Drug abuse potential evaluation in animals

Animal laboratory studies can provide useful information concerning the potential of drugs for abuse. Over the past 50 years, methods have been developed for use with animal subjects which allow the evaluation of pharmacological properties of drugs which are particularly relevant to their abuse. The methods for preclinical drug abuse liability testing are reviewed under six heading: (1) establishment of the degree of pharmacological equivalence to known drugs of abuse, (2) drug discrimination studies, (3) tests of tolerance and cross-tolerance, (4) tests of physical dependence capacity, (5) drug self-administration tests of reinforcing effects, and (6) evaluation of toxicity and performance impairment at self-administered doses. Preclinical studies can be helpful early in drug development to select lead compounds with low abuse potential for further study. In the case of new or already marketed medications, animal testing can often compliment and extend abuse liability evaluation in human subjects. The results of abuse potential evaluation studies can be useful in making decisions about the possible need for regulation under national and international drug laws, and thus play an important role in drug abuse prevention.

Preference procedures for testing the abuse liability of drugs in humans

Drug preference procedures may be used to assess the abuse liability of drugs in human volunteers. These procedures usually measure subjects' choice of a drug versus a placebo, under controlled, double-blind conditions. Depending upon the conditions of the studies (e.g. subject population, environmental conditions of testing), drugs that are known to be abused are also preferred in these laboratory procedures. To assess the abuse liability of an unknown test drug, the frequency of choice of the test drug is compared to choice of a standard drug with known abuse potential: preference for the test drug, relative to the standard, is then taken as an indicator of its relative potential for abuse. A number of methodological variables must be considered when designing and interpreting choice studies, including the characteristics of the subjects, the range of doses tested, whether choices were spaced widely apart (e.g. one a day) or closely together (e.g. one immediately after another), and whether the study was conducted under controlled laboratory conditions or in the subjects' normal daily environments. The conditions for testing the abuse liability of an unknown compound must be selected based on the existing literature with similar, known drugs of abuse.

Assessment of abuse liability of stimulant drugs in humans: a methodological survey

Stimulant drugs have been illicitly consumed for non-medical purposes for centuries. Within the past several decades procedures for assessing the abuse liability of a compound prior to marketing have been developed, which should limit the introduction of new stimulants with high abuse liability to the public. Drugs with high abuse liability are positive reinforcers in that they maintain behavior leading to their consumption. Assessment of abuse liability has traditionally involved a comparison of the profile of effects of a known drug of abuse to that of a drug with unknown abuse liability. With respect to stimulant drugs, amphetamine and cocaine may be considered prototypic stimulants with high abuse liability. Various procedures have been used to study abuse liability including the measuring of (1) subjective effects, (2) drug ‘liking’, (3) estimates of value, (4) the accuracy of drug identification, (5) the ability of trained subjects to discriminate one drug from another and (6) drug self-administration. In this paper the behavioral effects of a number of stimulant drugs will be compared along these dimensions and the effects along each dimension will be related to each drugs' known abuse liability. Dissociations among the various measures will also be described, and the implications of these dissociations discussed. We conclude that the best single assessment of abuse liability is obtained from drug self-administration studies, but that the most accurate assessment of abuse liability is obtained when the effects of a drug are evaluated along as many of these dimensions as possible.