Journal of Pharmacological and Toxicological Methods
Appraisal of state-of-the-artDrug discrimination: A versatile tool for characterization of CNS safety pharmacology and potential for drug abuse
Introduction
Preclinical assessment of abuse liability is regulated by several guidance documents, issued by the European Medicines Agency (EMA, 2006), the U.S. Food and Drug Administration (FDA, 2010), the International Conference of Harmonization (ICH, 2009), and the Japanese Ministry of Health Education and Welfare (MHLW, 1994). Compounds with central nervous system (CNS) activity, by design or as a side-effect, need abuse and dependence liability assessment, thereby explicitly including drugs designed for peripheral targets that may have the potential to enter the CNS, either as the parent compound or as a metabolite (see Swedberg, 2013, for a recent review and strategy). Guidelines also specify what types of data are expected to be included with the documentation submitted to regulatory agencies. These include data from self-administration, drug discrimination and physical dependence studies. These methods address abuse liability from different aspects utilizing methods with unique characteristics. By design, one major advantage of the drug discrimination method is the simultaneous and independent determination of the discriminative effect and effects on response rate, thereby controlling for factors potentially influencing the ability an animal to respond such as e.g. sedative or other motivational effects, and providing a measure to assess the specificity of the results (see Swedberg, 2013, Swedberg and Giarola, 2015). This paper presents the versatility of the drug discrimination technique to address central nervous system (CNS) effects in pharmacology and safety pharmacology of drugs using examples from several pharmacological classes. For detailed descriptions of the drug discrimination procedure basics and for further references on methodology, readers are referred to Swedberg and Giarola (2015) and sources cited therein.
A note on doses: discriminative effects typically occur at doses below those causing overt behavioral disturbances. Therefore, in drug discrimination experiments doses are typically escalated to levels at which response rates are substantially suppressed, particularly in the apparent absence of discriminative effects, to ensure that exposure to the drug has been high enough to reveal any behavioral effects to ascertain that behaviorally active doses have been achieved. Exceptions may occur when drug material is scarce, solubility is low, or when behavioral and toxic dose range margins are narrow and animal health becomes an issue. In addition to determining effects on response rates, blood samples can be collected in the experimental animals or in control groups. For further discussion, see Swedberg and Giarola (2015). The highest doses to be studied in abuse liability assessment are usually in the toxic range, since the major risk population are recreational users and drug abusers, and the doses needed to produce the effect desired determine the amount of drug taken. Therefore, a traditional application of the concept of safety margin is not appropriate (see e.g. Swedberg, 2013).
New safety pharmacology assays can be developed by training animals to discriminate novel candidate drugs from no drug, in order to determine psychoactivity. To the extent that the novel drug can function as a discriminative stimulus, the assay then needs to be characterized by investigating the mechanisms of action mediating the discrimination. To this end, drugs with known and characterized mechanisms of action and/or drugs of abuse are tested, as are potential antagonists. It is essential to investigate a wide dose range of test compounds so that discriminative as well as response rate effects can be determined (see Swedberg, 2013, for discussion).
Examples will be provided by discussing investigations on the novel analgesics flupirtine (ethyl-N-[2-amino-6-(4-fluorophenylmethylamino)pyridin-3-yl] carbamate) an α2 adrenergic antagonist, anpirtoline (6-chlor-2-(piperidyl-4-thio)-pyridine HCI, D-16949), a serotonin 1B (5-HT1B) agonist, and the mGluR5 antagonist AZD9272 (3-fluoro-5-(3-(5-fluoropyridin-2-yl)-1,2,4-oxadiazol-5-yl)benzonitrile).
Novel assays can be set up, characterized and used for in vivo screening. An example will be provided by data collected on the use of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) agonist 2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionate (ATPA) as a training drug with the aim to screen for novel AMPA antagonists.
Using characterized drug discrimination assays to optimize efficacy and/or potency in vivo by testing novel drugs in drug discrimination assays based on standard drugs with the same mechanism of action is a common way to use drug discrimination techniques in drug discovery and safety pharmacology efforts. An example will be provided by safety pharmacology data collected on the central nervous system (CNS) acting nicotinic acetylcholine receptor (nAChR) α4β2 subunit agonist NNC 90-0270 ((Z) 3-(3-methylisoxazol-4-yl)methylene-1-azabicyclo[2.2.2]octane).
Increasing the sensitivity of assays for detection of psychoactive properties in safety pharmacology and potential abuse liability may be achieved by using the intravenous route of administration to minimize influence of slow absorption and distribution, and by varying the speed of infusions to optimize brain penetration. Intravenous administration may also be useful in the detection and characterization of CNS effects of drugs designed for peripheral action and for which assessment of CNS penetration through measured brain levels of the novel drug may suggest “minimal” penetration. Examples will be provided by data collected on the peripheral μ-opioid peptides LEF553 (Tyr-d-Arg-Phe-Phe-NH2·2HCl) and LEF576 (frakefamide; l-tyrosyl-d-alanyl-p-fluoro-l-phenylalanyl-l-phenylalaninamide hydrochloride), and on the mixed anesthetic/analgesic sameridine (N-ethyl-1- hexyl-N-methyl-4-phenyl-4-piperidinecarboxamide hydrochloride).
The onset of the discriminative effects of drugs has been shown to not always be well predicted by pharmacokinetic observations, and from a safety pharmacology perspective may pose a CNS safety risk. An example will be provided using newly published data on the mGluR5 antagonist AZD9272.
Section snippets
Novel candidate drugs as training drugs
Investigating the potential of novel proprietary drugs to serve as training drugs enable the comparison of the novel compound with known drugs of abuse. Novel compounds that have been demonstrated to be dissimilar to those of known drugs of abuse after having been tested in the appropriate drug discrimination assays may still have psychoactive properties of relevance to safety pharmacology. The absence of psychoactive properties similar to those of drugs of abuse is not a guarantee that the
Mechanistic assay development using novel drugs as training drugs
Another example of studies of a novel drug that causes discriminative effects and therefore may itself have the potential to cause drug abuse were experiments conducted in the context of developing an assay for AMPA antagonists. Since AMPA antagonists like NMDA antagonists inhibit excitatory amino acid (EAA) receptors (Monaghan et al., 1989), and that NMDA antagonism had shown psychotomimetic effects (Koek et al., 1990), it was essential from a safety pharmacology perspective to develop an in
Characterized training drugs in drug candidate optimization
Findings suggesting a therapeutic role for nicotine in the treatment of Alzheimer's disease, Parkinson's disease and Tourette's syndrome (Benowitz, 1996, Lee, 1994, Jones et al., 1992), led to a search for novel nicotinic agents with an increased CNS selectivity and without nicotine's side effects, including abuse potential. The α4β2 nicotinic acetylcholine receptor (nAChR) subtype has been identified as a potential target for therapeutic indications (Arneric et al., 1995), which is also a
Increasing assay sensitivity by intravenous administration
The drug discrimination procedure is commonly used in several species to predict abuse liability, and the discriminative effects as well as the self-administration effects may depend on the speed at which a drug is administered when given intravenously. The rush is considered important to maintain intravenous drug abuse and the importance of the rate of infusion for psychoactive effects after intravenous administration has been shown previously for cocaine's ability to maintain
Determination of onset and duration of psychoactive action
Drug exposure levels in blood do not necessarily predict the onset of behavioral effects as shown and discussed in recent studies on mGluR5 antagonists (Swedberg & Raboisson, 2014). The discriminative effects occurred well before what could have been expected based on exposure levels suggesting caution against rigid predictions of pharmacological onset of effect based on drug exposure levels in blood. Rather it appears that the onset of psychoactive properties in animals and humans may depend
Conclusions
Traditionally, drug discrimination studies for assessment of psychoactive properties of drugs for safety pharmacology purposes have been targeted towards investigating novel compounds in assays based on well characterized compounds traditionally used in drug discrimination studies. For example, for assessment of drug abuse and drug dependence the focus has traditionally been on testing compounds against the standard drugs implicated in drug abuse settings, e.g. opioids, CNS stimulants,
Conflicts of interest
The studies described herein were conducted while the author was an employee of Novo Nordisk A/S, Måløv, Denmark, or Astra Pain Control, Södertälje, Sweden or AstraZeneca, Södertälje, Sweden. All studies were funded by the respective companies.
Ethics approvals
These studies were carried out in accordance with the Declaration of Helsinki and with the Guide for the Care and Use of Laboratory Animals as adopted and promulgated by the National Institutes of Health (Denmark).
All animal experiments were performed in accordance with the guidelines of The Swedish National Board for Laboratory Animals under a protocol approved by the Ethical Committee of Southern Stockholm, Sweden. Studies were carried out in accordance with the Declaration of Helsinki and
Acknowledgements
The author thanks Hanne Nielsen, Wei Liu and Marianne Schilder (NovoNordisk), and Maria Ståhlberg, Charlotte Velasquez and Pernilla Hammar (Astra Pain Control and AstraZeneca) for expert technical assistance over many years of work.
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2022, NeuropharmacologyCitation Excerpt :In a second study, it was tested in combination with cocaine, to verify that it does not potentiate cocaine's ICSS-enhancing effects. NLX-112 was also tested in a drug discrimination procedure, another behavioral assay that is commonly used to assess the misuse potential of compounds (Swedberg, 2016). In this assay, animals are trained to detect the interoceptive cue produced by a drug with known misuse potential, such as indirect dopaminergic agonists like cocaine or d-amphetamine, or opioid agonists such as morphine or fentanyl.
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2022, Advances in PharmacologyCitation Excerpt :For example, an abstinent individual with a history of OUD might be at a high risk of relapse following ingestion of a stimulant. Drug discrimination is an operant conditioning procedure often used alongside self-administration procedures to characterize abuse-related effects of drugs (see Swedberg, 2016; Bolin et al., 2018 for recent reviews). In a typical experiment, subjects are trained to discriminate the injection of a particular dose of drug (“training drug”) from the injection of the drug vehicle using differential reinforcement.
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2020, Drug and Alcohol DependenceCitation Excerpt :Because drugs with similar pharmacodynamic mechanisms of action produce similar interoceptive effects, they share discriminative stimulus effects in this procedure. This procedure has been used successfully to identify the pharmacological mechanisms of novel and existing drugs, as well as predict their abuse liability in humans (Cunningham and Callahan, 1994; Grant, 1999; Sanger, 1988; Swedberg, 2016; Walker, 2018). Technical features of drug discrimination procedures limit their translational value as a model of SUD.
Evaluating the abuse potential of psychedelic drugs as part of the safety pharmacology assessment for medical use in humans
2018, NeuropharmacologyCitation Excerpt :A number of excellent reviews have been written on the methodological aspects of drug-discrimination testing (Colpaert, 1995, 1999; Ator and Griffiths, 2003; Stolerman et al., 2011). In addition, the use of this technique as a tool for assessing abuse potential has also been the subject of a number of good reviews (see Mori et al., 2012; Mead, 2014; Swedberg, 2016) including those which discuss the relative strengths and weaknesses of drug-discrimination for this purpose (Moser et al., 2011; Mead, 2014). Following the emergence of the CDER/FDA (2017) guidelines that include a recommendation to conduct drug-discrimination testing under extinction, i.e., no food rewards for lever-pressing in testing sessions, Gauvin et al. (2016) have written a strong rebuttal of the FDA's thinking on the matter that is well worth reading.
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2018, Advanced Issue Resolution in Safety Pharmacology