Elsevier

Journal of Pharmacological and Toxicological Methods

Volume 81, September–October 2016, Pages 295-305
Journal of Pharmacological and Toxicological Methods

Appraisal of state-of-the-art
Drug discrimination: A versatile tool for characterization of CNS safety pharmacology and potential for drug abuse

https://doi.org/10.1016/j.vascn.2016.05.011Get rights and content

Abstract

Drug discrimination studies for assessment of psychoactive properties of drugs in safety pharmacology and drug abuse and drug dependence potential evaluation have traditionally been focused on testing novel compounds against standard drugs for which drug abuse has been documented, e.g. opioids, CNS stimulants, cannabinoids etc. (e.g. Swedberg & Giarola, 2015), and results are interpreted such that the extent to which the test drug causes discriminative effects similar to those of the standard training drug, the test drug would be further characterized as a potential drug of abuse.

Regulatory guidance for preclinical assessment of abuse liability 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) detail that compounds with central nervous system (CNS) activity, whether by design or not, need abuse and dependence liability assessment. Therefore, drugs with peripheral targets and a potential to enter the CNS, as parent or metabolite, are also within scope (see Swedberg, 2013, for a recent review and strategy). Compounds with novel mechanisms of action present a special challenge due to unknown abuse potential, and should be carefully assessed against defined risk criteria. Apart from compounds sharing mechanisms of action with known drugs of abuse, compounds intended for indications currently treated with drugs with potential for abuse and or dependence are also within scope, regardless of mechanism of action. Examples of such compounds are analgesics, anxiolytics, cognition enhancers, appetite control drugs, sleep control drugs and drugs for psychiatric indications.

Recent results (Swedberg et al., 2014; Swedberg & Raboisson, 2014; Swedberg, 2015) on the metabotropic glutamate receptor type 5 (mGluR5) antagonists demonstrate that compounds causing hallucinatory effects in humans did not exhibit clear discriminative effects when tested against classical drugs of abuse in drug discrimination studies, and were not self-administered by rats. However, these compounds did cause salient discriminative effects of their own in animals trained to discriminate them from no drug. Therefore, from a safety pharmacology perspective, novel compounds that do not cause discriminative effects similar to classical drugs of abuse, may still cause psychoactive effects in humans and carry the potential to maintain drug abuse, suggesting that proactive investigation of drug abuse potential is warranted (Swedberg, 2013).

These and other findings will be discussed, and the application of drug discrimination procedures beyond the typical standard application of testing novel compounds against known and well characterized reference drugs will be addressed.

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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