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

Is schedule IV suvorexant an appropriate reference drug of abuse to use in preclinical abuse liability testing in the rat?

The abuse potential of novel CNS-active drug candidates with low specificity for known receptors involved in abuse might be complex to test preclinically relative to an appropriate reference drug of abuse. Suvorexant, a Schedule IV dual orexin receptor antagonist was investigated for its potential use as a reference drug in Drug Discrimination Learning (DDL) studies. Firstly, toxicokinetic properties of suvorexant were determined in male and female rats after single oral doses of 160 and 325 mg/kg in MC and PEG400. Thereafter the subjective effects of suvorexant at 325 mg/kg versus vehicle were evaluated in a DDL paradigm and plasma exposures were measured. Mean maximum plasma exposures in male rats after a single dose of 325 mg/kg suvorexant were 2.5- (MC) to 10.5-fold (PEG400) the human exposure at supratherapeutic doses of 40 mg q.d. (Cmax:1.1 μM), and 4.9- (MC) to 20.8-fold (PEG400) the approved maximum human efficacious dose (20 mg q.d.; 0.557 μM). Training male rats at 325 mg/kg in the DDL study however did not result in discriminative stimulus generalisation versus respective vehicles. Suvorexant, a Schedule IV dual orexin receptor antagonist failed to serve as a robust reference drug of abuse in the DDL paradigm in rats despite appropriate exposures.

In Vivo and In Vitro Analyses of Novel Peptidomimetic Disruptors for the Serotonin 5-HT2C Receptor Interaction With Phosphatase and Tensin Homolog

Hypofunction of the serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) has been implicated in a variety of disorders including substance use disorders. As such, approaches to enhance 5-HT_2CR signaling display therapeutic potential. In the present study, we show that disruption of the 5-HT_2CR interaction with the protein phosphatase and tensin homolog (PTEN) via peptidomimetics enhances 5-HT_2CR-mediating signaling in vitro and potentiates selective 5-HT_2CR agonists in behavioral rodent models. Overall, the present study provides further evidence that 5-HT_2CR activity can be modulated through an allosteric protein-protein interaction. This work provides the groundwork for the continued exploration of protein-protein interactions that can allosterically modulate this critical receptor and other important G protein-coupled receptors (GPCRs) for new therapeutic development through mechanisms that may display clinical utility.

Selective activation of Dopamine D3 receptors and norepinephrine transporter blockade enhances sustained attention

Catecholamine transmitters dopamine (DA) and norepinephrine (NE) regulate prefrontal cortical (PFC) circuit activity and PFC-mediated executive functions. Accordingly, pharmacological agents that influence catecholamine neurotransmission exert prominent effects on cognition. Many such agents are used clinically to treat attention disorders. For example, methylphenidate blocks DA and NE reuptake and is the leading choice for attention deficit hyperactivity disorder (ADHD) treatment. Recently, we have designed SK609 - a selective small molecule agonist of the DA D3 receptor (D3R). In this study, we further characterized SK609's ability to selectively inhibit the reuptake of NE by NE transporters (NET). Our results indicate SK609 selectively inhibits NET with a K_i value of ∼500 nM and behaves as a NET substrate. Systemic dosing of SK609 (4 mg/kg; i.p.) in naïve rats produced a 300% and 160% increase in NE and DA, respectively, in the PFC as measured by microdialysis. Based on these neurochemical results, SK609 was tested in a PFC-dependent, visually-guided sustained attention task in rats. SK609 improved performance in a dose-dependent manner with a classical inverted-U dose response function with a peak effect at 4 mg/kg. SK609's peak effect was blocked by a pre-treatment with either the D2/D3R antagonist raclopride (0.05 mg/kg; i.p) or the alpha-1 adrenergic receptor antagonist prazosin (0.25 mg/kg; i.p), confirming a role for both DA and NE in promoting sustained attention. Additionally, SK609 improved sustained attention more prominently among low-performing animals. Doses of SK609 (2, 4, and 8 mg/kg) associated with cognitive enhancement did not produce an increase in spontaneous locomotor activity, suggesting a lack of side effects mediated by DA transporter (DAT) activity. These results demonstrate that the novel catecholaminergic modulator SK609 has the potential to treat sustained attention deficits without affecting DAT activity, distinguishing it from amphetamines and methylphenidate.

Safety pharmacology investigations on the nervous system: An industry survey

The Safety Pharmacology Society (SPS) conducted an industry survey in 2015 to identify industry practices as they relate to central, peripheral and autonomic nervous system ('CNS') drug safety testing. One hundred fifty-eight (158) participants from Asia (16%), Europe (20%) and North America (56%) responded to the survey. 52% of participants were from pharmaceutical companies (>1000 employees). Oncology (67%) and neurology/psychiatry (66%) were the most frequent target indications pursued by companies followed by inflammation (48%), cardiovascular (43%), metabolic (39%), infectious (37%), orphan (32%) and respiratory (29%) diseases. Seizures (67% of participants), gait abnormalities (67%), tremors (65%), emesis (56%), sedation (52%) and salivation (47%) were the most commonly encountered CNS issues in pre-clinical drug development while headache (65%), emesis/nausea (60%), fatigue (51%) and dizziness (49%) were the most frequent issues encountered in Phase I clinical trials. 54% of respondents reported that a standard battery of tests applied to screen drug candidates was the approach most commonly used to address non-clinical CNS safety testing. A minority (14% of all participants) reported using electroencephalography (EEG) screening prior to animal inclusion on toxicology studies. The most frequent group size was n=8 for functional observation battery (FOB), polysomnography and seizure liability studies. FOB evaluations were conducted in a dedicated room (78%) by blinded personnel (66%) with control for circadian cycle (55%) effects (e.g., dosing at a standardized time; balancing time of day across treatment groups). The rat was reported as the most common species used for seizure liability, nerve conduction and drug-abuse liability testing.

Perspectives and Trends in Pharmacological Approaches to the Modulation of Pain

Pharmacological approaches to our understanding and treatment of pain have had a long history and have traditionally relied on very few drugs that either have significant side effects and abuse liability, such as the nonsteroidal anti-inflammatory drugs or the opioids, respectively, or those that have been developed for other conditions such as the tricyclic antidepressants. The pathophysiology of pain is undoubtedly complex, complicated in part by the fact that it is not a singular condition, and has a variety of etiologies and a number of associated comorbidities that make treatment interventions challenging. Moreover, there are changes in the central nervous system during the course of the development of chronic pain that, in a manner parallel to neurodegenerative disorders, likely require different pharmacological approaches in the early stages of acute pain compared to those that would be effective when pain has become chronic. This chapter reviews the current status of the field of pain research focusing on some relatively underdeveloped areas, such as pain and its associated comorbidities, and the use of transgenic animals and drug self-administration procedures in the context of analgesic assessment. This chapter also incorporates more recent developments and emerging trends in the area of epigenetics, biomarkers, and the use of induced pluripotent stem cells for pharmacological evaluation, target identification, and validation. Recent progress in the study of "organs-on-a-chip" will also be included in this overview, setting expectations for future progress that integrates these advances for deeper insights into mechanisms, novel treatments, and facilitated efforts in drug discovery.

Serotonin 5-HT2 receptor interactions with dopamine function: implications for therapeutics in cocaine use disorder

Cocaine exhibits prominent abuse liability, and chronic abuse can result in cocaine use disorder with significant morbidity. Major advances have been made in delineating neurobiological mechanisms of cocaine abuse; however, effective medications to treat cocaine use disorder remain to be discovered. The present review will focus on the role of serotonin (5-HT; 5-hydroxytryptamine) neurotransmission in the neuropharmacology of cocaine and related abused stimulants. Extensive research suggests that the primary contribution of 5-HT to cocaine addiction is a consequence of interactions with dopamine (DA) neurotransmission. The literature on the neurobiological and behavioral effects of cocaine is well developed, so the focus of the review will be on cocaine with inferences made about other monoamine uptake inhibitors and releasers based on mechanistic considerations. 5-HT receptors are widely expressed throughout the brain, and several different 5-HT receptor subtypes have been implicated in mediating the effects of endogenous 5-HT on DA. However, the 5-HT2A and 5-HT2C receptors in particular have been implicated as likely candidates for mediating the influence of 5-HT in cocaine abuse as well as to traits (e.g., impulsivity) that contribute to the development of cocaine use disorder and relapse in humans. Lastly, new approaches are proposed to guide targeted development of serotonergic ligands for the treatment of cocaine use disorder.