Preclinical evaluation of the abuse potential of the analgesic bicifadine

Effects of repeated treatment with monoamine-transporter-inhibitor antidepressants on pain-related depression of intracranial self-stimulation in rats

RATIONALE

Synaptic neurotransmission with dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is terminated primarily by reuptake into presynaptic terminals via the DA, NE, and 5-HT transporters (DAT/NET/SERT, respectively). Monoamine transporter inhibitors constitute one class of drugs used to treat both depression and pain, and therapeutic effects by these compounds often require repeated treatment for days or weeks.

OBJECTIVES

The present study compared antinociceptive effects produced by repeated treatment with monoamine transporter inhibitors in a preclinical assay of pain-related depression of positively reinforced operant responding.

METHODS

Adult Sprague-Dawley rats equipped with microelectrodes targeting a brain-reward area responded for pulses of electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus that repeatedly depressed ICSS and also produced weight loss during 7 days of repeated acid administration.

RESULTS

Acid-induced depression of both ICSS and body weight were completely blocked by repeated pretreatment with the nonsteroidal anti-inflammatory drug ketorolac. The DAT-selective inhibitor bupropion also fully blocked acid-induced ICSS depression and weight loss throughout all 7 days of treatment. The NET-selective inhibitor nortriptyline and the SERT-selective inhibitor citalopram were generally less effective, but both drugs blocked acid-induced ICSS depression by the end of the 7-day treatment. Acid-induced depression of ICSS and body weight were not blocked by the kappa opioid receptor (KOR) agonist U69593 or the KOR antagonist norbinaltorphimine.

CONCLUSIONS

These results support effectiveness of bupropion to alleviate signs of pain-related behavioral depression in rats and further suggest that nortriptyline and citalopram produce significant but less reliable effects.

Effects of the triple monoamine uptake inhibitor amitifadine on pain-related depression of behavior and mesolimbic dopamine release in rats

Pain-related depression of behavior and mood is a key therapeutic target in the treatment of pain. Clinical evidence suggests a role for decreased dopamine (DA) signaling in pain-related depression of behavior and mood. Similarly, in rats, intraperitoneal injection of dilute lactic acid (IP acid) serves as a chemical noxious stimulus to produce analgesic-reversible decreases in both (1) extracellular DA levels in nucleus accumbens (NAc) and (2) intracranial self-stimulation (ICSS), an operant behavior reliant on NAc DA. Intraperitonial acid-induced depression of ICSS is blocked by DA transporter (DAT) inhibitors, but clinical viability of selective DAT inhibitors as analgesics is limited by abuse potential. Drugs that produce combined inhibition of both DA and serotonin transporters may retain efficacy to block pain-related behavioral depression with reduced abuse liability. Amitifadine is a "triple uptake inhibitor" that inhibits DAT with approximately 5- to 10-fold weaker potency than it inhibits serotonin and norepinephrine transporters. This study compared amitifadine effects on IP acid-induced depression of NAc DA and ICSS and IP acid-stimulated stretching in male Sprague-Dawley rats. Amitifadine blocked IP acid-induced depression of both NAc DA and ICSS and IP acid-stimulated stretching. In the absence of the noxious stimulus, amitifadine increased NAc levels of both DA and serotonin, and behaviorally, amitifadine produced significant but weak abuse-related ICSS facilitation. Moreover, amitifadine was more potent to block IP acid-induced depression of ICSS than to facilitate control ICSS. These results support consideration of amitifadine and related monoamine uptake inhibitors as candidate analgesics for treatment of pain-related behavioral depression.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Expression and treatment of pain-related behavioral depression

Pain is often associated with clinically relevant depression of behavior and mood, and relief of pain-related depression is a common goal of treatment in both human and veterinary medicine. In the development of pharmacological compounds to treat pain and related depression, preclinical studies may be used to evaluate the analgesic potential of new drugs. Such studies require reliable, accurate assays of pain-related behavioral depression in animals. Intracranial self-stimulation (ICSS) is a type of operant conditioning procedure that produces stable baseline behavioral response rates. The author reviews recent research on the use of ICSS to evaluate the expression and pharmacological modulation of pain-related behavioral depression in rats. Results suggest that assays of pain-depressed behavior using ICSS may serve as a useful new tool to improve the translation of preclinical findings to clinical results in analgesic drug development.

Effects of monoamine reuptake inhibitors in assays of acute pain-stimulated and pain-depressed behavior in rats

Pain is associated with stimulation of some behaviors (eg, withdrawal reflexes) but depression of many other behaviors (eg, feeding, locomotion, positively reinforced operant behavior). Drugs that block reuptake of serotonin, norepinephrine, and/or dopamine are widely used to treat depression, and they have also emerged as useful drugs for treatment of pain. This study compared effects of selective and mixed-action inhibitors of serotonin, norepinephrine, and/or dopamine reuptake in assays of acute pain-stimulated and pain-depressed behavior. Intraperitoneal injection of dilute acid served as a noxious stimulus to stimulate a writhing response or depress intracranial self-stimulation (ICSS) in Sprague Dawley rats. Selective reuptake inhibitors of serotonin (citalopram, clomipramine) and norepinephrine (nisoxetine, nortriptyline) and a mixed-action reuptake inhibitor of serotonin and norepinephrine (milnacipran) blocked acid-stimulated writhing but failed to block acid-induced depression of ICSS. Selective dopamine reuptake inhibitors (RTI-113 [3ß-(4-chlorophenyl)tropane-2ß-carboxylic acid phenyl ester hydrochloride], bupropion) and a triple reuptake inhibitor of dopamine, serotonin, and norepinephrine (RTI-112 [3ß-(3-methyl-4-chlorophenyl)tropane-2ß-carboxylic acid methyl ester hydrochloride]) blocked both acid-stimulated writhing and acid-induced depression of ICSS, although these drugs also produced an abuse-related facilitation of ICSS in the absence of the noxious stimulus. These results support further consideration of dopamine reuptake inhibitors as candidate analgesics, although abuse liability remains a concern.

PERSPECTIVE

Monoamine reuptake inhibitors are used to treat depression and some forms of pain. This study examined effects of monoamine reuptake inhibitors in a preclinical assay of pain-related behavioral depression. The results support further consideration of dopamine reuptake inhibitors as candidate analgesics under selected circumstances, although abuse liability remains a concern.

A review of the abuse potential assessment of atomoxetine: a nonstimulant medication for attention-deficit/hyperactivity disorder

RATIONALE

Treatment of attention-deficit/hyperactivity disorder (ADHD) has for many years relied on psychostimulants, particularly various formulations of amphetamines and methylphenidate. These are central nervous system stimulants and are scheduled because of their abuse potential. Atomoxetine (atomoxetine hydrochloride; Strattera®) was approved in 2002 for treatment of ADHD, and was the first nonstimulant medication approved for this disorder. It was classified as an unscheduled medication indicating a low potential for abuse. However, the abuse potential of atomoxetine has not been reviewed.

OBJECTIVES

In this article, we review the evidence regarding abuse potential of atomoxetine, a selective inhibitor of the presynaptic norepinephrine transporter, which is unscheduled/unrestricted in all countries where it is approved.

METHODS

Results from receptor binding, in vitro electrophysiology, in vivo microdialysis, preclinical behavioral, and human laboratory studies have been reviewed.

RESULTS

Atomoxetine has no appreciable affinity for, or action at, central receptors through which drugs of abuse typically act, i.e., dopamine transporters, GABA(A) receptors, and opioid μ receptors. In behavioral experiments in rodents, atomoxetine does not increase locomotor activity, and in drug discrimination studies, its profile is similar to that of drugs without abuse potential. Atomoxetine does not serve as a reinforcer in monkey self-administration studies, and human laboratory studies suggest that atomoxetine does not induce subjective effects indicative of abuse.

CONCLUSION

Neurochemical, preclinical, and early clinical studies predicted and supported a lack of abuse potential of atomoxetine, which is consistent with the clinical trial and postmarketing spontaneous event data in the past 10 years.

Amitifadine, a triple monoamine uptake inhibitor, reduces binge drinking and negative affect in an animal model of co-occurring alcoholism and depression symptomatology

The co-occurrence of alcoholism and depression is highly prevalent and difficult to treat. In an animal model of binge drinking that exhibits abstinence-induced behaviors reminiscent of negative affective states, the triple monoamine uptake inhibitor, amitifadine, produced a selective, dose dependent attenuation of binge drinking. Amitifadine also reversed abstinence-induced increases in the intracranial self-stimulation threshold, a model of anhedonia, and immobility in the forced swim test, reflecting behavioral despair. In view of the safety profile of amitifadine in humans, including low risk for weight gain, lack of sexual side effects, and low potential for abuse, we hypothesize that amitifadine will be effective in treating co-occurring alcoholism and depression.