Co-targeting the kappa opioid receptor and dopamine transporter reduces motivation to self-administer cocaine and partially reverses dopamine system dysregulation.

Cocaine disrupts dopamine (DA) and kappa opioid receptor (KOR) system activity, with long-term exposure reducing inhibiton of DA uptake by cocaine and increasing KOR system function. Single treatment therapies have not been successful for cocaine use disorder; therefore, this study focuses on a combination therapy targeting the dopamine transporter (DAT) and KOR. Sprague Dawley rats self-administered 5 days of cocaine (1.5 mg/kg/inf, max 40 inf/day, FR1), followed by 14 days on a progressive ratio (PR) schedule (0.19 mg/kg/infusion). Behavioral effects of individual and combined administration of phenmetrazine and nBNI were then examined using PR. Additionally, ex vivo fast scan cyclic voltammetry was then used to assess alterations in DA and KOR system activity in the nucleus accumbens before and after treatments. Chronic administration of phenmetrazine as well as the combination of phenmetrazine and nBNI-but not nBNI alone-significantly reduced PR breakpoints. In addition, the combination of phenmetrazine and nBNI partially reversed cocaine-induced neurodysregulations of the KOR and DA systems, indicating therapeutic benefits of targeting the DA and KOR systems in tandem. These data highlight the potential benefits of the DAT and KOR as dual-cellular targets to reduce motivation to administer cocaine and reverse cocaine-induced alterations of the DA system.

Fluorinated phenmetrazine "legal highs" act as substrates for high-affinity monoamine transporters of the SLC6 family.

A variety of new psychoactive substances (NPS) are appearing in recreational drug markets worldwide. NPS are compounds that target various receptors and transporters in the central nervous system to achieve their psychoactive effects. Chemical modifications of existing drugs can generate NPS that are not controlled by current legislation, thereby providing legal alternatives to controlled substances such as cocaine or amphetamine. Recently, 3-fluorophenmetrazine (3-FPM), a derivative of the anorectic compound phenmetrazine, appeared on the recreational drug market and adverse clinical effects have been reported. Phenmetrazine is known to elevate extracellular monoamine concentrations by an amphetamine-like mechanism. Here we tested 3-FPM and its positional isomers, 2-FPM and 4-FPM, for their abilities to interact with plasma membrane monoamine transporters for dopamine (DAT), norepinephrine (NET) and serotonin (SERT). We found that 2-, 3- and 4-FPM inhibit uptake mediated by DAT and NET in HEK293 cells with potencies comparable to cocaine (IC50 values < 2.5 µM), but display less potent effects at SERT (IC50 values >80 µM). Experiments directed at identifying transporter-mediated reverse transport revealed that FPM isomers induce efflux via DAT, NET and SERT in HEK293 cells, and this effect is augmented by the Na+/H+ ionophore monensin. Each FPM evoked concentration-dependent release of monoamines from rat brain synaptosomes. Hence, this study reports for the first time the mode of action for 2-, 3- and 4-FPM and identifies these NPS as monoamine releasers with marked potency at catecholamine transporters implicated in abuse and addiction. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Dissociable effects of the prodrug phendimetrazine and its metabolite phenmetrazine at dopamine transporters.

Phendimetrazine (PDM) is a clinically available anorectic and a candidate pharmacotherapy for cocaine addiction. PDM has been hypothesized to function as a prodrug that requires metabolism to the amphetamine-like monoamine transporter substrate phenmetrazine (PM) to produce its pharmacological effects; however, whether PDM functions as an inactive prodrug or has pharmacological activity on its own remains unclear. The study aim was to determine PDM pharmacological mechanisms using electrophysiological, neurochemical, and behavioral procedures. PDM blocked the endogenous basal hDAT (human dopamine transporter) current in voltage-clamped (-60 mV) oocytes consistent with a DAT inhibitor profile, whereas its metabolite PM induced an inward hDAT current consistent with a DAT substrate profile. PDM also attenuated the PM-induced inward current during co-application, providing further evidence that PDM functions as a DAT inhibitor. PDM increased nucleus accumbens dopamine levels and facilitated electrical brain stimulation reinforcement within 10 min in rats, providing in vivo evidence supporting PDM pharmacological activity. These results demonstrate that PDM functions as a DAT inhibitor that may also interact with the pharmacological effects of its metabolite PM. Overall, these results suggest a novel mechanism for PDM therapeutic effects via initial PDM DAT inhibition followed by PM DAT substrate-induced dopamine release.

Cocaine-like discriminative stimulus effects of phendimetrazine and phenmetrazine in rats.

Phendimetrazine is a clinically available anorectic and candidate medication for the treatment of cocaine addiction. Phendimetrazine can be metabolized to the amphetamine-like monoamine releaser phenmetrazine, but it is unclear if phendimetrazine functions as an inactive prodrug or might have activity on its own. As one method to address this issue, the present study compared the potency and time course of phendimetrazine and phenmetrazine to produce cocaine-like discriminative stimulus effects in adult, male rats (N=5) trained to discriminate cocaine (5.6 mg/kg, intraperitoneally) from saline in a two-key food-reinforced discrimination procedure. We hypothesized that, if metabolism to phenmetrazine was required for phendimetrazine effects, then phendimetrazine would be less potent and have a slower onset and offset of effects than phenmetrazine. Both phendimetrazine and phenmetrazine produced dose-dependent cocaine-like discriminative stimulus effects, and phendimetrazine was 7.8-fold less potent than phenmetrazine. However, the time courses of discriminative stimulus effects produced by phendimetrazine and phenmetrazine were similar, with peak effects at 10 min and offset by 100 min. These results show the effectiveness of phendimetrazine to rapidly produce cocaine-like behavioral effects in rats and support other nonhuman primate evidence to suggest that metabolism to phenmetrazine may not be required for phendimetrazine effects.

Effects of the dopamine/norepinephrine releaser phenmetrazine on cocaine self-administration and cocaine-primed reinstatement in rats.

RATIONALE: Like other monoamine releasers such as D-amphetamine, chronic treatment with phenmetrazine can attenuate cocaine self-administration in monkeys. OBJECTIVES: The present studies extended this finding to rodents and to cocaine-primed reinstatement, a putative laboratory animal model of relapse. METHODS: In experiment 1, rats self-administered food pellets or injections of 0.19 mg/kg cocaine (i.v.) under a progressive-ratio schedule. When responding was stable, subcutaneous osmotic pumps were implanted containing saline or (+)-phenmetrazine (25 or 50 mg/kg per day). In experiment 2, rats self-administered injections of 0.75 mg/kg cocaine under a fixed-ratio 1 schedule in daily 6-h sessions. When responding was stable, rats were removed from the self-administration environment for 7 days and treated continuously with saline, 5 mg/kg per day D-amphetamine or phenmetrazine (25 or 50 mg/kg per day) via osmotic pumps. Rats were then returned to the self-administration context while treatment continued, and responding was extinguished by removing response-contingent stimulus changes and cocaine injections. Once responding was extinguished, reinstatement tests were conducted using cocaine injections (10 mg/kg i.p.). RESULTS: Phenmetrazine decreased self-administration of cocaine, but not food pellets, during the 14-day treatment period; effects persisted for several days after treatment was discontinued. Moreover, cocaine-induced increases in responding during the reinstatement test were attenuated by D-amphetamine and both phenmetrazine doses. CONCLUSIONS: These results extend the study of the effects of phenmetrazine on cocaine self-administration to a rodent model, and provide further support for the use of monoamine releasers as agonist medications for cocaine abuse.

A generalized matching law analysis of cocaine vs. food choice in rhesus monkeys: effects of candidate 'agonist-based' medications on sensitivity to reinforcement.

BACKGROUND: We have previously demonstrated reductions in cocaine choice produced by either continuous 14-day phendimetrazine and d-amphetamine treatment or removing cocaine availability under a cocaine vs. food choice procedure in rhesus monkeys. The aim of the present investigation was to apply the concatenated generalized matching law (GML) to cocaine vs. food choice dose-effect functions incorporating sensitivity to both the relative magnitude and price of each reinforcer. Our goal was to determine potential behavioral mechanisms underlying pharmacological treatment efficacy to decrease cocaine choice. METHODS: A multi-model comparison approach was used to characterize dose- and time-course effects of both pharmacological and environmental manipulations on sensitivity to reinforcement. RESULTS: GML models provided an excellent fit of the cocaine choice dose-effect functions in individual monkeys. Reductions in cocaine choice by both pharmacological and environmental manipulations were principally produced by systematic decreases in sensitivity to reinforcer price and non-systematic changes in sensitivity to reinforcer magnitude. CONCLUSIONS: The modeling approach used provides a theoretical link between the experimental analysis of choice and pharmacological treatments being evaluated as candidate 'agonist-based' medications for cocaine addiction. The analysis suggests that monoamine releaser treatment efficacy to decrease cocaine choice was mediated by selectively increasing the relative price of cocaine. Overall, the net behavioral effect of these pharmacological treatments was to increase substitutability of food pellets, a nondrug reinforcer, for cocaine.

Effects of monoamine releasers with varying selectivity for releasing dopamine/norepinephrine versus serotonin on choice between cocaine and food in rhesus monkeys.

Monoamine releasers constitute one class of candidate medications for the treatment of cocaine abuse, and concurrent cocaine-versus-food choice procedures are potentially valuable as experimental tools to evaluate the efficacy and safety of candidate medications. This study assessed the choice between cocaine and food by rhesus monkeys during treatment with five monoamine releasers that varied in selectivity to promote the release of dopamine and norepinephrine versus serotonin (5HT) [m-fluoroamphetamine, (+)-phenmetrazine, (+)-methamphetamine, napthylisopropylamine and (±)-fenfluramine]. Rhesus monkeys (n=8) responded under a concurrent-choice schedule of food delivery (1-g pellets, fixed ratio 100 schedule) and cocaine injections (0-0.1 mg/kg/injection, fixed ratio 10 schedule). Cocaine choice dose-effect curves were determined daily during continuous 7-day treatment with saline or with each test compound dose. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice, and the highest cocaine doses (0.032-0.1 mg/kg/injection) maintained almost exclusive cocaine choice. Efficacy of monoamine releasers to decrease cocaine choice corresponded to their pharmacological selectivity to release dopamine and norepinephrine versus 5HT. None of the releasers reduced cocaine choice or promoted reallocation of responding to food choice to the same extent as when saline was substituted for cocaine. These results extend the range of conditions across which dopamine and norepinephrine-selective releasers have been shown to reduce cocaine self-administration.

Selective suppression of cocaine- versus food-maintained responding by monoamine releasers in rhesus monkeys: benzylpiperazine, (+)phenmetrazine, and 4-benzylpiperidine.

Monoamine releasers constitute one class of drugs currently under investigation as potential agonist medications for the treatment of cocaine dependence. The efficacy and safety of monoamine releasers as candidate medications may be influenced in part by their relative potency to release dopamine and serotonin, and we reported previously that releasers with approximately 30-fold selectivity for dopamine versus serotonin release may be especially promising. The present study examined the effects of the releasers benzylpiperazine, (+)phenmetrazine, and 4-benzylpiperidine, which have 20- to 48-fold selectivity in vitro for releasing dopamine versus serotonin. In an assay of cocaine discrimination, rhesus monkeys were trained to discriminate 0.4 mg/kg i.m. cocaine from saline in a two-key, food-reinforced procedure. Each of the releasers produced a dose- and time-dependent substitution for cocaine. 4-Benzylpiperidine had the most rapid onset and shortest duration of action. Phenmetrazine and benzylpiperazine had slower onsets and longer durations of action. In an assay of cocaine self-administration, rhesus monkeys were trained to respond for cocaine injections and food pellets under a second order schedule. Treatment for 7 days with each of the releasers produced a dose-dependent and selective reduction in self-administration of cocaine (0.01 mg/kg/injection). The most selective effects were produced by phenmetrazine. Phenmetrazine also produced a downward shift in the cocaine self-administration dose effect curve, virtually eliminating responding maintained by a 30-fold range of cocaine doses (0.0032-0.1 mg/kg/injection) while having only small and transient effects on food-maintained responding. These findings support the potential utility of dopamine-selective releasers as candidate treatments for cocaine dependence.

An in vivo voltammetric comparison of the effects of three psychomotor stimulants on electrically evoked neostriatal dopamine release.

The effects of three psychomotor stimulants (mazindol, beta-phenylethylamine and D-phenmetrazine) on electrically evoked neostriatal dopamine release were studied by in vivo voltammetry. Mazindol (10 mg/kg) enhanced release and this effect persisted after dopamine synthesis inhibition by alpha-methyl-p-tyrosine. beta-Phenylethylamine (100 mg/kg) caused a large decrease in stimulated dopamine release and exerted no effect after dopamine synthesis inhibition. D-Phenmetrazine (45 mg/kg) enhanced dopamine release on the first post-drug stimulation and also restored release after dopamine synthesis inhibition. Disruption of vesicular dopamine storage by Ro 4-1284 abolished electrically stimulated dopamine release. Only D-phenmetrazine was able to cause dopamine release following Ro 4-1284. These results imply different biochemical modes of action of these three stimulants.

The discriminative stimulus and subjective effects of d-amphetamine, phenmetrazine and fenfluramine in humans.

The discriminative stimulus (DS) and subjective effects of d-amphetamine (AMP), phenmetrazine (PMT) and fenfluramine (FFL) were studied in a group of normal healthy adults. Subjects (N = 27) were trained to discriminate between placebo and 10 mg AMP (PO). Fourteen of the subjects (discriminators) reliably learned the discrimination, whereas the other 13 did not. Nearly all discriminators labelled AMP as a stimulant, and AMP, relative to placebo, increased ratings of drug liking and general activity level, and produced typical stimulant-like subjective effects, as measured by the Profile of Mood States, the Addiction Research Center Inventory, and a series of visual analog scales. The discrimination accuracy of discriminators increased as a function of hour after drug ingestion, as did analog ratings of how certain subjects were that their discrimination responses were correct. Discriminators were tested with doses of PMT (25 and 50 mg) and FFL (20 and 40 mg) to determine whether the DS properties of these drugs would substitute for those of AMP. Both doses of PMT consistently substituted for AMP, and PMT produced subjective effects very similar to those of AMP. Conversely, neither dose of FFL consistently substituted for AMP, and FFL produced essentially no subjective effects. These findings are consistent with results from discrimination studies with other species, and provide further evidence of the validity of this procedure for studying the DS properties of drugs in humans.

Report from the University of Chicago Drug Abuse Research Center.

In summary, I have reviewed the results from 6 coded compounds we have evaluated as part of CPDD's efforts to expand into testing the dependence potential of anorectics and anxiolytics. I have reviewed a great deal of additional studies in order to place the results with the test drugs into a broader context and also to demonstrate our experience in evaluating these classes of drugs. Clearly the 4 anxiolytic drugs were well characterized by our tests. The two compounds designated as anorectics are still unknown to us. CPDD 5 appears amphetamine-like, is self-administered, and in both tests is 1/10th as potent as amphetamine. The results with CPDD 6, however, are less clear. In the drug discrimination studies, it only substitutes for amphetamine in some animals, and in monkeys has minimal reinforcing properties.

Influence of intravenous self-administered psychomotor stimulants on performance of rhesus monkeys in a multiple schedule paradigm.

Rhesus monkeys were trained to complete three multiple schedules. The schedules consisted of three components: a fixed interval (component 1), a variable interval (component 2), and a fixed ratio (component 3). During components 1 and 2, pressing lever 1 was always reinforced by food delivery. During component 3, pressing lever 2 resulted in either food delivery or intravenous infusions of saline solution, solutions of cocaine, of d-amphetamine, of phenmetrazine, or fenetylline. In schedule I, animals were presented with all three components independent of key-pressing behavior during components 1 and 2. In schedule II the availability of component 2 was dependent on completion of component 1. Component 3 was made available only on completion of component 2. Noncompletion of components 1 or 2 resulted in time-out of 15 and 10 min, respectively. Schedule III was identical with schedule II, except that in schedule III the completion of components was indicated only by a change in the lever lights. The influence of self-administered drugs on behavior in all three components was evaluated. Self-administration of psychomotor stimulants impaired the performance of animals and delayed completion of components 1 and 2 of schedules I, II, and III. The effects on behavior were similar with low drug intake in schedule III, moderate intake in schedule II, and high drug intake in schedule I. These effects were strong with self-administration of phenmetrazine, moderate with self-administration of cocaine and d-amphetamine, and weak with self-administration of fenetylline.

The effects of psychomotor stimulants on single-spatial alternation behavior in dogs.

Dogs were trained to pedal press for drinking water in a noncued, single-spatial alternation task. After the dogs were exhibiting stable performance at or above predetermined criteria levels, they were given three doses of four different drugs (methylphenidate, 0.2, 0.4, and 0.8 mg/kg; d-amphetamine, 0.15, 0.3 and 0.6 mg/kg; cocaine, 0.5, 1, and 2 mg/kg; and phenmetrazine, 0.6, 1.2 and 2.4 mg/kg). In general, all four drugs produced similar changes in performance. The number of correct responses was an especially sensitive indicator of drug effects. All four drugs also produced significant increases in both the average response latency and total session duration, but there were few significant changes in either the total number of responses or number of intertrial interval responses. Relative to d-amphetamine, the potencies of cocaine and phenmetrazine, but not methylphenidate, were generally higher for the measures of single-spatial alternation than for self-administration.

Relationship between anorectic and reinforcing properties of appetite suppressant drugs: implications for assessment of abuse liability.

A quantitative ratio measure was developed which permitted comparisons between the reinforcing and anorectic potency of eight phenylethylamine anorectics and cocaine in laboratory baboons. The ordering of these compounds based upon this ratio bears a reasonable correspondence to clinical drug evaluations. The measure may provide information for preclinical evaluation of relative abuse potential of anorectic drugs.

Open-field behavior after intravenous amphetamine analogues in rats.

A variety of behaviors were studied in an open-field setting after i.v. amphetamine (0.5, 2.0, 8.0 mg/kg), phenmetrazine (1.0, 4.0, 16.0 mg/kg), or fenfluramine (1.0, 4.0, 16.0 mg/kg). Amphetamine and phenmetrazine increased ambulation initially and rearing during the whole experiment, and decreased grooming. At 30 and 60 min, with the three higher doses of amphetamine, stereotyped behaviors interfered with and decreased both ambulation and groomin. Fenfluramine decreased ambulation, rearing, and grooming, and was the only drug to induce backing. The technique seems to be a simple and rapid method to establish dependence liability in amphetamine analogues. Interrater and test-retest reliability was established through ITV recordings.