Discriminative stimulus properties of amphetamine and other stimulants in lead-exposed and normal rats

Current FDA regulatory guidance on the conduct of drug discrimination studies for NDA review: Does the scientific literature support recent recommendations?

BACKGROUND

The Controlled Substances Staff of the Center for Drug Evaluation and Research at the US Food and Drug Administration and the Pharmaceutical Research Manufacturers Association (PhRMA) conducted a series of open forum dialog sessions between 2006 and 2016. A Cross Company Abuse Liability Council (CCALC) was formed during the process of this unique collaborative effort between Industry and Federal Regulators whose goals were to establish the development of standards for the preclinical screening of new molecular entities for schedule control actions required as part of every New Drug Application process. The draft guidance document was published and disseminated in 2010, which allowed for alternative approaches to each study protocol requirement needed for NDA review, if the approach satisfied the requirements of the applicable statutes and regulations (i.e., the controlled substance act). In a series of recent pre-study protocol reviews requested by confidential Pharmaceutical Sponsors of MPI Research, the CSS staff appeared to change its policy and set forth to require all drug discrimination study data to be generated under "extinction" test sessions. MPI Research is a Contract Research Organization acting on behalf of pharmaceutical companies and bound under separate confidentiality agreements.

PURPOSE

The purpose of this review is to highlight the data appearing in peer-reviewed scientific journals that do not support the regulatory administrative constraints on one specific testing methodology (extinction) to the exclusion of another (reinforced test sessions).

CONCLUSION

This mind shift represents a restrictive administrative policy by the FDA that is not supported by the published data.

Sensitized nucleus accumbens dopamine terminal responses to methylphenidate and dopamine transporter releasers after intermittent-access self-administration

Long-access methylphenidate (MPH) self-administration has been shown to produce enhanced amphetamine potency at the dopamine transporter and concomitant changes in reinforcing efficacy, suggesting that MPH abuse may change the dopamine system in a way that promotes future drug abuse. While long-access self-administration paradigms have translational validity for cocaine, it may not be as relevant a model of MPH abuse, as it has been suggested that people often take MPH intermittently. Although previous work outlined the neurochemical and behavioral consequences of long-access MPH self-administration, it was not clear whether intermittent access (6 h session; 5 min access/30 min) would result in similar changes. For cocaine, long-access self-administration resulted in tolerance to cocaine's effects on dopamine and behavior while intermittent-access resulted in sensitization. Here we assessed the neurochemical consequences of intermittent-access MPH self-administration on dopamine terminal function. We found increased maximal rates of uptake, increased stimulated release, and subsensitive D2-like autoreceptors. Consistent with previous work using extended-access MPH paradigms, the potencies of amphetamine and MPH, but not cocaine, were increased, demonstrating that unlike cocaine, MPH effects were not altered by the pattern of intake. Although the potency results suggest that MPH may share properties with releasers, dopamine release was increased following acute application of MPH, similar to cocaine, and in contrast to the release decreasing effects of amphetamine. Taken together, these data demonstrate that MPH exhibits properties of both blockers and releasers, and that the compensatory changes produced by MPH self-administration may increase the abuse liability of amphetamines, independent of the pattern of administration.

Methylphenidate amplifies the potency and reinforcing effects of amphetamines by increasing dopamine transporter expression

Methylphenidate (MPH) is commonly diverted for recreational use, but the neurobiological consequences of exposure to MPH at high, abused doses are not well defined. Here we show that MPH self-administration in rats increases dopamine transporter (DAT) levels and enhances the potency of MPH and amphetamine on dopamine responses and drug-seeking behaviours, without altering cocaine effects. Genetic overexpression of the DAT in mice mimics these effects, confirming that MPH self-administration-induced increases in DAT levels are sufficient to induce the changes. Further, this work outlines a basic mechanism by which increases in DAT levels, regardless of how they occur, are capable of increasing the rewarding and reinforcing effects of select psychostimulant drugs, and suggests that individuals with elevated DAT levels, such as ADHD sufferers, may be more susceptible to the addictive effects of amphetamine-like drugs.

Reinforcing, subject-rated, performance and physiological effects of methylphenidate and d-amphetamine in stimulant abusing humans

Methylphenidate has potential for abuse because it produces behavioural effects similar to those observed with other abused stimulants, such as d-amphetamine and cocaine. The aim of this study was to further characterize the abuse potential of oral methylphenidate relative to oral d-amphetamine. Ten drug-abusing volunteers were recruited to participate in this study, which consisted of seven dose conditions: methylphenidate (16, 32 and 48 mg), d-amphetamine (8, 16 and 24 mg) and placebo. The reinforcing effects of these drugs were assessed during a self-administration session (preceded by a sampling session for each condition) with a modified progressive-ratio procedure. Subject-rated, performance and physiological effects were assessed concurrently during both the sampling and self-administration sessions. The intermediate dose of methylphenidate and d-amphetamine increased responding significantly above placebo levels. Both methylphenidate and d-amphetamine produced dose-dependent increases in stimulant-like subject ratings (e.g. Active, Alert, or Energetic and High), but the effects of these drugs were not isomorphic. These findings are consistent with epidemiological data and previous findings from laboratory studies that suggest methylphenidate has at least some abuse potential.

Effects of training dose on amphetamine drug discrimination: dose-response functions and generalization to cocaine

Rats were trained to discriminate one of three doses of amphetamine (AM), 0.5, 1, or 2 mg/kg, from vehicle (VEH) in a two-lever, food-reinforced, drug-discrimination task. The purpose of the study was to investigate the nature of the shift of the dose-response curve and generalization to cocaine (COC) as a function of training dose. In order to preclude potential differences among the groups in stimulus control, the three training-dose groups were required to perform the discrimination at high and equivalent levels of accuracy. The shift of the dose-response functions to the right as a function of increasing training dose was not parallel. The slope decreased as training dose increased. There was a dose-dependent increase in AM lever responding to test doses of COC that tended to be affected by training dose. The results suggest that proper evaluation of training-dose effects requires that groups be trained to equivalent levels of stimulus control.

Assessing the abuse potential of methylphenidate in nonhuman and human subjects: a review

Methylphenidate (MPH) is widely used for the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children, adolescents, and adults. Methylphenidate is clearly effective for the treatment of ADHD, but there is controversy as to whether it has significant abuse potential like other psychostimulants (e.g., D-amphetamine and cocaine). In general, the drug is believed to be abused at rates much lower than those for other stimulants. The present review examines studies that investigated the behavioral pharmacological profile of methylphenidate and discusses how results from these studies address its abuse liability. Using MEDLINE search terms methylphenidate, drug discrimination, reinforcement, self-administration, subjective effects, subject-rated effects, abuse potential, and abuse liability, along with a review of the references from identified articles, 60 studies were located in which the reinforcing, discriminative-stimulus, or subjective effects of methylphenidate were directly assessed in nonhumans or humans. Forty-eight (80.0%) of the studies reviewed indicate that methylphenidate either functions in a manner similar to D-amphetamine or cocaine (e.g., functions as a reinforcer, substitutes fully in drug discrimination experiments), or produces a pattern of subjective effects suggestive of abuse potential. The results are discussed as they pertain to factors that may account for the apparent discrepancy in abuse rates between methylphenidate and other stimulants, including characterization of actual abuse rates, defining abuse and misuse, pharmacokinetic factors, and validity of abuse liability assays.

(-)Ephedrine and caffeine mutually potentiate one another's amphetamine-like stimulus effects

Using rats trained to discriminate 1 mg/kg of (+)amphetamine (ED50 = 0.4 mg/kg) from saline vehicle in a two-lever drug discrimination procedure, it was shown that (-)ephedrine (ED50 = 4.5 mg/kg), but not (+)ephedrine, substitutes for the (+)AMPH stimulus. It was also shown that caffeine (ED50 = 12.9 mg/kg) can substitute for (+)amphetamine in a dose-related fashion. Doses of (-)ephedrine and caffeine, which produced < or = 1% drug-appropriate responding when administered alone, were able to enhance each other's stimulus effects when administered in combination such that there was a twofold leftward shift in their respective dose-response curves. Furthermore, stimulus generalization occurred when a dose of caffeine that produced saline-appropriate responding when administered alone was administered in combination with (+)ephedrine. It would appear that low doses of (-)ephedrine and caffeine may mutually potentiate one another's stimulus effects in (+)AMPH-trained rats, and that a combination of caffeine and (+)ephedrine result in altered stimulus character when compared to comparable doses of either agent administered alone.

Drug discrimination training with low doses: maintenance of discriminative control

Procedures are reported that maintain control by the drug cue during and after drug discrimination training with lower doses that yield predominantly vehicle-appropriate choices. Twelve pigeons were trained to discriminate chlordiazepoxide (CDP) from saline using two-key (drug vs. vehicle) drug discrimination procedures. Intermixed within each block of 30 sessions were nine sessions of training with 8.0 mg/kg CDP, nine with one of seven lower training doses (4.0, 2.8, 2.0, 1.4, 1.0, 0.7, or 0.5 mg/kg CDP), and 12 with saline. The lower training dose was decreased across blocks. The three lowest training doses (1.0, 0.7, and 0.5 mg/kg CDP) yielded predominantly saline-appropriate choices but had no effect on discrimination of 8.0 mg/kg CDP or saline. Three doses (2.0, 1.4, and 1.0 mg/kg CDP) were retrained, and each yielded percentages of drug-appropriate choices nearly identical to those obtained during previous training. This drug discrimination procedure maintains control by the drug cue during and after training with vehicle-like doses of the training drug and may allow for repeated assessment of effects of low training doses.

Chronic lead exposure attenuates ethanol-induced hypoalgesia

Adult male rats were exposed to drinking fluid containing either 500 ppm lead acetate (group lead), or an equivalent concentration of sodium acetate (group control) for 61 days prior to pain reactivity testing using a tail-flick procedure. Rats were placed in restraining tubes for a 20 min acclimation period, and then baseline tail-flick latencies in response to a radiant heat source were measured. Subsequently, half the animals from each group were serially injected IP with either 1.0, 2.0, or 3.0 g/kg body weight of a 20% v/v ethanol solution, and the other half were injected with an equivalent volume of saline. Tail-flick latencies were reassessed at 20-min intervals over the next 2 h. Results indicated dose-dependent ethanol-induced hypoalgesia at all doses, but at the two higher doses the magnitude of the hypoalgesic response was significantly greater in the group control animals than in the group lead animals across the 2-h postinjection period. Results are discussed in terms of an attenuation of the pharmacological properties of ethanol by lead.

Drug discrimination by humans compared to nonhumans: current status and future directions

In drug discrimination (DD) procedures, behavior is differentially reinforced depending on the presence or absence of specific drug stimuli. The DD paradigm has been widely adopted by behavioral pharmacologists because of its specificity of stimulus control, concordance with drug action at cellular levels and its use as a preclinical model of subject-rated effects in humans. With the successful extension of DD to humans, a comparison of human and nonhuman DD will help place each in the context of the other. Twenty-eight studies of DD in humans are reviewed, including studies of amphetamine, opioid, benzodiazepine, caffeine, nicotine, marijuana and ethanol discriminative stimuli. Comparison of procedures between studies in humans and nonhumans reveals a common tradition, except the use of instructions appears to facilitate greatly DD acquisition in humans. Findings were qualitatively similar between humans and nonhumans. Potency relationships were quantitatively similar between humans and most, but not all, other species. Areas of human DD needing additional empirical evaluation include the influence of instructions, the effects of training dose and the effects of antagonists. Additionally, antihistamines, barbiturates, nicotine and marijuana are under-represented in human DD.

Chronic exposure to lead attenuates cocaine-induced behavioral activation

Adult, male rats were exposed to a diet containing 500 ppm (0.05%) lead for 105 days before testing for cocaine-related changes in activity using a Digiscan activity system. Behavioral testing occurred on 6 successive test days. Activity was recorded for 20 min prior to and 40 min after IP injections of either 10, 20, or 40 mg/kg cocaine HCl, with saline injections on the day preceding each drug test day. Cocaine-induced behavioral activation was evident in control diet animals for all three doses (10, 20, and 40 mg/kg). While 10 mg/kg cocaine HCl did not produce behavioral activation in lead-treated animals, both 20 and 40 mg/kg did result in increased activity comparable to that observed in control counterparts.

Postnatal lead exposure induces supersensitivity to the stimulus properties of a D2-D3 agonist

To examine the impact of lead (Pb) exposure during the ontogeny of dopaminergic (DA) systems on resultant DA function, rats were exposed postnatally (0-21 days of age) via the lactating dam to 0, 100 or 350 ppm Pb acetate in drinking water. At 2 months of age, the postnatally Pb-exposed rats were trained to discriminate the stimulus properties of either the D1 receptor agonist SKF38393 (6.0 mg/kg) or the D2-D3 receptor family subtype agonist quinpirole (0.05 mg/kg) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. In each training group, dose-effect curves describing drug lever responding to lower doses of the training drug and to preadministration of selective DA antagonists were obtained to examine Pb-induced changes in DA sensitivity, and doses of non-DA compounds were substituted to determine the specificity of any changes in DA sensitivity. In the D1/saline training condition, Pb exposure was not associated with any specific or consistent changes in DA sensitivity. In contrast, exposure to Pb was associated with D2-D3 receptor subtype supersensitivity as was indicated by significantly elevated levels of drug lever responding in the presence of quinpirole and haloperidol and to at least one dose of apomorphine. No differences in the dose-effect curves for either (+)-amphetamine or NMDA were observed in the D2-D3-trained control and Pb-exposed groups, but an increase in drug lever responding in the presence of pentobarbital was noted in the Pb-exposed group relative to control. Taken together, these findings are consistent with a Pb-induced functional D2-D3 supersensitivity possibly mediated via autoreceptors. Moreover, this functional D2-D3 supersensitivity necessarily represents a permanent effect of postnatal Pb exposure since both blood and brain Pb levels were negligible at the time drug discrimination training began.

Low level lead exposure increases sensitivity to the stimulus properties of dopamine D1 and D2 agonists

To examine the impact of Pb exposure on dopaminergic (DA) function, weanling rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water. At 3 months of age, the rats were trained to discriminate the stimulus properties of either the D1 agonist SKF38393 (3.0 mg/kg i.p.; D1/sal) or the D2 agonist quinpirole (0.05 mg/kg i.p., D2/sal) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. Lead-exposed rats learned the discriminations faster than respective controls. Moreover, they exhibited greater levels of drug lever responding to lower doses of the training drugs (D1/sal and D2/sal), and to selected doses of other direct and indirect DA agonists (D2/sal only), including apomorphine, cocaine and (+)-amphetamine, and less blockade of drug lever responding by haloperidol (D2/sal). Taken together, these findings are consistent with a generalized DA supersensitivity. There were no differential Pb effects when non-DA compounds including morphine, pentobarbital and MK-801 were substituted for the training drugs, indicating the selectivity of the DA effects in the context of these experiments, and the improbability of a non-specific behavioral causation. Pb-exposed rats in the D2/sal group also showed a pronounced enhancement of drug lever responding when NMDA was substituted for quinpirole, suggesting the possibility of a Pb-induced NMDA supersensitivity as well.

Discriminative stimulus effects of d-amphetamine, methylphenidate, and diazepam in humans

Eight male community volunteers, who reported current psychomotor stimulant use, were trained to discriminate between the presence and absence of orally administered d-amphetamine 30 mg. During daily experimental sessions, in which a single drug dose or placebo was tested, physiological and subjective measures were assessed and subjects indicated their discrimination by responding on an operant color-tracking procedure. During four test of acquisition sessions, discriminative responding indicated that all subjects learned the discrimination, and d-amphetamine produced physiological and subjective effects typical of psychomotor stimulants. Generalization testing then followed in which dose-response curves were determined for the following drugs: d-amphetamine (3.75, 7.5, 15 and 30 mg), diazepam (5, 10, 20 and 40 mg), and methylphenidate (7.5, 15, 30 and 60 mg). d-Amphetamine and methylphenidate produced dose-related increases in d-amphetamine-appropriate responding, whereas no dose of diazepam substituted for d-amphetamine in any subject. d-Amphetamine and methylphenidate produced a similar pattern of subjective changes, including increased ratings of euphoria and drug liking and decreased sedation. In contrast, diazepam increased subjective scales of sedation and dysphoria. These results are consistent with similar studies testing animals and humans and demonstrate the utility of human drug discrimination research as an integral component of drug abuse liability testing.

Discriminative stimulus effects of caffeine and benzphetamine in amphetamine-trained volunteers

The discriminative stimulus (DS) and subjective effects of caffeine (100 and 300 mg, PO) and benzphetamine (12.5 and 50 mg, PO) were studied in 18 normal human volunteers trained to discriminate between d-amphetamine (10 mg) and placebo. d-Amphetamine increased ratings of drug liking and activity level and produced a profile of subjective effects characteristic of amphetamine and related psychomotor stimulants. The DS effects of d-amphetamine generalized only partially to caffeine and benzphetamine; mean percent d-amphetamine-appropriate responding was 42 and 58 after 100 and 300 mg caffeine, respectively, and 17 and 56 after 12.5 and 50 mg benzphetamine, respectively. Neither dose of caffeine affected ratings of drug liking or activity level, but 300 mg caffeine did produce a profile of subjective effects that partially overlapped with that produced by d-amphetamine. Benzphetamine 50 mg, but not 12.5 mg, increased ratings of drug liking and activity level and produced a profile of subjective effects qualitatively similar to, but weaker than, that produced by d-amphetamine. For both caffeine and benzphetamine, a close relationship was observed between their subjective effects and their ability to substitute for the DS effects of d-amphetamine. These results correspond well with findings obtained from similar studies conducted with laboratory animals, providing further support for the reliability and validity of human drug discrimination paradigms.