Response to methylphenidate is not influenced by DAT1 polymorphisms in a sample of Brazilian adult patients with ADHD

In silico clinical trial evaluating lisdexamfetamine's and methylphenidate's mechanism of action computational models in an attention-deficit/hyperactivity disorder virtual patients' population

Introduction

Attention-deficit/hyperactivity disorder (ADHD) is an impairing psychiatric condition with the stimulants, lisdexamfetamine (LDX), and methylphenidate (MPH), as the first lines pharmacological treatment.

Methods

Herein, we applied a novel in silico method to evaluate virtual LDX (vLDX) and vMPH as treatments for ADHD applying quantitative systems pharmacology (QSP) models. The objectives were to evaluate the model's output, considering the model characteristics and the information used to build them, to compare both virtual drugs' efficacy mechanisms, and to assess how demographic (age, body mass index, and sex) and clinical characteristics may affect vLDX's and vMPH's relative efficacies.

Results and Discussion

We molecularly characterized the drugs and pathologies based on a bibliographic search, and generated virtual populations of adults and children-adolescents totaling 2,600 individuals. For each virtual patient and virtual drug, we created physiologically based pharmacokinetic and QSP models applying the systems biology-based Therapeutic Performance Mapping System technology. The resulting models' predicted protein activity indicated that both virtual drugs modulated ADHD through similar mechanisms, albeit with some differences. vMPH induced several general synaptic, neurotransmitter, and nerve impulse-related processes, whereas vLDX seemed to modulate neural processes more specific to ADHD, such as GABAergic inhibitory synapses and regulation of the reward system. While both drugs' models were linked to an effect over neuroinflammation and altered neural viability, vLDX had a significant impact on neurotransmitter imbalance and vMPH on circadian system deregulation. Among demographic characteristics, age and body mass index affected the efficacy of both virtual treatments, although the effect was more marked for vLDX. Regarding comorbidities, only depression negatively impacted both virtual drugs' efficacy mechanisms and, while that of vLDX were more affected by the co-treatment of tic disorders, the efficacy mechanisms of vMPH were disturbed by wide-spectrum psychiatric drugs. Our in silico results suggested that both drugs could have similar efficacy mechanisms as ADHD treatment in adult and pediatric populations and allowed raising hypotheses for their differential impact in specific patient groups, although these results require prospective validation for clinical translatability.

Molecular Characterisation of the Mechanism of Action of Stimulant Drugs Lisdexamfetamine and Methylphenidate on ADHD Neurobiology: A Review

Attention-deficit/hyperactivity disorder (ADHD) is a common childhood-onset neurodevelopmental disorder characterised by persistent inattention, hyperactivity and impulsivity. Moreover, ADHD is commonly associated with other comorbid diseases (depression, anxiety, bipolar disorder, etc.). The ADHD symptomatology interferes with subject function and development. The treatment of ADHD requires a multidisciplinary approach based on a combination of non-pharmacological and pharmacological treatments with the aim of ameliorating the symptomatology; among first-line pharmacological treatments are stimulants [such as methylphenidate (MPH) and lisdexamfetamine dimesylate (LDX)]. In this review we explored recent ADHD- and stimulants-related literature, with the aim of compiling available descriptions of molecular pathways altered in ADHD, and molecular mechanisms of current first-line stimulants MPH and LDX. While conducting the narrative review, we applied structured search strategies covering PubMed/MEDLINE database and performed handsearching of reference lists on the results of those searches. The aetiology and pathophysiology of ADHD are incompletely understood; both genetic and environmental factors have been associated with the disorder and its grade of burden, and also the relationship between the molecular mechanisms of pharmacological treatments and their clinical implications. The lack of comprehensive understanding of the underlying molecular pathology makes both the diagnosis and treatment difficult. Few published studies evaluating molecular data on the mechanism of action (MoA) of MPH and LDX on ADHD are available and most of them are based on animal models. Further studies are necessary to improve the knowledge of ADHD pathophysiology and how the MoAs of MPH and LDX differentially modulate ADHD pathophysiology and control ADHD symptomatology.

Genetics in the ADHD Clinic: How Can Genetic Testing Support the Current Clinical Practice?

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder with a childhood prevalence of 5%. In about two-thirds of the cases, ADHD symptoms persist into adulthood and often cause significant functional impairment. Based on the results of family and twin studies, the estimated heritability of ADHD approximates 80%, suggests a significant genetic component in the etiological background of the disorder; however, the potential genetic effects on disease risk, symptom severity, and persistence are unclear. This article provides a brief review of the genome-wide and candidate gene association studies with a focus on the clinical aspects, summarizing findings of ADHD disease risk, ADHD core symptoms as dimensional traits, and other traits frequently associated with ADHD, which may contribute to the susceptibility to other comorbid psychiatric disorders. Furthermore, neuropsychological impairment and measures from neuroimaging and electrophysiological paradigms, emerging as potential biomarkers, also provide a prominent target for molecular genetic studies, since they lie in the pathway from genes to behavior; therefore, they can contribute to the understanding of the underlying neurobiological mechanisms and the interindividual heterogeneity of clinical symptoms. Beyond the aforementioned aspects, throughout the review, we also give a brief summary of the genetic results, including polygenic risk scores that can potentially predict individual response to different treatment options and may offer a possibility for personalized treatment for the therapy of ADHD in the future.

Gene-Environment Correlation Between the Dopamine Transporter Gene (DAT1) Polymorphism and Childhood Experiences of Abuse

In the present study, we investigated the possible gene-environment correlation between the dopamine transporter gene (DAT1) polymorphism and childhood experiences of abuse and neglect. Genetic information was obtained from 1,442 male and 2,178 female twins and their siblings drawn from a Finnish population-based sample. The Childhood Trauma Questionnaire was used to measure the childhood experiences of abuse and neglect. In men, the DAT1 polymorphism was associated with having experienced sexual abuse in childhood, such that men with the 9R9R genotype reported less sexual abuse experiences than men with the 9R10R or the 10R10R genotypes. In women, there was an association between the DAT1 polymorphism and childhood experiences of emotional abuse, such that women with the 9R9R genotype reported less emotional abuse experiences than women with the 9R10R or 10R10R genotypes. No other associations between the DAT1 polymorphism and childhood experiences of abuse and neglect were found. In sum, the results suggested that some genetic components might predispose children to experience childhood abuse and neglect. Possible reasons for this association were discussed.

Exocytosis-related genes and response to methylphenidate treatment in adults with ADHD

Experimental studies have demonstrated that methylphenidate (MPH) modulates the synaptic vesicle trafficking and synaptotagmin-1 (SytI) mRNA levels. SytI is a regulatory protein of the SNARE complex, a neurotransmitter exocytosis mediator. Despite this evidence, most SNARE complex-related genes have never been evaluated in attention-deficit/hyperactivity disorder (ADHD) pharmacogenetics. This study evaluates, for we believe the first time, polymorphisms on the SNARE complex-related genes STX1A (rs2228607), VAMP2 (26bp Ins/Del) and SYT1 (rs1880867 and rs2251214) on the response to immediate-release methylphenidate (IR-MPH) in a naturalistic sample of adults with ADHD. The sample comprised 433 subjects, of which 272 (62.8%) have completed the short-term IR-MPH treatment (at least 30 days). The main outcome measure was the categorical variable of short-term response to IR-MPH based on the Swanson, Nolan and Pelham Rating Scale version 4 (SNAP-IV), and on the clinical global impression-improvement scale. Additional analyses evaluated the percentage of SNAP-IV symptom reduction for each dimension as well as short- and long- (7 years) term treatment persistence. SYT1-rs2251214 was associated with the categorical short-term response to IR-MPH (P=0.006, P_FDR=0.028), and with the percentage of inattention and oppositional defiant disorder symptoms reduction (P=0.007, P_FDR=0.028 and P=0.017, P_FDR=0.048, respectively). SYT1-rs2251214 was also associated with short-term treatment persistence (P=0.018, P_FDR=0.048), and with months of treatment (P=0.002, P_FDR=0.016) in the long-term protocol. Our findings suggest that SYT1-rs2251214 presents a broad influence in IR-MPH response variability in adults with ADHD, being involved with both symptom response and treatment persistence. If such findings are replicated, SytI could represent a key element in MPH pharmacodynamics in adults with ADHD.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

Attention-deficit hyperactivity disorder in adults: A systematic review and meta-analysis of genetic, pharmacogenetic and biochemical studies

The adult form of attention-deficit/hyperactivity disorder has a prevalence of up to 5% and is the most severe long-term outcome of this common disorder. Family studies in clinical samples as well as twin studies suggest a familial liability and consequently different genes were investigated in association studies. Pharmacotherapy with methylphenidate (MPH) seems to be the first-line treatment of choice in adults with attention-deficit hyperactive disorder (ADHD) and some studies were conducted on the genes influencing the response to this drug. Finally some peripheral biomarkers were identified in ADHD adult patients. We believe this work is the first systematic review and meta-analysis of candidate gene association studies, pharmacogenetic and biochemical (metabolomics) studies performed in adults with ADHD to identify potential genetic, predictive and peripheral markers linked specifically to ADHD in adults. After screening 5129 records, we selected 87 studies of which 61 were available for candidate gene association studies, 5 for pharmacogenetics and 21 for biochemical studies. Of these, 15 genetic, 2 pharmacogenetic and 6 biochemical studies were included in the meta-analyses. We obtained an association between adult ADHD and the gene BAIAP2 (brain-specific angiogenesis inhibitor 1-associated protein 2), even after Bonferroni correction, with any heterogeneity in effect size and no publication bias. If we did not apply the Bonferroni correction, a trend was found for the carriers allele 9R of dopamine transporter SLC6A3 40 bp variable tandem repeat polymorphism (VNTR) and for 6/6 homozygotes of SLC6A3 30 bp VNTR. Negative results were obtained for the 9-6 haplotype, the dopamine receptor DRD4 48 bp VNTR, and the enzyme COMT SNP rs4680. Concerning pharmacogenetic studies, no association was found for the SLC6A3 40 bp and response to MPH with only two studies selected. For the metabolomics studies, no differences between ADHD adults and controls were found for salivary cortisol, whereas lower serum docosahexaenoic acid (DHA) levels were found in ADHD adults. This last association was significant even after Bonferroni correction and in absence of heterogeneity. Other polyunsaturated fatty acids (PUFAs) such as AA (arachidonic acid), EPA (eicosapentaenoic acid) and DyLA (dihomogammalinolenic acid) levels were not different between patients and controls. No publication biases were observed for these markers. Genes linked to dopaminergic, serotoninergic and noradrenergic signaling, metabolism (DBH, TPH1, TPH2, DDC, MAOA, MAOB, BCHE and TH), neurodevelopment (BDNF and others), the SNARE system and other forty genes/proteins related to different pathways were not meta-analyzed due to insufficient data. In conclusion, we found that there were not enough genetic, pharmacogenetic and biochemical studies of ADHD in adults and that more investigations are needed. Moreover we confirmed a significant role of BAIAP2 and DHA in the etiology of ADHD exclusively in adults. Future research should be focused on the replication of these findings and to assess their specificity for ADHD.

Should we keep on? Looking into pharmacogenomics of ADHD in adulthood from a different perspective

A considerable proportion of adults with attention-deficit/hyperactivity disorder (ADHD) do not respond to the treatment with methylphenidate. This scenario could be due to inherited interindividual differences that may alter pharmacologic treatment response. In this sense, in 2012 we conducted a systematic search on PUBMED-indexed literature for articles containing information about pharmacogenomics of ADHD in adults. Five studies were found on methylphenidate pharmacogenomics and the only significant association was reported by one particular study. However, this single association with the SLC6A3 gene was not replicated in two subsequent reports. In the present review, although we could not find additional pharmacogenomics studies, we discuss these up-to-date findings and suggest new approaches for this field. Additionally, using systeomic-oriented databases, we provide a broad picture of new possible candidate genes as well as potential gene-gene interactions to be investigated in pharmacogenomics of persistent ADHD.

ADHD pharmacogenetics across the life cycle: New findings and perspectives

Attention-deficit/hyperactivity disorder (ADHD) is a complex and heterogeneous disorder, affecting individuals across the life cycle. Although its etiology is not yet completely understood, genetics plays a substantial role. Pharmacological treatment is considered effective and safe for children and adults, but there is considerable inter-individual variability among patients regarding response to medication, required doses, and adverse events. We present here a systematic review of the literature on ADHD pharmacogenetics to provide a critical discussion of the existent findings, new approaches, limitations, and recommendations for future research. Our main findings are: first, the number of studies continues to grow, making ADHD one of the mental health areas with more pharmacogenetic studies. Second, there has been a focus shift on ADHD pharmacogenetic studies in the last years. There is an increasing number of studies assessing gene-gene and gene-environment interactions, using genome-wide association approaches, neuroimaging, and assessing pharmacokinetic properties. Third and most importantly, the heterogeneity in methodological strategies employed by different studies remains impressive. The question whether pharmacogenetics studies of ADHD will improve clinical management by shifting from trial-and-error approach to a pharmacological regimen that takes into account the individual variability remains unanswered. © 2014 Wiley Periodicals, Inc.

Polymorphisms in the dopamine transporter gene are associated with visual hallucinations and levodopa equivalent dose in Brazilians with Parkinson's disease

The requirement for dopaminergic drugs in Parkinson's disease (PD) is highly variable. Visual hallucinations are a frequent and serious complication of chronic levodopa therapy. Polymorphisms in the DAT1 gene might affect the reuptake of dopamine in the synaptic cleft, but the influence of this variability on adverse effects or levodopa equivalent dose on PD patients is still poorly investigated. Therefore, the aim of the present study was to investigate DAT1 gene polymorphisms on levodopa equivalent dose and visual hallucination occurrence in PD patients. Altogether, 196 PD patients in treatment with at least 200 mg levodopa equivalent dose for at least 1 yr were included. These patients were genotyped for the -839 C > T and 3' VNTR DAT1 polymorphisms by PCR-based methodologies. Visual hallucinations occurred in 25.5% of the sample. After controlling for confounders, the dopamine transporter (DAT) -839 C allele was associated with visual hallucinations (prevalence ratio 2.5, 95% confidence intervals 1.13-5.5, p = 0.02). Levodopa equivalent dose was lower in carriers of the nine repeat allele of the DAT 3'UTR VNTR (741.2 ± 355.0 vs. 843.4 ± 445.7), explaining 21% of dose variability (p = 0.01). Our results support an effect of DAT1 polymorphisms in adverse effects of anti-Parkinsonian drugs and in levodopa equivalent dose usage.

Pharmacogenetics of response to methylphenidate in adult patients with Attention-Deficit/Hyperactivity Disorder (ADHD): a systematic review

Methylphenidate (MPH) is a first line option in the psychopharmacologic treatment of adults with Attention-Deficit/Hyperactivity Disorder (ADHD). However, there is a considerable proportion of adult patients who do not respond to treatment with MPH or discontinue drug therapy. Since effects of genetic variants in the response to MPH treatment might explain these negative outcomes, we conducted an electronic systematic search of MEDLINE-indexed literature looking for articles containing information about pharmacogenetics of ADHD in adults published until January, 2012. The keywords used were 'ADHD', 'Attention-Deficit/Hyperactivity Disorder' and 'gene' in combination with methylphenidate, amphetamine or atomoxetine. Only 5 pharmacogenetic studies on adult ADHD met inclusion criteria. The results evidenced that most findings obtained so far are negative, and all studies focused on MPH response. There is only one positive result, for a polymorphism at the dopamine transporter gene (DAT1) gene. The current state of the art in adult ADHD implies that pharmacogenetic tests are far from routine clinical practice. However, the integration of these studies with neuroimaging and neuropsychological tests may help to understand mechanisms of drug action and the pathophysiology of ADHD.

Meta-analysis of the association between dopamine transporter genotype and response to methylphenidate treatment in ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent childhood-onset neuropsychiatric disorder. Treatment with methylphenidate, which blocks dopamine and noradrenaline transporters, is clinically efficacious in reducing the symptoms of ADHD. However, a considerable proportion of patients show no or only insufficient response to methylphenidate. Following a pharmacogenetic approach, a number of studies have suggested that heterogeneity in treatment response across subjects might to some extent be due to genetic factors. In particular, a variable number tandem repeat (VNTR) polymorphism in the 3' untranslated region of the SLC6A3 gene, which codes for the dopamine transporter, has been considered as a predictor of treatment success. However, the literature has so far been inconsistent. Here we present results of a meta-analysis of studies investigating the moderating effect of the SLC6A3 VNTR on response to methylphenidate treatment in subjects with ADHD. Outcome measures from 16 studies including data from 1572 subjects were entered into a random-effects model. There was no significant summary effect for the SLC6A3 VNTR on the response to methylphenidate treatment (P>0.5) and no effect on specific symptom dimensions of hyperactivity/impulsivity and inattention (all P>0.2). However, in a subanalysis of naturalistic trials, we observed a significant effect of d=-0.36 (P=0.03), indicating that 10R homozygotes show less improvement in symptoms following treatment than the non-10/10 carriers. This meta-analysis indicates that SLC6A3 VNTR is not a reliable predictor of methylphenidate treatment success in ADHD. Our study leaves unanswered the question of whether other genetic polymorphisms or nongenetic factors may contribute to the observed heterogeneity in treatment response across ADHD subjects.

No significant association between genetic variants in 7 candidate genes and response to methylphenidate treatment in adult patients with ADHD

Results from pharmacogenetic investigations of methylphenidate (MPH) response in patients with ADHD are still inconsistent, especially among adults. This study investigates the role of genetic variants (SLC6A4, HTR1B, TPH2, DBH, DRD4, COMT, and SNAP25) in the response to MPH in a sample of 164 adults. Genes were chosen owing to previous evidence for an influence in ADHD susceptibility. No significant differences in allele or genotype frequencies between MPH responders and nonresponders were detected. In conclusion, our findings do not support an effect of these genes in the pharmacogenetics of MPH among adults with ADHD.

The genetics of attention deficit/hyperactivity disorder in adults, a review

The adult form of attention deficit/hyperactivity disorder (aADHD) has a prevalence of up to 5% and is the most severe long-term outcome of this common neurodevelopmental disorder. Family studies in clinical samples suggest an increased familial liability for aADHD compared with childhood ADHD (cADHD), whereas twin studies based on self-rated symptoms in adult population samples show moderate heritability estimates of 30-40%. However, using multiple sources of information, the heritability of clinically diagnosed aADHD and cADHD is very similar. Results of candidate gene as well as genome-wide molecular genetic studies in aADHD samples implicate some of the same genes involved in ADHD in children, although in some cases different alleles and different genes may be responsible for adult versus childhood ADHD. Linkage studies have been successful in identifying loci for aADHD and led to the identification of LPHN3 and CDH13 as novel genes associated with ADHD across the lifespan. In addition, studies of rare genetic variants have identified probable causative mutations for aADHD. Use of endophenotypes based on neuropsychology and neuroimaging, as well as next-generation genome analysis and improved statistical and bioinformatic analysis methods hold the promise of identifying additional genetic variants involved in disease etiology. Large, international collaborations have paved the way for well-powered studies. Progress in identifying aADHD risk genes may provide us with tools for the prediction of disease progression in the clinic and better treatment, and ultimately may help to prevent persistence of ADHD into adulthood.

Amphetamine-induced locomotion in a hyperdopaminergic ADHD mouse model depends on genetic background

We previously generated a knock-in mouse line with a cocaine-insensitive dopamine transporter (DAT-CI mice). These mice lost several behavioral responses to cocaine, but retained their response to amphetamine. DAT-CI mice are hyperdopaminergic due to reduced DAT function, and may thus be a good model for studying attention deficit hyperactivity disorder (ADHD). These mice had been behaviorally characterized while they were on a mixed genetic background. However as the colony was propagated over time, the mixed genetics were shifted toward a pure C57Bl/6J background--via a common breeding scheme known as "backcrossing." Several phenotypes appeared to have changed during this time frame. In this study, we investigated whether backcrossing altered the hyperlocomotive phenotype and behavioral responses to amphetamine, a drug used to treat ADHD. C57-congenic DAT-CI mice had high spontaneous locomotor activity that could be suppressed by low doses of amphetamine. Furthermore, their locomotion was not stimulated by very high doses of amphetamine (20mg/kg). After the reversion to a mixed genetic background by breeding with the 129 strain, the C57:129 hybrid DAT-CI mice displayed reduced basal locomotor activity compared to the C57-congenic mutant mice, and regained locomotor stimulation by high-dose amphetamine. The calming effect of amphetamine at low doses was retained in both strains. In summary, reduced DAT function in DAT-CI mice leads to a hyperdopaminergic state, and an ADHD-like phenotype in both strains. The data show that the genetic background of DAT-CI mice affects their locomotor phenotypes and their responses to amphetamine. Since the differences in genetic background between the strains of mice have a significant impact on the ADHD-like phenotype and the response to amphetamine, further study with these strains could identify the genetic underpinnings affecting the severity of ADHD-related symptoms and the treatment response.

Epigenetics in Developmental Disorder: ADHD and Endophenotypes

Heterogeneity in attention-deficit/hyperactivity disorder (ADHD), with complex interactive operations of genetic and environmental factors, is expressed in a variety of disorder manifestations: severity, co-morbidities of symptoms, and the effects of genes on phenotypes. Neurodevelopmental influences of genomic imprinting have set the stage for the structural-physiological variations that modulate the cognitive, affective, and pathophysiological domains of ADHD. The relative contributions of genetic and environmental factors provide rapidly proliferating insights into the developmental trajectory of the condition, both structurally and functionally. Parent-of-origin effects seem to support the notion that genetic risks for disease process debut often interact with the social environment, i.e., the parental environment in infants and young children. The notion of endophenotypes, markers of an underlying liability to the disorder, may facilitate detection of genetic risks relative to a complex clinical disorder. Simple genetic association has proven insufficient to explain the spectrum of ADHD. At a primary level of analysis, the consideration of epigenetic regulation of brain signalling mechanisms, dopamine, serotonin, and noradrenaline is examined. Neurotrophic factors that participate in the neurogenesis, survival, and functional maintenance of brain systems, are involved in neuroplasticity alterations underlying brain disorders, and are implicated in the genetic predisposition to ADHD, but not obviously, nor in a simple or straightforward fashion. In the context of intervention, genetic linkage studies of ADHD pharmacological intervention have demonstrated that associations have fitted the "drug response phenotype," rather than the disorder diagnosis. Despite conflicting evidence for the existence, or not, of genetic associations between disorder diagnosis and genes regulating the structure and function of neurotransmitters and brain-derived neurotrophic factor (BDNF), associations between symptoms-profiles endophenotypes and single nucleotide polymorphisms appear reassuring.

Adrenergic α2A receptor gene is not associated with methylphenidate response in adults with ADHD

Adrenergic α2A receptor gene (ADRA2A) is one of the most promising candidate genes for ADHD pharmacogenetics. Thus far, three studies have investigated the association between the ADRA2A -1291 C>G polymorphism and the therapeutic response to methylphenidate (MPH) in children with ADHD, all of them with positive results. The aim of this study is to investigate, for the first time, the association between three ADRA2A polymorphisms (-1291 C>G, -262 G>A, and 1780 C>T) and the response to MPH in adults with ADHD. The sample comprises 165 Brazilians of European descent evaluated in the adult ADHD outpatient clinic of the Hospital de Clínicas de Porto Alegre. The diagnostic procedures followed the DSM-IV criteria. Drug response was assessed by both categorical and dimensional approaches, through the scales Swanson, Nolan, and Pelham Rating scale version IV and the Clinical Global Impression-Severity Scale, applied at the beginning and after the 30th day of treatment. We found no evidence of association between the three ADRA2A polymorphisms and the therapeutic response to MPH treatment. Our findings do not support a significant role for the ADRA2A gene in ADHD pharmacogenetics, at least among adult patients.

Cognitive enhancement by drugs in health and disease

Attempts to improve cognitive function in patients with brain disorders have become the focus of intensive research efforts. A recent emerging trend is the use of so-called cognitive enhancers by healthy individuals. Here, we consider some of the effects - positive and negative - that current drugs have in neurological conditions and healthy people. We conclude that, to date, experimental and clinical studies have demonstrated relatively modest overall effects, most probably because of substantial variability in response both across and within individuals. We discuss biological factors that might account for such variability and highlight the need to improve testing methods and to extend our understanding of how drugs modulate specific cognitive processes at the systems or network level.