‘STIMULATING’ RATIONALE THERAPY IN ATTENTION-DEFICIT/HYPERACTIVITY DISORDER (ADHD)
Pharmacogenetics of psychostimulants in ADHD
Mario Masellis, Vincenzo S. Basile, and James L. Kennedy
*1. INTRODUCTION
Attention-Deficit/Hyperactivity Disorder (ADHD) is a highly prevalent childhood behavioural disease affecting approximately 3-5% of school-age children (A.P.A., 1994). It also poses a major problem for adults who were either first diagnosed later in life or have suffered from persisting symptoms from childhood (Faraone et al., 2000). The cardinal symptoms of ADHD may be classified into three major clusters – inattentiveness, hyperactivity, and impulsivity (A.P.A., 1994). These symptom clusters may coexist or may occur individually and must significantly
*
Toronto, Canada.
of Psychiatry, University of Toronto, Centre for Addiction and Mental Health, [email protected]. Vincenzo S. Basile, Departments of Neurology and Cognitive Neurology Unit, Room A 421, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 3M5; Tel: 416-480-6100 ext 2449; Fax: 416-480-4552; E-mail:
Psychiatry, University of Toronto, Toronto, Canada. James L. Kennedy, Department Neuroscience Research, Sunnybrook & Women’s College Health Science Centre, Mario Masellis, Departments of Neurology and Psychiatry, University of Toronto,
231
9. NEUROPSYCHOPHARMACOGENETICS:
interfere with academic, family, and social functioning of the afflicted individual.
Furthermore, onset must be ascertained to be prior to age seven. ADHD is a complex, multifactorial illness of unknown etiology with evidence suggesting a strong genetic component as determined by family, twin and adoption studies (Faraone et al., 1994; Gillis et al., 1992; Hechtman, 1994; Pauls, 1991).
Approximately 60 years ago, Bradley (1937) published the first report suggesting that psychostimulants have a unique calming effect on the behaviour of hyperactive children. Since then, there has been a wealth of research confirming the effectiveness of psychostimulant medications in controlling the hyperactive, impulsive, and inattentive symptoms of ADHD, and much work has been invested in determining their mechanism of action. As with all drugs, understanding the mechanism through which psychostimulants produce their therapeutic effects begins with the study of two important pharmacological parameters – pharmacokinetics (PK) and pharmacodynamics (PD).
In order for a drug to exert its biological effect, it must first accumulate in the tissue(s) where its pharmacological ‘target(s)’ is/are located. Pharmacokinetics explains how a drug achieves this by examining the drug concentration versus time relationships in an organism through mathematical formulations of its absorption a from site of administration, its distribution in tissues throughout the body, its metabolism by various enzyme systems, and its excretion from the body (ADME framework) (Greenblatt et al., 1995; Rowland et al., 1995). Once a drug is concentrated in the tissue, it interacts with the ‘target(s)’ through which the final biological effects are elicited (pharmacodynamic interaction). Specifically, an organism (Greenblatt et al., 1995; Rowland et al., 1995). The targets may include a variety of proteins, such as receptors, enzymes, transporters, ion channels, second messengers, among others. Alternatively, a drug may directly or indirectly interact with deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) to produce its biological effects.
We will now briefly review the pharmacokinetics and pharmacodynamics of methylphenidate (Ritalin
®), the most commonly prescribed psychostimulant used in over 90% of children diagnosed with ADHD in the U.S. (Kimko et al., 1999; Safer et al., 1988). This will be followed by a discussion of neuropsycho- pharmacogenetics and its application to ADHD using methylphenidate response as an example. Other less commonly used psychostimulants include dextro- 2002; Markowitz et al., 2001).
1.1. Pharmacokinetics of methylphenidate
Absorption of methylphenidate is complete and rapid with time to peak plasma concentrations (t
max) after oral administration being between 1 and 3 hours (Kimko et al., 1999). In hyperactive children, there is considerable person-to-person pharmacodynamics describes the drug concentration versus effect relationships in
amphetamine and pemoline, which have been reviewed elsewhere (Greenhill et al.,
variability in the absorption of methylphenidate [rate constant: 1.83 ± 1.5 h
-1for 0.34 mg/kg and 2.07 ± 1.0 h
-1for 0.65 mg/kg] (Shaywitz et al., 1982). Methylphenidate is highly lipid soluble and the extent of protein binding is low, so it undergoes rapid distribution throughout the body (Kimko et al., 1999). It is thus possible to attain high methylphenidate concentrations in the central nervous system (CNS) over a short duration of time.
In terms of metabolism and excretion in healthy adults and in children with ADHD, methylphenidate undergoes extensive stereoselective and first-pass clearance (Hubbard et al., 1989; Srinivas et al., 1993), with plasma concentrations of d-methylphenidate being greater than those of the
d
d l-isomer. The d-enantiomer is d d
responsible for the clinical effects of the drug (Srinivas et al., 1992). The predominant metabolic pathway is de-esterification of methylphenidate to form the corresponding carboxylic acid metabolites, d- and d d l-ritalinic acid, which have no known pharmacological activity (Kimko et al., 1999). These stereospecific metabolites undergo extensive urinary excretion.
Initially, it was thought that non-hepatic hydrolytic esterases, which are ubiquitously distributed throughout the body, were responsible for the de- esterification of methylphenidate (Kimko et al., 1999). More recent evidence suggests that the carboxylesterase isoenzyme, CES1A1, which is highly expressed in hepatic and gastrointestinal tissue, has high catalytic efficiency for both the d- and d d d l-enantiomers of methylphenidate; the catalytic efficiency for l-methylphenidate is 3.6- to 6-fold higher than for the d-enantiomer (Sun et al., 2004). The high catalytic d d efficiency of this enzyme together with the stereoselective and first-pass clearance of methylphenidate suggests a major role for CES1A1 in the metabolism of this drug.
Microsomal oxidation and conjugation appear to be minor pathways in the metabolism of methylphenidate accounting for less than 2% of the overall variance observed in its disposition (Kimko et al., 1999). Therefore, the role of microsomal cytochrome-P450 enzymes, responsible for the phase I metabolism of the majority of xenobiotics, appears to be minimal in the disposition of methylphenidate. This premise is supported by evidence suggesting that the polymorphic CYP2D6 enzyme does not play a prominent role in the metabolism of methylphenidate (DeVane et al., 2000).
1.2. Pharmacodynamics of methylphenidate
Although the precise mechanism of action of psychostimulants is not
understood, they are believed to act primarily on the dopaminergic and norepine-
phrinergic systems. Methylphenidate is an indirect agonist of central catechola-
minergic pathways. It facilitates the action of endogenous dopamine and norepinephrine
at their respective pre- and post-synaptic receptor sites. This occurs through three
mechanisms: inhibition of catecholamine reuptake, facilitation of catecholamine
release into the synaptic cleft, and inhibition of monoamine oxidase (MAO) activity
(Solanto, 1998). Specifically, methylphenidate has high affinity for and inhibits both
the dopamine and norepinephrine transporters, thereby blocking reuptake of the respective neurotransmitter (Seeman and Madras, 1998). Methylphenidate also promotes the release of dopamine from reserpine-sensitive storage pools (Solanto, 1998). Ultimately, methylphenidate alters the activity of the nigrostriatal and mesocorticolimbic dopaminergic systems, in addition to norepinephrinergic projections from the locus coeruleus to the cortex, all of which are thought to underlie improvement in ADHD symptoms. For a comprehensive review of the mechanisms of action of psychostimulants, refer to Seeman and Madras (1998), and Solanto (1998).
2. NEUROPSYCHOPHARMACOGENETICS: A SYNOPSIS
Before examining the molecular genetic basis underlying a variable trait such as response to methylphenidate, it is usually necessary to establish the existence of a heritable component. This is accomplished by undertaking epidemiologically based family, twin, and adoption studies from which estimates of trait heritability are derived. For a discussion of these approaches, refer to Gottesman (1991). These epidemiologically based methods are generally applied to determine whether genetic factors play a role in persistent phenotypes (Kalow et al., 1998).
In attempting to identify a heritable component for methylphenidate response, several considerations must be made. For pharmacological traits, it is important to keep in mind that 1) these are induced by the presence of the drug, and 2) these are
secondary to neuroleptic treatment, even after discontinuation of the offending agent (Kane et al., 1982). Furthermore, it is very difficult to find well-characterized families, all affected members of which have been taking the drug in question.
Despite these challenges, evidence for heritability of methylphenidate response can be accrued from several sources. Extrapolation to humans can be based on animal studies demonstrating that genetically different rat strains have variable responses to acute and chronically administered methylphenidate (Swerdlow et al., 2003; Yang et al., 2003). In addition, a small study in humans demonstrated that behavioural, biochemical and neuroendocrine responses to amphetamine were more highly correlated in monozygotic twins versus dizygotic twins suggesting heritability of these traits for a related psychostimulant (Nurnberger et al., 1982).
Kalow et al. (1998) have proposed that a comparison of both between and
within variabilities in drug (e.g., methylphenidate) response among unrelated
individuals could potentially be used to derive heritability estimates of response. In
applying their proposed methods, they suggest that large-scale, family-based
epidemiological studies may no longer be required to estimate the underlying
body. The latter premise does not always hold true as exemplified by the persis-
tence of tardive dyskinesia in patients who have developed this side effect
temporary, for the most part, existing when the drug is present in the patient’s
heritability of a pharmacological trait, such as response to a drug (Kalow et al., 1998).
Given that there is evidence of heritability in response to methylphenidate, how does one go about understanding the genetic mechanisms underlying this inter- individual variability in response? The field of pharmacogenetics seeks to understand the hereditary basis for variability in response and adverse drug reactions (ADRs) to pharmacologic agents among individuals. Recently, molecular genetic techniques have been applied to the study of variability in the effects of neuropsychopharmacotherapy forming the field known as neuropsycho- pharmacogenetics. Genetic variation or polymorphism of the enzymes that metabolize psychotropic drugs may account for variations in efficacy and toxicity observed in a population. A patient expressing a particular enzyme variant for rapid metabolism may display drug concentrations below the therapeutic plasma level; by contrast, a patient with a slowly metabolizing version of this enzyme may accumulate toxic drug concentrations. Alternatively, variations in the target sites for drugs (e.g., receptors, transporters) may be the reason why treatment failure of psychotropic agents occurs in some individuals but not in others with the same neuropsychiatric illness. Knowledge of the relationship between specific polymorphisms in genes encoding proteins involved in both a psychotropic drug’s pharmacokinetics and pharmacodynamics, and its clinical effects may lead to better drug design and to individualized pharmacotherapy. It is also important to note that disease severity, environmental, psychosocial and demographic factors may also contribute to variation in both drug efficacy and toxicity.
Clinical
Response & Side Effects
PHARMACOGENETICS
Pharmacology Genetics
Figure 1. Genetic make-up may contribute to predisposition to response and side effects in certain individuals and protection in others
have employed genetic association strategies. This method involves taking a group of unrelated patients with the neuropsychiatric illness in question, who have received appropriate pharmacotherapy, and assessing their degree of therapeutic response or presence of ADRs. Response is typically evaluated using psychometric measures, which provide indices of psychopathology and/or levels of functioning. Quality of life measures may also be used in the assessment of response. Therapeutic response can either be defined categorically based on an appropriate threshold value or it can be treated as a continuous measure. Depending on the type of ADR to the treatment in question, these can also be dichotomously measured (i.e., adverse effect, present or absent) or their severity may be quantified using a validated, reliable and responsive rating instrument. Candidate genes are then selected according to the degree with which their expressed proteins pharmacologically interact with the drug at hand. It is a requirement of the genetic association approach that there is prior evidence suggesting a role for the candidate gene in the phenotype of interest (Crowe, 1993).
Polymorphisms within the candidate genes must then be identified using various techniques [for example, single-stranded conformation polymorphism (SSCP) analysis, denaturing gradient gel electrophoresis (DGGE), and/or direct DNA sequencing]. The polymorphisms may alter the amino acid sequence of the protein and therefore affect its function; be located in promoter or regulatory regions of the gene and therefore affect expression of its protein; be located within the third base pair of a codon and therefore may be silent (does not alter amino acid sequence), affect mRNA stability, or introduce a splice site; or be located within an intron and have no known function. Polymorphisms having functional consequences or altering protein levels are obviously more interesting to examine than ones which are silent or non-functional, and in studying the former, one can increase the prior probability of detecting a valid association (Malhotra et al., 1999). The rationale for examining silent or non-functional polymorphisms is that they may be in linkage disequilibrium with functional variants elsewhere in the gene.
Once candidate gene polymorphisms are identified, the group of patients are then genotyped, and differences among the distribution of the genetic variants (alleles) in responders/non-responders or those with/without ADRs may then be statistically compared using the chi-square test or Fisher’s Exact Test. Alternatively, the mean changes in psychometric measure or ADR rating instrument, after treatment, may be grouped according to genotype and compared using the parametric analysis of variance (ANOVA) or the non-parametric Kruskall-Wallis H test. Information at the DNA level can potentially be maximised by combining allelic data from several polymorphisms across the gene (haplotype analysis). These methodologies allow the identification of genetic factors which account for some of the variance in the phenotype of drug response or side effect profile.
To date, the majority of studies in the field of neuropsychopharmacogenetics
3. NEUROPSYCHOPHARMACOGENETICS APPLIED TO ADHD
Why examine the pharmacogenetics of response and ADRs to psycho-stimulant to psychostimulants. There is an average response rate of about 75% to a given psychostimulant, as identified across several controlled outpatient trials (Solanto, 1998). In a key study conducted at the NIMH, Elia et al. (1991) demonstrated that approximately 25% of individuals have a clinically meaningful response to either methylphenidate or dextroamphetamine. Second, there is substantial variability among individuals in adverse effects to psychostimulants. These include: insomnia, weight loss, decreased appetite, abdominal pain, headache, irritability, anxiety, and proneness to crying, and increases in heart rate, diastolic and systolic blood pressure (Barkley et al., 1990; Kelly et al., 1988). It is unclear whether this variability in response and ADRs is predominantly pharmacokinetic or pharmacodynamic (Kimko et al., 1999). More likely, the inter-individual differences may be accounted for by a combination of both of the aforementioned factors as well as environmental influences. Third, there is evidence of heritability of this trait as discussed in the preceding section. Pharmacogenetic studies of psychostimulant therapy may help to elucidate the extent to which pharmacokinetic and pharmacodynamic factors play a role in predicting this inter-individual variation. Moreover, in identifying pharmacogenetic markers, predicted responders to a particular psychostimulant could be treated preferentially, while those less likely to respond or having propensity to develop adverse effects could be offered other possible treatments.
In considering candidate genes to examine with respect to the pharmacogenetics of methylphenidate, obvious choices include genes which express proteins involved PK perspective, identification of polymorphism within the carboxylesterase enzyme, CES1A1, and association with measures of response to methylphenidate in ADHD will be a priority in the near future. To our knowledge, however, no polymorphisms in this gene have been identified.
include numerous players from the dopamine and norepinephrine systems. To this end, a discussion of ‘direct’ and ‘indirect’ candidate genes is warranted. As previously proposed (Masellis et al., 2000), ‘direct’ candidate genes include those which express proteins having a direct pharmacological interaction with the drug in question. In the case of response and ADRs to methylphenidate, a ‘direct’ candidate gene of paramount importance to study is the dopamine transporter (DAT1 (( ), given that methylphenidate binds to and directly inhibits its expressed protein. In support of DAT1 as a candidate gene, DAT1 knockout mice are hyperactive as compared to wild type mice (Giros et al., 1996). Perhaps the strongest evidence comes from humans suffering from ADHD, where Krause et al. (2000) demonstrated using brain Single Photon Emission Computed Tomography (SPECT) that unmedicated patients had an elevation of striatal DAT1 density which decreased after methylphenidate therapy? First, there is documented evidence of inter-individual variation in response
Key candidate genes involved in the pharmacodynamics of methylphenidate
in the pharmacokinetics and pharmacodynamics of this psychostimulant. From the
treatment. Furthermore, numerous genetic association studies have demonstrated positive associations between ADHD and polymorphism within DAT1 (Comings et al., 1996; Cook et al., 1995; Daly et al., 1999; Gill et al., 1997; Waldman et al., 1998). From the norepinephrine system, the norepinephrine transporter (NET (( T T ) is also an important ‘direct’ candidate gene to study given the affinity of methylphenidate and other psychostimulants for its protein product.
Other candidate genes to examine in the phenotype of inter-individual variation in response and ADRs to methylphenidate include those expressing proteins involved in catecholamine biosynthesis and metabolism, as well as those involved in both extraneuronal and intraneuronal signal transduction. Examples include: tyrosine hydroxylase, dopamine β-hydroxylase, monoamine oxidase (MAO), catechol-O- methyl transferase (COMT) and the five dopamine receptor subtypes. For a review, refer to Cook (1999). Genes expressing these proteins may be considered ‘indirect’
candidate genes according to our current state of knowledge, given that their expressed proteins do not appear to have a ‘direct’ pharmacological interaction with methylphenidate. Rather they may be ‘indirectly’ involved in the mechanism of methylphenidate through secondary changes in catecholamine levels following inhibition of the dopamine transporter. It is important to distinguish between ‘direct’
and ‘indirect’ candidate genes, as ones directly involved in the mechanism of action of methylphenidate will likely account for a substantial proportion of the variance in response, as well as increasing the prior probability of detecting a valid association (Masellis et al., 2000).
Winsberg and Comings (1999) were the first to examine the relationship
between polymorphism in several candidate genes from the dopamine system and
polymorphisms in DAT1, the dopamine D4 receptor gene (DRD4 (( ), and the dopamine
D2 receptor gene (DRD2 (( ) in a sample of 30 African-American children, who had
received DSM-IIIR diagnoses of ADHD and who underwent a prospective, open-
label trial of methylphenidate therapy. DRD4 is an important ‘indirect’ candidate
gene to examine with regard to methylphenidate response in light of replicated
evidence suggesting that the 7-repeat allele is associated with ADHD (Barr et al.,
2000; LaHoste et al., 1996; Muglia et al., 2000; Rowe et al., 1998; Sunohara et al.,
2000; Swanson et al., 1998; Tahir et al., 2000). The three polymorphisms examined
were the Taq1 A1/A2 polymorphism located 3’ to DRD2, the 48 bp repeat
polymorphism located in the coding region of the third intracytoplasmic loop of
DRD4, and the variable number tandem repeat (VNTR) polymorphism located in the
3’-untranslated region (3’-UTR) of DAT1. They described a significant association
between the 10/10 genotype of the DAT1 VNTR and non-response to
methylphenidate in their sample. No significant associations were observed between
the DRD2 and DRD4 polymorphisms and variation in response to methylphenidate,
although their sample size did not have adequate power to detect an association with
these. This finding further confirms the role of the dopamine transporter as the site
of action of methylphenidate. Furthermore, the authors speculated that binding of
inter-individual variation in response to methylphenidate. They examined
methylphenidate to the dopamine transporter might be altered in those carrying the 10/10 genotype versus 9/10, 8/10, and 5/9 genotypes.
Although there were several limitations with this initial study as described in Winsberg and Comings (1999) and Cook (1999), it does provide preliminary evidence suggesting that neuropsychopharmacogenetics can be effectively applied to the pharmacological treatment of childhood psychiatric diseases, which was the topic of discussion in a comprehensive review article published by Anderson and Cook (2000). Since then, several other studies have provided support for this finding. In a naturalistic study of 50 Brazilian youths with ADHD, Roman et al.
(2002) also found that the 10/10 genotype predicted a non-response to methylphenidate. Kirley et al. (2003) found an association between transmission of the 10-repeat allele of the DAT1 VNTR and positive response to methylphenidate in a family-based sample of 119 Irish children with ADHD, who were retrospectively assessed for response. In addition, a recent study presented by Stein et al. (2002) demonstrated that homozygosity for the 9-repeat allele of DAT1 was associated with poor response to methylphenidate.
Although these studies support an association between DAT1 and methylphenidate response, the results are not always in the same direction, i.e., the same allele/genotype predicts negative response in one sample and positive response in another. One explanation for this may be that this polymorphism is not causative and is only in linkage disequilibrium with another functional polymorphism in DAT1. Alternatively, methodological differences such as power issues, and retrospective versus prospective nature of assessment, among others, may account for the inconsistent findings. These studies also provide support for the concept of distinguishing between ‘direct’ and ‘indirect’ candidate genes as contributors to the phenotype of response and ADRs to psychotropic medications, given that the significant associations observed in these studies were between DAT1 and response to methylphenidate.
Several other studies examining responses to amphetamine also demonstrate
significant association with the DAT1. Using a double-blind, randomized, cross-over
design, Lott et al. (2004) demonstrated that, in 96 healthy controls, subjective
responses to d-amphetamine in a challenge paradigm were no different from placebo d d
in 9/9 homozygotes for the DAT1 polymorphism, whereas 9/10 heterozygotes and
10/10 homozygotes described feelings of anxiety and euphoria. Applying similar
stringent research-design methodologies to clinical samples of ADHD should
improve neuropsychopharmacogenetic studies in the future. Another study
demonstrated that nine or fewer repeats of the DAT1 VNTR predicted persistence of
psychosis secondary to methamphetamine for greater than one month after drug
discontinuation (Ujike et al., 2003). An important study in a pilot sample of eight
boys suffering from ADHD, who were treated with methylphenidate, documented
significantly higher regional cerebral blood flow using SPECT in medial frontal and
left basal ganglia in 10/10 homozygotes versus those without this genotype (Rohde
et al., 2003). All of the aforementioned studies of responsiveness to psychostimulant
therapy and association with the DAT1 VNTR polymorphism are supported by data suggesting this polymorphism regulates expression of DAT1 (Fuke et al., 2001; Mill et al., 2002).
Although DAT1 appears to be one of the critical targets of methylphenidate and other psychostimulants, there is one study to suggest that polymorphism within the norepinephrine transporter (NET (( T T ) also predicts response to methylphenidate in ADHD. In a sample of 35 Chinese Han youths with ADHD, Yang et al. (2004) documented that the G1287A polymorphism in NET was associated with T improvement in hyperactive-impulsive subscale scores after methylphenidate treatment. Specifically, individuals with the G-allele demonstrated a higher reduction in hyperactive-impulsive symptoms versus those homozygous for the A-allele. Interestingly, none of the other scales demonstrated change according to genotype raising the possibility that endophenotypes (‘phenotypes within’) such as subscale scores in ADHD may be regulated differentially by genes. The topic of endophenotypes in ADHD will be more comprehensively discussed in the following section.
4. DEFINING RESPONSE: THE CHALLENGES
One of the main reasons for the inconsistent results among neuropsycho-
pharmacogenetic studies is difficulties in what comprises the definition or
characterization of response. Phenotypes of response to neuropsychotropic
medications are extremely complex to measure because of a number of potential
sources of error. First, symptoms of neuropsychiatric disease are subjectively
described by either the patient or by a caretaker or observed and interpreted by the
clinician. Despite the use of ‘objective’ psychometric measures that have previously
demonstrated validity and reliability in quantifying these symptoms, ultimately the
subjective nature of the initial description or interpretation will add ‘noise’ or error
into any subsequent statistical analyses of response. Second, the high variability
among clinicians/raters will further confound the determination of response to a
neuropsychotropic drug. Third, there are several different psychometric measures
available to assess response to a drug and this leads to significant heterogeneity
among neuropsychopharmacogenetic studies. To provide a contrast, an example of a
medical phenotype will be discussed. There have been numerous studies that have
consistently replicated the finding that polymorphism within the ȕ
2-adrenoceptor
predicts vascular and airway responses to ȕ
2-agonist therapy (reviewed in Evans
et al., 2003). We postulate that the high replicability of these findings is due to the
use of standardized investigations that directly provide measures of airway- and
vaso-reactivity at baseline and after administration of the drug. Several other
confounds contribute to the inconsistent results among neuropsychopharmacogenetic
studies and these are reviewed in detail elsewhere (Masellis et al., 2000).
This leads into a discussion of endophenotypes that are being developed to more accurately determine response to methylphenidate in ADHD. As applied to drug response, endophenotypes are latent traits (e.g., physiological, cognitive, or radiographic) that are related to the global phenotype of response to a drug but are also more closely linked to underlying genetic factors (Leboyer et al., 1998). Thus, they may help in ‘bridging the gap’ between genotype and phenotype and improve both validity and reliability of neuropsychopharmacogenetic studies. To this end, three major topics will be covered including cognitive, neurophysiological and functional neuroradiographic endophenotypes in ADHD.
Two meta-analyses examining neuropsychological features of ADHD in children and adults have been published which demonstrate that overall cognitive ability is diminished in ADHD as compared to normal healthy controls (Frazier et al., 2004; Hervey et al., 2004). Although generalized impairments were noted, the underlying cognitive domains that were most prominently affected include attention, behavioural inhibition, working memory, and executive function (Frazier et al., 2004; Hervey et al., 2004). It is likely that deficits in attention and executive function translate into global deficits in ADHD as these measures are critical for performing most tasks during neuropsychological testing. Therefore, it may be extrapolated that deficits involving frontal-subcortical circuits of attention, behavioural inhibition, working memory, and executive function may be the most responsive measures to psychostimulant therapy. Indeed, these cognitive domains have been explored as cognitive endophenotypes in ADHD (Nigg et al., 2004;
Seidman et al., 2000; Slaats-Willemse et al., 2003; Westerberg et al., 2004).
Furthermore, Mollica et al. (2004) has successfully applied statistical decision rules to cognitive and behavioural domains in children with ADHD to assess responsiveness to psychostimulant therapy. They demonstrated a high sensitivity and specificity of their methods in determining a favourable response, and methodo- logies such as these will likely play an important role for future pharmacogenetic studies in ADHD.
From a neurophysiological perspective, several studies have been conducted
using electroencephalography (EEG) and event-related potentials as endophenotypes
in assessing responsiveness to methylphenidate. Ozdag et al. (2004) demonstrated
that auditory event-related potentials assessing parietal P3 latency and amplitude, as
well as frontal P3 amplitude were significantly different in children with ADHD
versus controls at baseline. These differences normalized after methylphenidate
treatment suggesting that abnormalities in signal detection and discrimination as
well as information processing in ADHD may be improved by methylphenidate
therapy (Ozdag et al., 2004). Another study employing EEG in 36 children with
ADHD documented that, in responders to methylphenidate, frontal beta-activity was
increased and that this was significantly correlated with improvement on a
continuous performance test and parent-rated measures of attention and
hyperactivity (Loo et al., 2004). Taking this one step further, Loo et al. (2003)
conducted an association study of the DAT1 VNTR polymorphism which
demonstrated that 10/10 homozygotes performed more poorly on a vigilance task and had increased central and parietal beta-activity, decreased right frontal theta- activity as well as lower theta:beta ratios. This suggested that neurophysiological as well as clinical measures are correlated with each other during treatment with ADHD and that genotype status of the DAT1 VNTR may contribute to some of the variance in this relationship. This supports the use of neurophysiological endophenotypes of psychostimulant response in pharmacogenetic studies.
Functional neuroradiographic studies also show great promise as an endophenotype in assessing response to psychostimulants. A recent functional magnetic resonance imaging (f (( MRI) study demonstrated that unmedicated ff adolescents with ADHD recruited the left ventral basal ganglia significantly less than healthy controls during a test of divided attention (Shafritz et al., 2004). Once these patients were given a challenge dose of methylphenidate, there was a notable increase in activity in this brain area which approached that of the control group (Shafritz et al., 2004).
5. FUTURE DIRECTIONS
To date, neuropsychopharmacogenetics has been applied to the study of several phenotypes related to neuropsychopharmacologic therapy. Examples include:
serotonin receptor genes and therapeutic response to clozapine in psychosis (Arranz
et al., 1998; Masellis et al., 1998; Masellis et al., 2001; Masellis et al., 1995); the
serotonin transporter gene and antidepressant treatment response in depression
(Smeraldi et al., 1998) and obsessive-compulsive disorder (Billett et al., 1997); and
dopamine receptor and cytochrome-P450 genes and predisposition to developing
tardive dyskinesia in schizophrenia (Basile et al., 1999; Basile et al., 2000). In
reviewing these studies, it is evident that results across studies that have been
replicated are, for the most part, inconsistent. This is likely the result of
methodological heterogeneity across studies, which is expected given that the field
of neuropsychopharmacogenetics is in its early stages. However, in order for the
field to progress, methodological consistency must be achieved not only for the basis
of comparison among studies, but more importantly to allow for collaborative efforts
that combine samples. Sample size limitations that decrease the power of
analysis can be overcome through these collaborative efforts. Methodological
considerations include the establishment of discrete inclusion/exclusion criteria
regarding patient diagnosis and sample characterization; consensus regarding the use
of particular psychiatric rating instruments should be established a priori (Rietschel
et al., 1999). Furthermore, studies should be designed specifically for the
identification of genetic susceptibility to pharmacogenetic traits. To date, studies
have used samples obtained from prospective or retrospective clinical trials with
subsequent analysis of genetic hypotheses. The current efforts of the National
Institutes of Health to fund pharmacogenetic studies are positive steps forward in achieving these goals (NIH, 1998a; NIH, 1998b).
The limitations inherent in genetic association strategies are well described in the literature however the problem of population stratification provides the greatest challenges. Population stratification can be defined as false positive and negative results that occur due to differences in allele frequencies among different subpopulations of the sample in question. The development of family based association strategies (transmission disequilibrium test: TDT), which control for population stratification biases would be useful in pharmacogenetic studies (Thomson, 1995). TDT based approaches are particularly relevant with regard to pharmacogenetic studies of psychostimulant response, and therapy in other childhood psychiatric diseases because of the increased availability of parents in obtaining samples for parent-offspring trios.
Future areas of research will include further clarification of the roles of other attentional circuits in the CNS, namely the norepinephrinergic and cholinergic systems, which are already being targeted in drug discovery for ADHD. Beane and Marrocco (2004) have suggested that there is a reduced level of norepinephrinergic facilitation of the cholinergic system in ADHD. This is proposed to be on the basis of: 1) reduced activity of locus coeruleus neurons, 2) impaired synthesis of norepinephrine, 3) supersensitive or upregulated Į-2 epinephrinergic autoreceptors on terminals of locus coeruleus neurons, 4) altered regulation of norepinephrine release, or combinations thereof (reviewed in Beane et al., 2004). Interestingly, one of the newest drugs approved by the FDA in the U.S. for use in ADHD is the non- stimulant, atomoxetine. This drug selectively inhibits the pre-synaptic norepinephrine transporter (NET (( T T ) thus elevating synaptic levels of norepinephrine (Corman et al., 2004). From a pharmacogenetic perspective, it would be important to examine polymorphism within NET and also within CYP2D6, since this T cytochrome-P450 isoenzyme is the main metabolizer of atomoxetine. There is also preliminary evidence supporting a role for cholinergic-facilitating agents in ADHD.
Nicotine patches have shown efficacy in treating symptoms of ADHD (Levin et al., 1996; Shytle et al., 2002). A small case series of treatment-resistant youths with ADHD demonstrated a benefit of the cholinesterase inhibitor, donepezil (Wilens et al., 2000). Furthermore, a double-blind, placebo-controlled, randomized, crossover trial of transdermal ABT-418, a nicotinic analogue, in 32 adults with DSM-IV diagnosed ADHD showed significant improvement over placebo (Wilens et al., 1999). Pharmacogenetic studies targeting candidate genes from the cholinergic system may be useful in further clarifying the mechanisms of action of these potential drugs for ADHD.
may eventually lead to a simple, fast, and inexpensive DNA test to identify response
status and predisposition to ADRs of individual patients. In doing so, a decision on
Neuropsychopharmacogenetic research has great potential to improve the treat-
ment of childhood, adolescent, and adult neuropsychiatric disease, and thus ultimately
the quality of life for patients suffering from these disorders. This line of research
whom to initiate treatment and which drug and dose to use can be made in advance.
This may spare the patient significant side effects and maximize response. This research may also aid in elucidating the mechanism of action of psychotropic medications, and thus may help in designing new, more efficacious therapeutic agents.
6. ACKNOWLEDGEMENTS
We are thankful to Professor Sandra E. Black for her insightful comments. This work was supported by a Canadian Institutes of Health Research (MT15007) grant.
MM is supported by the Royal College Clinician Investigator/Scientist Program at the University of Toronto. JLK is supported by a NARSAD Independent Investigator Award.
7. REFERENCES
Anderson, G. M., Cook, E. H., 2000, Pharmacogenetics. Promise and potential in child and adolescent psychiatry, Child Adolesc Psychiatr Clin N Am. 9: 23-42.
Arranz, M. J., Munro, J., Sham, P., Kirov, G., Murray, R. M., Collier, D. A., Kerwin, R. W., 1998, Meta- analysis of studies on genetic variation in 5-HT2A receptors and clozapine response, Schizophr Res. 32: 93-9.
Barkley, R. A., McMurray, M. B., Edelbrock, C. S., Robbins, K., 1990, Side effects of methylphenidate in children with attention deficit hyperactivity disorder: a systemic, placebo-controlled evaluation, Pediatrics. 86: 184-92.
Barr, C. L., Wigg, K. G., Bloom, S., Schachar, R., Tannock, R., Roberts, W., Malone, M., Kennedy, J. L., 2000, Further evidence from haplotype analysis for linkage of the dopamine D4 receptor gene and attention-deficit hyperactivity disorder, Am J Med Genet. 96: 262-7.
Basile, V. S., Masellis, M., Badri, F., Paterson, A. D., Meltzer, H. Y., Lieberman, J. A., Potkin, S. G., Macciardi, F., Kennedy, J. L., 1999, Association of the MscI polymorphism of the dopamine D3 receptor gene with tardive dyskinesia in schizophrenia, Neuropsychopharmacology. 21:
17-27.
Basile, V. S., Ozdemir, V., Masellis, M., Walker, M. L., Meltzer, H. Y., Lieberman, J. A., Potkin, S. G., Alva, G., Kalow, W., Macciardi, F. M., Kennedy, J. L., 2000, A functional polymorphism of the cytochrome P450 1A2 (CYP1A2) gene: association with tardive dyskinesia in schizophrenia, Mol Psychiatry. 5: 410-7.
Beane, M., Marrocco, R. T., 2004, Norepinephrine and acetylcholine mediation of the components of reflexive attention: implications for attention deficit disorders, Prog Neurobiol. 74: 167-81.
Billett, E. A., Richter, M. A., King, N., Heils, A., Lesch, K. P., Kennedy, J. L., 1997, Obsessive compulsive disorder, response to serotonin reuptake inhibitors and the serotonin transporter gene, Mol Psychiatry. 2: 403-6.
Bradley, C., 1937, The behavior of children receiving benzedrine, Am J Psychiatry. 94: 577-85.
Comings, D. E., Wu, S., Chiu, C., Ring, R. H., Gade, R., Ahn, C., MacMurray, J. P., Dietz, G., Muhleman, D., 1996, Polygenic inheritance of Tourette syndrome, stuttering, attention deficit hyperactivity, conduct, and oppositional defiant disorder: the additive and subtractive effect of the three dopaminergic genes–DRD2, D beta H, and DAT1, Am J Med Genet. 67: 264-88.
Cook, E. H., Jr., 1999, The early development of child psychopharmacogenetics, J Am Acad Child Adolesc Psychiatry. 38: 1478-81.
Cook, E. H., Jr., Stein, M. A., Krasowski, M. D., Cox, N. J., Olkon, D. M., Kieffer, J. E., Leventhal, B. L., 1995, Association of attention-deficit disorder and the dopamine transporter gene, Am J Hum Genet. 56: 993-8.
American Psychiatric Association, 1994, Diagnostic and statistical manual of mental disorder, 4th ed.
Corman, S. L., Fedutes, B. A., Culley, C. M., 2004, Atomoxetine: the first nonstimulant for the management of attention-deficit/hyperactivity disorder, Am J Health Syst Pharm. 61: 2391-9.
Crowe, R. R., 1993, Candidate genes in psychiatry: an epidemiological perspective,Am J Med Genet. 48:
74-7.
Daly, G., Hawi, Z., Fitzgerald, M., Gill, M., 1999, Mapping susceptibility loci in attention deficit hyperactivity disorder: preferential transmission of parental alleles at DAT1, DBH and DRD5 to affected children, Mol Psychiatry. 4: 192-6.
DeVane, C. L., Markowitz, J. S., Carson, S. W., Boulton, D. W., Gill, H. S., Nahas, Z., Risch, S. C., 2000, Single-dose pharmacokinetics of methylphenidate in CYP2D6 extensive and poor metabolizers, J Clin Psychopharmacol. 20: 347-9.
Elia, J., Borcherding, B. G., Rapoport, J. L., Keysor, C. S., 1991, Methylphenidate and dextroamphetamine treatments of hyperactivity: are there true nonresponders?, Psychiatry Res.
36: 141-55.
Evans, W. E., McLeod, H. L., 2003, Pharmacogenomics–drug disposition, drug targets, and side effects, N Engl J Med.dd 348: 538-49.
Faraone, S. V., Biederman, J., 1994, Genetics of ADHD, Child Adolesc Psychiatr Clin North America. 3:
285-301
Faraone, S. V., Biederman, J., Spencer, T., Wilens, T., Seidman, L. J., Mick, E., Doyle, A. E., 2000, Attention-deficit/hyperactivity disorder in adults: an overview, Biol Psychiatry. 48: 9-20.
Frazier, T. W., Demaree, H. A., Youngstrom, E. A., 2004, Meta-analysis of intellectual and neuropsychological test performance in attention-deficit/hyperactivity disorder, Neuro- psychology. 18: 543-55.
Fuke, S., Suo, S., Takahashi, N., Koike, H., Sasagawa, N., Ishiura, S., 2001, The VNTR polymorphism of the human dopamine transporter (DAT1) gene affects gene expression, Pharmacogenomics J.
JJ 1: 152-6.
Gill, M., Daly, G., Heron, S., Hawi, Z., Fitzgerald, M., 1997, Confirmation of association between attention deficit hyperactivity disorder and a dopamine transporter polymorphism, Mol Psychiatry. 2: 311-3.
Gillis, J. J., Gilger, J. W., Pennington, B. F., DeFries, J. C., 1992, Attention deficit disorder in reading- disabled twins: evidence for a genetic etiology, J Abnorm Child Psychol. 20: 303-15.
Giros, B., Jaber, M., Jones, S. R., Wightman, R. M., Caron, M. G., 1996, Hyperlocomotion and indifference to cocaine and amphetamine in mice lacking the dopamine transporter, Nature.
379: 606-12.
Gottesman, I. I., 1991, Schizophrenia Genesis. The Origins of Madness, W.H. Freeman and Company, New York
Greenblatt, D. J., Harmatz, J. S., von Moltke, L. L., Shader, R. I. (1995). Pharmacokinetics and Pharmaco-dynamics. In Psychopharmacology: The Fourth Generatiion of Progress, ed.
Bloom, F. E., Kupfer, D. J., pp. 849-58. New York: Raven Press.
Greenhill, L. L., Pliszka, S., Dulcan, M. K., Bernet, W., Arnold, V., Beitchman, J., Benson, R. S., Bukstein, O., Kinlan, J., McClellan, J., Rue, D., Shaw, J. A., Stock, S., 2002, Practice parameter for the use of stimulant medications in the treatment of children, adolescents, and adults, J Am Acad Child Adolesc Psychiatry. 41: 26S-49S.
Hechtman, L., 1994, Genetic and neurobiological aspects of attention deficit hyperactive disorder: a review, J Psychiatry Neurosci. 19: 193-201.
Hervey, A. S., Epstein, J. N., Curry, J. F., 2004, Neuropsychology of adults with attention- deficit/hyperactivity disorder: a meta-analytic review, Neuropsychology. 18: 485-503.
Hubbard, J. W., Srinivas, N. R., Quinn, D., Midha, K. K., 1989, Enantioselective aspects of the disposition of dl-threo-methylphenidate after the administration of a sustained-release formulation to children with attention deficit-hyperactivity disorder, J Pharm Sci. 78: 944-7.
Kalow, W., Tang, B. K., Endrenyi, L., 1998, Hypothesis: comparisons of inter- and intra-individual variations can substitute for twin studies in drug research, Pharmacogenetics. 8: 283-9.
Kane, J. M., Smith, J. M., 1982, Tardive dyskinesia: prevalence and risk factors, 1959 to 1979, Arch Gen Psychiatry. 39: 473-81.
Kelly, K. L., Rapport, M. D., DuPaul, G. J., 1988, Attention deficit disorder and methylphenidate: a multi-step analysis of dose-response effects on children’s cardiovascular functioning, Int Clin Psychopharmacol. 3: 167-81.
Kimko, H. C., Cross, J. T., Abernethy, D. R., 1999, Pharmacokinetics and clinical effectiveness of methylphenidate, Clin Pharmacokinet. 37: 457-70.
Kirley, A., Lowe, N., Hawi, Z., Mullins, C., Daly, G., Waldman, I., McCarron, M., O’Donnell, D., Fitzgerald, M., Gill, M., 2003, Association of the 480 bp DAT1 allele with methylphenidate response in a sample of Irish children with ADHD, Am J Med Genet B Neuropsychiatr Genet.
121: 50-4.
Krause, K. H., Dresel, S. H., Krause, J., Kung, H. F., Tatsch, K., 2000, Increased striatal dopamine transporter in adult patients with attention deficit hyperactivity disorder: effects of methyl- phenidate as measured by single photon emission computed tomography, Neurosci Lett. 285:
107-10.
LaHoste, G. J., Swanson, J. M., Wigal, S. B., Glabe, C., Wigal, T., King, N., Kennedy, J. L., 1996, Dopamine D4 receptor gene polymorphism is associated with attention deficit hyperactivity disorder [see comments], Mol Psychiatry. 1: 121-4.
Leboyer, M., Bellivier, F., Nosten-Bertrand, M., Jouvent, R., Pauls, D., Mallet, J., 1998, Psychiatric genetics: search for phenotypes, Trends Neurosci. 21: 102-5.
Levin, E. D., Conners, C. K., Sparrow, E., Hinton, S. C., Erhardt, D., Meck, W. H., Rose, J. E., March, J., 1996, Nicotine effects on adults with attention-deficit/hyperactivity disorder, Psychopharmacology (Berl). 123: 55-63.
Loo, S. K., Hopfer, C., Teale, P. D., Reite, M. L., 2004, EEG Correlates of Methylphenidate Response in ADHD: Association With Cognitive and Behavioral Measures, J Clin Neurophysiol. 21:
457-64.
Loo, S. K., Specter, E., Smolen, A., Hopfer, C., Teale, P. D., Reite, M. L., 2003, Functional effects of the DAT1 polymorphism on EEG measures in ADHD, J Am Acad Child Adolesc Psychiatry. 42:
986-93.
Lott, D. C., Kim, S. J., Cook, E. H., de Wit, H., 2004, Dopamine Transporter Gene Associated with Diminished Subjective Response to Amphetamine, Neuropsychopharmacology.
Malhotra, A. K., Goldman, D., 1999, Benefits and pitfalls encountered in psychiatric genetic association studies, Biol Psychiatry. 45: 544-50.
Markowitz, J. S., Patrick, K. S., 2001, Pharmacokinetic and pharmacodynamic drug interactions in the treatment of attention-deficit hyperactivity disorder, Clin Pharmacokinet. 40: 753-72.
Masellis, M., Basile, V., Meltzer, H. Y., Lieberman, J. A., Sevy, S., Macciardi, F. M., Cola, P., Howard, A., Badri, F., Nothen, M. M., Kalow, W., Kennedy, J. L., 1998, Serotonin subtype 2 receptor genes and clinical response to clozapine in schizophrenia patients, Neuropsychopharmacology.
19: 123-32.
Masellis, M., Basile, V. S., Meltzer, H. Y., Lieberman, J. A., Sevy, S., Goldman, D. A., Hamblin, M. W., Macciardi, F. M., Kennedy, J. L., 2001, Lack of association between the T/C 267 serotonin 5-HT6 receptor gene (HTR6) polymorphism and prediction of response to clozapine in schizophrenia., Schizophrenia Research. 47: 49-58.
Masellis, M., Basile, V. S., Ozdemir, V., Meltzer, H. Y., Macciardi, F. M., Kennedy, J. L., 2000, Pharmacogenetics of antipsychotic treatment: lessons learned from clozapine, Biol Psychiatry.
47: 252-66.
Masellis, M., Paterson, A. D., Badri, F., Lieberman, J. A., Meltzer, H. Y., Cavazzoni, P., Kennedy, J. L., 1995, Genetic variation of 5-HT2A receptor and response to clozapine, Lancet. 346: 1108.
Mill, J., Asherson, P., Browes, C., D’Souza, U., Craig, I., 2002, Expression of the dopamine transporter gene is regulated by the 3’ UTR VNTR: Evidence from brain and lymphocytes using quantitative RT-PCR, Am J Med Genet. 114: 975-9.
Mollica, C. M., Maruff, P., Vance, A., 2004, Development of a statistical approach to classifying treatment response in individual children with ADHD, Hum Psychopharmacol. 19: 445-56.
Muglia, P., Jain, U., Macciardi, F., Kennedy, J. L., 2000, Adult attention deficit hyperactivity disorder and the dopamine D4 receptor gene, Am J Med Genet. 96: 273-7.
Nigg, J. T., Blaskey, L. G., Stawicki, J. A., Sachek, J., 2004, Evaluating the endophenotype model of ADHD neuropsychological deficit: results for parents and siblings of children with ADHD combined and inattentive subtypes, J Abnorm Psychol. 113: 614-25.
NIH, 1998a, Mechanisms underlying variations in drug responses,www.nih.gov/grants/guide/pa-files/PA- 99-016.htmllpps. 1-10.
GM-099-004.htmllpps . 1-18.
Nurnberger, J. I., Jr., Gershon, E. S., Simmons, S., Ebert, M., Kessler, L. R., Dibble, E. D., Jimerson, S.
S., Brown, G. M., Gold, P., Jimerson, D. C., Guroff, J. J., Storch, F. I., 1982, Behavioral, biochemical and neuroendocrine responses to amphetamine in normal twins and ‘well-state’
bipolar patients, Psychoneuroendocrinology. 7: 163-76.
Ozdag, M. F., Yorbik, O., Ulas, U. H., Hamamcioglu, K., Vural, O., 2004, Effect of methylphenidate on auditory event related potential in boys with attention deficit hyperactivity disorder, Int J Pediatr Otorhinolaryngol. 68: 1267-72.
Pauls, D. L., 1991, Genetic factors in the expression of attention-deficit hyperactivity disorder, J Child Adolesc Psychopharm. 1: 353-60.
Rietschel, M., Kennedy, J. L., Macciardi, F. M., DeLisi, L., Meltzer, H. Y., The Consensus Group for Outcome Measures in Psychoses for Pharmacogenetic Studies, 1999, Pharmacogenetics in psychosis, the first consensus conference, Schizophrenia Research. 37: 191-6.
Rohde, L. A., Roman, T., Szobot, C., Cunha, R. D., Hutz, M. H., Biederman, J., 2003, Dopamine transporter gene, response to methylphenidate and cerebral blood flow in attention- deficit/hyperactivity disorder: a pilot study, Synapse. 48: 87-9.
Roman, T., Szobot, C., Martins, S., Biederman, J., Rohde, L. A., Hutz, M. H., 2002, Dopamine transporter gene and response to methylphenidate in attention-deficit/hyperactivity disorder, Pharmacogenetics. 12: 497-9.
Rowe, D. C., Stever, C., Giedinghagen, L. N., Gard, J. M., Cleveland, H. H., Terris, S. T., Mohr, J. H., Sherman, S., Abramowitz, A., Waldman, I. D., 1998, Dopamine DRD4 receptor polymorphism and attention deficit hyperactivity disorder, Mol Psychiatry. 3: 419-26.
Rowland, M., Tozer, T. N., 1995, Clinical Pharmacokinetics. Concepts and Applications, Williams &
Wilkins, Philadelphia.
Safer, D. J., Krager, J. M., 1988, A survey of medication treatment for hyperactive/inattentive students, Jama. 260: 2256-8.
Seeman, P., Madras, B. K., 1998, Anti-hyperactivity medication: methylphenidate and amphetamine, Mol Psychiatry. 3: 386-96.
Seidman, L. J., Biederman, J., Monuteaux, M. C., Weber, W., Faraone, S. V., 2000, Neuropsychological functioning in nonreferred siblings of children with attention deficit/hyperactivity disorder, J Abnorm Psychol. 109: 252-65.
Shafritz, K. M., Marchione, K. E., Gore, J. C., Shaywitz, S. E., Shaywitz, B. A., 2004, The effects of methylphenidate on neural systems of attention in attention deficit hyperactivity disorder, Am J Psychiatry. 161: 1990-7.
Shaywitz, S. E., Hunt, R. D., Jatlow, P., Cohen, D. J., Young, J. G., Pierce, R. N., Anderson, G. M., Shaywitz, B. A., 1982, Psychopharmacology of attention deficit disorder: pharmacokinetic, neuroendocrine, and behavioral measures following acute and chronic treatment with methylphenidate, Pediatrics. 69: 688-94.
Shytle, R. D., Silver, A. A., Wilkinson, B. J., Sanberg, P. R., 2002, A pilot controlled trial of transdermal nicotine in the treatment of attention deficit hyperactivity disorder, World J Biol Psychiatry. 3:
150-5.
Slaats-Willemse, D., Swaab-Barneveld, H., de Sonneville, L., van der Meulen, E., Buitelaar, J., 2003, Deficient response inhibition as a cognitive endophenotype of ADHD, J Am Acad Child Adolesc Psychiatry. 42: 1242-8.
Smeraldi, E., Zanardi, R., Benedetti, F., Di Bella, D., Perez, J., Catalano, M., 1998, Polymorphism within the promoter of the serotonin transporter gene and antidepressant efficacy of fluvoxamine, Mol Psychiatry. 3: 508-11.
NIH, 1998b, Pharmacogenetics research network and database, www.nih.gov/grants/guide/rfa-files/RFA-
Solanto, M. V., 1998, Neuropsychopharmacological mechanisms of stimulant drug action in attention- deficit hyperactivity disorder: a review and integration, Behav Brain Res. 94: 127-52.
Srinivas, N. R., Hubbard, J. W., Korchinski, E. D., Midha, K. K., 1993, Enantioselective pharmacokinetics of dl-threo-methylphenidate in humans, Pharm Res. 10: 14-21.
Srinivas, N. R., Hubbard, J. W., Quinn, D., Midha, K. K., 1992, Enantioselective pharmacokinetics and pharmacodynamics of dl-threo-methylphenidate in children with attention deficit hyperactivity disorder, Clin Pharmacol Ther. 52: 561-8.
Stein, M. A., Sarampote, C., Waldman, I., Seymour, K., Robb, A., Colon, C., et al., 2002, Dopamine transporter genotype affects stimulant response according to parent ratings. Presented at 49th Annual Meeting of Child and Adolescent Psychiatry, San Francisco.
Sun, Z., Murry, D. J., Sanghani, S. P., Davis, W. I., Kedishvili, N. Y., Zou, Q., Hurley, T. D., Bosron, W.
F., 2004, Methylphenidate is stereoselectively hydrolyzed by human carboxylesterase CES1A1, J Pharmacol Exp Ther. 310: 469-76.
Sunohara, G. A., Roberts, W., Malone, M., Schachar, R. J., Tannock, R., Basile, V. S., Wigal, T., Wigal, S. B., Schuck, S., Moriarty, J., Swanson, J. M., Kennedy, J. L., Barr, C. L., 2000, Linkage of the dopamine D4 receptor gene and attention-deficit/hyperactivity disorder, J Am Acad Child Adolesc Psychiatry. 39: 1537-42.
Swanson, J. M., Sunohara, G. A., Kennedy, J. L., Regino, R., Fineberg, E., Wigal, T., Lerner, M., Williams, L., LaHoste, G. J., Wigal, S., 1998, Association of the dopamine receptor D4 (DRD4) gene with a refined phenotype of attention deficit hyperactivity disorder (ADHD): a family- based approach, Mol Psychiatry. 3: 38-41.
Swerdlow, N. R., Shoemaker, J. M., Platten, A., Pitcher, L., Goins, J., Crain, S., 2003, Heritable differences in the effects of amphetamine but not DOI on startle gating in albino and hooded outbred rat strains, Pharmacol Biochem Behav. 75: 191-7.
Tahir, E., Yazgan, Y., Cirakoglu, B., Ozbay, F., Waldman, I., Asherson, P. J., 2000, Association and linkage of DRD4 and DRD5 with attention deficit hyperactivity disorder (ADHD) in a sample of Turkish children, Mol Psychiatry. 5: 396-404.
Thomson, G., 1995, Mapping disease genes: family-based association studies, American Journal of Human Genetics. 57: 487-98.
Ujike, H., Harano, M., Inada, T., Yamada, M., Komiyama, T., Sekine, Y., Sora, I., Iyo, M., Katsu, T., Nomura, A., Nakata, K., Ozaki, N., 2003, Nine- or fewer repeat alleles in VNTR polymorphism of the dopamine transporter gene is a strong risk factor for prolonged methamphetamine psychosis,Pharmacogenomics J. JJ 3: 242-7.
Waldman, I. D., Rowe, D. C., Abramowitz, A., Kozel, S. T., Mohr, J. H., Sherman, S. L., Cleveland, H.
H., Sanders, M. L., Gard, J. M., Stever, C., 1998, Association and linkage of the dopamine transporter gene and attention- deficit hyperactivity disorder in children: heterogeneity owing to diagnostic subtype and severity, Am J Hum Genet. 63: 1767-76.
Westerberg, H., Hirvikoski, T., Forssberg, H., Klingberg, T., 2004, Visuo-spatial working memory span:
a sensitive measure of cognitive deficits in children with ADHD, Neuropsychol Dev Cogn C Child Neuropsychol. 10: 155-61.
Wilens, T. E., Biederman, J., Spencer, T. J., Bostic, J., Prince, J., Monuteaux, M. C., Soriano, J., Fine, C., Abrams, A., Rater, M., Polisner, D., 1999, A pilot controlled clinical trial of ABT-418, a cholinergic agonist, in the treatment of adults with attention deficit hyperactivity disorder, Am J Psychiatry. 156: 1931-7.
Wilens, T. E., Biederman, J., Wong, J., Spencer, T. J., Prince, J. B., 2000, Adjunctive donepezil in attention deficit hyperactivity disorder youth: case series, J Child Adolesc Psychopharmacol.
10: 217-22.
Winsberg, B. G., Comings, D. E., 1999, Association of the dopamine transporter gene (DAT1) with poor methylphenidate response, J Am Acad Child Adolesc Psychiatry. 38: 1474-7.
Yang, L., Wang, Y. F., Li, J., Faraone, S. V., 2004, Association of norepinephrine transporter gene with methylphenidate response, J Am Acad Child Adolesc Psychiatry. 43: 1154-8.
Yang, P. B., Amini, B., Swann, A. C., Dafny, N., 2003, Strain differences in the behavioral responses of male rats to chronically administered methylphenidate, Brain Res. 971: 139-52.
