Carboxylesterase 1 Genetic Variation and Methylphenidate in ADHD

Psychostimulants are the first-line pharmacotherapy for Attention deficit-hyperactivity
disorder (ADHD), and MPH accounts for approximately 50% of all stimulant usage. There has
been an ~10-fold increase in MPH prescribing since 1990, with 18 million prescriptions
dispensed in 2010, including 1.9 million new starts on MPH, making it the 5th most commonly
prescribed medication to children ages 2 -11 and the single most frequently prescribed
medication of any type in those aged 12-17 years. The annual exposure of pediatric patients
to MPH is extremely high and MPH is among the most commonly prescribed chronic use oral
medications for US children. Despite nearly 60 years of accrued clinical experience with MPH,
the significant inter-individual variability in MPH pharmacokinetics (PK), pharmacodynamics
(PD) and adverse effects is inadequately explained and unpredictable. Up to 35% of ADHD
patients do not respond satisfactorily to MPH therapy, and an even larger percentage
discontinues treatment despite persistent ADHD. During clinical trials of MPH in
treatment-naïve patients, a significant number suffer from adverse effects that are severe
and persistent enough to require dose decreases or even study withdrawal. Moreover, some
severe adverse drug reactions (ADRs) - including sudden cardiac death - have been associated
with MPH, although the precise reasons for these associations remain elusive and
controversial. Research efforts have been made to identify genetic biomarkers associated with
MPH therapeutic outcomes, almost exclusively focusing on genes related to MPH PD (e.g.,
dopamine transporter gene [DAT1, SLC6A3]). Unfortunately, findings from these studies have
been somewhat inconsistent, equivocal or even contradictory, and they do not explain the
variability in the PK of MPH. Pharmacogenomic studies in MPH-treated children have not
assessed the influence of genes associated with individual variability in PK in relation to
clinical response. Carboxylesterase-1 (CES1), an abundant hepatic enzyme encoded by the
polymorphic CES1 gene, is the sole hepatic enzyme catalyzing the metabolism (i.e., hydrolytic
deactivation) of MPH. CES1 expression and activity are known to vary substantially among
individuals. The first clinically significant CES1 variant G143E (rs71647871), discovered in
the study team's lab during the course of a healthy volunteer MPH PK study, led to gross
impairments in MPH metabolism. This variant has been unequivocally shown in vitro and in
clinical studies to lead to significantly impaired metabolism of MPH and other known CES1
substrates. The study team has established the minor allele frequency (MAF) of the G143E
variant as 3-4% in the general population. Accordingly, with ~1.9 million new starts of MPH
annually, an estimated 133,000 pediatric patients (i.e. G143E carrier's frequency 6-8%) with
a genetically impaired ability to metabolize/deactivate the drug will receive it - exposing
them to high systemic concentrations of MPH and any attendant risks or toxicities. In
addition, the study team's in vitro studies have revealed that another common CES1 variant
D203E (rs2307227) exhibited significantly impaired activity on MPH metabolism, although the
effects on D203E on clinical response are in need of further elucidation. Furthermore,
despite recent intensive research on CES1 pharmacogenetics, the functions of a large number
of additional CES1 variants remain undetermined.

The study team's hypothesis is that the CES1 variants, such as the G143E and D203E, can
significantly alter the expression and/or activity of CES1, thereby influencing the
metabolism and disposition of MPH. These influences will be directly investigated in relation
to MPH therapeutic response and tolerability in ADHD patients.

Inclusion Criteria:

- Youth ages 6-17 years with ADHD as a primary diagnosis

- Participants that are healthy, nonsmokers and are not pregnant

- Participants are receiving or about to receive methylphenidate for treatment of ADHD
from their care providers

- Newly enrolled youth who have the targeted CES1 variants

- Youth with banked samples who have the targeted CES1 variants and had agreed to be
re-contacted for future studies

- Age/sex matched controls with ADHD but without the targeted variants will be eligible
for inclusion in the 8 hour PK study

Exclusion Criteria:

- Participants that do not have ADHD as a primary diagnosis

- Participants that do not want, require, or are not healthy enough for medication
treatment with MPH for ADHD per the clinical judgment of the treating and study
clinicians

- Participants that are smokers or, are pregnant