Ms. H, age 30, presents to the outpatient clinic for a follow-up visit, where she reports difficulty paying attention to conversations, starting and completing tasks, and meeting deadlines. These challenges occur at work and home. Her psychiatric history includes attention-deficit/hyperactivity disorder (ADHD), major depressive disorder, and generalized anxiety disorder. Approximately 10 years ago, she underwent Roux-en-Y gastric bypass surgery. Following surgery, Ms. H’s care team prescribed liquid formulations of medications whenever possible to minimize malabsorption. Ms. H may be a rapid metabolizer; she says the effects of her prescribed stimulants only last briefly, so she has to frequently redose. As a result, she often runs out of her monthly stimulant allotment earlier than expected.
Ms. H’s current medications include dextroamphetamine/amphetamine immediate-release (IR) 30 mg 3 times daily, atenolol 50 mg/d, and escitalopram oral solution 10 mg/d. Previous unsuccessful medication trials for her ADHD include methylphenidate IR 20 mg 3 times daily and lisdexamfetamine 70 mg/d. Ms. H reports that when her responsibilities increased at work or home, she took methylphenidate IR 20 mg up to 6 times daily to relieve her symptoms.
In the United States, ADHD affects an estimated 4.4% of adults age 18 to 44.1 The actual rate may be higher, however, as recent research has called into question the hypothesis that approximately 50% of cases of childhood ADHD remit by adulthood.2 Prevalence estimates relying on DSM-IV criteria (which were designed with children in mind) can underestimate this condition in adults. Newer data suggest that up to 90% of individuals with ADHD in childhood continue to experience significant ADHD symptoms into adulthood.2
Unless contraindications are present, methylphenidate or amphetamine-based stimulants are the medications of choice for treating adult ADHD.3 Many formulations of both medications are available,4 which allows clinicians to better tailor therapy to each patient’s pharmacokinetics and daily schedule. Although there can be differences in response and tolerability, methylphenidate and amphetamine offer comparable efficacy and a similar adverse effect profile.5
Because amphetamine is more potent than methylphenidate, clinicians commonly start treatment with an amphetamine dose that is one-half to two-thirds the dose of methylphenidate.6 While both classes of stimulants inhibit the reuptake of dopamine and norepinephrine into presynaptic neurons, amphetamines also promote the release of dopamine and norepinephrine from their storage sites in presynaptic nerve terminals.3
Methylphenidate
Methylphenidate IR has an average onset of action of 30 to 45 minutes and its effects last approximately 3 to 4 hours. The extended-release (XR) formulations have varying onsets of action, with durations of action up to 12 hours (Table 13,7).4 The XR products usually immediately release a certain percentage of the medication, eliminating the need for an additional IR tablet. One methylphenidate XR product (Jornay) as well as serdexmethylphenidate/dexmethylphenidate (Azstarys) offer durations of action of 24 to 36 hours. Methylphenidate is primarily metabolized by carboxylesterase 1 (CES1) to the inactive metabolite ritalinic acid. Most of the medication (60% to 80%) is excreted in the urine as ritalinic acid.4 Theoretically, genetic variations in the CES1 and concomitant use of medications that compete with or alter this pathway may impact methylphenidate pharmacokinetics.8 However, plasma levels have not yet shown to be helpful in guiding treatment selection or dosing.4
Amphetamine
Dextroamphetamine/amphetamine IR has an average onset of action of 30 to 45 minutes and its effects last approximately 4 to 6 hours. XR formulations have varying onsets of action, with durations of action up to 13 hours (Table 23,7,9).4 One XR product, mixed salts of single amphetamine entity (Mydayis), has a duration of action of 16 hours. In XR formulations, a certain percentage of the medication is typically released immediately, eliminating the need for an additional IR tablet. Amphetamine is primarily metabolized by cytochrome P450 (CYP) 2D6 hydroxylation and oxidative deamination. Genetic variability in amphetamine metabolism may be relevant due to CYP2D6 polymorphisms. Ultra-rapid metabolizers might need higher doses, while poor metabolizers might require smaller amounts and may be more susceptible to adverse effects.4 However, there is currently insufficient data supporting gene/medication concentration relationships. As is the case with methylphenidate, plasma levels have not yet shown to be helpful in guiding treatment selection or dosing.6
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