Attention-Deficit Hyperactivity Disorder

Abstract

Throughout this decade, there has been significant research into pharmacotherapies for attention-deficit hyperactivity disorder (ADHD). This article considers the efficacy and safety of five of the more novel long-acting pharmacological treatments recently approved by the FDA for marketing in the US for paediatric ADHD, along with an alpha(2)-adrenoceptor agonist in preparation. Reviewed treatments include the non-stimulant atomoxetine, three novel extended-release (XR) stimulant preparations: dexmethylphenidate, lisdexamfetamine dimesylate and the methylphenidate transdermal system (TDS), and the recently approved XR alpha(2)-adrenoceptor agonist, guanfacine. Dexmethylphenidate XR is a stimulant treatment in a single isomer form, and has an efficacy and tolerability similar to two doses of immediate-release (IR) dexmethylphenidate when taken 4 hours apart, but is dosed at half of the usual d,l-methylphenidate dose. Dexmethylphenidate XR utilizes a beaded bimodal release, with 50% initially released and another 50% released 4 hours later to provide benefit lasting up to 10-12 hours. Lisdexamfetamine was the first stimulant treatment approved as a prodrug, whereby the single isomer d-amfetamine remains pharmacologically inactive until activated by cleaving the lysine. Its efficacy and tolerability are generally consistent with that of XR mixed amfetamine salts, with this activation method and more consistent absorption generally resulting in up to an 11- to 13-hour benefit. The methylphenidate TDS patch utilizes skin absorption to provide predictable and uniform delivery of methylphenidate when worn for 9 hours/day. The efficacy and tolerability of the methylphenidate TDS patch is generally consistent with that of osmotic-controlled release oral system (OROS) methylphenidate, providing benefit for about 11-12 hours. Because of their formulation, lisdexamfetamine and methylphenidate each have an onset of effect at about 2 hours after administration. An adjustable wear time for the methylphenidate TDS patch accommodates related adverse effects, but its disadvantages are frequent skin irritation and the need to remember to take the patch off. Atomoxetine is the first non-stimulant treatment approved by the FDA and employs weight-based dosing up to 1.4 mg/kg/day. Benefit is generally observed within 2-8 weeks of initiation and is considered to have a lesser therapeutic effect than that of stimulants. A recent parallel-group controlled study found that atomoxetine (up to 1.8 mg/kg/day) and OROS methylphenidate both improved ADHD symptoms, although subjects receiving OROS methylphenidate had a significantly better response. Interestingly, treatment-naive children had a similar beneficial response to atomoxetine as those receiving OROS methylphenidate. Subsequent crossover treatment revealed a subgroup of youths who did not respond well to OROS methylphenidate but did respond to atomoxetine. Also identified was a larger than expected subgroup who did not respond well to either active treatment, confirming the need to continue the pursuit of novel treatments. As of September of 2009, guanfacine in XR form is the first alpha(2)-adrenoceptor agonist to gain approval to treat ADHD, approved for the treatment of 6- to 17-year olds. A second alpha(2)-adrenoceptor agonist, clonidine, is in development as a potential XR treatment for paediatric ADHD. IR clonidine has a fast onset and short half-life, with its use historically limited by somnolence. Although early formulations did not improve inattention well, recent evidence suggests that clonidine XR may have potential use as monotherapy or in extending benefit when taken with a stimulant. Guanfacine has a more specific neuronal action and a longer action than that of clonidine. The approved dosing of guanfacine XR 1 to 4 mg daily generally provides symptom benefit lasting 8-14 hours, and up to 24 hours in some children and adolescents receiving a higher dose. Such recent developments and ongoing study of additional potential pharmacological interventions may lead to additional future treatment options for children with ADHD.