Efficacy and Safety of SHP465 Mixed Amphetamine Salts in the Treatment of Attention-Deficit/Hyperactivity Disorder in Adults: Results of a Randomized, Double-Blind, Placebo-Controlled, Forced-Dose Clinical Study

OBJECTIVE The objective of this randomized, double-blind, placebo-controlled study was to evaluate the efficacy and safety of SHP465 mixed amphetamine salts (MAS) in adults with attention-deficit/hyperactivity disorder (ADHD). METHODS Eligible adults [aged 18-55 years; meeting the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition ADHD criteria; baseline ADHD Rating Scale with Adult Prompts (ADHD-RS-AP) total scores ≥28] were randomized 1:1:1 to placebo or forced-dose SHP465 MAS (12.5 or 37.5 mg/day) for 4 weeks. The ADHD-RS-AP total score change from baseline to week 4 (primary endpoint) and Clinical Global Impressions-Improvement score at week 4 (key secondary endpoint) were assessed using linear mixed-effects models for repeated measures. Other efficacy endpoints were changes from baseline to week 4 on the ADHD-RS-AP hyperactivity/impulsivity and inattentiveness subscales and the percentage of participants categorized as improved on the dichotomized Clinical Global Impressions-Improvement. Safety and tolerability assessments were treatment-emergent adverse events, vital sign and weight changes, Columbia-Suicide Severity Rating Scale responses, and electrocardiogram results. RESULTS Of 369 screened participants, 275 were randomized (placebo, n = 91; 12.5 mg/day of SHP465 MAS, n = 92; 37.5 mg/day of SHP465 MAS, n = 92) and 236 completed the study (placebo, n = 80; 12.5 mg/day of SHP465 MAS, n = 80; 37.5 mg/day of SHP465 MAS, n = 76). Least-squares mean (95% confidence interval) treatment differences at week 4 significantly favored SHP465 MAS over placebo for the ADHD-RS-AP total score change from baseline [12.5 mg/day: -8.1 (-11.7, -4.4), effect size = 0.67; 37.5 mg/day: -13.4 (-17.1, -9.7), effect size = 1.11; both p < 0.001] and Clinical Global Impressions-Improvement score [12.5 mg/day: -0.8 (-1.1, -0.4), effect size = 0.68; 37.5 mg/day: -1.2 (-1.6, -0.9), effect size = 1.11; both p < 0.001]. Treatment differences for the change from baseline at week 4 favored 12.5 and 37.5 mg/day of SHP465 MAS, respectively, over placebo on the ADHD-RS-AP hyperactivity/impulsivity (both nominal p < 0.001; effect size = 0.56 and 0.91) and inattentiveness (both nominal p < 0.001; effect size = 0.70 and 1.19) subscales. At the final on-treatment assessment, the percentage of participants categorized as improved on Clinical Global Impressions-Improvement was higher with both SHP465 MAS doses than with placebo (both nominal p < 0.001). Treatment-emergent adverse events reported (>5%) with SHP465 MAS were decreased appetite, dry mouth, insomnia, headache, anxiety, initial insomnia, irritability, and bruxism. Severe treatment-emergent adverse events and treatment-emergent adverse events leading to discontinuation, respectively, were reported by 8 and 12 participants (placebo, n = 2 and 0; 12.5 mg/day SHP465 MAS, n = 1 and 7; 37.5 mg/day SHP465 MAS, n = 5 and 5). At the final on-treatment assessment, mean ± standard deviation increases from baseline were observed with 12.5 and 37.5 mg/day of SHP465 MAS for pulse (3.3 ± 10.52 and 7.1 ± 11.48 bpm) and blood pressure (systolic 0.2 ± 7.24 and 1.7 ± 9.99 mmHg; diastolic 1.0 ± 7.46 and 2.8 ± 7.90 mmHg) and decreases were observed for weight (-0.97 ± 1.523 and -1.65 ± 2.333 kg), body mass index (-0.33 ± 0.519 and -0.56 ± 0.777 kg/m2), and Fridericia corrected QT interval (-3.0 ± 10.72 and -1.6 ± 13.70 ms). No participant in any treatment group had a positive response for on-study Columbia-Suicide Severity Rating Scale assessments. CONCLUSIONS SHP465 MAS was superior to placebo in reducing ADHD symptoms, with a safety profile consistent with other long-acting stimulants. ClinicalTrials.gov Registry Number: NCT02604407.