Central nervous system (CNS) clinical trials come with a structural disadvantage. Success rates are lower, timelines are longer and recruitment remains one of the biggest causes of delay and termination. In fact, patient recruitment issues drive one of the leading causes of halted CNS clinical studies.
So what actually breaks these studies? Protocol burden, vulnerable populations, patient and caregiver hesitation, and site limitations quietly derail timelines long before endpoints are even analyzed. This case study in pediatric ADHD shows what happens when we tackled those risks early and translate that understanding into targeted strategies, instead of reacting to them later.
The reality of pediatric CNS trial recruitment
A Phase II randomized, double-blind, placebo-controlled ADHD study in children aged 6 to 12 was conducted across 35 sites in the United States, targeting 200 participants. The study evaluated an innovative treatment option in a patient population where additional therapeutic options remain needed, particularly for children who do not respond optimally to existing ADHD therapies or cannot tolerate them.
Recruitment and retention presented significant challenges. Participation required families to consider enrollment in a placebo-controlled study, including washout from existing ADHD medication before baseline, and the possibility of receiving placebo during the trial.
Recruitment had to happen during the school year. This timing was critical because the school schedule provided a consistent structure for children with ADHD, creating a more reliable context for observing behavior and completing endpoint assessments. That same routine also helped improve study medication compliance and study visit attendance, both critical to keeping patients engaged through the trial. However, the requirement to recruit during the school year further narrowed the recruitment window.
Many parents and caregivers were also understandably reluctant to alter their child's treatment regimen during the school year, particularly given the possibility of placebo assignment. This raised concerns about symptom control, school performance and daily functioning during the study period.
The study also required repeated site visits, behavioral assessments, caregiver involvement and sustained adherence over several weeks, creating considerable time and logistical commitment for families. An optional pharmacokinetic (PK) sub-study involving multiple blood draws introduced further burden in this pediatric population. Together, these clinical, operational and caregiver-related challenges made patient recruitment and retention particularly demanding.
From recruitment problem to study design problem
Most sponsors treat recruitment as something to fix once enrollment slows down, but in fact recruitment success depends on decisions made much earlier: how the protocol is structured, how much burden it places on patients and sites and how participation feels from a parent’s perspective. This is where a thorough clinical study feasibility assessment can identify potential recruitment barriers before the protocol is finalized.
Our approach starts long before recruitment begins. Recruitment and retention are treated as study design challenges, not afterthoughts. That thinking often starts well before study start, during protocol development and early clinical planning, where strategic consultancy helps sponsors make more informed design decisions.
By exploring opportunities to optimize the study design and implementing strategies to reduce patient and site burden, the study team improved feasibility and recruitability in this pediatric CNS study while preserving the scientific objectives. This allowed the study to become more predictable, helping protect timelines, budgets and development program. The focus in this case study moved to one principle: reduce friction at every step of the journey.
Localized recruitment strategies that actually convert
Instead of relying on centralized campaigns and a one-size-fits-all recruitment model, we worked closely with sites to develop and implement locally tailored recruitment strategies. Sites were empowered to drive recruitment efforts using channels that resonated with their communities, including radio, print, and especially social media platforms, because trust drives participation and trust is built locally. Families tend to be more responsive to familiar voices and relevant messaging rather than generic campaigns.
Sites were also encouraged to enroll multiple eligible children from the same family where possible. This helped boost enrollment numbers while making participation more practical for families, as visits could be coordinated around a shared schedule.
This shift from centralized control to site-level ownership created stronger engagement, improved recruitment efficiency and supported a more consistent flow of pre-screened patients.
Reducing patient burden in CNS trial design
Participation decisions in pediatric CNS trials are influenced by practical, logistical and emotional considerations, as much as by the study itself. To address this, we introduced a stepwise commitment model. Patients and parents first enrolled in the main study without being overwhelmed by the study procedures. Only later, once they were comfortable with participation, were they invited to join the PK sub-study.
At the same time, visit schedules were adapted to reduce inconvenience. Blood sampling was spread across multiple visits instead of requiring extended in-clinic waiting periods. These changes did not alter the scientific objectives of the study, but they reduced participation burden and improved the overall participant experience. In pediatric CNS trials, small adjustments that make participation more manageable can have a meaningful impact on enrollment and retention.
Improving retention in long and complex CNS studies
Retention is where many CNS trials fail silently. Patients enroll, but keeping them engaged throughout the study can be equally difficult. In this study, dropout rates increased, driven primarily by concerns about placebo assignment and loss to follow-up. Rather than accepting this as inevitable, we took a proactive approach. Close collaboration between the sponsor, study teams and sites allowed continuous monitoring of retention trends and timely adjustments throughout study conduct, an important aspect of effective clinical trial management.
When dropout rates rose above expectations, the study team proactively enrolled an additional 34 subjects to offset the higher-than-expected attrition and protect overall enrollment targets. An open-label extension was also positioned as an incentive to complete the trial. At the same time, sites increased their communication with families, reinforcing engagement and helping maintain participation throughout the study.
Clinical trial outcome
Despite the complexity, the study reached full enrollment ahead of schedule in around 3.5 months and 45% of patients agreed to participate in the PK sub-study, despite the additional burden. Moreover, while retention remained a challenge, proactive measures helped keep the trial on track, with approximately 72% of enrolled subjects completing the study and contributing to the successful collection of study data.
In a pediatric CNS population, achieving this level of participation and enrollment efficiency is far from typical. Effective study design and successful management of recruitment, retention and operational challenges ensured the study enrolled the required participants and generated the data needed to meaningfully evaluate the efficacy and safety of the investigational treatment, supporting continued drug development.
Recruitment delays, protocol burden and patient dropouts are not isolated issues. They are symptoms of the same root problem: misalignment between study design and real-world execution. Addressing these challenges requires integrated clinical research expertise across planning, site selection, recruitment and study execution.
When recruitment is built into the protocol, when patient burden is actively managed, CNS clinical trials become more predictable. That predictability matters, because in a therapeutic area where timelines stretch and failure rates remain high, control is what ultimately protects both budgets and development programs.
Successful CNS development requires a CRO partner who can identify and address study risks early, balance scientific objectives with patient feasibility and support informed development decisions. By integrating study design, recruitment, retention, and execution, we help sponsors turn CNS study complexity into clarity and clinical data into smarter development decisions.
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