Establishing a Core Outcome Set for Creatine Transporter Deficiency and Guanidinoacetate Methyltransferase Deficiency.

Creatine transporter (CTD) and guanidinoacetate methyltransferase (GAMT) deficiencies are rare inborn errors of creatine metabolism, resulting in cerebral creatine deficiency. Patients commonly exhibit intellectual and developmental disabilities, often accompanied by behavior problems, delayed speech, seizures, and motor impairments. There is currently no efficacious treatment for CTD, while the current management for GAMT requires lifelong treatment with a protein restricted diet and intake of high amounts of oral supplements. Efforts to develop effective, sustainable treatments for these disorders are limited by the lack of clinical and patient-derived meaningful outcomes. A core outcome set (COS) can facilitate consensus about outcomes for inclusion in studies. Unfortunately, patient and caregiver perspectives have historically been overlooked in the COS development process, thus limiting their input into the outcome selection. We partnered with caregivers and health professionals to establish the first COS for CTD and GAMT. The COS developed includes seven outcomes ("Adaptive Functioning", "Cognitive Functioning", "Emotional Dysregulation", "MRS Brain Creatine", "Seizure/Convulsions", "Expressive Communication", and "Fine Motor Functions") for both CTD and GAMT, and an additional outcome for GAMT ("Serum/Plasma Guanidinoacetate") that are important to stakeholders and consequently should be considered for measurement in every clinical trial. Caregivers were valued partners throughout the COS development process, which increased community engagement and facilitated caregiver empowerment. We expect this COS will ensure a patient-centered approach for accelerating drug development for CTD and GAMT, make clinical trial results comparable, minimize bias in clinical trial outcome selection, and promote efficient use of resources.