Baseline characteristics and feasibility of clinical outcome measures in CDKL5 deficiency disorder: The CANDID observational study

OBJECTIVE: CDKL5 deficiency disorder (CDD) is a rare X-linked developmental and epileptic encephalopathy caused by loss-of-function variants in the CDKL5 gene. Preclinical experiments using enzyme replacement or gene therapies show promise and could be transformative therapies. This precompetitive consortium sought to harmonize nonseizure clinical endpoint selection for efficacy trials. Clinical Assessment of Neurodevelopmental Measures in CDD (CANDID) is an ongoing study evaluating the feasibility and suitability of neurocognitive tests and functioning scales in CDD patients. METHODS: CANDID is a 3-year, longitudinal, noninterventional global study involving children and adults with CDD. On-site and remote visits include clinical, behavioral, developmental, and quality of life assessments. RESULTS: We enrolled 112 patients (111 included in analyses); mean age = 8.3 years (range 16 seizures; six were seizure-free. Median seizure onset was at 1.5 months (range = 0-66). Patients used an average of 2.6 antiseizure medications at baseline. The most frequent comorbidities included gastrointestinal hypomotility, muscle tone abnormalities, and sleep disorders. Gross Motor Function Measure-88 (GMFM-88) scores indicated a floor effect in crawling, standing, and walking across all ages. Vineland-3 and Bayley-4 scores could be derived in most, with receptive language, interpersonal relationships, and fine and gross motor scores increasing with age. Bruni sleep questionnaire identified sleep initiation, sleep-awake transition, and excessive somnolence as the most disrupted components across all age groups. The mean Quality of Life Inventory-Disability total scores ranged from 53% to 64%, the independence domain being the most impacted. SIGNIFICANCE: The scales in the CANDID study capture disease-related deficits and phenotype variability in CDD. Floor effects in subdomains aligned with disease severity. The GMFM-88 lacks granularity, and its operational limitations make it unsuitable for CDD trials. Baseline analyses demonstrate the feasibility and potential value of most selected scales, supporting their use in optimizing trial design and endpoint selection for future CDD clinical trials.