Background: IPI (anti-CTLA4) is approved for treatment of advanced MEL in adults, and more information on efficacy and safety of IPI in younger patients (pts) is needed. Methods: Pts from study CA184-178 (NCT01696045) age 12 to <18 yrs with previously treated or untreated, unresectable stage III or IV malignant MEL received up to 4 doses of IPI at 3 or 10 mg/kg Q3W. Key exclusion criteria were ocular MEL, active brain metastases, or autoimmune disease. Primary endpoints were 1-yr overall survival (OS) and safety. Results: From April 2013 to June 2016, 12 pts received either IPI 3 mg/kg (n=4) or 10 mg/kg (n=8); 2 did not meet study criteria and were not treated. For the 3 and 10 mg/kg groups, median age was 13 yrs and 15 yrs, 50% and 63% were male, 25% and 13% had elevated baseline lactate dehydrogenase, and 75% and 50% had prior systemic therapy, respectively. Median number of IPI doses received was 4.0 (range: 2–4) for 3 mg/kg and 3.0 (range: 1–4) for 10 mg/kg. At 1 yr, 3/4 pts on 3 mg/kg and 5/8 pts on 10 mg/kg were alive (Table). Two pts on 10 mg/kg had partial response and 1 pt from each group had stable disease (Table). One pt had ongoing partial response >2 yrs without further treatment. Treatment-related grade 1-4 adverse events were reported in 2/4 and 7/8 pts in 3 and 10 mg/kg groups, respectively. There was 1 grade 3-4 immune-mediated adverse reaction with 3 mg/kg (hepatitis) and 5 with 10 mg/kg (hepatitis [2] and pyrexia [2] were most common). The study was stopped early due to slow accrual. Conclusions: At >1 yr of follow-up, IPI demonstrated activity in MEL pts aged 12 to <18 yrs, with a safety profile consistent with that observed in adults. Our trial highlights the difficulties of enrolling children with rare diseases in clinical trials for drugs that are approved in adults, suggesting adolescents should be included earlier in adult trials of promising new drugs. Clinical trial information: NCT01696045

NR=not reached.