Geoerger B, Kang HJ, Yalon-Oren M, et al. - Researchers undertook this interim analysis of an ongoing phase 1–2 open-label trial, KEYNOTE-051, to not only establish the recommended phase 2 dose of pembrolizumab but also to verify its safety as well as antitumour activity in advanced paediatric cancer. They recruited children aged 6 months to 17 years at 30 hospitals established in Australia, Brazil, Canada, France, Germany, Israel, Italy, South Korea, Sweden, the UK, and the USA. To establish the recommended phase 2 dose, they used pharmacokinetics and dose-limiting toxicities. Consistent with experience in adult patients, the good tolerability as well as encouraging antitumour activity of pembrolizumab was evident in paediatric patients with relapsed or refractory Hodgkin lymphoma. The recommended phase 2 dose was 2 mg/kg every 3 weeks. In most of the paediatric tumour types, the low antitumour activity of pembrolizumab was seen, and only a few rare PD-L1-positive tumour types exhibited responses. This indicates the non-sufficiency of PD-L1 expression alone as a biomarker for the choice of paediatric patients who are likely to respond to PD-1 checkpoint inhibitors.