RET and neuroblastoma: We have previously shown that the combination of RET inhibition with 13-cis-retinoic acid, a vitamin A analogue currently used for maintenance therapy in children with neuroblastoma,35 demonstrated synergistic efficacy against neuroblastoma in preclinical models.29,36 To determine whether 13-cis-retinoic acid also enhanced the efficacy of regorafenib, neuroblastoma cell lines were treated with regorafenib alone and in combination with 13-cis-retinoic acid.