Background: Aberrant RNA splicing contributes to oncogenic transcriptional dysregulation in pediatric brain tumors. SM09419 is a potent pan-CLK/DYRK inhibitor that modulates alternative splicing and Wnt-related signaling pathways. SM09419 is a closely structurally related analog of Cirtuvivint (SM08502), another CLK/DYRK inhibitor from Biosplice Therapeutics, TenaRx, Inc., which has been studied in two completed Phase I clinical trials for treatment in adult solid tumors (NCT03355066 and NCT05084859), and is currently being studied in adults with AML or MDS (NCT06484062), Soft tissue sarcomas (NCT07032285), Ovarian cancer (NCT06856499) and Small cell lung cancer (NCT07155200).. The objective of this study is to evaluate antitumor efficacy as a single CLK/DYRK targeting agent in different molecular subtypes of medulloblastoma (MB) in a panel of patient-derived orthotopic xenograft (PDOX) models. Methods: Eleven MB PDOX models including G3 (ICb-1572MB, ICb-1595MB, ICb-2555MB, ICb-5301MB), SHH (ICb-5610MB, ICb-3854MB, ICb-984MB, ICb-4989MB) and WNT (ICb-1192MB, ICb-1140MB, ICb-S1218MB) were treated with SM09419 (25 mg/kg, oral, once daily for 21 days). Changes of animal survival times were assessed by Gehan-Breslow-Wilcoxon tests. Cell proliferation (Ki-67) and (apoptosis cleaved-PARP1, cleaved-Caspase-3) was quantitatively evaluated by Visiopharm-aided immunohistochemistry. Results: Significant extension of survival times were generated by SM09419 in three MB models, increasing median survival times from 27 days (control) to 36 days (treated) (33.3%) ( P = 0.0143) in ICb-1572MB (G3); from 38.0 to 46.0 days (21.1%) ( P = 0.0029) in ICb-5610MB (SHH) and from 154.0 days to 219.0 Days (42.2%) ( P = 0.0359) in ICb-3854MB (SHH), accompanied by a significant reduction of Ki-67 ( P < 0.05) and increased trends in cleaved-PARP1 and cleaved-Caspase-3 levels ( P > 0.05) in ICb-1572MB and ICb-5610MB. No significant changes of animal survival times were detected in 7 models ( P > 0.05), while the last model exhibited decreased survival times ( P < 0.05) following drug treatment. Conclusions: SM09419 produced significant therapeutic efficacy a subset of MB PDOX models characterized by decreased proliferative activity. Our findings highlight the biological heterogeneity among MB subtypes and support continued investigation of splicing-modulatory CLK/DYRK inhibitors as potential targeted therapies for pediatric brain tumors.