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Overcoming resistance to Sonic Hedgehog inhibition by targeting p90 ribosomal S6 kinase in pediatric medulloblastoma Pambid, Mary Rose


Molecular subtyping has allowed for the beginning of personalized treatment in children suffering from medulloblastoma (MB). However, resistance inevitably emerges against these therapies, particularly in the Sonic Hedgehog (SHH) subtype. We discovered that children with SHH subtype have inferior outcomes relative to other MB subtypes underscoring the need to identify new therapeutic targets. High content screening of a 129 compound library identified agents that inhibited SHH MB growth. The lead molecular target was p90 ribosomal S6 kinase (RSK). Its levels were characterized by immunoblotting and qRT-PCR. Comparisons were made to human neural stem cells (hNSC). Impact of inhibiting RSK with the small molecule BI-D1870 or siRNA was assessed in growth assays (monolayer, neurosphere, and soft agar). NanoString was used to detect RSK in a cohort of 58 patients with MB. To determine BI-D1870 pharmacokinetics/pharmacodynamics, 100 mg/kg was I.P. injected into mice and tissues were collected at various time points. Daoy, ONS76, UW228, and UW426 MB cells were exquisitely sensitive to BI-D1870 but unresponsive to SHH inhibitors. RSK inhibition had no effect on hNSCs. Anti-tumour growth corresponded with inactivated RSK in MB cells. Inhibiting RSK with siRNA or BI-D1870 suppressed growth, induced apoptosis, and sensitized cells to SHH agents. Notably, RSK2-4 was expressed in SHH patients as well as the other subtypes. In mice, BI-D1870 was well-tolerated and crossed the blood-brain barrier (BBB). RSK inhibitors are promising because their target RSK is found in SHH MB patients. They also induce high levels of apoptosis in only MB cells. Importantly, BI-D1870 crosses the BBB, acting as a scaffold for the development of more long-lived RSK inhibitors.

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