CNS Penetration and the Blood-Brain Barrier

Dr. Lakshmi Nayak asks Dr. Motoo Nagane, as a neurosurgeon and neuro-oncologist, about the pharmacologic importance of CNS penetration. Dr. Nagane explains that within the contrast-enhancing tumor the blood-brain barrier is disrupted, so even large or poorly penetrant molecules such as rituximab may reach and treat enhancing disease. However, because PCNSL is highly infiltrative, tumor cells extend into non-enhancing regions where the barrier remains intact, and drugs with poor penetration may miss the residual cells that drive recurrence. He illustrates this with R-CHOP, whose components largely fail to penetrate the CNS and which has not shown a clear survival benefit in PCNSL. He cites pharmacokinetic data indicating measurable CSF penetration for tirabrutinib, supporting its CNS exposure as a pharmacologic advantage. He notes that tirabrutinib is currently the only BTK inhibitor approved for relapsed/refractory PCNSL in Japan, that other BTK inhibitors are neither approved nor routinely available for this indication there, and that ongoing Japanese studies, including a maintenance-therapy trial, are centered on tirabrutinib alone.