C5aR1 Inhibition Alleviates Cranial Radiation–Induced Cognitive Decline

Cranial radiotherapy (RT) for brain cancers leads to an irreversible decline in cognitive function without an available remedy. Radiation-induced cognitive decline (RICD) is a particularly pressing problem for brain cancer survivors who live long after RT. Radiation-induced neuroinflammation and gliosis lead to excessive loss of synaptic integrity and cognitive dysfunction. Using intact and brain cancer-bearing mouse models, we show here that inhibiting anaphylatoxin complement C5a receptor (C5aR1) signaling is neuroprotective against RICD. Both genetic loss in a C5ar1 knockout mouse and pharmacologic inhibition using the orally active, brain-penetrant C5aR1 antagonist PMX205 reversed RICD, leading to neurocognitive improvements in object recognition memory and memory consolidation tasks. Inhibiting the C5a-C5aR1 axis reduced microglial activation, astrogliosis, and synaptic loss in the irradiated brain. Importantly, C5aR1 blockade in two syngeneic, orthotopic glioblastoma-bearing mouse models protected against RICD and elevated gene signatures associated with neuroprotection without interfering with the therapeutic efficacy of RT in reducing tumor volume in vivo. As PMX205 has been found to be safe in clinical trials with healthy individuals, C5aR1 inhibition is a translationally feasible approach to address RICD, an unmet medical need. SIGNIFICANCE: Complement cascade activation by cranial radiotherapy for brain cancers mediates radiation-induced neuroinflammation, synaptic loss, and cognitive dysfunction that can be alleviated by inhibiting the complement C5a-C5aR1 signaling axis, providing a tractable therapeutic approach.