P4–246: Passive Immunotherapy with M266 Results in Altered CSF Aβ

CSF Aβ is an important biomarker for Alzheimer's disease (AD). No physical barrier exists between the interstitial fluid of the brain and CSF and hence concentrations within one compartment are thought to mirror the other. The objective of the current studies was to determine whether CSF Aβ is altered after peripheral injection of the anti–Aβ central domain antibody m266. As ∼ 0.1 percent of peripheral antibody enters the CNS, we were interested in distinguishing between the total Aβ pool (antibody bound plus the free unbound) from the “free” Aβ (non–IgG bound pool). We developed a solid–phase immunodepletion assay that could accurately quantify the free Aβ pool and performed a proof of concept study with rats dosed intravenously with 5 mg/kg of biotinylated m266 (m266B). CSF collected from rats 24–hours post dose had significantly increased total Aβ and significantly decreased “free” Aβ. To define the time course of the CSF Aβ alterations, rats were dosed with a single injection of 1, 5, or 10mg/kg m266B and subsequently analyzed at 4, 8, 24, 48, 120, and 240 hours post dose. Multiple analytes were studied in this experiment including CSF and plasma total Aβ ×–40, ×–42, m266B, and free Aβ (CSF only). The large time course study demonstrated several m266B dose–dependent and time–dependent alterations in the CSF compartment. Exposure of m266B in CSF was dose–dependent and reached equilibrium with the periphery at ∼ 0.12%. The concentrations of total Aβ in CSF increased to ∼ 200% of control, while the concentrations of free Aβ significantly decreased. The most profound effects were observed for Aβ ×–42, wherein the free concentration was decreased by roughly 50%. These studies demonstrate that peripheral injection of m266B results in significant alterations of CSF Aβ. Additionally, the presence of the therapeutic antibody in the CSF results in a complex mixture of CNS Aβ that is either bound or unbound (free) to the m266B antibody.