Methoxy-X04, a derivative of Congo red and Chrysamine-G, is a brain-permeable fluorescent probe for amyloid-β (Aβ).

In in vitro binding studies, compared with Chrysamine-G (Ki = 25.3 nM), Methoxy-X04 exhibited similar binding affinity for Aβ fibrils (Ki = 26.8 nM) [1]. On brain slices of 12-month-old APP-PS1 (dE9) mice, 145 μM Methoxy-X04 was used for labeling fibrillary amyloid deposits [3].

In PS1/APP mice, a single i.v. injection of 5 to 10 mg/kg Methoxy-X04 distinguished individual plaques from brain fluorescent images, within 30 to 60 min after treatment. A single i.p. injection of 10 mg/kg Methoxy-X04 also produced high contrast images of plaques and cerebrovascular amyloid in PS1/APP mouse brains [1].