Background Carbon-11-labelled Pittsburgh compound B (11C-PiB) PET is a marker of cortical fibrillar amyloid-β load in vivo. We used 11C-PiB PET to investigate whether bapineuzumab, a humanised anti-amyloid-β monoclonal antibody, would reduce cortical fibrillar amyloid-β load in patients with Alzheimer's disease. Methods Patients with mild-to-moderate Alzheimer's disease were randomly assigned to receive intravenous bapineuzumab or placebo in a ratio of seven to three in three ascending dose groups (0·5, 1·0, or 2·0 mg/kg). Each dose group was enrolled after safety review of the previous group. Randomisation was by interactive voice response system; masking was achieved with numbered kit allocation. Patients, investigators, study site personnel, sponsor staff, and carers were masked to treatment. Patients received up to six infusions, 13 weeks apart, and had 11C-PiB PET scans at baseline and at weeks 20, 45, and 78. The primary outcome was the difference between the pooled bapineuzumab group and the pooled placebo group in mean change from screening to week 78 in 11C-PiB cortical to cerebellar retention ratio averaged across six cortical regions of interest. Analysis was by modified intention to treat. This study is registered with EudraCT , number 2004-004120-12 ; ISRCTN17517446. Findings 28 patients were assigned to bapineuzumab (n=20) or placebo (n=8). 19 patients in the bapineuzumab group and seven in the placebo group were included in the modified intention-to-treat analysis. Estimated mean 11C-PiB retention ratio change from baseline to week 78 was −0·09 (95% CI −0·16 to −0·02; p=0·014) in the bapineuzumab group and 0·15 (95% CI 0·02 to 0·28; p=0·022) in the placebo group. Estimated mean difference in 11C-PiB retention ratio change from baseline to week 78 between the bapineuzumab group and the placebo group was −0·24 (95% CI −0·39 to −0·09; p=0·003). Differences between the bapineuzumab group and the placebo group in the individual regions of interest were similar to the overall mean difference. Adverse events were typically mild to moderate in severity and transient. Two patients in the 2·0 mg/kg bapineuzumab group had transient cerebral vasogenic oedema. Interpretation Treatment with bapineuzumab for 78 weeks reduced cortical 11C-PiB retention compared with both baseline and placebo. 11C-PiB PET seems to be useful in assessing the effects of potential Alzheimer's disease treatments on cortical fibrillar amyloid-β load in vivo. Funding Elan Pharmaceuticals and Wyeth Research.

11C-PiB PET assessment of change in fibrillar amyloid-β load in patients with Alzheimer's disease treated with bapineuzumab: a phase 2, double-blind, placebo-controlled, ascending-dose study