“It’s sort of an anti-climactic thing,” admitted Klunk, who co-developed PIB with chemist Chester Mathis. Since human studies with the radiotracer began six years ago, PIB has been used on more than 2,000 subjects at 40 research centers worldwide. “But this was the last piece,” Klunk told ARF. “The final nail, if you will.”

To build a more solid case for PIB as a tool for monitoring amyloid changes during disease progression, first author Milos Ikonomovic and colleagues used PIB-PET to peer into the brain of a 64-year-old woman with severe AD. After her death 10 months later, the researchers removed her brain for biochemical analyses on frozen sections from the right hemisphere and, from the left hemisphere, histological studies. The team also collected brain tissue postmortem from 27 other patients who did not undergo PIB-PET imaging while alive but were diagnosed with AD at autopsy.

In histological studies of brain samples from the autopsy cases, the scientists found that 6-CN-PIB, a highly fluorescent PIB analog, specifically labeled amyloid-β (Aβ) plaques across multiple brain regions in patterns very similar to those of anti-Aβ antibodies. To assess PIB’s binding specificity more quantitatively, the team performed ELISA and observed a direct correlation between [3H]PIB binding and levels of insoluble Aβ peptide in postmortem frontal and occipital cortices. Importantly, the researchers found no significant correlation between [3H]PIB binding and soluble Aβ, which does not form plaques.

From the AD patient who underwent PIB-PET imaging 10 months before death, the scientists were able to compare in vivo PIB uptake with region-matched postmortem measurements of amyloid pathology in the same brain. To make these comparisons, the team probed some 25 regions of interest (ROIs)—1x1-cm tissue cubes prepared from multiple brain regions displaying distinctive patterns of Aβ plaques and neurofibrillary tangles, the emblematic markers of AD. Experiments to quantitate PIB binding, Aβ plaque load, and Aβ peptide levels in these ROIs were then compared with PIB uptake during PIB-PET imaging. An MRI scan taken just prior to PIB-PET imaging allowed the researchers to define specific brain areas that correspond to the postmortem ROIs.

“The in vivo PIB signal correlates very well with the amyloid load in the brain determined at autopsy,” Klunk said of the typical AD case study. Based on their findings, he and his colleagues propose that PIB binding is highly specific for insoluble, fibrillar Aβ but not for neurofibrillary pathology. This conclusion was bolstered by the experiments using autopsy samples from the other 27 AD patients who did not undergo PIB-PET scanning while alive. PIB “behaves the same way in all these AD cases,” Klunk said, “so the inferences we’re making from this one patient seem to be representative.”

Christopher Rowe, director of the Centre for PET at Austin Health in Melbourne, Australia, agreed. "This paper is the icing on the cake for the validation of what is proving to be an extraordinarily good PET tracer," he wrote in an e-mail to ARF (see further comments below).

En route to its much-hoped-for entry into the clinical realm as an AD diagnostic marker, PIB is being used as a readout for drug effect in human trials. A team led by Agneta Nordberg at the Karolinska Institute in Stockholm, Sweden, has just published the first such study—using PIB to track brain amyloid load in 20 patients with mild AD during the course of treatment with phenserine. Based on their findings, which appeared in the Annals of Neurology last month (Kadir et al., 2008), PIB can be useful in evaluating other anti-amyloid drug therapies, Nordberg told ARF. However, one factor in the current trial that might hamper sound assessment of PIB’s effectiveness is that phenserine is an acetylcholinesterase inhibitor whose Aβ-reducing activity is only a secondary effect (see comment below). Also ongoing are small, unpublished trials in Europe using PIB to measure amyloid load in AD patients receiving immunotherapy with AAB-001 (bapineuzumab), a humanized monoclonal antibody that binds to and clears Aβ peptide (see ARF clinical trials update).

While Rowe describes PIB’s march toward clinical use as “tantalizingly close,” Klunk concedes that the real value of PIB will depend on the eventual availability of much more effective AD treatments than are available now. “If you don’t have a drug to treat the disease, there’s really no point screening for it,” Klunk said, noting that a typical PET scan costs $1,500 to $2,000 dollars. “But if you have a drug…that’s a drop in the bucket.”—Esther Landhuis.

Esther Landhuis is a science journalist in Dublin, California.

References:
Ikonomovic MD, Klunk WE, Abrahamson EE, Mathis CA, Price JC, Tsopelas ND, Lopresti BJ, Ziolko S, Bi W, Paljug WR, Debnath ML, Hope CE, Isanski BA, Hamilton RL, Dekosky ST. Post-mortem correlates of in vivo PIB-PET amyloid imaging in a typical case of Alzheimer's disease. Brain. 2008 Mar 12; [Epub ahead of print] Abstract

Kadir A, Andreasen N, Almkvist O, Wall A, Forsberg A, Engler H, Hagman G, Lärksäter M, Winblad B, Zetterberg H, Blennow K, Långström B, Nordberg A. Effect of phenserine treatment on brain functional activity and amyloid in Alzheimer's disease. Ann Neurol. 2008 Feb 25; [Epub ahead of print] Abstract