Headline from marketwatch, page-3

The AZN drug is a BACE inhibitor which acts on the amyloid plaques. AZN is currently looking at a combined Phase 2/3 trial. As many of us believe, and has been shown by other trials, however, removing the plaques is not thought to be the key. That by the time the plaques are formed there has already been significant neuro-degeneration that is thought to be irreversible. The beauty of the MPAC theory is that it stops the progression prior to formation of the plaques and heads off the neurological damage. The issue for PBT2 in the IMAGINE trial is not that the drug didn't work but that the biomarker endpoints that were selected were inappropriate to see effects of the treatment. Here's a little background on the AZN drug from ALZforum.

Background
AZD3293 is an inhibitor of BACE1, the β-secretase sheddase that cleaves the APP protein to release APP's C99 fragment. This fragment then becomes a substrate for subsequent γ-secretase cleavage and Aβ peptide generation. The rationale is that inhibiting BACE1 will reduce amyloid-related toxicity in Alzheimer's disease. AZD3293 is one of several BACE1 inhibitors currently in development. Preclinical data have not been formally published. AZD3293 is administered in solution. Findings In December 2012, AstraZeneca started a Phase 1 study that evaluated the safety and pharmacological effects of single doses of oral AZD3293 in 72 healthy volunteers. Doses ranged from 1–1,000 mg. This trial ended in May 2013, but results have not been reported. In August 2013, a second Phase 1 study started. This one will test multiple ascending doses of AZD3293 solution in 56 healthy elderly volunteers and patients with mild to moderate Alzheimer's disease for safety, tolerability, pharmacokinetics, and effects on biomarkers in plasma and CSF.

The thing that PBT2 has is the ability to treat both HD and AD. HD affords orphan designation (speedier path to approval, reductions in costs for trials, etc) that AD does not. Additionally, having two applications for the same drug will allow use of safety and tolerability data from trials from AD and HD to support the HD NDA.