Dose ranging for trials through biomarkers of drug effects.

To test the hypothesis that excess amyloid deposition is an essential step in the pathophysiology of Alzheimer's disease appropriate biomarkers are essential in selecting agents that modify amyloid formation or clearance. Cerebrospinal fluid concentrations of relevant analytes and PET measures of total brain load have been developed. These are directly applied to testing whether drugs reduce various soluble forms of amyloid as well as whether they enhance elimination of material deposited in brain parenchyma. The ideal profile of a drug that can fully test the amyloid hypothesis can be understood in terms of effects on currently available and future biomarkers. Dose selection for clinical trials should require a quantitative threshold effect on the most relevant biomarker.