Exploratory studies: implications for drug development in Alzheimer's disease

The process of drug development involves cycles of learning and confirming. In the learning phase of the cycle, data from studies are used to generate hypotheses; later studies are designed specifically to confirm these hypotheses. There is a growing realization that exploratory studies early in drug development are critical for the design of large confimatory clinical trials. For example, patients often differ from healthy volunteers in their tolerance of CNS compounds, so investigators should determine the maximum tolerated dose (MTD) in the patient population though a "bridging study" prior to designing Phase II efficacy trials. By performing well-controlled safety/tolerability studies with a small number of patients on an inpatient basis, researchers can avoid exposing large numbers of study participants to the wrong dose. Bridging studies also provide information on the approximate frequencies of various types of adverse events, which is very helpful in designing outpatient trials to confirm the safety profile. A second type of exploratory study, a "dynabridge study", is an early pharmacokinetic/pharmacodynamic (PK/PD) study in patients to measure drug concentrations and activities in the central compartment (assessed by sampling the CSF). Dynabridge studies can be used to optimize the dosing regimen, to confirm that the drug is affecting the targeted receptor, and to measure correlations between drug activity and proposed surrogate markers.