Use of dried blood spots in drug development: pharmacokinetic considerations.

Dried blood spots are increasingly being used in drug development. This commentary considers the pharmacokinetic issues that arise and compares these with those attached to plasma, the mainstay matrix. A common implicit use of these matrices is as a surrogate for plasma water, and to this extent, the critical assumption made is constancy in fraction unbound for plasma and, additionally for blood, constancy of hematocrit and blood cell affinity of compound. Often, these assumptions are reasonable and either matrix suffices, but not always. Then the value of one over the other matrix depends on the magnitude of the blood-to-plasma concentration ratio of drug, its clearance, and the cause of the deviation from constancy. Additional considerations are the kinetics of distribution within blood and those arising when the objective is assessment or comparison of bioavailability. Most of these issues can be explored and addressed in vitro prior to the main drug development program.