Pharmacokinetic concepts - drug binding, apparent volume of distribution and clearance.

Blood flow rate-limited physiological pharmacokinetic models have been used to examine the relationship between apparent volume of distribution and clearance or, more specifically between drug binding in blood, eliminating regions or noneliminating regions and clearance. The influence of binding on drug elimination depends on the driving force concentration in the eliminating region. In most instances this is likely to be free drug concentration in the region. Under these conditions, the results indicate that apparent volume of distribution and drug clearance from the blood should be treated as independent pharmacokinetic variables. Volume of distribution per se has no effect on clearance or on average steady-state blood levels. Drug binding in nonvascular regions(i.e. tissue binding) seems to be of limited importance except as a determinant of half-life. Although changes in tissue binding will affect partition coefficient and apparent volume of distribution, such changes will have no effect on average steady-state blood levels of either total or free drug.