Oxaprozin dose proportionality

Twelve normal subjects each received single 300-, 600-, and 1200-mg oral doses of oxaprozin according to a three-period crossover design. Total drug plasma concentrations did not increase in proportion to the dose administered. Total clearance (CIo) and volume of distribution (Vd) increased with dose, though elimination t1 2 remained unchanged. The fraction of unbound oxaprozin in plasma (fup) varied linearly with total plasma concentration: it increased from 0.068 per cent at 10 micrograms/ml to 0.180 per cent at 170 micrograms/ml. A parameter fup was therefore introduced to express the average degree of unbound drug plasma for a given dose, and to allow the calculation of unbound volume of distribution (Vdu) and intrinsic clearance (CIi) as if binding were constant. Even though fup increased with dose, the overall binding in the body (fub approximately 0.52 per cent) was relatively stable. Neither Vdu nor CIi changed with dose; hence, unbound oxaprozin kinetics can be considered to be linear. Protein binding had no effect on unbound oxaprozin plasma levels within the given dose range, and there was a one-to-one proportionality between the dose administered and the unbound drug concentration in plasma.