New pharmacokinetic methods. II: Determination of the presence or absence of product inhibition in drugs with nonlinear pharmacokinetics.
We show that for drugs metabolized by one enzyme the slope of a plot of serum concentration (C) versus 1/clearance (CL) is linear with a value of 1/Vmax in the presence of substrate saturation and may be linear (rarely) or curved (usually) with a slope always greater than 1/Vmax in the presence of substrate saturation and product inhibition (competitive, noncompetitive, or uncompetitive) when the serum concentration of product varies with the serum concentration of substrate. Serum concentration, CL, and Vmax were determined for a group of six subjects receiving phenytoin monotherapy using three approaches. With each approach: 1) a plot C versus 1/CL was linear (r greater than or equal to 0.738, P less than .01); 2) the slope of this regression line did not differ significantly (P greater than .30) from 1/Vmax. We conclude: 1) our method is a simple and useful method for determination of mechanism of a drug's nonlinear pharmacokinetics (substrate saturation versus substrate saturation and product inhibition), 2) phenytoin has nonlinear pharmacokinetics due to substrate saturation only in man.
Browne
T R
TR
Department of Neurology, Boston University School of Medicine, MA.
Szabo
G K
GK
Walsh
C T
CT
Schumacher
G E
GE
Evans
J E
JE
Evans
B A
BA
D006801:Humans; D008433:Mathematics; D008722:Methods; D010599:Pharmacokinetics; D010672:Phenytoin
D010672:Phenytoin