From the Pharmacology Dept., Pars Biopharmacy Research Co., P. O. Box 14515-717, Tehran
Abstract: (3895 Views)
Adjustment of phenytoin dosage in patients is very difficult due to its nonlinear
metabolism and patient to patient variation in its kinetics. It has been
recommended that the dosage of phenytoin should be adjusted according to its
plasma concentration and requirements of the patients. Therefore, the present
study was carried out to identify the various factors which may influence the
plasma level of this drug. The phenytoin plasma concentration was determined in
91 patients with steady-state concentrations according to the EMIT method. In a
further 14 patients, who had recieved at least two different doses of phenytoin, the
Km, and Vmax of phenytoin metabolism were determined according to Mullen's
direct linear plot. The results of this study showed that the plasma level of
phenytoin was below the therapeutic level in 62 (68.2%) of the patients and above
the therapeutic level in 8 (8.8%). Statistical analysis did not show any correlation
between plasma level and factors such as sex, age, or type of drug administered.
Only a small correlation was found between dosage and plasma level. The K", of
phenytoin metabolism in the group studied was found to be in the range of 1.8-
26j..Lg/mI and that of V max in the range of 5.33-13.88 mg/kg/day. The mean values
of Km, (8.4±1.7µg/mI) and V max (7.3±O.S8 mg/kg/day) were slightly higher than
reported values in the literature (S.7±2.9 µg/mI and 5.9±1.2 mg/kg/day,
respectively). However, this difference was not statistically significant.