Effect of phenytoin on the pharmacokinetics of doxorubicin and doxorubicinol in the rabbit.

Doxorubicin is metabolized extensively to doxorubicinol by the ubiquitous aldoketoreductase enzymes. The extent of conversion to this alcohol metabolite is important since doxorubicinol may be the major contributor to cardiotoxicity. Aldoketoreductases are inhibited in vitro by phenytoin. The present study was conducted to examine the effect of phenytoin on doxorubicin pharmacokinetics. Doxorubicin single-dose pharmacokinetic studies were performed in 10 New Zealand White rabbits after pretreatment with phenytoin or phenytoin vehicle (control) infusions in crossover fashion with 4-6 weeks between studies. Infusions were commenced 16 h before and during the course of the doxorubicin pharmacokinetic studies. Phenytoin infusion was guided by plasma phenytoin estimation to maintain total plasma concentrations between 20 and 30 micrograms/ml. Following doxorubicin 5 mg/kg by i.v. bolus, blood samples were obtained at intervals over 32 h. Plasma doxorubicin and doxorubicinol concentrations were measured by HPLC. The mean plasma phenytoin concentrations ranged from 17.4 to 33.9 micrograms/ml. Phenytoin infusion did not alter doxorubicin pharmacokinetics. The elimination half-life and volume of distribution were almost identical to control. Clearance of doxorubicin during phenytoin administration (60.9 +/- 5.8 ml/min per kg, mean +/- SE) was similar to that during vehicle infusion (67.5 +/- 5.4 ml/min per kg). Phenytoin administration was associated with a significant decrease in doxorubicinol elimination half-life from 41.0 +/- 4.8 to 25.6 +/- 2.8 h. The area under the plasma concentration/time curve (AUC) for doxorubicinol decreased significantly from 666.8 +/- 100.4 to 491.5 +/- 65.7 n.h.ml-1. These data suggest that phenytoin at clinically relevant concentrations does not alter the conversion of doxorubicin to doxorubicinol in the rabbit. The reduction in the AUC for doxorubicinol caused by phenytoin appears to be due to an increased rate of doxorubicinol elimination. Phenytoin or similar agents may have the effect of modifying doxorubicinol plasma concentrations by induction of doxorubicinol metabolism rather than by inhibition of aldoketoreductase enzymes.

Phenytoin pharmacokinetics in the rabbit: evidence of rapid autoinduction.

Phenytoin pharmacokinetics were studied during continuous intravenous infusion in 12 New Zealand white rabbits. The mean clearance at 40 hours (5.1 +/- 1.1 ml/min per kg; mean +/- SE) was significantly greater than that at 16 hours (3.1 +/- 0.5 ml/min per kg; p less than 0.05). These data suggest that with chronic administration, autoinduction of metabolism results in an increase in the rate of phenytoin clearance in the rabbit.