Increased clearance of phenytoin during infectious mononucleosis

The effects of an interferon inducer, polyriboinosinic polyribocytidylic acid on cytochrome P-450 dependent hepatic progesterone metabolism

A time course study on the effects of an interferon inducing agent, polyriboinosinic polyribocytidylic acid (poly rIrC) on the hepatic cytochrome P-450 dependent progesterone metabolism was performed. Administration of a single dose of poly rIrC (10 mg/kg i.p.) to adult male Wistar rats caused a time dependent effect on liver weight, microsomal protein and total cytochrome P-450 levels, as well as the 16 alpha and 6 beta hydroxylation of progesterone. The response was multiphasic, with a maximal depression of both hydroxylase activity 48 hours post-injection, followed by enhanced activity at 72 hours and subsequent return to control activity twenty-four hours later. A second less dramatic rise in the activities followed, bringing the 16 alpha and 6 beta hydroxylase activity to 159% and 141% of their respective control values by 336 hours, at which point of time, the trend appeared to be still on the rise. The enhanced activity at 72 hours was preceded by an increase in serum cortisol and corticosterone levels, the ability of which to enhance the activity of 6 beta hydroxylation of steroids may partly explain the phenomenon.

Predictive performance of pharmacokinetic methods for phenytoin dosing: a multi-center evaluation in 282 patients with epilepsy

A retrospective study was conducted in 282 patients with epilepsy to assess the predictive performance of pharmacokinetic methods for individualizing dosage of phenytoin. Two population-based dosing methods (population clearance method and bayesian feedback method) and one individual-based method (the so-called linearized Michaelis-Menten method) were evaluated, when applicable, for single-point and/or 2-point dose predictions of phenytoin. In single-point predictions, we found a generally low percentage of dose calculations falling inside the +/- 10% range (48.9% and 51.1% for the population clearance and the bayesian methods, respectively). In 2-point predictions, the bayesian method was 'accurate' (dose within the +/- 10% range) in approximately 54.3% or 55.0% of cases (depending on the particular method of implementation adopted). An even worse percentage of 'accurate' dose predictions (38.3%) was obtained by using the linearized Michaelis-Menten method. Our data do not confirm results from previous studies indicating a generally good performance of pharmacokinetic methods for predicting phenytoin dosage.

Alteration in the disposition and metabolism of S(-)-propranolol in rats with active respiratory viral infection

Viral illness elicited marked changes in the disposition and metabolism of S(-)-propranolol in rats during the course of a recent viral outbreak in our rodent colony. A slower rate of gastrointestinal absorption and a much higher clearance of orally-administered S(-)-propranolol were observed. The apparent increase in metabolic clearance is partly attributed to a decrease in the serum protein binding of S(-)-propranolol. A direct stimulation of microsomal oxidative metabolism was also suggested by an increase in urinary recovery of the monohydroxyl metabolites of S(-)-propranolol. In addition, a greater portion of the phenolic metabolites were present in urine as the glucuronic acid conjugates, which may be explained by a concomitant stimulation in conjugative reaction. These observations are unusual in that existing reports indicate that viral illness in man and animals generally leads to an inhibition of oxidative drug metabolism.

A comparison of phenytoin dosing methods in private practice seizure patients

In an attempt to evaluate the accuracy of phenytoin (PHT) pharmacokinetic dosage adjustments in a private practice setting, three single dose-concentration pair methods and three multiple point PHT pharmacokinetic dosing methods were studied. Dose and concentration data from 28 patients seen in a private neurology practice were utilized for the study. From a comparison of these methods in private practice seizure patients, it appears that the Bayesian feedback method may be the most accurate in making routine PHT dosage adjustments, perhaps by minimizing the contribution of unknown variables within the Bayesian approach.

Vaccine-drug interactions

Interaction between a vaccine and a drug has been reported only with influenza vaccine and four drugs (aminopyrine, phenytoin sodium, theophylline, and warfarin sodium), and with BCG vaccine and theophylline. Some of these interactions still are unconfirmed. The underlying mechanism of the interaction is thought to be due to the vaccine (as an interferon-inducer) inactivating the hepatic cytochrome P-450 system; this results in depressed drug metabolism and reduced clearance. Because this is a nonspecific mechanism it could occur with other vaccines and it is possible that other vaccine-drug interactions are unrecognized. The clinical significance of vaccine-drug interactions is not fully determined; available evidence suggests that adverse reactions to warfarin or theophylline are rare after influenza vaccination and their possibility should not deter physicians from vaccinating those elderly patients at risk from influenza. Physicians should, however, monitor the response of these patients to medication in the immediate period following vaccination in case an adverse reaction occurs.