The effects of phenobarbital, chloral betaine, and glutethimide administration on warfarin plasma levels and hypoprothrombinemic responese in man

Warfarin Interaction With Hepatic Cytochrome P-450 Enzyme-Inducing Anticonvulsants

Initiation of cytochrome P-450 (CYP)-inducing anticonvulsant medications during warfarin therapy may decrease anticoagulant effect and necessitate frequent warfarin dose adjustments to maintain therapeutic response measured by the international normalized ratio (INR). Clinical information regarding interactions between warfarin and these medications is limited. This study investigated warfarin dose and INR response following CYP-inducing anticonvulsant initiation among chronic warfarin users. This retrospective, pre-post study included patients ≥18 years who were receiving chronic warfarin therapy and who initiated carbamazepine, oxcarbazepine, phenobarbital, or phenytoin between January 1, 2006, and December 31, 2013. Mean weekly warfarin dose/INR ratio and mean weekly warfarin dose were compared in the 90 days pre- and days post-anticonvulsant initiation periods. Of the 57 included patients, 34 (60%), 15 (26%), 6 (11%), and 2 (3%) patients purchased a prescription for carbamazepine, phenytoin, oxcarbazepine, and phenobarbital, respectively. Mean age was 70 years, 59% were female, and the majority were receiving chronic warfarin therapy for atrial fibrillation (39%) or venous thromboembolism (26%). The ratio of mean warfarin dose and INR increased significantly between the pre- and post-anticonvulsant initiation periods (from 13 mg/INR to 18 mg/INR, respectively, P ≤ .001) as did the mean weekly warfarin dose (from 33 mg to 37 mg, P = <.001). Warfarin dose and dose/INR ratio significantly increased after carbamazepine initiation (both P < .001), while oxcarbazepine, phenobarbital, and phenytoin initiation did not significantly affect warfarin dosing. Our results support the presence of a clinically meaningful interaction between warfarin and carbamazepine. Frequent INR monitoring and warfarin dose escalation are recommended in this setting.

Interindividual variability in inhibition and induction of cytochrome P450 enzymes

Drug interactions have always been a major concern in medicine for clinicians and patients. Inhibition and induction of cytochrome P450 (CYP) enzymes are probably the most common causes for documented drug interactions. Today, many pharmaceutical companies are predicting potential interactions of new drug candidates. Can in vivo drug interactions be predicted accurately from in vitro metabolic studies? Should the prediction be qualitative or quantitative? Although some scientists believe that quantitative prediction of drug interactions is possible, others are less optimistic and believe that quantitative prediction would be very difficult. There are many factors that contribute to our inability to quantitatively predict drug interactions. One of the major complicating factors is the large interindividual variability in response to enzyme inhibition and induction. This review examines the sources that are responsible for the interindividual variability in inhibition and induction of cytochrome P450 enzymes.

Identification of a novel transcriptional silencer in the protein-coding region of the human CYP2C9 gene

A novel regulatory element (27 bp) which confers transcriptional repression was identified within the protein-coding region immediately after the translation start codon in the human cytochrome P450 (CYP) 2C9 gene. Deletion of this element increased transcriptional activity in HepG2 cells by transient transfection assay. Nuclear protein extracts from HepG2 cells and human liver were found in electrophoretic mobility shift assays to bind specifically to the 27 bp element. A putative binding protein was partially purified by DNA-affinity chromatography and was determined by Southwestern blotting to have a molecular weight of approx. 100 kDa. Studies with mutated competitor oligonucleotides established that binding of the nuclear protein to the 27 bp cis-element was dependent upon two 6 bp direct repeats (5'-CTTGTG-3') that were separated by three bases. It is possible that this novel cis-acting element may be involved in the negative regulation of CYP2C9.

Phenobarbital minimally alters plasma concentrations of losartan and its active metabolite E-3174

Losartan, a selective angiotensin II (AT1) receptor antagonist for hypertension, is metabolized to an active carboxylic acid metabolite, E-3174, which has a longer half-life. To investigate the effects of induction of cytochrome P450 on the metabolism of losartan, we evaluated the effects of phenobarbital on the plasma profiles of losartan and E-3174 in 15 healthy male subjects. Ten subjects received a single 100 mg oral dose of losartan before and during phenobarbital administration (100 mg/day for 16 days), and five subjects received losartan before and during placebo. Urinary excretion of 6-beta-hydroxycortisol (relative to 17-hydroxycorticosteroids) was measured as an endogenous marker of cytochrome P450 induction. The geometric mean area under the plasma concentration-time curve ratios (with/without phenobarbital and 90% confidence intervals) for losartan and its metabolite (E-3174) were 0.795 (0.723, 0.875) and 0.799 (0.778, 0.820), respectively, indicating that phenobarbital treatment significantly but to a clinically minor extent reduced plasma concentrations of losartan and E-3174 (p<0.01). Half-life values of losartan and E-3174 were unchanged. The ratio of 6-beta-hydroxycortisol to 17-hydroxycorticosteroids doubled in the phenobarbital group (p < 0.001) and did not change appreciably in the placebo group.

Mechanisms of action of aminoglutethimide as endocrine therapy of breast cancer

During the last decade aminoglutethimide has been recognised as a valuable alternative in endocrine therapy for advanced breast cancer. Although some side effects do occur, most often these are initial effects which subside within a few weeks, and cessation of therapy is not usually indicated. Aminoglutethimide was originally introduced as an inhibitor of steroidogenesis in the adrenal cortex. It was soon recognised, however, that inhibition of the non-glandular aromatase, blocking the conversion of androgenic prohormones to oestrogens, was more important, resulting in decreased blood levels of oestrogens. In this review the role of aromatase inhibition as the only important aspect of the mechanism of action of aminoglutethimide is challenged. Evidence has accumulated during the last few years that aminoglutethimide is a most potent inducer of microsomal enzymes. In addition to the pharmacological implications this has (suggesting important interactions), it also points to the possibility that levels of oestrogens are decreased due to accelerated metabolism of these hormones. Based on new experimental data, and also clinical work with alternative aromatase inhibitors, it appears that the antitumour activity of aminoglutethimide may be due to both aromatase inhibition and accelerated metabolism of oestrogens. This seriously challenges the importance of aromatase inhibition alone as a strategy in endocrine therapy of breast cancer, and furthermore suggests that accelerated metabolism of key hormones is an alternative strategy to be explored.

Warfarin-griseofulvin interaction

A 61-year-old man with a mitral valve replacement who had been therapeutically anticoagulated with warfarin for the previous 12 months had a decrease in his prothrombin time after starting treatment with griseofulvin. Over a three-month period his warfarin dose was gradually increased. An overall increase of 41 percent was needed to maintain therapeutic anticoagulation. A reduction in the griseofulvin dose resulted in an increase in the prothrombin time and, subsequently, a lowering of the warfarin dose. No other potential causes for the decreased response to warfarin were identified. While there previously has only been limited documentation of this interaction, it appears that a substantial decrease in the response to warfarin can occur following initiation of griseofulvin therapy. Immediate adjustment of the warfarin dose at the onset of griseofulvin therapy is not needed. However, the prothrombin time should be monitored at weekly intervals and the dose of warfarin increased according to the patient's response.

Pros and cons of aminoglutethimide for advanced postmenopausal breast cancer

In a phase II clinical trial, 38 postmenopausal women with advanced breast cancer were treated with aminoglutethimide and replacement hydrocortisone. All women had previously received up to 4 modalities of endocrine therapy. Seventeen patients had also been treated with cytostatic drugs. Twenty-five percent of the 29 evaluable patients experienced objective tumor regression, lasting from 11 to more than 18 months. In 29% the disease was stabilized for 3 to more than 15 months. Toxicity was significant, necessitating drug withdrawal in 3 patients. One patient died within 3 weeks of therapy from multiple perforated gastric ulcers. Two patients developed herpes zoster within 4 weeks of treatment. Many side effects were minor and transient. However, treatment resulted in overt primary hypothyroidism in 25% of the evaluable patients and in a strongly increased need of acenocoumarin in all 3 patients on anticoagulant therapy.

Decreased systemic clearance of caffeine due to cimetidine

1 Five normal subjects received pre-treatment with cimetidine 200 mg three times daily and 400 mg at night for 6 days, or matching placebo. 2 Caffeine (300 mg) was given orally before any treatment and at the beginning of the last day of each treatment course. Treatments were randomly allocated and separated by at least one week. 3 A significant reduction occurred in the systemic clearance of caffeine and the half-life was prolonged as determined from measurement of caffeine in plasma and saliva. No change occurred in the apparent volume of distribution. 4 The oral bioavailability of caffeine was found to be complete in the one subject studied. 5 It is suggested that cimetidine inhibits the microsomal metabolism of caffeine. Although the steady state plasma caffeine would increase by approximately 70%, it is unlikely that this would produce adverse clinical effects.

Comparative pharmacokinetics of coumarin anticoagulants. XLVI: Effect of treatment of phenobarbital on pharmacokinetics of (S)-(-)-warfarin in rats

This investigation was carried out (a) to determine if the enzyme inductive effect produced by phenobarbital reduces the interindividual variability in the biotransformation of a drug, as suggested in the literature; (b) to test whether the intrinsic clearance of free drug, for drugs exhibiting restrictive clearance, reflects the activity of drug-metabolizing enzyme systems; and (c) to determine if enzyme induction affects the apparent volume of distribution of a drug that tends to concentrate in the liver. Twelve pairs of adult male rats, matched with respect to their serum warfarin free fraction, received an intravenous injection of (S)-(-)-warfarin, 0.6 mg/kg, after four daily injections of either saline solution or phenobarbital (75 mg/kg). Phenobarbital treatment increased both the total and intrinsic clearance of (S)-(-)-warfarin almost threefold but did not reduce the coefficient of variation of the intrinsic clearance. Serum protein binding of (S)-(-)-warfarin was not affected by phenobarbital treatment. The biological half-life of warfarin and the duration of its anticoagulant effect were reduced substantially by treatment with phenobarbital. Consistent with pharmacokinetic theory, the relationship between total clearance and the free fraction of warfarin in serum remained approximately linear, but the slope of the regression line was increased for the animals treated with phenobarbital.

The features of hepatic enzyme induction with glutethimide in man

1 Sequential measurements of D-glucaric acid excretion were made in six healthy volunteers before, during and after 3 weeks' daily medication with glutethimide 500 mg. 2 There was a rapid rise in D-glucaric acid excretion within 2 days of starting medication and a rapid decline when it was stopped. 3 Antipyrine clearance and indocyanine green clearance were measured before and at the end of the 3 weeks' medication. 4 There was a 55% increase in antipyrine clearance but no change in indocyanine green clearance. 5 There was no correlation between antipyrine clearance and D-glucaric acid excretion. 6 Glutethimide causes rapid enzyme induction in man without concomitant rise in hepatic blood flow.

GLC determination of warfarin in human plasma

A specific and quantitative GLC method for warfarin in human plasma is described. The procedure uses papaverine as the internal standard and involves a dichloroethane extraction of the acidified specimen. The organic extract is evaporated, and the evaporated extract is dissolved in 50 microliter of chloroform. Aliquots of 2-3 microliter are injected into a gas chromatograph equipped with a flame-ionization detector. The sensitivity of the method is such that 0.3 microgram of intact warfarin can be detected in 1 ml of plasma. Statistical analyses indicate a recovery of 97.26 +/- 1.89% SD. The procedure was successfully applied to plasma drug level studies in humans.

Enzyme induction by drugs. Its relevance to clinical biochemistry

The properties of the hepatic microsomal, drug detoxicating, enzyme system are reviewed with particular reference to its inducibility. Induction is modified by various factors of which the diet is particularly important. A theoretical model system for induction has been proposed and this is discussed. There are a number of methods for assessing microsomal enzyme induction in man, none of which is ideal. Nevertheless, induction is a well recognized phenomenon in man and has a bearing on the metabolism of a number of endogenous substances. The effect of induction on steroid metabolism and the relationship between inducers, vitamin D, and metabolic bone disease is discussed. Bilirubin metabolism is affected by changes in microsomal enzyme activity and inducers have been used therapeutically in some cases of hyperbilirubinaemia. The relationship between drugs and the hepatic porphyrias is reviewed. The hepatic microsomal enzyme system is but one of many inducible enzymes and the role of induction in general in metabolic regulation is emphasised.

Interactions of CNS drugs--hypnotics and sedatives

The physicochemical properties of the barbiturates and a brief review of microsomal enzyme induction introduces this literature review of the interactions of sedative and hypnotic drugs. Food has been shown to delay absorption of barbiturates; barbiturates, in turn, may interfere with the absorption of griseofulvin, dicumarol, and folic acid. Barbiturate-produced enzyme induction may result in interactions with the oral anticoagulants, the anticonvulsants, vitamin D, bilirubin, digitoxin, doxycycline, and perhaps other drugs. The problem of additive central nervous system depression and the relative lack of documented evidence is considered. Finally, the interactions of nonbarbiturate hypnotics, such as glutethimide, chloral hydrate, and others are reviewed.

Drug interactions and clinical pharmacokinetics

Some drugs influence the gastro-intestinal absorption, distribution , metabolism or renal excretion of other drugs, i.e., processes involved in pharmacokinetic interactions. The clinical consequences of pharmacokinetic drug-drug interactions will be either an increase or a decrease in known therapeutic or toxic effects of the interacting drug. In order to evaluate the importance of drug interaction affecting gastro-intestinal absorption, it is necessary to distinguish between interactions which alter the rate of absorption of another drug and those which alter the amount of drug absorbed. Many drugs displace other drugs from their protein binding sites in vitro. This may cause an increase in the pharmacological effect of the displaced drug. However, much discrepancy exists between in vitro findings. In some cases, the enhanced effect only seems to be a temporary phenomenon. The degree of protein binding and the size of apparent volume of distribution (Vd) must also be taken into consideration. Perhaps the importance of interaction involving protein binding has been overemphasized. Barbiturates, glutethimide, rifampicin and phenytoin increase the rate of drug metabolism in man. The most important interactions reported are between oral anti-coagulants and barbiturates. After withdrawal of these hepatic microsomal enzyme inducing drugs, it takes 2 to 3 weeks before the rate of drug metabolism reaches the pretreatment level. In this period, risk of haemorrhage exists. Induction seems to be dose-dependent, but not all persons are inducible. Many drugs compete for the drug metabolising enzyme system in the liver and consequently some drugs inhibit the biotransformation of other drugs. The time course of these interactions depends on the pharmacokinetic properties of the drug involved, and these interactions also seem to be dose-dependent. The most important of such interactions, clinically involved the oral sulphonylurea hypoglycaemic drugs and the antiepilepic drug phenytoin. Drugs are eliminated by urinary excretion through three mechanisms: glomerular filtration, tubular reabsorption, and active tubular secretion. The most important interactions seem to be those involving competition for tubular secretion.

Continued anticoagulation in oral surgery procedures

Anticoagulants are used in a number of clinical settings for the prevention and treatment of thromboembolic disease. The oral surgeon is seeing an increasing number of these anticoagulated patients with oral and dental disease. A technique is presented which minimizes the nonanticoagulated time for a patient requiring an oral surgical procedure. The technique requires hospitalization and should be used only for patients who are in danger of thromboembolism.

The effects of allopurinol and clofibrate on the elimination of coumarin anticoagulants in man

The effects of chronic treatment with allopurinol and clofibrate on the elimination from plasma of warfarin and dicoumarol were examined in man. In addition, the binding of warfarin to human serum proteins was measured with and without clofibrate and its metabolite chlorophenoxyisobutyric acid (CPIB), both present in therapeutic concentrations. Treatment with allopurinol and clofibrate did not alter the rate of elimination of warfarin from plasma. In addition, clofibrate and CPIB caused no significant displacement of warfarin from serum proteins. This evidence supports the conclusion that the clinically significant potentiation of warfarin activity by clofibrate in man is due to an interaction at the receptor site. In contrast, treatment with allopurinol resulted in significant prolongation of the plasma half-life of dicoumarol in one of three subjects. These data are consistent with inhibition of the metabolism of dicoumarol by allopurinol in some individuals.

Clinical implications of warfarin interactions with five sedatives

The intensity, uniformity and time course of anticoagulant interference by phenobarbital, secobarbital, glutethimide, chloral hydrate and methaqualone were systematically investigated in 16 patients receiving coumarin therapy. Each subject received an individualized fixed daily dose of warfarin and served as his own pre- and postsedative treatment control. Prothrombin times were measured four times weekly during five long-term experiments. Anticoagulant inhibition was observed during the administration of phenobarbital, secobarbital and glutethimide; there was no significant change in prothrombin test results during the trials of chloral hydrate and methaqualone. Barbiturates and glutethimide should not be administered to patients receiving coumarin drugs. The concurrent use of drugs from these groups is decreasing according to a survey of 200 hospital medical records. Chloral hydrate and methaqualone interact pharmacologically with orally administered anticoagulant agents, but the effect is not clinically significant. It is concluded that chloral hydrate and methaqualone may be administered safely without additional caution in prothrombin test monitoring during oral anticoagulant therapy.

Abnormal drug metabolism after barbiturate and paracetamol overdose

Drug-metabolizing capacity has been assessed by serial measurements of the plasma antipyrine half life in 11 patients with severe barbiturate intoxication and in 17 patients with acute hepatic necrosis due to paracetamol overdosage. Drug metabolism was strikingly enhanced after barbiturate over-dosage, and this effect was still present six weeks later. In contrast the antipyrine half life was greatly prolonged in patients with paracetamol-induced acute hepatic necrosis but returned to normal or near-normal values within seven to 21 days.

Changes in human drug metabolism after long-term exposure to hypnotics

The influence of the newer, non-barbiturate hypnotics Mandrax (diphenhydramine-methaqualone) and nitrazepam on drug-metabolizing capacity was assessed and compared with the effect of amylobarbitone, a known inducer of drug-metabolizing enzymes. Plasma antipyrine and phenylbutazone half-lives and urinary output of 6beta-hydroxycortisol were used as indices. Volunteer subjects were exposed to therapeutic amounts of these agents and, in the case of Mandrax and barbiturates, further studies were carried out in dependent patients.Mandrax but not nitrazepam increased the rate of drug metabolism, presumably by enzyme induction. The degree of induction was comparable with that produced by hypnotic doses of amylobarbitone. The Mandrax-dependent and barbiturate-dependent patients were the fastest metabolizers studied. It is concluded that drug interactions resulting from interference with drug metabolism are as likely to occur with Mandrax as with barbiturates. On the other hand, it is unlikely that such drug interactions would occur with nitrazepam.

Metabolic interactions among environmental chemicals and drugs

It is evident that metabolic interactions can occur among drugs, insecticides, food additives, carcinogenic hydrocarbons, and a variety of environmental chemicals. A common denominator governing these effects is the versatile nature of the liver microsomal enzymes that metabolize chemicals with diverse structures and biological activities, and the fact that these enzymes can be stimulated or inhibited by other chemicals administered simultaneously. The discovery of these particular enzymes in the 1950's laid the groundwork for the current research on metabolic interactions. Such research provides information that is helpful in the evaluation of the safety and efficacy of drugs and environmental chemicals, and suggests new directions for further research. Some examples are as follows.