Interaction of commonly prescribed drugs and warfarin

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.

Induction of liver cytochrome P-450 in mice by warfarin. Comparison of warfarin-, phenobarbitone-, and cobalt-induced hepatic microsomal protein patterns by PAGE after partial purification on octyl-sepharose CL-4B

A rapid method is presented to separate mouse liver cytochrome P-450 from other components of the microsomal monooxygenase system and to increase specific activity by hydrophobic interaction chromatography on Octyl-Sepharose CL-4B by a factor of between 3.8 and 5.3. In addition it is shown that varieties of cytochrome P-450 can be separated from each other by Octyl-Sepharose CL-4B. After oral applications of 120 mg/kg warfarin once daily for three days SDS-PAGE analysis of the partially purified cytochrome P-450 fraction revealed a protein pattern in the 50 Kd region that is practically indistinguishable from that after conventional phenobarbitone pretreatment. On the other hand, cobalt pretreatment results in a different pattern that is distinguished from that of normals as well as from that of phenobarbitone- and warfarin-pretreated mice. From these results in conjunction with the previous finding of increased drug metabolic activity after warfarin pretreatment it is concluded that warfarin elicits phenobarbitone-like induction of the hepatic monooxygenases in mice.

Trauma and oral anticoagulants

Patients on oral anticoagulants who present to the emergency department following trauma pose a special problem. Careful attention must be given to exclude any sites of overt or occult bleeding, particularly in the CNS, where there may be no external evidence of injury. A PT or thrombotest should be obtained as minimum laboratory workup in all cases. In specific situations, emergency reversal of anticoagulation must be undertaken.

[The influence of diazepam (valium) on the effect of phenprocoumon (marcumar) (author's transl)]

In 10 patients who were treated with phenprocoumon (Marcumar) for thrombembolic diseases, we examined the influence of the combined therapy with diacepam (Valium) on the effect of the anticoagulant. After stopping the therapy with diacepam we saw a significant increase of the prothrombinetime (Quick) beyond the therapeutic level of 25%. By increasing the dosis of phenprocoumon the therapeutic level was achieved again not until after 8 days. The dosis needed for maintaining this level was on the average 32% higher than during the time of the therapy in which diacepam was used. Because of the increased thrombembolic risk, when going beyond the therapeutic level of the prothrombinetime, this interaction is of clinical importance.

The effect of furosemide and bumetanide on warfarin metabolism and anticoagulant response

In a study of 11 normal subjects who received either furosemide or bumetanide, response to warfarin was unaffected by either diuretic. The mean prothrombin times after a single oral dose of 50 mg warfarin were 18.8 +/- 0.7 sec before and 19.6 +/- 1.7 sec during furosemide administration, and 18.3 +/- 1.4 sec before and 19.7 +/- 1.3 sec during bumetanide administration. Mean plasma warfarin half-lives were 43.2 +/- 2.8 hr before and 37.8 +/- 1.7 hr during furosemide treatment and 39.9 +/- 9.5 hr before and 37.4 +/- 2.4 hr during bumetanide treatment. Hypoprothrombinemic responses, plasma warfarin concentrations, and warfarin half-lives were not significantly influenced by treatment with either diuretic. Both loop diuretics furosemide and bumetanide can be safely administered during warfarin anticoagulant therapy without risk of clinically significant interaction.

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.

Clinical experience with an oral anticoagulant in children

Nineteen cardiac patients were given warfarin sodium to prevent thrombosis and embolism. Empirical dosage resulted in long hospital stays to obtain a stable therapeutic prothrombin time. Using a dosage of 0 therefore 5-0 therefore 7 mg/kg, therapeutic levels were achieved in a much shorter time. When prothrombin time was excessive, it was successfully reduced with vitamin K or plasma. A haematoma in one case and haematuria in another were easily controlled. Warfarin sodium appears to be appropriate and effective for children requiring anticoagulants within the above dosage limits and with meticulous follow-up.

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.

N-desmethyldiazepam and amylobarbitone sodium as hypnotics in anxious patients. Plasma levels, clinical efficacy and residual effects

Drug plasma levels of N-desmethyldiazepam (10 and 20 mg/day given at night) and amylobarbitone sodium (200 mg/day given at night) were determined in 45 patients after 1 and 7 days of treatment together with estimates of hypnotic and residual effects. No relationships were found between clinical rati ngs of hypnotic effect and drug plasma levels, while significant correlations were observed between some performance taska and drug plasma concentrations of amylobarbitone sodium, and of the higher dose of N-desmethyldiazepam.

Drug interactions in patients on long-term oral anticoagulant and antihypertensive adrenergic neuron-blocking drugs

The occurrence of drug interactions was studied in outpatients on anticoagulant or adrenergic neuron-blocking drugs. In the anticoagulant group (254 patients) a theoretical potential drug interaction with warfarin was noted in 33% of patients with drugs supplied by prescription and in 30% of patients with drugs taken as self-medication, though a probable adverse drug interaction was documented in only 9% of patients. The deficiency in the transfer of information about current drug therapy and the lack of awareness of patients about the hazards of anticoagulant therapy was clear.In the 64 patients on the adrenergic neuron-blocking drugs guanethidine, bethanidine, or debrisoquine a theoretical potential drug interaction was noted in 34%, but a probable adverse drug interaction was documented in only three patients. Further studies are required of other well-recognized groups of patients on drugs which are known to interact to assess the relevance and clinical importance of the formidable lists of interactions which are now available to doctors who prescribe drugs.

Interactions of benzodiazepines with warfarin

Administration of nitrazepam (10 mg nightly), diazepam (15 mg/day), and chlordiazepoxide (15 and 30 mg/day) had no effect on steady-state plasma warfarin concentrations, the plasma half-life of warfarin, or anticoagulant control in patients and it appears safe to prescribe these agents to patients on long-term oral anticoagulants. Urinary excretion of 6 beta-hydroxycortisol, however, increased in two out of five patients given chlordiazepoxide. In rats pretreatment with chlordiazepoxide (40 mg/kg for four days) caused stimulation of liver microsomal enzyme activity but neither diazepam nor nitrazepam had this effect.

Psychopharmacologic management of anxiety in the cardiac patient

Of 253 patients who had suffered acute myocardial infarction ( MI), 88 percent received daytime sedation while hospitalized. Sixty percent of all prescriptions were for phenobarbital, 30 percent for chlordiazepoxide, and 6 percent for meprobamate. In addition, 79 percent of patients received night time hypnotic medication. Sixty-six percent of such prescriptions were for short-acting barbiturates, 21 percent for chloral hydrate, and 7 percent for diphenhydramine. Multiple drug use was common.

The barbiturates, while inexpensive sedative agents, are of questionable efficacy in reducing anxiety and probably produce only general CNS depression. A narrow margin of safety, liability to produce habituation, and antagonism of warfarin-type anticoagulants are other disadvantages. The benzodiazepine tranquilizers, whose antianxiety efficacy is more clearly established, do not produce general CNS depression, have a wide margin of safety, and do not interact with oral anticoagulants.

Antidepressant agents, phenothiazine tranquilizers, and propranolol were used infrequently or not at all. The risks associated with the use of these agents in patients with acute myocardial infarction are considerable and should be weighed carefully before they are administered.