Difficulties in anticoagulation management during coadministration of warfarin and rifampin

Management of Prosthetic Mitral Valve Infective Endocarditis in a Patient With Congenital Heart Disease

We present the case of an adult patient with Kartagener's syndrome, multiple prior sternotomies, and recurrent prosthetic valve endocarditis, a scenario without clear guidelines to direct management. Ultimately, the team elected for medical management given the high mortality risk associated with surgery; the patient responded to antibiotic therapy.

Warfarin-Rifampin-Gene (WARIF-G) Interaction: A Retrospective, Genetic, Case-Control Study

Warfarin is extensively metabolized by cytochrome P450 2C9 (CYP2C9). Concomitant use with the potent CYP2C9 inducer, rifampin, requires close monitoring and dosage adjustments. Although, in theory, warfarin dose increase should overcome this interaction, most reported cases over the last 50 years have not responded even to high warfarin doses, but some have responded to modest doses. To investigate the genetic polymorphisms' impact on this unexplained interpatient variability, we performed genotyping of CYP2C9, VKORC1, and CYP4F2 for warfarin and rifampin concomitant receivers from 2016 to 2022 at Hamad Medical Corporation, Doha, Qatar. We identified and included 36 patients: 22 responders and 14 nonresponders. Warfarin-responders were significantly more likely to have one or more warfarin-sensitizing CYP2C9/VKORC1 alleles than nonresponders (odds ratio = 23.2, 95% confidence interval = 3.2-195.6; P = 0.0001). The mean genetic-based pre-interaction calculated dose was significantly lower for responders than for nonresponders (P < 0.001); and was negatively correlated with warfarin sensitivity index (WSI) (r = -0.58; P = 0.0002). The median percentage time in therapeutic range and mean WSI were significantly higher in the warfarin-sensitizing CYP2C9/VKORC1 alleles carriers than noncarriers (P = 0.017 and 0.0004, respectively). Whereas the warfarin-sensitizing CYP2C9/VKORC1 genotypes were associated with modest on-rifampin warfarin dose requirements, the noncarriers would have required more than double these doses to respond. Warfarin-sensitizing CYP2C9/VKORC1 genotypes and low genetic-based warfarin calculated doses were associated with higher warfarin sensitivity and better anticoagulation quality in patients receiving rifampin concomitantly.

Safety and effectiveness of anticoagulation with non-vitamin K antagonist oral anticoagulants and warfarin in patients on tuberculosis treatment

Anti-tuberculosis treatment can cause significant drug-drug interaction and interfere with effective anticoagulation. However, there is a lack of evidence and conflicting data on the optimal oral anticoagulation in patients treated for tuberculosis. We investigated the safety and effectiveness of anticoagulation with non-vitamin K antagonist oral anticoagulants (NOACs) and warfarin in patients on anti-tuberculosis treatment. Patients on concomitant oral anticoagulation and anti-tuberculosis treatment including rifampin were identified from the Korean nationwide healthcare database. Subjects were censored at discontinuation of either anticoagulation or rifampin. The outcomes of interest were major bleeding, death, and ischemic stroke. A total 2090 patients (1153 on warfarin, 937 on NOAC) were included. NOAC users, compared to warfarin users, were older, had a lower prevalence of hypertension, heart failure, ischemic stroke, and aspirin use and a higher prevalence of cancer, with no significant differences in CHA₂DS₂-VASc or HAS-BLED scores. There were 18 major bleeding events, 106 deaths, and 50 stroke events during a mean follow-up of 2.9 months. After multivariable adjustment, the use of NOAC was associated with a lower risk of incident ischemic stroke (HR 0.51, 95% CI 0.27-0.94), while there was no significant difference in risk for major bleeding or death compared with warfarin. These results suggest that NOACs have better effectiveness for stroke prevention and similar safety compared with warfarin in patients on concomitant anti-tuberculosis treatment. This is the first study assessing the safety and effectiveness of NOACs compared to warfarin in this clinical scenario.

Successful Anticoagulation With Warfarin After Switching From Rifampin to Rifabutin

PURPOSE

A case of a patient receiving warfarin for pulmonary embolism (PE) concomitantly with rifampin for treatment of active pulmonary tuberculosis (PTB) is presented. A successful clinical intervention whereby the patient achieved therapeutic anticoagulation after switching to an alternative rifamycin antibacterial, rifabutin, is described.

SUMMARY

The drug-drug interaction between warfarin and rifampin is well known and documented. However, to our knowledge, no case reports of the interaction between warfarin and rifabutin have been published, and literature describing this interaction is lacking. We describe the case of a 27-year-old African American female referred to a pharmacist-managed anticoagulation clinic for treatment of PE with warfarin. The patient was also being treated for active tuberculosis with rifampin, isoniazid, pyrazinamide, and ethambutol. Warfarin was initiated and over the course of 1 month was continuously increased to a total weekly dose (TWD) of 140 mg without ever achieving the target international normalized ratio (INR) of 2 to 3. In an attempt to reach the target INR, rifampin was switched to rifabutin to minimize the drug-drug interaction with warfarin. Six days after this switch, the target INR was achieved with a lower warfarin TWD of 115 mg. Rifabutin interacts with warfarin to a lesser degree than rifampin and may be considered as an alternative in patients taking warfarin who require treatment with a rifamycin.

CONCLUSION

For patients in whom therapeutic anticoagulation with warfarin has been difficult, the use of rifabutin may be considered in place of rifampin when the concomitant use of a rifamycin is required.

A Case of Tuberculosis-related Cerebral Venous Sinus Thrombosis and Pulmonary Thromboembolism Successfully Treated with Edoxaban

A 22-year-old woman was admitted to the hospital with complaints of headache and vomiting. Radiological examinations revealed cerebral sinus venous thromboses, pulmonary thromboembolism, and cavities in the left upper lung. Pulmonary tuberculosis was diagnosed based on sputum and gastric aspirate culture. Heparin followed by warfarin was administered. Anti-tuberculosis agents including rifampicin were also initiated. Since the effect of warfarin did not reach the therapeutic level because of interaction with rifampicin, edoxaban was administered and thromboses were ameliorated. This report illustrates rare thrombotic complications in a TB-induced hypercoagulable state and the potential benefits and safety of edoxaban in combination with rifampicin.

Venous thromboembolic disease in adults admitted to hospital in a setting with a high burden of HIV and TB

BACKGROUND

HIV and tuberculosis (TB) independently cause an increased risk for venous thromboembolic disease (VTE): deep vein thrombosis (DVT) and/or pulmonary embolism (PE). Data from high HIV and TB burden settings describing VTE are scarce. The Wells' DVT and PE scores are widely used but their utility in these settings has not been reported on extensively.

OBJECTIVES

To evaluate new onset VTE, compare clinical characteristics by HIV status, and the presence or absence of TB disease in our setting. We also calculate the Wells' score for all patients.

METHODS

A prospective cohort of adult in-patients with radiologically confirmed VTE were recruited into the study between September 2015 and May 2016. Demographics, presence of TB, HIV status, duration of treatment, CD4 count, viral load, VTE risk factors, and parameters to calculate the Wells' score were collected.

RESULTS

We recruited 100 patients. Most of the patients were HIV-infected (n=59), 39 had TB disease and 32 were HIV/TB co-infected. Most of the patients had DVT only (n=83); 11 had PE, and 6 had both DVT and PE. More than a third of patients on antiretroviral treatment (ART) (43%; n=18/42) were on treatment for <6 months. Half of the patients (51%; n=20/39) were on TB treatment for <1 month. The median (interquartile range (IQR)) DVT and PE Wells' score in all sub-groups was 3.0 (1.0 - 4.0) and 3.0 (2.5 - 4.5), respectively.

CONCLUSION

HIV/TB co-infection appears to confer a risk for VTE, especially early after initiation of ART and/or TB treatment, and therefore requires careful monitoring for VTE and early initiation of thrombo-prophylaxis.

Pulmonary Tuberculosis With Saddle Pulmonary Embolism and Deep Vein Thrombosis: A Rare Case Report

Pulmonary tuberculosis is a common endemic disease in developing countries but its thrombogenic tendency is not well-studied and established yet. Pulmonary embolism is rarely reported in Mycobacterium tuberculosis infection. There are reports stating the relation of pulmonary embolism (PE) and deep vein thrombosis (DVT) with a severe infection of tuberculosis but no data is available to establish a mutual association between pulmonary tuberculosis and pulmonary thromboembolism. Herein, we report the case of a 51-year-old male who presented with a one-month history of productive cough, shortness of breath, and fever associated with chills and night sweating. He reported an 8 kg weight loss in the last month. He was found to have pulmonary tuberculosis. On further investigations for leg swelling and tachycardia. Deep vein thrombosis and sub-massive saddle bilateral pulmonary embolism were diagnosed, which was treated with thrombolysis therapy (alteplase). He responded well to initial therapy and was discharged on anticoagulation with anti-tuberculous therapy (ATT).

Effect of rifampicin on anticoagulation of warfarin: A case report

BACKGROUND

The drug interaction between warfarin and rifampicin is widely known, but there are still some difficulties in managing the combination of the two drugs.

CASE SUMMARY

A patient with brucellosis received strict monitoring from a Chinese pharmacist team during combination of warfarin and rifampicin. The dose of warfarin was increased to 350% in 3 mo before reaching the lower international normalized ratio treatment window. No obvious adverse reaction occurred during the drug-adjustment period. This is the first case report of long-term combined use of rifampicin and warfarin in patients with brucellosis and valve replacement in China based on the Chinese lower warfarin dose and international normalized ratio range.

CONCLUSION

Anticoagulation for valve replacement in Chinese patients differs from that in other races. Establishment of a pharmacist clinic provides vital assistance in warfarin dose adjustment.

Genetic Polymorphism Effect on Warfarin-Rifampin Interaction: A Case Report and Review of Literature

Warfarin-rifampin interaction has been reported since the 1970s. Due to rifampin's strong induction of CYP2C9, most cases could not attain the target international normalized ratio (INR) despite warfarin dose escalation. Genetic polymorphisms determine up to 50% of warfarin dose variability. A 38-year-old woman was started on warfarin and rifampin for cerebral venous sinus thrombosis and pulmonary tuberculosis. Over six weeks, the daily warfarin dose was increased from 3 to 10 mg to attain three consecutive in-clinic therapeutic INRs. She completed three complications-free months of warfarin treatment with time in therapeutic range (TTR) of 46%. We performed retrospective genetic testing to determine the patient's CYP2C9, CYP4F2, and VKORC1 genotypes and whether they had affected the interaction outcome. The analysis revealed that the subject carries CYP2C9*3*3 and VKORC1-1639 (GA) mutations, classifying her as a slow metabolizer and, hence, highly warfarin-sensitive. This was reflected on how the case responded to a relatively lower dose than previously reported cases that did not achieve the target on warfarin daily doses up to 35 mg. This is the first report addressing the genotype effect on this interaction. Patients with genetic variants requiring low warfarin doses are more likely to respond at a feasible dose while on rifampin. Future studies to evaluate warfarin-rifampin-gene interaction are warranted.

A case series of the rifampin-warfarin drug interaction: focus on practical warfarin management

PURPOSE

To provide practical guidance by providing weekly descriptions of warfarin requirements for the onset and offset of the rifampin-warfarin interaction.

METHODS

A retrospective chart review within an outpatient Anticoagulation Clinic (AC). Patients were eligible for the onset phase provided they had known ambulatory-based warfarin steady-state requirements prior to rifampin initiation. For the offset phase, warfarin must be managed by the AC following rifampin discontinuation. Each phase was described separately with warfarin proportionate dose changes (median, IQR) for weeks 1, 2, and 4 as well as the change required to reach warfarin steady state.

RESULTS

Ten patients with 11 courses of warfarin-rifampin were included. For onset, clinicians should anticipate proportionate warfarin dose increases of 30-80% from week 1 to week 2 and a further 20-100% from week 2 to 4, with an overall warfarin dose increase of 165% (IQR 99, 227) to reach steady state at 30 days. For offset, clinicians should anticipate proportionate warfarin dose decreases of 15-25% for both week 1 and 2, and a further 20% for both week 3 and 4, resulting in an overall warfarin decrease of 67% (IQR - 70, - 58) to reach steady state at 4 weeks for most patients.

CONCLUSION

Close monitoring with at least twice weekly INRs for weeks 1 to 2 of both phases is needed to respond to substantially changing warfarin dose requirements. While inter- and intra-patient variability for proportionate warfarin dose changes for both the onset and offset of this drug interaction exists, our data provides general guidance.

Two Strategies for the Dosage of Acenocoumarol Co-Administered with Rifampicin in Staphylococcal Prosthetic Valve Endocarditis

According to current European Society of Cardiology guidelines, for staphylococcal prosthetic valve endocarditis, rifampicin should be one of the drugs used. However, there is a concomitant need for vitamin K antagonists in patients with mechanical prostheses. It is widely known that rifampicin interacts with vitamin K antagonists (VKA), and this interaction makes it difficult to maintain the INR (international normalized ratio) value in the therapeutic range. We present two clinical cases of staphylococcal prosthetic valve endocarditis patients. Two different strategies for dealing with adverse drug interactions have been applied. In the first case, the dose of warfarin was up-titrated until the optimal INR value was obtained. In the second case, due to the history of labile INR values, a decision was made to modify the dosage of warfarin, taking into account pharmacological aspects of drug interactions.

Pharmacoepidemiologic Screening of Potential Oral Anticoagulant Drug Interactions Leading to Thromboembolic Events

Drug-drug interactions (DDIs) with oral anticoagulants may lead to under-anticoagulation and increased risk of thromboembolism. Although warfarin is susceptible to numerous DDIs, few studies have examined DDIs resulting in thromboembolism or those involving direct-acting oral anticoagulants (DOACs). We aimed to identify medications that increase the rate of hospitalization for thromboembolic events when taken concomitantly with oral anticoagulants. We conducted a high-throughput pharmacoepidemiologic screening study using Optum Clinformatics Data Mart, 2000-2016. We performed self-controlled case series studies among adult users of oral anticoagulants (warfarin, dabigatran, rivaroxaban, apixaban, and edoxaban) with at least one hospitalization for a thromboembolic event. Among eligible patients, we identified all oral medications frequently co-prescribed with oral anticoagulants as potential interacting precipitants. Conditional Poisson regression was used to estimate rate ratios comparing precipitant exposed vs. unexposed time for each anticoagulant-precipitant pair. To minimize within-person confounding by indication for the precipitant, we used pravastatin as a negative control object drug. Multiple estimation was adjusted using semi-Bayes shrinkage. We screened 1,622 oral anticoagulant-precipitant drug pairs and identified 226 (14%) drug pairs associated with statistically significantly elevated risk of thromboembolism. Using pravastatin as the negative control object drug, this list was reduced to 69 potential DDI signals for thromboembolism, 33 (48%) of which were not documented in the DDI knowledge databases Lexicomp and/or Micromedex. There were more DDI signals associated with warfarin than DOACs. This study reproduced several previously documented oral anticoagulant DDIs and identified potential DDI signals that deserve to be examined in future etiologic studies.

2019 EACTS Expert Consensus on long-term mechanical circulatory support

Long-term mechanical circulatory support (LT-MCS) is an important treatment modality for patients with severe heart failure. Different devices are available, and many-sometimes contradictory-observations regarding patient selection, surgical techniques, perioperative management and follow-up have been published. With the growing expertise in this field, the European Association for Cardio-Thoracic Surgery (EACTS) recognized a need for a structured multidisciplinary consensus about the approach to patients with LT-MCS. However, the evidence published so far is insufficient to allow for generation of meaningful guidelines complying with EACTS requirements. Instead, the EACTS presents an expert opinion in the LT-MCS field. This expert opinion addresses patient evaluation and preoperative optimization as well as management of cardiac and non-cardiac comorbidities. Further, extensive operative implantation techniques are summarized and evaluated by leading experts, depending on both patient characteristics and device selection. The faculty recognized that postoperative management is multidisciplinary and includes aspects of intensive care unit stay, rehabilitation, ambulatory care, myocardial recovery and end-of-life care and mirrored this fact in this paper. Additionally, the opinions of experts on diagnosis and management of adverse events including bleeding, cerebrovascular accidents and device malfunction are presented. In this expert consensus, the evidence for the complete management from patient selection to end-of-life care is carefully reviewed with the aim of guiding clinicians in optimizing management of patients considered for or supported by an LT-MCS device.

Diffuse Alveolar Hemorrhage Caused by Warfarin after Rifampicin Discontinuation

An 83-year-old man under warfarin therapy presented for assessment of prolonged prothrombin time and cough. High-resolution computed tomography findings of the chest showed diffuse alveolar hemorrhage. His international normalized ratio (INR) was 11.89. He had been treated with rifampicin for a persistent infection, but this had been discontinued about two months before admission. Rifampicin suppresses the anticoagulant activity of warfarin, which can lead to a need for increased doses of warfarin to achieve and maintain a therapeutic INR. More frequent INR monitoring is needed even after discontinuing rifampicin.

Thromboembolism in Mycobacterium tuberculosis Infection: Analysis and Literature Review

Background

Tuberculosis is associated with hypercoagulation; however, there are few reports of cases thromboembolism and tuberculosis at the same time in the real world. The purpose of this study was to report the incidence and clinical course of thromboembolism in patients diagnosed with tuberculosis.

Materials and Methods

We retrospectively analyzed the data of patients who were diagnosed with both tuberculosis and thromboembolism including pulmonary thromboembolism (PTE) or deep vein thrombosis (DVT) at Seoul National University Boramae Medical Center from January 2000 through March 2015.

Results

Among the 7905 tuberculosis patients, 49 (0.6%) exhibited PTE, DVT, or both at or after the time of tuberculosis diagnosis. All patients treated for tuberculosis started with isoniazid, ethambutol, rifampicin, and pyrazinamide. Eight patients were switched to treatment with second-line medication because of resistance or adverse events. About half of the patients (n = 21, 44.7%) had thrombosis at the time of tuberculosis diagnosis. Of 48 patients treated for thromboembolism, 36 received warfarin. A total of 20 patients improved symptom caused by thrombosis, and 10 patients were confirmed cure by image study such as computed tomography or doppler ultrasonography. Eight patients who were treated with warfarin had persistent thrombosis. Five patients (10.2%) experienced major bleeding that required hospitalization. All of these bleeding events were associated with warfarin therapy.

Conclusions

Careful attention to PTE/DVT is needed at the time of diagnosis of tuberculosis and during anti-tuberculosis therapy. Warfarin therapy administered with anti-tuberculosis medication requires frequent monitoring to prevent major bleeding.

Drug-drug interactions in an era of multiple anticoagulants: a focus on clinically relevant drug interactions

Oral anticoagulants are commonly prescribed but high risk to cause adverse events. Skilled drug interaction management is essential to ensure safe and effective use of these therapies. Clinically relevant interactions with warfarin include drugs that modify cytochrome 2C9, 3A4, or both. Drugs that modify p-glycoprotein may interact with all direct oral anticoagulants, and modifiers of cytochrome 3A4 may interact with rivaroxaban and apixaban. Antiplatelet agents, nonsteroidal anti-inflammatory drugs, and serotonergic agents, such as selective serotonin reuptake inhibitors, can increase risk of bleeding when combined with any oral anticoagulant, and concomitant use should be routinely assessed. New data on anticoagulant drug interactions are available almost daily, and therefore, it is vital that clinicians regularly search interaction databases and the literature for updated management strategies. Skilled drug interaction management will improve outcomes and prevent adverse events in patients taking oral anticoagulants.

Drug-drug interactions in an era of multiple anticoagulants: a focus on clinically relevant drug interactions

Oral anticoagulants are commonly prescribed but high risk to cause adverse events. Skilled drug interaction management is essential to ensure safe and effective use of these therapies. Clinically relevant interactions with warfarin include drugs that modify cytochrome 2C9, 3A4, or both. Drugs that modify p-glycoprotein may interact with all direct oral anticoagulants, and modifiers of cytochrome 3A4 may interact with rivaroxaban and apixaban. Antiplatelet agents, nonsteroidal anti-inflammatory drugs, and serotonergic agents, such as selective serotonin reuptake inhibitors, can increase risk of bleeding when combined with any oral anticoagulant, and concomitant use should be routinely assessed. New data on anticoagulant drug interactions are available almost daily, and therefore, it is vital that clinicians regularly search interaction databases and the literature for updated management strategies. Skilled drug interaction management will improve outcomes and prevent adverse events in patients taking oral anticoagulants.

Warfarin-Rifampin Drug Interaction in a Pediatric Patient

Rifampin has been documented to significantly attenuate the effect of warfarin in adult patients. No data have been presented on the use of rifampin and warfarin in a pediatric patient. We report an extreme case of increased warfarin metabolism in a pediatric patient who was concomitantly receiving rifampin, despite receiving other medications that significantly decrease warfarin metabolism. The inhibitory effect of rifampin on warfarin therapy may be amplified in pediatric patients.

Induction of human cytochrome P450 3A4 by the irreversible myeloperoxidase inactivator PF-06282999 is mediated by the pregnane X receptor

1. 2-(6-(5-Chloro-2-methoxyphenyl)-4-oxo-2-thioxo-3,4-dihydropyrimidin-1(2H)-yl) acetamide (PF-06282999) is a member of the thiouracil class of irreversible inactivators of human myeloperoxidase enzyme and a candidate for the treatment of cardiovascular disease. PF-06282999 is an inducer of CYP3A4 mRNA and midazolam-1'-hydroxylase activity in human hepatocytes, which is consistent with PF-06282999-dose dependent decreases in mean maximal plasma concentrations (C_max) and area under the plasma concentration time curve (AUC) of midazolam in humans following 14-day treatment with PF-06282999. 2. In the present study, the biochemical mechanism(s) of CYP3A4 induction by PF-06282999 was studied. Incubations in reporter cells indicated that PF-06282999 selectively activated human pregnane X receptor (PXR). Treatment of human HepaRG cells with PF-06282999 led to ∼14-fold induction in CYP3A4 mRNA and 5-fold increase in midazolam-1'-hydroxylase activity, which was nullified in PXR-knock out HepaRG cells. TaqMan® gene expression analysis of human hepatocytes treated with PF-06282999 and the prototypical PXR agonist rifampin demonstrated increases in mRNA for CYP3A4 and related CYPs that are regulated by PXR. 3. Docking studies using a published human PXR crystal structure provided insights into the molecular basis for PXR activation by PF-06282999. Implementation of PXR transactivation assays in a follow-on discovery campaign should aid in the identification of back-up compounds devoid of PXR activation and CYP3A4 induction liability.

Challenges in achieving a target international normalized ratio for deep vein thrombosis among HIV-infected patients with tuberculosis: a case series

Background

Tuberculosis (TB) and HIV are among the risk factors for deep vein thrombosis (DVT). There are several challenges in the management of DVT patients with TB-HIV co-infection including drug-drug interactions and non-adherence due to pill burden.

Methods

HIV infected patients starting treatment for TB were identified and followed up two weekly. Cases of DVT were diagnosed with Doppler ultrasound and patients were initiated on oral anticoagulation with warfarin and followed up with repeated INR measurements and warfarin dose adjustment.

Results

We describe 7 cases of TB and HIV-infected patients in Uganda diagnosed with DVT and started on anticoagulation therapy. Their median age was 30 (IQR: 27–39) years and 86 % were male. All patients had co-medication with cotrimoxazole, tenofovir, lamivudine and efavirenz and some were on fluconazole. The therapeutic range of the International Normalization Ratio (INR) was difficult to attain and unpredictable with some patients being under-anticoagulated and others over-anticoagulated. The mean Time in Therapeutic Range (TTR) for patients who had all scheduled INR measurements in the first 12 weeks was 33.3 %. Only one patient among those with all the scheduled INR measurements had achieved a therapeutic INR by 2 weeks. Four out of seven (57 %) of the patients had at least one INR above the therapeutic range which required treatment interruption. None of the patients had major bleeding.

Conclusion

We recommend more frequent monitoring and timely dose adjustment of the INR, as well as studies on alternative strategies for the treatment of DVT in TB-HIV co-infected patients.

Rifampin-warfarin interaction in a mitral valve replacement patient receiving rifampin for infective endocarditis: a case report

INTRODUCTION

Warfarin therapy is associated with many drug interactions that may cause a significant alteration in its anticoagulant effect. Rifampin is a widely used antimicrobial that has major interactions with several medications including warfarin due to its strong P-glycoprotein and liver enzyme inducer activity especially on CYP2C9, CYP3A4, CYP1A2 and CYP2C19.

PRESENTATION

We report a case of a 34-year-old Srilankan female chronically treated with warfarin for her mitral valve replacement. The patient developed infective endocarditis and was started on a 6-week treatment with rifampin along with other antibiotics. Warfarin dose was increased from 52.5 to 210 mg/week over the course of the rifampin therapy, however, the INR remained subtherapeutic throughout the whole period and reached 2.4 by the end of rifampin therapy.

DISCUSSION AND EVALUATION

Anticoagulation management was challenging in the period following the end of rifampin therapy as well, and multiple dose adjustments starting with an increase and followed by reduction were required till she was stable on an 80 mg/week warfarin dose 5 weeks post-rifampin therapy.

CONCLUSION

Our findings suggest the importance of close monitoring of warfarin therapy during and after the use of rifampin to minimize the risks of under and over-anticoagulation and improve the safety and efficacy of warfarin therapy.

Potential drug interaction between warfarin and linezolid

OBJECTIVE

Warfarin is known to interact with many drugs; however, there are currently no descriptions of an interaction with linezolid in the literature. It was recently brought to our attention, however, that several warfarin-medicated patients have experienced an increase in the prothrombin time international normalized ratio (PT-INR) following the administration of linezolid. We therefore performed a retrospective survey in order to investigate the possibility of an interaction between warfarin and linezolid.

METHODS

The survey items included age, gender, underlying disease, type of surgery, type of infectious disease, duration of linezolid administration, laboratory values and the dose of warfarin. The PT-INR was observed over time before treatment and at days 4 or 5 and 10, completion and one week after the end of concomitant therapy. Patients The subjects included six patients who were recovering from recent heart-related surgery.

RESULTS

The PT-INR increased from 1.62±0.32 before concomitant linezolid administration to 3.00±0.83 at day 4 or 5 after concomitant administration (p<0.01) and significantly decreased from 1.65±0.45 at the completion of the regimen to 1.26±0.1 one week later (p<0.05). With respect to the relationship between the dose of warfarin and the PT-INR in five cases, the PT-INR increased following concomitant linezolid treatment in all cases.

CONCLUSION

Although it has been reported that linezolid does not influence the metabolism or protein binding of warfarin, our data showed potential drug interactions between warfarin and linezolid. Our data suggest that PT-INR monitoring after the completion of concomitant warfarin and linezolid therapy is important.

Describing the profile of patients on concurrent rifampin and warfarin therapy in western Kenya: a case series

Background

Rifampicin’s ability to induce hepatic enzymes is responsible for causing a clinically significant drug interaction with warfarin. Little data exists to guide clinicians on managing this interaction, especially in Sub-Saharan Africa where many patients are exposed to this combination due to a higher burden of tuberculosis.

Objective

The objective of the case series is to provide insight to practicing clinicians of the unique dynamics of this drug interaction in resource-constrained settings. The case series will provide details on commonly encountered scenarios and the dosage adjustments required to maintain a therapeutic INR.

Methods

A retrospective chart review was conducted of patients attending the Moi Teaching and Referral Hospital anticoagulation clinic in Eldoret, Kenya. Patients were included if they had a history of concurrent rifampicin and warfarin therapy and a minimum follow up of 2 months. Descriptive statistics were used to explain the demographic characteristics, time to therapeutic INR and average weekly warfarin dose. The inference on proportions test was conducted to compare the time in the therapeutic range (TTR) for patients on concurrent rifampicin to the rest of the patients not receiving rifampicin in the clinic.

Results

Of the 350 patient charts evaluated, 10 met the inclusion criteria. The median percentage increase of the weekly warfarin dose from baseline was 15.7 %. For the patients in this analysis, the median TTR was 47 %.

Discussion

Patients on concurrent therapy should be rigorously monitored with regular INR checks and warfarin dosage adjustments. Empiric dosage adjustments of warfarin should be avoided but patient characteristics can aid in understanding the alterations seen in INR.

Rifampicin-warfarin interaction leading to macroscopic hematuria: a case report and review of the literature

Background

Rifampicin remains one of the first-line drugs used in tuberculosis therapy. This drug´s potential to induce the hepatic cytochrome P450 oxidative enzyme system increases the risk of drug-drug interactions. Thus, although the presence of comorbidities typically necessitates the use of multiple drugs, the co-administration of rifampicin and warfarin may lead to adverse drug events. We report a bleeding episode after termination of the co-administration of rifampicin and warfarin and detail the challenges related to international normalized ratio (INR) monitoring.

Case presentation

A 59-year-old Brazilian woman chronically treated with warfarin for atrial fibrillation (therapeutic INR range: 2.0-3.0) was referred to a multidisciplinary anticoagulation clinic at a university hospital. She showed anticoagulation resistance at the beginning of rifampicin therapy, as demonstrated by repeated subtherapeutic INR values. Three months of sequential increases in the warfarin dosage were necessary to reach a therapeutic INR, and frequent visits to the anticoagulation clinic were needed to educate the patient about her pharmacotherapy and to perform the warfarin dosage adjustments. The warfarin dosage also had to be doubled at the beginning of rifampicin therapy. However, four weeks after rifampicin discontinuation, an excessively high INR was observed (7.22), with three-day macroscopic hematuria and the need for an immediate reduction in the warfarin dosage. A therapeutic and stable INR was eventually attained at 50% of the warfarin dosage used by the patient during tuberculosis therapy.

Conclusions

The present case exemplifies the influence of rifampicin therapy on warfarin dosage requirements and the increased risk of bleeding after rifampicin discontinuation. Additionally, this case highlights the need for warfarin weekly monitoring after stopping rifampicin until the maintenance dose of warfarin has decreased to the amount administered before rifampicin use. In particular, patients with cardiovascular diseases and active tuberculosis represent a group with a substantial risk of drug-drug interactions. Learning how to predict and monitor drug-drug interactions may help reduce the incidence of clinically significant adverse drug events.

Absence of a Pharmacokinetic Interaction between Etanercept and Warfarin

Etanercept, a soluble recombinant human tumor necrosis factor receptor (TNFr) fusion protein, is effective and well tolerated in the treatment of rheumatoid arthritis (RA), juvenile rheumatoid arthritis (JRA), psoriatic arthritis (PsA), and ankylosing spondylitis (AS). The primary objective of this study was to investigate the potential pharmacokinetic and pharmacodynamic interaction between a single dose of R- and S-enantiomers of warfarin and multiple doses of etanercept after administration of warfarin and etanercept alone and together. In a nonrandomized, three-period study, 12 healthy male subjects received a single oral 25-mg dose of warfarin after an overnight fast, followed by twice-weekly 25-mg subcutaneous doses of etanercept for seven doses. The last dose of etanercept was administered concurrently with a second dose of warfarin. Serial blood samples for plasma warfarin concentration measurement and international normalized ratio (INR) assessment were collected before and up to 144 hours after dose administration. Serial blood samples for serum etanercept concentration measurement were collected before and up to 60 hours after the sixth dose and 264 hours after the seventh dose. Etanercept did not affect the pharmacokinetics and pharmacodynamics of warfarin. All ratios of maximum serum concentration (C(max)) and area under the serum concentration versus time curve (AUC) for pharmacokinetics (R- and S-enantiomers of warfarin) and INR fell within the confidence interval of 0.8 to 1.25. Warfarin also did not cause a clinically significant alteration in the pharmacokinetics of etanercept. In conclusion, coadministration of etanercept and warfarin would not be expected to change the pharmacokinetics of either medication; therefore, no dosage adjustment is needed in cases in which warfarin and etanercept are coadministered.

Methods for the quantitative evaluation and prediction of CYP enzyme induction using human in vitro systems

IMPORTANCE OF THE FIELD

For successful drug development, it is important to investigate the potency of candidate drugs causing drug-drug interactions (DDI) during the early stages of development. The most common mechanisms of DDIs are the inhibition and induction of CYP enzymes. Therefore, it is important to develop co.mpounds with lower potencies for CYP enzyme induction.

AREAS COVERED IN THIS REVIEW

The aim of the present paper is to present an overview of the current knowledge of CYP induction mechanisms, particularly focusing on the transcriptional gene activation mediated by pregnane X receptor, aryl hydrocarbon receptor and constitutive androstane receptor. The adoptable options of in vitro assay methods for evaluating CYP induction are also summarized. Finally, we introduce a method for the quantitative prediction of CYP3A4 induction considering the turnover of CYP3A4 mRNA and protein in hepatocytes based on the data obtained from a reporter gene assay.

WHAT THE READER WILL GAIN

In order to predict in vivo CYP enzyme induction quantitatively based on in vitro information, an understanding of the physiological induction mechanisms and the features of each in vitro assay system is essential. We also present the estimation method of in vivo CYP induction potency of each compound based on the in vitro data which are routinely obtained but not necessarily utilized maximally in pharmaceutical companies.

TAKE HOME MESSAGE

It is desirable to select compounds with lower potencies for the inductive effect. For this purpose, an accurate prioritization procedure to evaluate the induction potency of each compound in a quantitative manner considering the pharmacologically effective concentration of each compound is necessary.

Performance outcomes of a pharmacist-managed anticoagulation clinic in the rural, resource-constrained setting of Eldoret, Kenya

BACKGROUND

It is recommended that warfarin therapy should be managed through an anticoagulation monitoring service to minimize the risk of bleeding and subsequent thromboembolic events. There are few studies in Sub-Saharan Africa that describe warfarin management in spite of the high incidence of venous thromboembolism (VTE) and rheumatic heart disease.

OBJECTIVE

To examine the feasibility of the Moi Teaching and Referral Hospital anticoagulation monitoring service and compare its performance with clinics in resource-rich settings.

METHODS

A retrospective chart review compared the percentage time in the therapeutic range (TTR) and rates of bleeding and thromboembolic events to published performance targets using the inference on proportions test. Wilcoxon's rank sum analyses were used to establish predictors of TTR.

RESULTS

For the 178 patients enrolled, the mean TTR was 64.6% whereas the rates of major bleeds and thromboembolic events per year were 1.25% and 5%, respectively. In the primary analysis, no statistically significant differences were found between the results of TTR, major bleeds and thromboembolic events for the clinic and published performance rates. In the secondary analysis, having an artificial heart valve and a duration of follow-up of > 120 days were positively associated with a higher TTR (P < 0.05) whereas venous thromboembolism, history of tuberculosis, HIV and a duration of follow-up of < 120 days were associated with having a lower TTR (P < 0.05).

CONCLUSIONS

The performance of the MTRH anticoagulation clinic is non-inferior to published metrics on the performance of clinics in resource-rich settings.

Optimizing use of current anticoagulants

Although warfarin has been the mainstay of oral anticoagulation therapy for decades, evidence-based methods for improving the quality of warfarin therapy remain underused. The arrival of new anticoagulants that do not require routine laboratory monitoring and lack the significant dietary and drug interaction potential that are seen with warfarin is an important evolutionary step in the management of thromboembolic disease. However, it will be years before the efficacy and long-term safety of these new agents are defined. Newer oral anticoagulants will be more expensive than generic warfarin. This article examines various approaches to optimize the clinical use of warfarin. For patients able to achieve stable anticoagulation control, warfarin remains an important therapeutic option, delivering similar clinical outcomes at a fraction of the cost to the health care system.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Drug Interactions Between Antibiotics and Select Maintenance Medications: Seeing More Clearly Through the Narrow Therapeutic Window of Opportunity

OBJECTIVE

Infections often occur while treating patients with long-term medications for chronic illnesses. Treating these infections with systemic antibiotics often leads to pharmacokinetic and pharmacodynamic interactions between the antimicrobials and one or more of the maintenance medications. Previously optimized long-term regimens may become either subtherapeutic or super-therapeutic, with deleterious consequences. This article discusses some of the most significant and commonly encountered antibiotic drug interactions that may occur with medications with "narrow therapeutic windows" including warfarin, phenytoin, carbamazepine, theophylline, and the two calcineurin inhibitors. Given the logistics of many consultant pharmacists' practices, it may not always be possible for them to react prospectively when these combinations are prescribed at their facilities. Therefore, there are several things the pharmacist can do: provide regular and comprehensive inservice raining on this topic, be available as needed to answer patient-specific questions, and provide readily available charts and other educational materials that help identify and characterize these important interactions.

DATA SOURCES

A Medline search of the English literature was performed in October/November 2003, going back to 1980 for the commonly used antibiotics and drug interactions stated in this text. In some cases, cross referencing of articles reviewed also led to older publications. Textbooks dealing with drug interactions also were used as initial sources. However, whenever possible, any data quoted within the text were verified from the original research paper.

STUDY SELECTION

Pharmacokinetic studies, case reports, and general review articles published in the English medical literature were all selected for review. In cases where review articles were cited that summarize groups of data from previous original research papers, the authors made the best possible effort to verify the accuracy by referring to the original research papers.

DATA SYNTHESIS

Because of the breadth of the topic in terms of all the antibiotics discussed, the interacting medications that pertained to each antibiotic, and the lack of homogeneity among the various types of papers (most of which were case reports), most analyses include broad-based summaries based on the aggregate findings of the authors.

CONCLUSION

The addition of antibiotics to a stabilized medical regimen can result in either potentiation or antagonism of the clinical effects of narrow-therapeutic-window medications such as warfarin, phenytoin, theophylline, calcineurin inhibitors, carbamazepine, and numerous other agents. As usual in the clinical arena, awareness is the first step in appropriate management of these encounters.

Update on the interaction of rifampin and warfarin

A 79-year-old man with a history of deep vein thrombosis and pulmonary embolism received anticoagulation therapy with warfarin 5 mg daily for 8 months. He was diagnosed with osteomyelitis and underwent partial metatarsal resection of his right foot. After surgery, antibiotics were initiated, including ertapenem sodium 1 g intravenously every 24 hours, vancomycin 1400 mg intravenously every 24 hours, and rifampin 300 mg by mouth twice daily. Achieving a therapeutic level of anticoagulation was difficult despite escalating doses of warfarin, because of the interaction with rifampin. A 5- to 6-fold increase in warfarin dose was prescribed to reach therapeutic international normalized ratios (INRs), but even these increases were insufficient to maintain his INR in the therapeutic range. After rifampin was discontinued, warfarin doses were gradually reduced over the next 2 months. When concurrent warfarin-rifampin therapy is necessary, vigilant monitoring is imperative and significant increases in warfarin doses are likely.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

Roles of rifampicin in drug-drug interactions: underlying molecular mechanisms involving the nuclear pregnane X receptor

Rifampicin, an important drug in the treatment of tuberculosis, is used extensively despite its broad effects on drug-drug interactions, creating serious problems. The clinical importance of such interactions includes autoinduction leading to suboptimal or failed treatment. The concomitantly administered effects of rifampicin on other drugs can result in their altered metabolism or transportation that are metabolised by cytochromes P450 or transported by p-glycoprotein in the gastrointestinal tract and liver. This review paper summarises recent findings with emphases on the molecular mechanisms used to explain these broad drug-drug interactions. In general, rifampicin can act on a pattern: rifampicin activates the nuclear pregnane X receptor that in turn affects cytochromes P450, glucuronosyltransferases and p-glycoprotein activities. This pattern of action may explain many of the rifampicin inducing drug-drug interactions. However, effects through other mechanisms have also been reported and these make any explanation of such drug-drug interactions more complex.

Bosentan and warfarin interaction

OBJECTIVE

To report a case of decreased international normalized ratio (INR) in a patient receiving warfarin and bosentan.

CASE SUMMARY

A 35-year-old African American woman with a history of primary pulmonary hypertension managed with warfarin, diltiazem, and hydrochlorothiazide was initiated on bosentan therapy. The patient's INR had been stable and within therapeutic range (goal 2.0-3.0) for the previous 3 months with warfarin 27.5 mg/wk, but became subtherapeutic after 10 days of bosentan therapy. Addition of over-the-counter medications, herbal products, vitamins, or dietary changes was denied. The INR remained subtherapeutic for 5 weeks despite weekly warfarin dose increases. After these 5 weeks of dosage increases, the INR became supratherapeutic for 3 weeks, resulting in a subsequent dosage decrease. The resultant warfarin dose required to maintain a therapeutic INR was 45 mg/wk, a 63.6% dosage increase after the initiation of bosentan.

DISCUSSION

This case shows that a clinically significant interaction between bosentan and warfarin may exist. An objective causality assessment revealed that the interaction was probable. Although the possibility of this interaction has been noted, no previously documented occurrence of this interaction has been identified.

CONCLUSIONS

Bosentan may significantly decrease the anticoagulant properties of warfarin. The INR should be monitored more frequently when bosentan is initiated, adjusted, or discontinued in patients taking warfarin.