[Switching from acenocoumarol to phenprocoumon: step in personalised anticoagulation?]

Acenocoumarol and phenprocoumon are vitamin K antagonists (VKA) with average half-lives of 11 hours and 160 hours, respectively. They are used to treat and prevent thrombosis in mechanical cardiac valve replacement, atrial fibrillation and venous thromboembolism. There are historical regional differences in preferred VKA in the Netherlands. Safe and effective treatment requires the international normalized ratios (INRs) to be in the therapeutic range, and stable. Theoretically, the longer-acting phenprocoumon would yield a higher time in therapeutic range (TTR) and lower INR variability. In practice, switching from acenocoumarol to phenprocoumon eventually improves INR variability and in some patients TTR as well. However, during the preceding transition period, INRs are more often volatile and supratherapeutic. Furthermore, switching to an alternative VKA could weaken integrated care, as other healthcare providers are less experienced with it. Healthcare providers must coordinate an intended switch with the anticoagulation clinic.

The effect of different oral anticoagulants on diphenylhydantoin (DPH) and tolbutamide metabolism

The effect of bishydroxycoumarin, phenprocoumon, warfarin and phenindione on the metabolism of diphenylhydantoin (DPH) and tolbutamide has been studied in 54 patients. The half-lives of DPH and tolbutamide in blood following i.v. injections were studied in 33 patients before and after one week of anticoagulant treatment. Bishydroxycoumarin increased the mean half-life values of DPH from 8.8 to 37.4 hours and of tolbutamide from 4.9 to 17.5. Phenprocoumon prolonged DPH half-life from a mean value of 9.9 to 14.0 hours but did not change the tolbutamide half-life. Warfarin and phenindione did not affect DPH or tolbutamide half-lives. Steady state concentration studies in 21 patients showed a rise in serum DPH during bishydroxycoumarin and phenprocoumon treatment but not during treatment with warfarin and phenindione. A rise in serum tolbutamide was noted during treatment with bishydroxycoumarin. These findings suggest that bishydroxycoumarin inhibits the betabolism of DPH and tolbutamide and that phenprocoumon inhibits DPH metabolism. No effect on DPH and tolbutamide metabolism could be demonstrated following administration of warfarin and phenindione.

Genetic polymorphisms of cytochrome P450 2C9 causing reduced phenprocoumon (S)-7-hydroxylationin vitroandin vivo

The effect of cytochrome P450 (CYP) 2C9 polymorphisms on the stereoselective biotransformation of the oral anticoagulant phenprocoumon (PPC) to inactive, monohydroxylated metabolites was studied in vitro and in vivo. In human liver microsomes, the (S)-7-hydroxylation--being the major metabolic pathway--was significantly compromised in a gene-dose-dependent manner in samples expressing the CYP2C9*2 or CYP2C9*3 allele. The CYP2C9*3/*3 genotype corresponded to an almost fourfold lower (S)-7-hydroxylation rate than CYP2C9*1/*1 (wild-type). The intrinsic clearance of human recombinant CYP2C9*2 and CYP2C9*3 for the (S)-7-hydroxylation was 28.9 and 50.9% lower than of CYP2C9*1, respectively. The area under the plasma concentration-time curve (AUC) of PPC metabolites after oral intake of 12 mg racemic PPC was significantly lower in volunteers expressing the CYP2C9*2 or CYP2C9*3 allele. Increasing plasma AUC metabolic ratios (parent compound/metabolite) in CYP2C9*2 and CYP2C9*3 variant allele carriers were found for each hydroxylation reaction and the CYP2C9*3/*3 genotype corresponded to an about 10-fold higher metabolic ratio of PPC (S)-7-hydroxylation relative to CYP2C9*1/*1. CYP2C9 polymorphisms cause a markedly compromised PPC (S)-7-hydroxylation. However, PPC metabolism appears overall less influenced by CYP2C9 genotype compared with other oral anticoagulants and it may thus be a valuable alternative for therapeutic anticoagulation of patients expressing CYP2C9 variant alleles.

Risk of major bleeding during concomitant use of antibiotic drugs and coumarin anticoagulants

BACKGROUND

Coumarin anticoagulants are prone to drug-drug interactions. For example, antibiotic drugs may enhance the anticoagulant effect of coumarins. However, whether such interactions are associated with an increased risk of bleeding, and if so, how frequently this occurs remains unknown.

OBJECTIVE

To assess the risk of major bleeding associated with the concomitant use of antibiotic drugs and coumarin anticoagulant therapy.

METHODS

We analyzed a retrospective cohort study including all users of acenocoumarol or phenprocoumon in the PHARMO Record Linkage System (age range: 40-80 years). All patients were followed up until end of last coumarin treatment, hospitalization for bleeding, death, or end of study period. For each patient, the number of days on either coumarins alone, or on coumarins in combination with antibiotic drugs was determined. From these data, the relative risks of major bleeding were calculated.

RESULTS

A total of 52,102 users of acenocoumarol and 7885 users of phenprocoumon met the inclusion criteria of our study cohort and contributed 139,159 person-years of follow-up. During follow-up, 838 patients (1.4%) were hospitalized for a bleeding while taking coumarins. Of the 62 different antibiotics taken by study members, 19 were associated with a bleeding episode. Of these, 10 were associated with a statistically significant increased bleeding risk. The relative risk of bleeding was three to five for doxycycline, amoxicillin, amoxicillin/clavulanic acid, ciprofloxacin, cotrimoxazole, azithromycin and pheneticillin, nine for tetracycline and 43 for cefradine and neomycin.

CONCLUSION

Based on relative risks and incidence of use, amoxicillin (alone or with clavulanic acid) and doxycycline are the main antibiotic drugs associated with major bleeding when used in combination with coumarin.

Quality of anticoagulation with unfractionated heparin plus phenprocoumon for the prevention of thromboembolic complications in cardioversion for non-valvular atrial fibrillation. Sub-analysis from the Anticoagulation in Cardioversion using Enoxaparin (ACE) trial

INTRODUCTION

Anticoagulation in cardioversion for atrial fibrillation is performed using unfractionated heparin and oral anticoagulants. TEE-guided cardioversion, after achievement of therapeutic anticoagulation (1-3 days), may be an alternative to the traditional procedure (3-week anticoagulation followed by cardioversion). The quality of anticoagulation in atrial fibrillation has not been investigated in a randomised trial with TEE-guided cardioversion. We analysed respective data from the ACE trial on the quality of conventional anticoagulation, where most participating centres chose the TEE-guided approach.

MATERIALS AND METHODS

In a randomised, prospective, multicentre trial, we analysed the efficacy of unfractionated heparin plus phenprocoumon in 248 patients on an intention-to-treat basis. There were 2373 evaluable anticoagulation measurements (out of 2925 measurements) and 4 categories of anticoagulation quality (under-, target, over- and severe over-anticoagulation). Of patients with evaluable measurements, 88% received short-term anticoagulation (4 weeks) in TEE-guided cardioversion.

RESULTS

The median time to achieve therapeutic anticoagulation (aPTT> or =60 and <80 s or INR> or =2 and <3) was 3 days. Anticoagulation values were out of therapeutic range in 69.5% of measurements during 4- or 7-week follow-up, and never within therapeutic range in 10% of patients. Of the 15 primary endpoints observed (death, thromboembolism and major bleeding complications), only 3 were in patients with anticoagulation measurements within therapeutic range.

CONCLUSIONS

In this study setting, with predominance of 4 weeks anticoagulation in TEE-guided cardioversion for atrial fibrillation, therapeutic anticoagulation was reached within 3 days using conventional anticoagulation. Despite careful dose adjustments, anticoagulation was out of therapeutic range in almost 70% of total measurements and 80% of primary endpoints.

Patients with an ApoE epsilon4 allele require lower doses of coumarin anticoagulants

OBJECTIVE

Vitamin K is an essential cofactor for the synthesis of several blood coagulation factors. It has been suggested that the apolipoprotein E (ApoE) genotype has profound effects on vitamin K status. Therefore, we investigated whether this common genetic polymorphism influenced dose requirements and effects of coumarin anticoagulants.

METHODS

We did a cohort study in 1637 patients from an outpatient anticoagulation clinic treated with acenocoumarol or phenprocoumon.

RESULTS

To attain the same level of anticoagulation, patients with genotype epsilon4/epsilon4 and genotype epsilon3/epsilon4 required respectively 3.4 mg (95%CI: -6.0 to -0.9) and 0.8 mg (95%CI: -1.6 to 0.1) acenocoumarol per week less than patients with genotype epsilon3/epsilon3. Patients homozygous for the epsilon2 allele required 3.5 mg (95%CI: 0.1 to 6.9) acenocoumarol per week more than patients with genotype epsilon3/epsilon3. The acenocoumarol maintenance dose showed a gene dose effect of the epsilon4 allele, but not of the epsilon2 allele. No significant dose difference was observed for phenprocoumon, possibly because of low numbers.

CONCLUSION

The ApoE genotype affects the dose requirements of acenocoumarol.

The risk of overanticoagulation in patients with heart failure on coumarin anticoagulants

Heart failure has been identified as a risk factor for increased coumarin anticoagulant responsiveness in several small-scale experiments. Epidemiological studies quantifying the risk of overanticoagulation by heart failure in a non-selected population on coumarins are scarce. Therefore, we investigated whether patients with heart failure have an increased risk of overanticoagulation and determined the effect of incidental heart failure on coumarin dose requirements. A cohort study of all patients was performed from an outpatient anticoagulation clinic treated with acenocoumarol or phenprocoumon between 1 January 1990 and 1 January 2000. All cohort members were followed until the first occurrence of an international normalized ratio (INR) > or = 6.0, the last INR assessment, death, loss to follow-up, or end of the study period. Of the 1077 patients in the cohort, 396 developed an INR > or = 6.0. The risk of overanticoagulation was 1.66 [95% confidence interval (CI): 1.33-2.07] for cases of prevalent heart failure and 1.91 (95%CI: 1.31-2.79) for incidental cases. The decrease in dose requirements in patients with incidental heart failure showed a significant trend from the fifth INR measurement preceding the date of incidental heart failure to the third measurement after this date. Heart failure is an independent risk factor for overanticoagulation. Therefore, patients with heart failure should be closely monitored to prevent potential bleeding complications.

Effects of cytochrome P450 2C9 polymorphisms on phenprocoumon anticoagulation status

OBJECTIVE

Our objective was to assess whether there is an association between the presence of allelic variants of the gene for cytochrome P450 (CYP) 2C9 and anticoagulation problems during the initial phase of phenprocoumon treatment.

METHODS

A prospective follow-up study was performed at 2 anticoagulation clinics in The Netherlands. Included subjects started phenprocoumon during the study period, had their first check of the international normalized ratio (INR) on the third or fourth day of therapy, and had an indication for the low therapeutic range (INR, 2.0-3.5). CYP2C9 genotypes ( CYP2C9*1 , CYP2C9*2 , and CYP2C9*3 ) were assessed, and data on indication, INR checks, comedication, and comorbidity were collected.

RESULTS

After genotyping, 284 subjects were available for analysis. Of these, 186 (65.5%) were homozygous carriers of the CYP2C9 wild-type allele ( CYP2C9*1/*1 ), 61 (21.5%) were carriers of the CYP2C9*2 allele, and 37 (13.0%) were carriers of the CYP2C9*3 allele. Compared with homozygous CYP2C9*1/*1 subjects, carriers of CYP2C9*2 or *3 had an increased risk of severe overanticoagulation (INR >6.0). The hazard ratio for CYP2C9*2 versus CYP2C9*1/*1 was 3.09 (95% confidence interval [CI], 1.56 to 6.13; P=.001), and the hazard ratio for CYP2C9*3 versus CYP2C9*1/*1 was 2.40 (95% CI, 1.03 to 5.57; P=.042). Carriers of CYP2C9*2 also had a lower chance to achieve stability in the follow-up period. The hazard ratio for CYP2C9*2 versus CYP2C9*1/*1 was 0.61 (95% CI, 0.43 to 0.85; P=.003). Carriers of the CYP2C9*2 or *3 allele needed a significantly lower phenprocoumon dosage compared with homozygous CYP2C9*1/*1 subjects.

CONCLUSION

The presence of at least 1 CYP2C9*2 or *3 allele in phenprocoumon users is associated with an increased risk of severe overanticoagulation. Similar to warfarin and acenocoumarol, phenprocoumon had a lower dosage requirement in carriers of CYP2C9*2 or *3 compared with that in CYP2C9 wild-type subjects.

Effects of vitamin K antagonist phenprocoumon on activated partial thromboplastin time measurement of direct thrombin inhibitors

The activated partial thromboplastin time (aPTT) is currently the most common test used to measure the anticoagulation intensity of heparins and direct thrombin inhibitors (DTIs). Vitamin K antagonists variably affect aPTT reagents. Interactions between heparin and DTIs occur during concurrent therapy. Three DTIs (lepirudin, argatroban, melagatran) and one unfractionated heparin (liquemin) were added to normal plasma (NP) samples (n = 23) and to vitamin K antagonist plasma (VKAP) samples (n = 23) of patients treated with phenprocoumon. Lepirudin and argatroban were added at concentrations from 300 to 3000 ng/ml, melagatran from 30 to 1000 ng/ml, and unfractionated heparin from 0.016 to 0.48 IU/ml. Wave parameters of clotting time and aPTT ratio curves were evaluated by multivariate analysis for inhibitors, aPTT reagents and NP and VKAP samples. Normal ranges resulting from NP samples were 34.5 +/- 1.0 s with Pathromtin SL and 33.9 +/- 0.8 s with Platelin LS. Normal ranges using VKAP were 52.8 +/- 2.6 s (Pathromtin SL) and 44.2 +/- 1.1 s (Platelin LS) (P < 0.0001). Variance analysis showed that inhibitors, plasmas (NP versus VKAP) and reagents influenced the wave characteristics of aPTT (s) (P < 0.0001) and aPTT ratios (P < 0.0001). Distinct statistical differences between aPTT reagents on one hand and normal versus vitamin K antagonist plasma on the other hand make a comparison of reported aPTT results difficult, especially during overlapping therapy with vitamin K antagonists.

Role of dietary vitamin K intake in chronic oral anticoagulation: prospective evidence from observational and randomized protocols

PURPOSE

The potential association between vitamin K intake and coagulation instability has been explored primarily in case reports and small, retrospective, uncontrolled studies. We prospectively evaluated the effects of dietary vitamin K intake on anticoagulation parameters.

METHODS

In an observational protocol, clinical characteristics and vitamin K intake, assessed semiquantitatively, were evaluated in 39 outpatients who made 230 visits to our anticoagulation clinic. In a randomized crossover protocol, 12 patients with stable anticoagulation underwent 4-day in-hospital dietary interventions, 1 to 2 weeks apart, providing a 500% increase and an 80% decrease in vitamin K intake relative to the baseline level.

RESULTS

Univariate analysis of the observational data demonstrated a progressive, statistically significant inverse association between the vitamin K intake score and different levels of anticoagulation. Multivariate logistic regression analysis showed that vitamin K intake was independently associated with both overcoagulation and undercoagulation. In the randomized protocol, the international normalized ratio increased from 2.6 +/- 0.5 at baseline to 3.3 +/- 0.9 at day 7 (P = 0.005) in subjects on the vitamin K-depleted diet and decreased from 3.1 +/- 0.8 at baseline to 2.8 +/- 0.6 at day 4 (P = 0.04) in those on the vitamin K-enriched diet.

CONCLUSION

Our prospective data strengthen the concept that the interaction between vitamin K and coumarin is a clinically relevant, major independent factor that interferes with anticoagulation stability.

Identification of cytochromes P 450 2C9 and 3A4 as the major catalysts of phenprocoumon hydroxylation in vitro

OBJECTIVE

This in-vitro study aimed at an identification of cytochrome P(450) (CYP) enzymes catalysing the (S)- and (R)-hydroxylation of the widely used anticoagulant phenprocoumon (PPC) to its major, inactive metabolites.

METHODS

Relevant catalysts were identified by kinetic, correlation and inhibition experiments using human liver microsomes and recombinant enzymes.

RESULTS

Kinetics revealed (S)-7-hydroxylation as quantitatively most important. Biphasic Eadie-Hofstee plots indicated more than one catalyst for the 4'-, 6- and 7-hydroxylation of both enantiomers with mean K(m1) and K(m2) of 144.5+/-34.9 and 10.0+/-6.49 microM, respectively. PPC hydroxylation rates were significantly correlated with CYP2C9 and CYP3A4 activity and expression analysing 11 different CYP-specific probes. Complete inhibition of PPC hydroxylation was achieved by combined addition of the CYP3A4-specific inhibitor triacetyloleandomycin (TAO) and a monoclonal, inhibitory antibody (mAb) directed against CYP2C8, 9, 18 and 19, except for the (R)-4'-hydroxylation that was, however, inhibited by ~80% using TAO alone. (S)-PPC hydroxylation was reduced by approximately 2/3 and approximately 1/3 using mAb2C8-9-18-19 and TAO, respectively, but (R)-6- and 7-hydroxylation by approximately 50% each. Experiments with mAbs directed against single CYP2C enzymes clearly indicated CYP2C9 as a major catalyst of the 6- and 7-hydroxylation for both enantiomers. However, CYP2C8 was equally important regarding the (S)-4'-hydroxylation. Recombinant CYP2C8 and CYP2C9 were high-affinity catalysts (K(m) <5 microM), whereas CYP3A4 operated with low affinity (K(m) >100 microM).

CONCLUSION

CYP2C9 and CYP3A4 are major catalysts of (S)- and (R)-PPC hydroxylation, while CYP2C8 partly catalysed the (S)-4'-hydroxylation. Increased vigilance is warranted when PPC treatment is combined with substrates, inhibitors, or inducers of these enzymes.

Effects of lepirudin, argatroban and melagatran and additional influence of phenprocoumon on ecarin clotting time

INTRODUCTION

Direct thrombin inhibitors (DTI) prolong the ecarin clotting time (ECT). Oral anticoagulants (OA) decrease prothrombin levels and thus interact with actions of DTIs on the ECT method during concomitant therapy.

MATERIALS AND METHODS

Actions of lepirudin, argatroban and melagatran on ECT were investigated in normal plasma (NP) and in plasma of patients (n=23 each) on stable therapy with phenprocoumon (OACP). Individual line characteristics were tested statistically.

RESULTS

Control ECT in OACP was prolonged compared to NP (50.1+/-0.9 vs. 45.7+/-0.8 s; p<0.001). Lepirudin prolonged the ECT linearly. Argatroban and melagatran delivered biphasic dose-response curves. OA showed additive effects on the ECT of lepirudin but not of argatroban and melagatran. Both in NP and OACP, the first and second slopes of melagatran were steeper compared to argatroban (primary analysis; p<0.001). When using the same drug, slopes in OACP were steeper than in NP (secondary analysis; p<0.001). At similar molar concentrations, the crossing points of both slopes were significantly higher with melagatran (323.1+/-11.0 s in NP and 333.2+/-8.2 s in OACP) than with argatroban (219.6+/-14.7 and 248.4+/-15.2 s) corresponding to ratios of 7.1+/-0.2 and 6.7+/-0.2 (melagatran) vs. 4.8+/-0.3 and 4.9+/-03 with argatroban (p<0.0001).

DISCUSSION

The patterns of interactions between vitamin K antagonists and DTI effects are different for bivalent (increase of slope without affecting linearity) and monovalent inhibitors (slight increase or alteration of nonlinear slopes), but there are also differences between the two monovalent inhibitors on thrombin inhibition as determined by ECT.

[Clinical problems with oral anticoagulation -- 3 case reports]

Two patients with severe bleeding complications under oral anticoagulant treatment are presented, in one case caused by pharmacokinetic drug interference (phenylbutazone), in the other by genetic predisposition to bleeding induced by coumarin anticoagulants. Another patient with decreasing INR due to drug interference (rifampicin) is presented as well. The possibility of drug interferences with coumarin anticoagulants has to be anticipated, whenever the medication of an orally anticoagulated patient is changed. A founder mutation of the factor IX propeptide constitutes a genetic predisposition to bleeding in patients put on coumarins. Its presence should be excluded in any patient suffering from hemorrhagic complications after starting anticoagulation when INR values are in the target range.

Effect of phenprocoumon on monitoring of lepirudin, argatroban, melagatran and unfractionated heparin with the PiCT method

Prothrombinase-induced clotting time (PiCT) is a clotting-time test for heparins and direct thrombin inhibitors to reduce drawbacks of aPTT. Effects of the direct thrombin inhibitors lepirudin, argatroban, melagatran and of unfractionated heparin (UFH) were investigated in normal and oral anticoagulant plasma samples. Lepirudin showed potentiating interferences with phenprocoumon effects. Melagatran, argatroban and UFH delivered distinct linear additive effects in both plasma sample groups. PiCT ratio reduces differences between both groups with UFH, and argatroban inhibitor-receptor-binding mode plays a role in interaction patterns.

Overanticoagulation associated with combined use of antifungal agents and coumarin anticoagulants

BACKGROUND

Several case reports have associated combined use of coumarins and antifungal agents with overanticoagulation. However, we are not aware of epidemiologic studies that quantify the risk of overanticoagulation caused by antifungal agents. We conducted a follow-up study in a large population-based cohort to investigate the antifungal agents that are associated with overanticoagulation during therapy with coumarins.

METHODS

All patients in the Rotterdam Study who were treated with acenocoumarol or phenprocoumon in the study period from April 1, 1991, through Dec 31, 1998, and for whom international normalized ratio data were available were followed up until an INR > or =6.0, the end of their treatment, death, or the end of the study period. Proportional hazards regression analysis was used to estimate the risk of an INR > or =6.0 in relation to concomitant use of an oral anticoagulant and antifungal agents, after adjustment for several potentially confounding factors such as age, gender, hepatic dysfunction, malignancies, and heart failure.

RESULTS

Of the 1124 patients in the cohort, 351 had an INR > or =6.0. The incidence rate was 6.9 per 10,000 treatment days. Oral miconazole most strongly increased the risk of overanticoagulation with an adjusted relative risk of 36.6 (95% confidence interval [CI], 12.4-108.0).

CONCLUSIONS

In this study among outpatients of an anticoagulant clinic who were on a regimen of coumarins, oral miconazole was especially strongly associated with overanticoagulation. Awareness of these drug interactions and more frequent monitoring of INR values during the initial stages of treatment with some antifungal drugs in patients taking coumarins may minimize the risk of bleeding complications. The concurrent use of miconazole and coumarins should be discouraged.

The quality of oral anticoagulation before, during and after a period of patient self-management

The study analyzes the quality of anticoagulation during a 3-year follow-up on patients who were treated by an anticoagulation clinic (ACS) for 1 year (Phase I), performed weekly self-management of anticoagulation (PSM) after a specific training for another year (Phase II) and finally returned to be treated by the anticoagulation clinic (ACS) for a third year (Phase III). The mean fraction of INR values within therapeutic target range was higher in Phase II (0.69 +/- 0, 11) compared to Phases I (0.40 +/- 0.20) and III (0.56 +/- 0.18; p < 0.05). Time spent in therapeutic target range was higher in Phase II (0.70 +/- 0.10) compared to Phases I (0.43 +/- 0.25) and III (0.60 +/- 0.17; p < 0.05). Mean square deviation from target value was lower in Phase II (0.39 +/- 0.17) compared to Phases I (0.81 +/- 0.44) and III (0.64 +/- 0.39, p = 0.05). Thus, the quality of anticoagulation during Phase II (PSM) was significantly better compared to Phases I (ACS) and III (ACS) in all endpoints tested. This shows that the quality of oral anticoagulation deteriorates again if patient self-management is stopped and patients return to conventional treatment. Furthermore, the quality of anticoagulation was better in Phase III (post-PSM) compared with Phase I (pre-PSM) although the type of treatment was identical in both phases (ACS). This suggests that the increased patient empowerment and enhanced compliance acquired during PSM (Phase II) might have a positive impact on the quality of anticoagulation, even when patients return to the conventional treatment (ACS).

No clinically relevant drug interaction between paracetamol and phenprocoumon based on a pharmacoepidemiological cohort study in medical inpatients

OBJECTIVE

A recent case control study suggests that paracetamol at a dosage above 1300 mg/day increases the anticoagulant effects of warfarin. In many European countries, phenprocoumon and not warfarin is used as an anticoagulant. However, the effects of paracetamol on the anticoagulant effects of phenprocoumon have so far not been evaluated.

METHODS

This study is based on the data recorded prospectively between 1996 and 1998 in two Swiss teaching hospitals in the pharmacoepidemiological database of the SAS/CHDM project. As a first step, all phenprocoumon-treated patients with at least one international normalized ratio (INR) determination at least 4 days after cohort entry were selected and evaluated concerning paracetamol co-administration. The "paracetamol group" included only patients receiving at least 1300 mg/day paracetamol on the 3 days preceding the INR determination, whereas the comparison group contained only patients without paracetamol exposure during hospital stay. Thereafter, INR values of the paracetamol and the comparison groups were compared.

RESULTS

The paracetamol and the comparison groups included 54 and 180 patients, respectively. Patients' characteristics in both groups were comparable. Patients in the paracetamol group received on average 2220 +/- 651 mg/day paracetamol. The median duration of paracetamol exposure was 5 days. The median difference in INR values between the paracetamol and the comparison groups was -0.3 (with a 99% confidence interval of -0.6, 0.1).

CONCLUSIONS

These results suggest that paracetamol co-administration at a dosage of 2000-2500 mg/day for 3 days has no clinically relevant effects on the anticoagulant effects of phenprocoumon.

In vitro bleeding test with PFA-100-aspects of controlling individual acetylsalicylic acid induced platelet inhibition in patients with cardiovascular disease

OBJECTIVES

This study investigated the usefulness and practicability of a platelet function analyzer (PFA-100(TM), DADE-Behring, Germany) to determine individual platelet inhibition in patients treated with acetylsalicylic acid (ASA).

BACKGROUND

Patients with coronary artery disease (CAD) routinely and during angioplasty (PTCA) receive standard doses of ASA to avoid acute coronary syndromes and abrupt vessel closures without information of the individual efficacy of platelet inhibition.

METHODS

With the PFA-100(TM) a standardized bleeding time is measured. Whole-blood anticoagulated with 3.2% sodium citrate is aspirated through a capillary ( solidus in circle 200 microm) and through an aperture ( solidus in circle 147 microm). The time until occlusion of the aperture (closure time, CT) by a stable platelet plug induced by shear stress, collagen and epinephrine (COLL/EPI-CT) or shear stress, collagen and adenosine 5'-diphosphate (COLL/ADP-CT) is determined. To examine the usefulness of the PFA-100(TM) as a rapid bedside test and the individual effect of ASA, closure time was measured in healthy individuals (n=17), in patients with stable CAD (n=19) and in patients undergoing PTCA (n=8).

RESULTS

Patients with stable CAD and regular medication with 100 mg ASA per day for at least 3 month showed shorter COLL/ADP-CT in comparison to healthy individuals who took only one single dose of 100 mg ASA. Of the patients with CAD 63% had a COLL/EPI-CT within normal range suggesting a low or no response to ASA. Also only 50% of the patients undergoing PTCA reached the expected COLL/EPI-CT>300 s after an additive single dose of 500 mg ASA intravenously. Neither heparin, phenprocoumon, sex nor different blood sampling methods seem to influence the measurements relevantly.

CONCLUSIONS

This pilot study indicates that with the PFA-100(TM) test device a simple and quick measurement of an in vitro bleeding time is possible. It is able to detect an increase in the bleeding time after a single dose of ASA 100 mg in healthy subjects, reflecting a sensitive detection of ASA induced changes in platelet inhibition respective activation. Differences in the individual response to ASA could be observed in healthy subjects, patients with stable CAD and patients undergoing PTCA. Further studies should validate the PFA-100(TM) with standard methods to determine ASA response in patients with cardiovascular disease and investigate implications for treatment and outcome in this patient group.

Effect of lysine clonixinate on the pharmacokinetics and anticoagulant activity of phenprocoumon

The effect of the non-steroidal anti-inflammatory drug lysine clonixinate ([2-(3-chloro-o-toluidino)nicotinic acid]-L-lysinate, CAS 55837-30-4) on the pharmacokinetics and anticoagulant activity of phenprocoumon (4-hydroxy-3-(1-phenylpropyl)-coumarin, CAS 435-97-2) was investigated in an open, randomised, two-fold, cross-over study in 12 healthy male volunteers. These subjects received a single dose of 18 mg phenprocoumon without or with concomitant treatment with lysine clonixinate (125 mg five times a day for 3 days before and 13 days after ingestion of a single dose of phenprocoumon). Pharmacokinetic parameters of phenprocoumon following oral administration were: CL/f: 0.779 +/- 0.157 ml/min, half-life of elimination: 147.2 +/- 19.9 h; free fraction in serum: 0.51 +/- 0.20%. These parameters were not significantly altered by concomitant treatment with lysine clonixinate. Prothrombin time increased from 13.3 +/- 1.3 s (at time 0) to 17.7 +/- 2.7 s following phenprocoumon and from 13.3 +/- 1.2 s to 18.0 +/- 2.2 s following combined administration. Prothrombin time returned to the pretreatment values 240 h after administration of phenprocoumon. The integrated effect (AUEC0-288 h) was identical following both treatments (4.303 +/- 461 and 4.303 +/- 312 s x h for phenprocoumon alone and phenprocoumon with lysine clonixinate, respectively). Thus, lysine clonixinate administered in therapeutic doses does not affect the pharmacokinetics and anticoagulant activity of phenproxoumon.

Drug interaction: omeprazole and phenprocoumon

BACKGROUND

Oral anticoagulants like the coumarin derivatives are characterised by a particularly narrow therapeutic range. Any interfering co-medication can pose a challenge to establishing a stable anticoagulant dosage regimen and thus present a serious risk for the patient. Here we describe two cases of patients on anticoagulant therapy that suggest possible drug interactions between phenprocoumon and the proton pump inhibitor omeprazole.

RESULTS

In both patients, the International Normalised Ratio (INR) increased beyond the therapeutic range after they were given omeprazole for the treatment of gastrointestinal symptoms. The INR returned to therapeutic levels after the phenprocoumon dose was reduced (Case 1) and omeprazole discontinued (Case 2).

CONCLUSIONS

The increased anticoagulant activity of phenprocoumon observed in the two described cases and the known interaction potential of omeprazole suggest that the clearance of phenprocoumon may have been reduced due to competitive inhibition of its degradation by omeprazole. Drug interactions may be one of the reasons for the difficulties encountered with some patients in establishing a stable anticoagulant therapy. Physicians should carefully review any concurrently used medication and if possible opt for drugs with a low interaction potential.

Quantitative determination of PEG-hirudin in human plasma using a competitive enzyme-linked immunosorbent assay

Polyethylene glycol (PEG) coupled r-hirudin mutein is determined by biological methods-the coagulation system. In the present study, a competitive enzyme-linked immunosorbent assay (ELISA) is described that permits the measurement of PEG r-hirudin. The ELISA system adopts a rabbit IgG antibody to quantitatively detect PEG-hirudin in human plasma. A PEG-hirudin calibration curve ranged from 50 to 7000 ng/mL. The limit of detection was 87 ng/mL. The intraassay coefficients of variation (CV) ranged between 16 and 21%, and interassay CV between 8 and 22% for low and high PEG-hirudin concentrations, respectively. The recovery of the compound in plasma was between 96 and 111.5%. The interindividual differences between 100 and 5000 ng/mL PEG-hirudin were between 12 and 22%. The correlation of the concentration of PEG-hirudin determined with the ELISA and the ecarin clotting time was r = 0.902. No interactions between unfractionated heparin, low molecular-weight heparin, or phenprocoumon and PEG-hirudin were observed in the ELISA. Deficiencies of thrombin or antithrombin as well as low, normal, and high fibrinogen levels did not interfere with the assay. It is concluded that the ELISA determines the concentration of PEG-hirudin and is not influenced by other major anticoagulants or by plasma levels of some coagulation proteins.

Prediction of pharmacokinetic drug/drug interactions from In vitro data: interactions of the nonsteroidal anti-inflammatory drug lornoxicam with oral anticoagulants

CYP2C9 is involved in the metabolism of the oral anticoagulants warfarin, phenprocoumon, and acenocoumarol. It is also responsible for the 5'-hydroxylation of the nonsteroidal anti-inflammatory drug lornoxicam. Therefore, lornoxicam and the oral anticoagulants are potential inhibitors of their metabolism. Their inhibitory potency was investigated in microsomes from six human livers. An approach to predict pharmacokinetic interactions of lornoxicam from in vitro inhibition data was developed. Where possible, the forecasts were verified by comparison with data from clinical interaction studies. The following increases in steady-state plasma concentrations or areas under the plasma concentration-time curve of the oral anticoagulants by concomitant lornoxicam medication were predicted (values in parentheses are for healthy volunteers): (S)-warfarin, 1. 58-fold (1.32-fold for racemate); racemic-acenocoumarol, 1.28-fold (1.09-fold); (R)-acenocoumarol, 1.10-fold (1.0-fold); racemic-phenprocoumon, 1.11-fold (1.18-fold); and (S)-phenprocoumon, 1.13-fold (1.24-fold). Lornoxicam 5'-hydroxylation was competitively inhibited in vitro by both phenprocoumon (K(i) = 1.2 +/- 0.4 microM) and acenocoumarol (K(i) = 5.5 +/- 3.5 microM). The present results indicate that relatively close predictions of the interactions of lornoxicam with oral anticoagulants from in vitro data are possible under the assumption that hepatic lornoxicam concentrations are similar to its total plasma concentrations. The degree of pharmacokinetic interactions exhibited by oral anticoagulants and lornoxicam is dependent on the respective contribution of CYP2C9 to their total clearance.

Structural forms of phenprocoumon and warfarin that are metabolized at the active site of CYP2C9

Possible reasons for the observed differences in metabolic behavior and drug interaction liability between the structurally similar oral anticoagulants warfarin and phenprocoumon were explored. Incubating (S)-phenprocoumon with human liver microsomes and cDNA-expressed CYP2C9 and determining its metabolism both in the absence and presence of the CYP2C9 inhibitor, sulfaphenazole, confirmed that phenprocoumon is a substrate for CYP2C9. Comparing the metabolic behavior of (S)- and (R)-warfarin, (S)- and (R)-phenprocoumon, and fixed structural mimics of the various tautomeric forms [(S)- and (R)-4-methoxyphenprocoumon, (S)- and (R)-2-methoxyphenprocoumon, (S)- and (R)-4-methoxywarfarin, (S)- and (R)-2-methoxywarfarin, and 9(S)- and 9(R)-cyclocoumarol] available to these two drugs with expressed CYP2C9 provides compelling evidence indicating that the ring closed form of (S)-warfarin and the ring opened anionic form of (S)-phenprocoumon are the major and specific structural forms of the two drugs that interact with the active site of CYP2C9. The conclusion that (S)-warfarin and (S)-phenprocoumon interact with CYP2C9 in very different structural states provides a clear basis for the significant differences observed in their metabolic profiles. Moreover, in accord with a previously established CoMFA model these results are consistent with the hypothesis that the active site of CYP2C9 possesses at least two major substrate binding sites, a pi-stacking site for aromatic rings and an ionic binding site for organic anions. An additional electrostatic binding site also appears to contribute to the orientation of coumarin analogs in the CYP2C9 active site by interacting with the C2-carbonyl group of the coumarin nucleus.

Opposite effects of lornoxicam co-administration on phenprocoumon pharmacokinetics and pharmacodynamics

OBJECTIVE

To investigate the effect of co-administration of the non-steroidal anti-inflammatory drug (NSAID) lornoxicam on the pharmacokinetics of (R)- and (S)-phenprocoumon and their effect on factor II and VII activities.

METHODS

Six healthy male volunteers completed an open crossover study. Plasma concentrations of (R)- and (S)-phenprocoumon and activities of coagulation factors II and VII were measured after a single oral dose of 9 mg phenprocoumon racemate. In the second session, lornoxicam administration was started 3 days before phenprocoumon administration and continued twice daily until the last blood sample was drawn.

RESULTS

Lornoxicam co-administration resulted in a statistically significant increase of the area under the concentration-time curve (AUC) of the more potent (S)-isomer of phenprocoumon from a median value of 100 (range 68-146) mg x h x 1(-1) to 124 (92-239) mg x h x 1(-1). For the (R)-isomer, the AUC increase from 96 (70-142) mg x h x 1(-1) in the absence to 108 (75-155) mg x h x 1(-1) in the presence of lornoxicam was not statistically significant. In a model-based analysis, an increase of (S)-phenprocoumon and (R)-phenprocoumon bioavailability of 14% [95% CI (9%, 19%)] and 6% (2%, 10%) and a decrease of their clearances by 15% (8%, 21%) and 6% (0%, 13%) was obtained. Lornoxicam co-administration did not influence the free fractions of (R)- or (S)-phenprocoumon. Contrary to what was expected from the changes in pharmacokinetics, a statistically significant decrease in the effect of phenprocoumon on factor II and VII activity was observed for the sessions with lornoxicam co-administration. For factor VII, lornoxicam was found to increase the concentration causing half-maximal effect (C50) of phenprocoumon by 70% [95% CI (38%, 111%)].

CONCLUSION

Co-administration of lornoxicam at the upper limit of recommended doses mainly altered the pharmacokinetics of the more potent (S)-isomer and to a lesser degree those of (R)-phenprocoumon. Despite these changes in pharmacokinetics, a decrease of the effect on factor II and VII activity was observed. These results suggest that in the case of lornoxicam co-administration in a patient treated with phenprocoumon the prothrombin time should be monitored closely.

Lack of interaction between tramadol and coumarins

The objective of this study was to investigate whether the central analgesic tramadol influences the effects of the coumarin anticoagulant phenprocoumon during multiple-dose administration. Nineteen patients receiving long-term anticoagulant therapy who had been in a stable hypothrombinemic state for at least 3 months completed a double-blind, placebo-controlled, crossover study. Tramadol was administered in the usual therapeutic dose of 50 mg three times daily. The average daily phenprocoumon dose was identical for individual patients in both treatment periods. The equivalence ratio (tramadol/placebo) of the international normalized ratio (INR) values was 0.99 (90% confidence interval 0.89-1.10), thus fulfilling predetermined bioequivalence criteria (0.70-1.43). Therefore, tramadol does not affect INR in patients being treated with phenprocoumon. These data suggest a lack of interaction between tramadol and coumarin anticoagulants.

General pharmacology of meloxicam--Part II: Effects on blood pressure, blood flow, heart rate, ECG, respiratory minute volume and interactions with paracetamol, pirenzepine, chlorthalidone, phenprocoumon and tolbutamide

The pharmacodynamic properties of meloxicam, a new nonsteroidal antiinflammatory drug (NSAID), that go beyond those typical of an NSAID were examined. The extent to which meloxican shows NSAID-like interactions with paracetamol, pirenzepine, chlorthalidone, phenprocoumon and tolbutamide was also investigated. In the dose range studied, meloxicam had no influence on the blood pressure of the unanaesthetized rat, blood flow, heart rate, ECG and respiratory minute volume of the anaesthetized cat or on the blood pressure, heart rate and respiratory minute volume of the anaesthetized dog. The acute toxicity level of meloxicam after oral and parenteral administration to the rat and mouse proved considerably lower than that of indomethacin. Meloxicam showed excellent tissue tolerability following parenteral administration. The effects against inflammatory pain and acute antiexudative effects of meloxicam were enhanced by simultaneous low doses of paracetamol. Pirenzepine showed an antagonistic effect on the ulcerogenicity of meloxicam in the rat stomach. The diuretic effect of chlorthalidone in the rat was not influenced by high doses of meloxicam. The effect of phenprocoumon in the rat was enhanced by high doses of meloxicam. However, the hypoglycaemic effect of tolbutamide in the rabbit was not influenced by meloxicam.

Unstable anticoagulation in the course of amitriptyline treatment

A potential drug-drug interaction between the tricyclic antidepressant amitriptyline and the oral anticoagulant phenprocoumon, causing intensified hypo- and/or hyperprothrombinemic effects, was investigated. In seven patients simultaneously receiving amitriptyline and phenprocoumon the course of the Quick values and the amitriptyline and phenprocoumon dosages were registered. The resulting data represented graphically was additionally compared with data and diagrams gained from a control group of seven phenprocoumon patients not receiving amitriptyline. Whereas in the control group the average Quick values (or the target International Normalized Ratio) lay within the required therapeutic range, massive fluctuations were seen in the amitriptyline-treated patients. These fluctuations did not disappear until the amitriptyline medication was discontinued. Whether our findings are actually due to a clinically relevant drug-drug interaction needs to be investigated in further controlled studies based on a larger number of patients, the more so as no comparable investigations and only few references to this subject are to be found in the literature. Should an amitriptyline influence on the frequently prescribed coumarin derivative be proven, an increased risk of rethrombosis or bleeding complications in patients receiving both amitriptyline and phenprocoumon would appear to be indicated.

Lack of pharmacodynamic and pharmacokinetic interaction between pantoprazole and phenprocoumon in man

OBJECTIVE

Pantoprazole is a selective proton pump inhibitor characterized by a low potential to interact with the cytochrome P450 enzymes in man. Due to the clinical importance of an interaction with anticoagulants, this study was carried out to investigate the possible influence of pantoprazole on the pharmacodynamics and pharmacokinetics of phenprocoumon.

METHODS

Sixteen healthy male subjects were given individually adjusted doses of phenprocoumon to reduce prothrombin time ratio (Quick method) to about 30-40% of normal within the first 5-9 days and to maintain this level. The individual maintenance doses remained unaltered from day 9 on and were administered until day 15. Additionally, on study days 11-15, pantoprazole 40 mg was given per once daily. As a pharmacodynamic parameter, the prothrombin time ratio was determined on days 9 and 10 (reference value) and on days 14 and 15 (test value), and the ratio test/reference was evaluated according to equivalence criteria.

RESULTS

The equivalence ratio (test/reference) for prothrombin time ratio was 1.02 (90% confidence interval 0.95-1.09), thus fulfilling predetermined bioequivalence criteria (0.70-1.43). The pharmacokinetic characteristics AUC0-24h and Cmax of S(-)- and R(+)-phenprocoumon were also investigated using equivalence criteria. Equivalence ratios and confidence limits of AUC0-24h and of Cmax of S(-)-phenprocoumon (0.93, 0.87-1.00 for AUC0-24h; 0.95, 0.88-1.03 for Cmax) and of R(+)-phenprocoumon (0.89, 0.82-0.96; 0.9, 0.83-0.98) were within the accepted range of 0.8-1.25.

CONCLUSION

Pantoprazole does not interact with the anticoagulant phenprocoumon on a pharmacodynamic or pharmacokinetic level. Concomitant treatment was well tolerated.

In vitro model to test the thrombogenicity of coronary stents

Thrombotic occlusion is a major complication limiting the application of stents in coronary arteries. In an in vitro model we investigated the thrombogenicity of different stent materials and several medical regimens to prevent thrombotic occlusion. Experiments were conducted in a closed system of silicon tubing with circulating citrated platelet rich plasma of healthy volunteers (n = 7) and of patients (n = 7 for each condition). Patients were either treated with phenprocoumon or with high or low dose heparin in combination with aspirin alone (100 mg) or aspirin (990 mg) plus dipyridamole (225 mg). After placement of tantalum wire stents into the system platelet aggregates were visible after 13.5 +/- 3.0 min, and occlusion occurred after 15.0 +/- 3.5 min. Similarly, with implanted stainless steel stents aggregation was seen after 13.0 +/- 3.5 min and thrombosis occurred after 14.5 +/- 3.5 min (p < 0.001 vs control without stent). Microscopic examination revealed combined platelet fibrin thrombi occluding the lumen. Platelet components predominately covered stent wires, particularly at crossing points. In all experiments high-dose heparin prevented platelet aggregate formation and stent occlusion independently of additional aspirin or aspirin plus dipyridamole; perfusion time > 60 min (p < 0.001 vs no heparin). Low-dose heparin could not prevent clotting. With aspirin alone aggregates were visible after 16.0 +/- 4.0 min and clotting occurred after 23.0 +/- 5.0 min. In combination with dipyridamole aggregates were visible after 15.5 +/- 5.0 min and clotting after 21.0 +/- 4.0 min (NS vs aspirin alone). Phenprocoumon prevented platelet aggregate formation and stent occlusion; perfusion time > 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Views of cardiac-valve prosthesis patients and their dentists on anticoagulation therapy

By administering a questionnaire to 253 patients with cardiac-valve prostheses (89.3% responding), and another to 136 of their attending dentists (79% responding), the level of knowledge among both groups of anticoagulant therapy in connection with dental treatment was investigated. The cardiothoracic department monitored all anticoagulation therapies. Of the anticoagulated patients, 96.6% were able to state their medication (94.1% received phenprocoumon); and of 86 dentists with patients on anticoagulation treatment, 94% were aware of their patients' medication. All 20 dentists stating that their patients did not receive anticoagulants were correct. The great majority (98%) of the dentists employed a special measure to reduce the risk of bleeding associated with invasive dental procedures, most commonly (86%) referring patients to their general practitioner or hospital department for adjustment of the anticoagulant therapy. Around 60% of the dentists considered extractions and operations to require measures to reduce the risk of bleeding complications. We recommend referral of patients to the attending physician for adjustment of anticoagulation to a target International Normalized Ratio (INR) of 4.0 or possibly 3.0 before undergoing dental procedures involving the risk of bleeding. Additional reduction of the bleeding risk can be obtained by local application of an inhibitor of fibrinolysis (tranexamic acid).

Nitric oxide production depends on preceding tetrahydrobiopterin synthesis by endothelial cells: selective suppression of induced nitric oxide production by sepiapterin reductase inhibitors

Using murine vascular endothelial cells expressing both constitutive and inducible nitric oxide synthases (cNOS and iNOS), we explored the feasibility of suppressing cytokine-induced nitric oxide (NO) production without affecting constitutive NO production by inhibition of the tetrahydrobiopterin (BH4) biosynthesis. We show in this study that in endothelial cells cytokine/endotoxin-activated BH4 synthesis precedes the induction of NO generation. Using the sepiapterin reductase inhibitors phenprocoumon or dicumarol as BH4 synthesis inhibitors, we achieved a pronounced and selective suppression of induced NO production in cytokine-activated endothelial cells. Addition of exogenous BH4, but not sepiapterin, restored NO production in the presence of the inhibitors. Despite profound inhibition of the BH4 biosynthesis, constitutive NO synthesis was not affected, thereby demonstrating the selectivity and specificity of the inhibitors. Suppression of enhanced NO production by sepiapterin reductase inhibitors such as cumaroles could provide pharmacologic means for therapeutic interventions in NO-mediated pathophysiologic events.

Effect of long-term oral anticoagulant treatment on mortality and cardiovascular morbidity after myocardial infarction. Anticoagulants in the Secondary Prevention of Events in Coronary Thrombosis (ASPECT) Research Group

The use of long-term oral anticoagulant treatment after myocardial infarction remains controversial because of conflicting findings on mortality in previous trials and the increased risk of bleeding associated with anticoagulants. We have carried out a randomised, placebo-controlled, double-blind, multicentre trial in 3404 hospital survivors of myocardial infarction. Eligible patients were randomly assigned to anticoagulant (nicoumalone or phenprocoumon) or placebo treatment within 6 weeks of discharge. The target prothrombin time was 2.8-4.8 international normalised ratio. During mean follow-up of 37 (range 6-76) months there were 170 deaths among 1700 anticoagulant-treated patients and 189 in 1704 placebo-treated patients (hazard ratio 0.90 [95% CI 0.73-1.11]). Anticoagulant treatment led to significant reductions by comparison with placebo treatment in recurrent myocardial infarction (114 vs 242 patients; hazard ratio 0.47 [0.38-0.59]) and cerebrovascular events (37 vs 62; 0.60 [0.40-0.90]). Major bleeding complications were seen in 73 patients who received anticoagulants and 19 who received placebo. We conclude that long-term oral anticoagulant treatment after myocardial infarction in low-risk patients has a limited effect on mortality but achieves substantial benefit by reducing the risk of cerebrovascular events and recurrent myocardial infarction.

[Measurement of prothrombin fragment F1+2 for monitoring the anticoagulant effect of phenprocoumon]

It was the aim of this study to ascertain whether the plasma level of prothrombin fragment F1+2 is a suitable indicator of the anticoagulant effect of coumarin derivatives. The F1+2 levels were measured in 164 patients (100 women, 64 men; mean age 63.3 [34-83] years) in whom stable anticoagulation had been achieved with phenprocoumon, comparing the results with those obtained in healthy subjects (28 women, 19 men; mean age 54.6 [25-72] years) without anticoagulation. There was a significant reduction (P < 0.0005) in F1+2 plasma levels with oral anticoagulation (0.45 + 0.23 vs. 0.67 + 0.32 nmol/l). Even on a low degree of anticoagulation (international normalized ratio [INR] < 2.0) the F1+2 value was reduced to within the normal range (0.32-1.2 nmol/l). These results indicate that changes in the plasma level of prothrombin fragment F1+2 are directly dependent on the degree of oral anticoagulation and that this measure seems suitable for the monitoring of the anticoagulant effect. This is also true for oral anticoagulation of mild degree (INR < 2.0) in which the effect cannot be satisfactorily measured by the thromboplastin time (Quick test).

Regulation of the L-arginine-dependent and tetrahydrobiopterin-dependent biosynthesis of nitric oxide in murine macrophages

Nitric oxide is a recently discovered biomolecule with a broad range of actions. The present study investigated the regulation of nitric oxide synthase activity by dexamethasone and the cofactor tetrahydrobiopterin in murine macrophages. The influence of the tetrahydrobiopterin biosynthesis inhibitors 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP cyclohydrolase I, and phenprocoumon, an inhibitor of sepiapterin reductase, on the synthesis of nitric oxide was investigated. Dexamethasone decreased the nitric oxide production due to direct inhibition of the induction of nitric oxide synthase and of GTP cyclohydrolase. Substitution of tetrahydrobiopterin via sepiapterin could not overcome the dexamethasone-mediated inhibition. 2,4-Diamino-6-hydroxypyrimidine abolished nitric oxide synthesis and synergized with dexamethasone, completely eliminating nitric oxide production. Phenprocoumon inhibited production of nitric oxide via interference with later steps of tetrahydrobiopterin biosynthesis. An exogenous supply of tetrahydrobiopterin through sepiapterin led to a further increase of nitric oxide production, even in fully activated macrophages. The amount of nitric oxide produced by murine macrophages is therefore limited by the amount of tetrahydrobiopterin present in the cells. Inhibitors of tetrahydrobiopterin biosynthesis could provide a novel approach for therapy of pathological conditions mediated by nitric oxide, such as septic shock.

Platelet-induced thrombin generation time: a new sensitive global assay for platelet function and coagulation. Method and first results

A new sensitive test--platelet-induced thrombin generation time (PITT)--is described, in which the formation of thrombin in partially anticoagulated platelet-rich plasma (PRP) leads to aggregation immediately followed by coagulation of PRP. 0.6 ml PRP are rotated in a disk-shaped cuvette within the light beam of a photometer. In PITT, platelets stick to the cuvette wall and, mediated by a large PRP/surface/air interface at the cuvette wall, are activated and participate in thrombin formation which leads to aggregation and clotting. The times from onset of rotation until aggregation (Ta) and until coagulation (Tc) of the PRP samples are recorded. PITT was very sensitive and detected low concentrations of unfractionated heparin (0.01 IU/ml) in vitro. PITT parameters were significantly prolonged ex vivo 2 h after oral administration of acetylsalicylic acid (0.5 g) and after single subcutaneous injections of heparin (5,000 IU). Patients receiving phenprocoumon prophylaxis had markedly prolonged Ta and Tc values (longer than 20 min, n = 23). Patients with recent thrombotic episodes had markedly shorter values than healthy volunteers. PITT may become a very sensitive global test to detect mild hemorrhagic disorders, to monitor the effects of antithrombotic drugs and to detect patients with a risk of vascular occlusions.

Stereoselective effect of phenprocoumon enantiomers on the binding of benzodiazepines to human serum albumin

The effect of phenprocoumon enantiomers on the stereoselective binding of 3-substituted 1,4-benzodiazepines to human serum albumin (HSA) was studied by chromatography on HSA-Sepharose column. (S)-Phenprocoumon exerts stereoselective allosteric interaction on the binding of benzodiazepines. The structural requirements of enhanced stereoselectivities are similar to those found previously with (S)-warfarin.

Effects of the oral anticoagulant phenprocoumon on blood coagulation and thrombogenesis induced by rabbit aorta subendothelium exposed to flowing human blood: role of dose and shear rate

We investigated the effect of oral anticoagulation on thrombogenesis induced by the subendothelium of rabbit aorta. Eighteen healthy volunteers underwent a 2-week treatment with the oral coumarin preparation phenprocoumon to a target international normalized ratio (INR) of 5. By using an ex-vivo perfusion chamber system, the interaction between flowing blood and exposed subendothelium was measured at low (50 sec-1) and high (650 sec-1) wall shear rates. The low shear rate simulated blood flow in venous vessels and the high shear rate simulated blood flow in arterial vessels. Deposition of fibrin, platelets, and platelet thrombi on subendothelium was quantified by morphometric and immunologic techniques. Fibrin deposition prevailed at the low shear rate (183 +/- 57 ng/mm2 vs 46 +/- 16 ng/mm2, low vs high shear rate; mean +/- SEM; p less than 0.01). In contrast, the interaction of platelets with subendothelium was more intense at high shear rates when compared with low shear rates, as indicated by higher platelet adhesion (44% +/- 2% vs 9% +/- 1% coverage of subendothelium with platelets, p less than 0.001) and platelet thrombus volumes (3.6 +/- 0.4 microns 3/microns 2 vs 1.3 +/- 0.3 microns 3/microns 2, p less than 0.001). Fibrin deposition on subendothelium was substantially reduced even at a low intensity of anticoagulation (reduction by 60% at INR 2.1 and by 75% at INR 3.7) and was abolished after high coumarin doses (INR 5.2). In contrast, a significant inhibition of platelet thrombus formation could be achieved only by high doses of phenprocoumon (INR 5.2). Our data indicate that relatively low doses of oral anticoagulants (INR between 2 and 3.5) substantially inhibit the fibrin formation on subendothelium prevailing at venous shear conditions, whereas a high intensity of anticoagulation (INR 4 to 5) is necessary to inhibit the formation of platelet-rich thrombi prevailing at arterial shear conditions.

Influence of anticoagulants on growth and vitamin K production of intestinal bacteria

The influence of anticoagulants on growth and vitamin K production of intestinal bacteria was studied. Bacteroides vulgatus and Escherichia coli were grown in vitro in the presence of increasing amounts of warfarin, phenprocoumon and acenocoumarol. It was found that growth of B. vulgatus was inhibited under anaerobic conditions whereas growth of E. coli under aerobic conditions was not inhibited. A specific inhibition of vitamin K biosynthesis was not observed in either case. It is concluded that therapeutic doses of anticoagulants are unlikely to affect growth or vitamin K production of intestinal bacteria.

The interaction of the prostaglandin E derivative rioprostil with oral anticoagulant agents

The prostaglandin E1 analogue rioprostil was tested for potential interaction with oral anticoagulant therapy in healthy male volunteers. The effect of rioprostil (0.3 mg, twice a day) was investigated on acenocoumarol (10 mg per subject, n = 7) and phenoprocoumon (0.2 mg/kg, n = 6) single-dose pharmacokinetics and pharmacodynamics. Plasma levels of thrombotest, prothrombin (factor II), and factor VII activities were assayed. Rioprostil, 7 days pretreatment, did not affect control parameters of blood coagulation activity. During the rioprostil period the effect of phenprocoumon on thrombotest and factor VII activities was significantly weaker (p less than 0.02, ANOVA) compared with the control experiment. The effect of acenocoumarol on thrombotest activity was significantly weaker at 24 and 31 hours. None of the pharmacokinetic parameters tested were affected by rioprostil medication. The findings suggest that prostaglandins, at least those of the E series, attenuate the anticoagulant action of the oral anticoagulant agents by a mechanism not related to any pharmacokinetic interaction.

Microsomal warfarin binding and vitamin K 2,3-epoxide reductase

Rat liver microsomal 4-hydroxycoumarin binding was studied by assaying specific [14C]warfarin binding. Microsomes of warfarin-sensitive rats contained about 40 pmole of specific binding sites per mg of microsomal protein. There was no difference for R- or S-[14C]warfarin. Neither was there any difference between the enantiomers of acenocoumarol and phenprocoumon to prevent the in vitro racemic [14C]warfarin binding. Pretreatment of the microsomes with dithiothreitol, the in vitro reductor for microsomal vitamin K epoxide reductase activity, reduced the warfarin binding. Vitamin K epoxide nor vitamin K affected the warfarin binding. Microsomes of the Welsh warfarin resistant genotype showed weak warfarin binding properties. The Scottish resistant variant, on the other hand, did not differ from sensitive microsomes. Warfarin binding was reduced in microsomes of rats to which S-warfarin was administered. The reduction in warfarin binding was linear with the inhibition of microsomal vitamin K epoxide reductase activity and was linear with the amount of S-warfarin present in the microsomes. The results show the microsomal 4-hydroxycoumarin binding to be related to the target enzyme vitamin K epoxide reductase.