Trends in antithrombotic drug use and adherence to non-vitamin K oral anticoagulants in the Netherlands

Vitamin K antagonists (VKA) have been used as anticoagulants for the past 70 years [1]. In the Netherlands acenocoumarol and phenprocoumon are widely used, whereas warfarin is most frequently used worldwide [2]. VKAs are highly effective in treating and preventing thromboembolic events. However, they pose difficulties in clinical practice as they have a narrow therapeutic range and delayed onset or offset of effect. Furthermore, they show a high dosage variation between individuals as well as within subjects over time. This is related to various factors such as age, body weight, genetic polymorphisms, comorbidity, co-medication, dietary vitamin K intake, and alcohol use [1, 3, 4]. Consequently VKA treatment has to be monitored closely to ensure effective anticoagulation without causing life-threatening bleeding. However, monitoring is difficult in clinical practice. A recent meta-analysis in the United States showed that patients receiving warfarin for stroke prevention were within the therapeutic range only 55 % of the time [5]. These disadvantages prompted the development of new oral anticoagulants with a rapid onset of therapeutic effect and fewer drug interactions [6, 7]. Furthermore, it is desirable that fixed doses can be used without the need for constant coagulation monitoring [1, 8‐11]. Among these new non-vitamin K oral anticoagulants (NOACs), dabigatran specifically targets the coagulation enzyme thrombin, whereas rivaroxaban and apixaban inhibit factor Xa. In the Netherlands, dabigatran and rivaroxaban were licensed in 2008 for prophylaxis of venous thromboembolism in orthopaedic surgery for short-term use up to 35 days after surgery [12, 13]. When their effectiveness and safety were sufficiently proven in various randomised controlled trials [14‐18], marketing approval of the NOACs was expanded in 2011 to prophylaxis for embolism related to nonvalvular atrial fibrillation [12].

An important task for pharmacovigilance is the monitoring of new drugs after their approval, including drug-related problems such as potentially harmful drug combinations and patient adherence. After NOACs were introduced as an alternative to VKAs [19], some concerns regarding their use were highlighted. First, while the effects of VKAs can be reversed by vitamin K, there is currently no specific antidote for the anticoagulant effects of NOACs [1, 10, 20]. Second, because of their short half-lives, NOACs are less effective when doses are missed [1]. Third, in the Netherlands, monitoring of VKAs is facilitated by a network of regional anticoagulation clinics that monitor about 400,000 VKA users [21], and evidence suggests this improves the time spent in the therapeutic range [5], and leads to cost savings [22, 23]. NOAC use is not monitored so closely. Fourth, although the cited advantage of NOACs is that no routine monitoring is required, knowledge of the precise anticoagulant status may be needed in patients before emergency surgery or other invasive procedures [6]. Little is known about NOAC use, their use with other antithrombotic agents and adherence [8].

NOAC use increased from 18 patients in 2008 to 29,687 in 2013. Figure 1 shows the numbers of NOAC users during the study period, stratified by short- and long-term use. From 2011, the number of long-term users increased steeply and exceeded short-term users in 2013. In comparison, the total number of VKA users in 2013 was 484,024 in the Netherlands. Table 1 compares the characteristics of users of antithrombotic drug classes in 2013, based on the data provided by 1697 community pharmacies (86 % of the totally 1974 community pharmacies in the Netherlands in that year). Thus NOAC users made up 6 % of all users VKA users in 2013. NOAC users were slightly younger than VKA users (mean age 70.3 vs. 73.5 years) and used fewer co-medications, except for antiarrhythmic drugs (used in 15.7 % of long-term NOAC users compared with 7.8 % of VKA users) and beta blockers (used in 62.6 % of long-term NOAC users compared with 54.7 % in VKA users). The majority (97 %) of NOAC users were first prescribed a NOAC by a medical specialist, and 3 % by a GP.

Table 2 shows the concomitant use of antithrombotic drugs and switching between drug classes per calendar year, with the absolute numbers standardized to the total number of Dutch community pharmacies in the corresponding calendar year. With the marked increase in NOAC use, the number of users with concomitant use of NOACs and other anticoagulants also increased. Although the total number was proportionately small, the number of patients was substantial: in 2013, a combination of NOACs with VKAs was dispensed to 820 subjects (2.7 % of NOAC users), while NOACs and ASA were dispensed to 446 subjects (1.5 % of NOAC users), compared with 11,207 receiving VKA and ASA (2.3 % of VKA users). Concomitant clopidogrel or prasugrel use occurred in 149 NOAC patients (0.5 % of NOAC users) compared with concomitant use in 6822 VKA patients (1.4 % of VKA users). Concomitant use of three antithrombotic drugs (NOACs with ASA and clopidogrel or prasugrel) was detected in 13 (0.05 %) patients, while 671 (0.1 %) VKA users also used two APAs concomitantly. Switching to a NOAC occurred more frequently than vice versa: 3502 subjects switched from a VKA to a NOAC, and 1552 from a NOAC to a VKA; 4850 subjects switched from an APA to a NOAC, and 2193 vice versa.

In 2013, the mean drug-taking compliance for NOACs was 84.2 % compared with 87.3 % for APA (Table 3). The percentage of subjects with a PDC higher than 80 % was 75.9 % for NOACs in chronic use compared with 80.2 % for APAs.

NOAC use in the Netherlands markedly increased during the study period: in 2013 there were 29,687 NOAC users compared to 484,024 VKA users. NOACs may be prescribed as an alternative for patients who did not start with VKAs because of the monitoring required, or for subjects who could not achieve stable anticoagulation with VKA treatment [27]. The latter reason is supported by the fact that switching from a VKA to a NOAC occurred more than twice as often as vice versa. Consistent with the guidelines, NOAC treatment was predominantly initiated by medical specialists and only occasionally by GPs. NOAC users were slightly younger and differed in the use of co-medications. With growing NOAC use, the prevalence of potentially harmful drug combinations increased: 820 subjects were detected in 2013 with concomitant use of NOACs and VKAs, a non-rational and potentially risky combination according to the guidelines [13]. Other combinations of antithrombotic drugs may have rational use for a certain duration. To examine this further, longitudinal research is needed together with information on dietary intake and diagnosis [28].

Poor adherence has been shown to be the single most important source of variations in the effects of all classes of drug treatment [29]. In clinical practice persistence with rivaroxaban was high in patients with atrial fibrillation [30]. This study showed a discontinuation rate of 15 % in the first year of treatment with rivaroxaban, mainly due to bleeding complications and other side effects. Our analysis shows that one in four NOAC users had a PDC lower than 80 % compared with one in five APA users. As we calculated adherence rates only in subjects with periods of NOAC use longer than for anticoagulation therapy in orthopaedic surgery, we aimed to address patients on lifelong NOAC therapy. Our strategy could either overestimate adherence (by falsely excluding early cessation in lifelong therapy) or underestimate it (e.g., by falsely including physician-initiated discontinuations because of major bleeding) [7]. The cut-off for adherence of a PDC of 80 % was chosen according to published studies [31]. However, this might be too low for NOACs which have short half-lives of between 7 and 17 h, and where there is a greater impact of a missed dose in achieving steady anticoagulation compared with VKAs [1, 7]. Consequently, a smaller proportion of NOAC users than VKA users may achieve effective anticoagulation in clinical practice. The impact of adherence to NOACs on clinical outcomes in daily practice is not known as yet [7, 32]. It was suggested that the advantage of NOACs in not requiring regular laboratory monitoring may translate into a lower awareness of the need to assess adherence [32]. Thus NOACs may not be suitable replacements for VKA in patients with adherence issues [1, 33]. Comparisons of adherence to VKAs and NOACs in clinical trials showed comparable poor results for both drug classes [14‐16]. However, in a review of a healthcare claims database in Canada, rivaroxaban was associated with a significantly lower risk of nonadherence compared with warfarin (hazard ratio for cessation = 0.66) [34]. This could be different from the situation in Netherlands because of the therapeutic VKA monitoring by specialised anticoagulation clinics [35]. To ensure an optimal antithrombotic effect, continuous and regular intake of NOACs is necessary [32, 33]. A number of barriers to optimal NOAC use have been identified, such as a lack of knowledge of stroke risk on the one hand side and of bleeding risk on the other [36]. A combination of strategies from prescribers and pharmacists was proposed to provide tailored support for optimal use of NOACs [32, 37]. Consequently, prescribers and pharmacists should address adherence to NOACs during each patient consultation [33]. Pharmacists can contribute to better adherence [33] and have been shown to improve adherence by use of dispensing data to detect patients who did not return for their medication in time [31].

A limitation of estimating adherence from dispensing data is the lack of information on when doses were taken or missed [7]. Thus our results could overestimate adherence in daily practice, especially when the temporal sequence of the doses is essential in delivering stable anticoagulation by a drug with a short half-life of 8 to 11 h [38]. Another shortcoming of our study was the lack of information on the indication for prescribing the drugs. In order to distinguish between short- and long-term use, we used periods of drug use with cutoff points as given in the current guidelines, but this may have introduced some misclassification. However, as we used the same criteria for APAs to compare adherence, this is unlikely to have caused differential misclassification. A further shortcoming of our study was that we could not compare NOAC adherence with VKA adherence because of the lack of information on individual VKA dosages in the pharmacy data. Therefore, adherence to APAs was chosen as the best alternative for an antithrombotic drug class. The validity of dispensing data for calculating adherence measures depends on the loyalty of patients to the same pharmacy. In the Netherlands, pharmacy-shopping behaviour is low [39], and furthermore we excluded ‘drop-in’ patients. Finally, our cross-sectional study design for a comprehensive description of annual NOAC use since their introduction into the market did not allow us to evaluate the duration of antithrombotic treatment at the patient level over time to assess the duration of specific antithrombotic therapy and combinations. A strength of our study was the availability of national dispensing data, which enabled us to describe the use of antithrombotic drugs in the Netherlands in the past 5 calendar years.

Our findings show a marked increase in the use of NOACs by outpatients in recent years in the Netherlands. The study indicated that a substantial number of patients require additional management to improve the safe and effective use of NOACs in clinical practice, with a reduction in non-rational combinations of antithrombotic drugs, and improved adherence.