Thromboembolic and neurologic sequelae of discontinuation of an antihyperlipidemic drug during ongoing warfarin therapy

Warfarin and antihyperlipidemics are commonly co-prescribed. Some antihyperlipidemics may inhibit warfarin deactivation via the hepatic cytochrome P450 system. Therefore, antihyperlipidemic discontinuation has been hypothesized to result in underanticoagulation, as warfarin metabolism is no longer inhibited. We quantified the risk of venous thromboembolism (VTE) and ischemic stroke (IS) due to statin and fibrate discontinuation in warfarin users, in which warfarin was initially dose-titrated during ongoing antihyperlipidemic therapy. Using 1999–2011 United States Medicaid claims among 69 million beneficiaries, we conducted a set of bidirectional self-controlled case series studies—one for each antihyperlipidemic. Outcomes were hospital admissions for VTE/IS. The risk segment was a maximum of 90 days immediately following antihyperlipidemic discontinuation, the exposure of interest. Time-varying confounders were included in conditional Poisson models. We identified 629 study eligible-persons with at least one outcome. Adjusted incidence rate ratios (IRRs) for all antihyperlipidemics studied were consistent with the null, and ranged from 0.21 (0.02, 2.82) for rosuvastatin to 2.16 (0.06, 75.0) for gemfibrozil. Despite using an underlying dataset of millions of persons, we had little precision in estimating IRRs for VTE/IS among warfarin-treated persons discontinuing individual antihyperlipidemics. Further research should investigate whether discontinuation of gemfibrozil in warfarin users results in serious underanticoagulation.


Results
In our dataset of over 69 million beneficiaries, we identified 629 subjects who: a) concomitantly used warfarin and an antihyperlipidemic of interest; b) experienced at least one venous thromboembolism / ischemic stroke outcome during observation time; and c) met all other inclusion criteria. Subjects were predominantly female (65.0%) and non-Hispanic Caucasian (56.9%), with a median age of 69.1 years. Subjects contributed 93,764 person-days of observation, 84,208 (89.8%) of which were in a non-hospital setting. The median and mean observation period length was 3.3 and 4.9 months, respectively. The risk, non-risk, and indeterminate risk segments accounted for 12.5%, 84.3%, and 3.2% of person-days, respectively. Subject characteristics stratified by antihyperlipidemic of interest are presented in Table 1. Note that there were ten or fewer persons constituting the cerivastatin, fluvastatin, and pitavastatin cohorts; therefore, self-controlled case series conditional Poisson regression models were not run for these agents.

Discussion
We examined rates of venous thromboembolism/ischemic stroke among users of warfarin upon discontinuation of concomitantly-prescribed antihyperlipidemic drugs. Using rigorous pharmacoepidemiologic methods and a dataset of healthcare claims from tens of millions of persons, we did not identify any statistically significant confounder-adjusted associations between antihyperlipidemic discontinuation and our composite outcome. Our findings' limited precision prohibits us from drawing definitive conclusions; this is a notable limitation. However, it is worth noting that adjusted incidence rate ratios vs. pravastatin were less than one for fenofibrate and nearly always less than one for each statin under study. Results when combining antihyperlipidemics of interest by likelihood of cytochrome P450 2C9 inhibition using two different groupings resulted in substantial increases in precision and still failed to demonstrate a statistically significant association. Because the adjusted incidence rate ratio for gemfibrozil vs. pravastatin was 1.59, one could hypothesize that discontinuing gemfibrozil might increase one's risk of venous thromboembolism/ischemic stroke. This finding aligns with our prior demonstration that warfarin plus gemfibrozil (vs. pravastatin) results in a 50% increased risk of gastrointestinal bleeding and intracranial hemorrhage 27 , a result of overanticoagulation. Therefore, if warfarin is dose-titrated in the presence of gemfibrozil exposure, a lower dose of warfarin may be required to reach the desired level of anticoagulation. If gemfibrozil therapy is subsequently discontinued, this may place the patient at risk for sequelae of underanticoagulation from suboptimal warfarin dosing. This may emphasize the importance of calls to monitor the level of anticoagulation upon initiating and discontinuing drugs that may inhibit warfarin's metabolism 6,28,29 , including certain antihyperlipidemics 6 .
To our knowledge, no prior population-based comparative safety study has investigated clinical outcomes associated with the discontinuation of individual statins and/or fibrates in users of a coumarin derivative. With respect to a surrogate endpoint, Zhelyazkova-Savova et al. examined changes in laboratory measures in a cross-sectional study elucidating potential statin drug interactions (Appendix Table 3) 30 . Among 69 Bulgarian inpatients concomitantly-exposed to acenocoumarol and a statin, one (1.4%) individual experienced a 22.6% reduction in their international normalized ratio (3.1 to 2.4) upon discontinuation of atorvastatin 30 . In a cross-sectional study examining a potential drug interaction between warfarin and amiodarone (a non-statin, non-fibrate precipitant drug that inhibits the metabolism of warfarin via cytochrome P450, similar to some statins  Medicare enrolled at start of observation   Table 3) 31 . It is possible that amiodarone discontinuation during warfarin use may be more clinically relevant than the warfarin-antihyperlipidemic offset drug-drug interaction examined herein, particularly with respect to statin discontinuation. This warrants further investigation. Our study has notable strengths. It is the first population-based comparative safety study to examine clinical sequelae of an offset drug interaction among warfarin users. We utilized a self-controlled study design, prespecified a reference exposure, and controlled for time-varying covariates to minimize confounding. We conducted numerous secondary analyses to further elucidate the association between exposure and outcome and to test the     Figure 1 presents primary findings of study. Black squares represent crude incidence rate ratios. White circles represent confounder-adjusted incidence rate ratios. Values for incidence rate ratios and 95% confidence intervals are presented in Appendix Table 1.
robustness of findings to assumptions of the self-controlled design. Finally, components of our outcome definition had high positive predictive values and moderate-to-high sensitivities.
Our study also has limitations. First, despite using a dataset of over 69 million individuals, we had limited statistical power. Second, we lacked access to biosamples and therefore could not examine the impact of genetic cytochrome P450 polymorphisms. Third, we lacked data on adherence to dispensed warfarin and antihyperlipidemic prescriptions. Fourth, we lacked access to results of laboratory orders (e.g. international normalized ratio values); such findings are not included in Centers for Medicare and Medicaid Services data. Fifth, administrative databases may poorly capture some lifestyle behaviors and nonprescription therapies that affect venous thromboembolism and/or ischemic stroke risk; yet, such factors seem unlikely to differ substantially by antihyperlipidemic exposure group. Finally, our results may not be generalizable beyond a United States Medicaid population. Nevertheless, this population was specifically chosen because of its inherent vulnerability and inclusion of large numbers of women and minorities-groups typically understudied. Biologic associations identified in Medicaid populations are often replicated in commercially insured populations and vice versa 32 .
Drug interactions with warfarin are a major public health concern. Nearly all existing population-based studies of warfarin interactions and clinical outcomes have examined risk periods defined by commencement of concomitant use of warfarin and a precipitant drug. In contrast, we examined putative offset drug-drug interactions-defined by discontinuation of the precipitant drug-hypothesizing that de-inhibition of warfarin's hepatic metabolism would lead to serious sequelae of underanticoagulation. We did not identify a clear relationship between discontinuing antihyperlipidemics and clinical events. The potential safety signal that concomitant users of warfarin and gemfibrozil may be at increased risk for venous thromboembolism/ischemic stroke upon discontinuation of gemfibrozil requires further study. The mechanism underlying this possible offset interaction also needs further elucidation, but is unlikely to solely involve a pharmacokinetic interaction mediated by cytochrome P450 inhibition or displacement of binding from plasma proteins.

Methods
Overview and study population. We conducted bidirectional self-controlled case series studies of adult users of warfarin experiencing the composite outcome of venous thromboembolism/ischemic stroke. Although the phrase "case series" within self-controlled case series may seem to imply the absence of a comparator, the design is actually a rigorous, reproducible, controlled epidemiologic method 33 ; it is the cohort analogue of the better known case-crossover design 34 . In a self-controlled case series study, individuals serve as their own referent, therefore eliminating confounding by time-invariant factors 35 . This is a major advantage over traditional cohort and case-control approaches, yet is accompanied by the following key assumptions: the occurrence of an outcome should not appreciably affect subsequent exposures; outcome rates are constant within intervals; and outcomes must be independently recurrent or rare 35 . The study's underlying cohorts, one for each of ten antihyperlipidemics of interest, consisted of episodes of new warfarin use initiated during ongoing therapy for that antihyperlipidemic. Study data included demographic, enrollment, and healthcare claims from the United States Medicaid programs of California, Florida, New York, Ohio, and Pennsylvania from 1999-2011 32 . These states comprise ~38% of the national enrollment 36 , with the 13-year dataset recording the experience of more than 69 million cumulative enrollees and nearly 222 million person-years of observation. Because a substantive proportion of Medicaid beneficiaries are co-enrolled in Medicare 37-39 , we included Medicare claims to ascertain a more complete picture of their healthcare 40,41 . We linked these datasets to the Social Security Administration Death Master File to supplement death dates included in Medicaid and Medicare enrollment files.
Study cohorts. Separate cohorts were constructed for each antihyperlipidemic of interest, serving as bases for each self-controlled case series study. For persons 18-100 years of age, we utilized National Drug Codes and days' supply values on prescription claims to build episodes of warfarin exposure. We allowed a 7-day grace period between contiguous warfarin prescriptions (and at the end of the terminal warfarin prescription) to account for imperfect adherence. This approach was repeated for each antihyperlipidemic of interest, thereby allowing us to identify persons concomitantly exposed to warfarin and an antihyperlipidemic. We then identified each person's first concomitant use episode during which the antihyperlipidemic drug was initiated at least 30 days prior to warfarin; this ensured that warfarin was initially dose titrated while hepatic cytochrome P450 isozymes were already inhibited by the antihyperlipidemic (if applicable). Further, as the self-controlled case series design is a "case-only" approach, each person under study was required to experience an outcome during their observation period (defined below). If fewer than 10 persons constituted a given cohort, the antihyperlipidemic was excluded from further study.
Observation and pre-observation baseline periods. For each cohort member meeting inclusion criteria, their observation period included all person-days of the warfarin episode that defined concomitancy. The observation period began upon warfarin initiation and was censored upon the earliest of: a) the end of the warfarin episode initially defining concomitancy (defined by exhausting days' supply [plus terminal grace period] or switching to a different oral anticoagulant); b) a dispensing for an antihyperlipidemic other than that initially defining concomitancy; c) a >7-day gap in Medicaid enrollment; d) the end of the study dataset; and e) death. Note that occurrence of an outcome did not censor observation time. This helped uphold the key self-controlled case series assumption of no event-dependent censoring 42 and thereby avoided introducing bias of an unpredictable direction 43 .
A baseline period was defined as the 180 days immediately before the observation period. It was required to be devoid of: a) a >7-day gap in Medicaid enrollment; b) a procedure code indicative of hepatic cytochrome P450 2C9 or vitamin K epoxide reductase complex genotyping-suggestive that the prescriber used genetics to guide warfarin dose titration; and c) a prescription claim for warfarin or any other oral anticoagulant-otherwise, a prescriber may not dose-titrate but rather reinstitute an old, tolerated dose of warfarin.

Categorizing observation period follow-up time.
Person-days within each observation period were assigned to mutually-exclusive risk, non-risk, and indeterminate risk segments-with risk segments commensurate with a biologically plausible time frame during which antihyperlipidemic discontinuation in the presence of warfarin may be expected to increase risk of the outcome 44,45 . The risk segment consisted of a maximum of 90 person-days immediately following the end of the antihyperlipidemic episode initially defining concomitancy. The indeterminate risk segment consisted of a maximum of 90 person-days immediately following the risk segment. These segments could be <90 days in length if censored (described above) or if the antihyperlipidemic initially defining concomitancy was re-initiated. The non-risk segment consisted of all other person-days of observation time not assigned to a risk or indeterminate risk segment. Therefore, non-risk segments could occur both before and after the risk segment, consistent with a bidirectional self-controlled case series design; this standard approach helped to minimize exposure trend bias 46 . Of note, each observation period was not required to have person-days in all segments, yet only observations with both risk and non-risk segments contributed to the estimation of the incidence rate ratio for the association of interest. See Fig. 2 for a graphical representation of concomitant use episodes potentially eligible for inclusion.
Exposure of interest and covariates. The exposure of interest was the antihyperlipidemic drug initially defining concomitancy that was subsequently discontinued in the presence of warfarin. Agents included atorvastatin, cerivastatin, fenofibrate, fluvastatin, gemfibrozil, lovastatin, pitavastatin, pravastatin, rosuvastatin, and simvastatin. As dictated by the methodologic approach, each of the exposures of interest was examined in a separate self-controlled case series study. Pravastatin served as a prespecified control precipitant 5 referent because it is a SCIEnTIfIC REPORTs | (2017) 7:18037 | DOI:10.1038/s41598-017-18318-6 negligible inhibitor of hepatic cytochrome P450 isozymes 47 involved in the metabolism of warfarin 18 and thus not expected to interact pharmacokinetically. Consequently, the discontinuation of pravastatin during ongoing warfarin therapy (dose-titrated while pravastatin was on board) would not be expected to lead to underanticoagulation.
The self-controlled case series design implicitly controls for time-invariant covariates 48 . We therefore considered only time-varying covariates as potential confounders. We included, in each regression model, covariates from the following broad categories: a) major non-chronic risk factors for venous thromboembolism or ischemic stroke; b) drugs that may increase the risk of venous thromboembolism, ischemic stroke, or both 49 ; c) major non-chronic diseases that may affect coagulation; d) drugs that may affect coagulation; e) drugs that may interact with warfarin 50,51 ; and f) therapeutic drug monitoring for warfarin (i.e., an order for an international normalized ratio). We added a time-varying covariate for average daily warfarin dose in a secondary analysis; this was relegated to a secondary analysis since warfarin dose is difficult to ascertain from prescription dispensings. See Appendix Table 4 for additional detail.
Outcome of interest. The composite outcome was hospitalization for venous thromboembolism or ischemic stroke-serious sequelae of underanticoagulation-identified by International Classification of Diseases 9 th Revision Clinical Modification discharge diagnosis codes on inpatient claims. Operational definitions [52][53][54][55][56] , including quantitative measures of algorithm performance, are presented in Table 3.
Analytic approach and statistical analysis. For each self-controlled case series study, we constructed an analytic file in which the unit of observation was the person-day of observation time. The dependent variable was an indicator for outcome. Independent variables included a unique subject identifier, the subject's observation period, the segment within the observation period, and other time-varying covariates discussed above. The primary analysis examined outcome incidence during the entire risk segment(s) vs. incidence during the non-risk segment(s). We used conditional Poisson regression models to estimate incidence rate ratios and 95% confidence intervals 33,48 . We conducted numerous secondary analyses (Appendix Table 5) to examine the robustness of our findings and assess potential violations of the design's underlying assumptions as a measure of good practice 35 . We calculated ratios of incidence rate ratios in which the effect estimate for each antihyperlipidemic of interest was in the numerator and the effect estimate for pravastatin (as the prespecified referent) was in the denominator.
Analyses were conducted using SAS (SAS Institute Inc.: Cary, NC). The research described herein was approved via expedited mechanism by the institutional review board of the University of Pennsylvania. As the research was determined to be no greater than minimal risk, the board issued a waiver of informed consent. Methods related to human subjects research were developed and carried out in accordance with relevant guidelines and regulations.
Data availability. Data that support study findings are available from the Centers for Medicare and Medicaid Services. Restrictions apply to the availability of these data, which were used by this study under a permissive data use agreement, and so are not publicly available. However, data may be available from the authors upon reasonable request and with permission from the Centers for Medicare and Medicaid Services.