International normalized ratio and warfarin dose trends within the first week after initiation of warfarin therapy in the postimplantation period were represented in Figure 2. As shown, CYP2C9 genotype groups did not have an effect on daily warfarin dosing and INR levels early after initiation of therapy. However, VKORC1 genotype was significantly associated with daily warfarin dosing and INR levels. Patients carrying two copies of variant allele (AA) had significantly higher INR values and lower warfarin doses required within the first week of initiation of therapy compared with AG and GG patients. Days to target INR did not have any correlation with serum creatinine, serum protein, serum albumin, or total bilirubim (p = not significant [NS] for all comparisons).
Effect of CYP2C9 and VKORC1 Variants on Time in Therapeutic Range and INR Variability
Time in therapeutic window was 30.4% in the early postimplantation (initial 30 days) and 57.2% in the outpatient setting in the overall patient cohort. Risk of over-anticoagulation (INR >3.0) was 6.7% in the early and 12.4% in the outpatient setting. Consistent with the noted differences in early INR trajectories, time in sub-therapeutic INR range was significantly lower in patients with rare VKORC1 variants in the early postimplant period (Figure 3A). However, TTR was not different among genotype groups during the outpatient follow-up (Figure 3B). Outpatient INR variance was also comparable between patients carrying CYP2C9 (0.44 for wild-type vs. 0.52 for rare variants) or VKORC1 (0.46 for GG, 0.41 for GA, and 0.32 for AA) variants (p = NS).
Heparin Coverage in Genotype Groups
CYP2C9 variants did not have an effect on number of aPTT testing or percentage of therapeutic aPTT during initiation of warfarin therapy. However, patients with rare VKORC1 variants had a trend toward less number of aPTT levels checked (13.9 ± 8.0 vs. 18.5 ± 10.6, p = 0.057), and significantly lower percentage time spent at therapeutic aPTT (28.2% vs. 42.7%, p = 0.019) compared with patients with wild-type genotype.
We investigated the role of warfarin metabolism genotype in 80 patients with CF-LVADs. The principal findings of this study are as follows: 1) Genotyping for CYP2C9 (*1, *2, and *3) and VKORC1 (–1639 G>A) variants are high-yield in European–American CF-LVAD recipients, because these variants occur at higher frequencies in this population; 2) VKORC1 (–1639 G>A) genotype successfully predicts INR response and warfarin dose requirements in the early postimplantation period. Taken together, these results suggest a potential utility of warfarin genotype testing in CF-LVAD patients.
Previous data exploring the relationship between warfarin genotype variants and INR dose response in CF-LVAD recipients is limited to one retrospective cohort of 65 patients.9 Although these authors demonstrated that the presence of any mutant allele was associated with an enhanced warfarin response, the limited sample size of this analysis precluded exploration of the impact of individual variants (e.g., CYP2C9 vs. VKORC1). Our analysis indicates that genetic variants in VKORC1, but not in CYP2C9 gene (*2 or *3), confer an effect on early response to warfarin therapy in CF-LVADs patients. This finding corroborates with prior studies suggesting a larger contribution of VKORC1 to warfarin dosing and early INR response compared with CYP2C9 genotype.10–12 Although precise mechanisms for this finding are unknown, it is likely that other variants in CYP2C9 that are routinely not tested may contribute to warfarin metabolism by regulating the function of this highly polymorphic gene.13 Moreover, only a limited number of patients (n = 5) in our study were compound heterozygous or homozygous for variant alleles (*2 or *3), which limits our ability to demonstrate differences in dosing patterns. Nevertheless, irrespective of presence or absence of clinical risk factors, VKORC1 genotype alone successfully predicted early INR response in CF-LVAD patients in a dose-dependent manner.
Genotype frequencies differed significantly for CYP2C9 and VKORC1 genes across racial groups of CF-LVAD patients, which is consistent with previous reports.14 Compared with European–American population, African–Americans had significantly lower frequencies of rare alleles in CYPC9 and VKORC1 genes. Moreover, these variants explain less variability for individuals of African–American descent, suggesting that testing of additional genetic variants may improve predictive power of genotyping in this population. A recent genome-wide association study identified a novel CYP2C9 gene variant (rs12777823), which significantly predicts warfarin dosing variability in the African–American population independent of CYP2C9 *2, *3, or *11 variants.15 Despite its low prevalence, VKORC1 1639 G>A genotype predicts similar dose requirements across all racial groups.16
Higher risk of overall hemorrhagic complications on warfarin therapy in has been previously reported in non-LVAD patients carrying rare variants (in particular CYP2C9 *3); however, such a relationship was not demonstrated for GI bleeding events.17–20 In CF-LVAD patients, GI bleeding and epistaxis typically occur months after initial implantation, at which point the warfarin dosing and INR levels stabilize for the majority of patients. However, these patients are potentially at higher risk for surgical bleeding complications and need for reoperation in the early postimplantation period. Conversely, warfarin sensitive patients may also be predisposed to increased risk of thrombosis during initiation of therapy, because elevation of INR in this period is primarily due to the rapid reductions in factor VII and may not truly reflect the full antithrombotic effect of warfarin, which does not occur until the prothrombin levels are reduced.21–23 The half-lives of factor VII and prothrombin are 4 and 60 hours, respectively, so warfarin depletes the former much more rapidly than the latter. Although temporal levels and activity of coagulation factors during initiation of therapy have not been investigated in detail for specific genotype groups, previous warfarin dosing experiments indicate that higher warfarin loading doses do not accelerate the reduction in prothrombin levels despite inducing a more rapid decline in Factor VII and elevation in INR levels.24,25 This in combination with unopposed rapid reductions in anticoagulant protein C and S levels may contribute to a higher procoagulant state observed during initiation of warfarin therapy in patients who are loaded with higher warfarin doses, and similarly in patients who are genetically sensitive to warfarin (i.e., patients with the rare VKORC1 variant).
The early procoagulant effect of warfarin was recently highlighted in large population-based study that demonstrated an increased risk of ischemic stroke associated with initiation of warfarin therapy.26 Similarly, CF-LVAD patients may particularly be vulnerable to nidus formation and/or clot propagation during the early pro-coagulant state, especially when heparin dosing and adjustments tend to be variable. In fact, our analysis showed that patients with VKORC1 rare variants are less likely to reach stable therapeutic aPTT levels. This is likely due to the relatively shorter time spent on intravenous heparin and the limited opportunity to optimize dosing, which may place these patients at an increased risk for thrombosis. Although clinical outcomes were not investigated in this report because of limited sample size, it is plausible that warfarin genotype and resultant warfarin response may have an effect on both bleeding and clotting complications in LVAD patients.
International normalized ratio variability may predict warfarin adverse events independent of TTR,27 however we were unable to demonstrate differences in outpatient INR variability among genotype groups. Another consideration is that whether INR is a reliable measure of coagulation status in CF-LVAD patients, similar to patients with cirrhosis who exhibit increased thrombotic complications despite elevated INR levels.28 Using advanced coagulation measures such as specific factor antigen and/or activity levels may be more informative in this population. This is particularly relevant for CF-LVAD patients because of high incidence of right-heart failure, liver dysfunction, and chronic malnutrition, all of which may impact INR levels by modulating the vitamin K metabolism.
Novel oral anticoagulants (NOACs) such as direct thrombin inhibitors or Factor Xa inhibitors are not yet recommended for use in CF-LVAD patients. Although NOACs are shown to be effective and safe in patients with atrial fibrillation, data regarding use of NOACs in CF-LVAD patients are scarce. In addition, given the absence of specific NOAC antidotes, strategies for the reversal of the anticoagulant effects are limited, which may be problematic in CF-LVAD patients with high incidence of bleeding events. Nevertheless, a recent crossover study performed in seven Heartmate II patients suggested that dabigatran use was safe and effective in this population.29 Larger follow-up studies are required to demonstrate safety and efficacy of NOACs in CF-LVAD patients compared with warfarin anticoagulation.
There are several limitations of our analysis that warrant further discussion. First, warfarin genotyping during the study period was collected at the discretion of the treating physician. As such, this lack of a systematic approach to testing may have introduced bias into the study population. Despite this limitation, we feel that our robust sample size of 80 patients is still adequate to describe the effect of warfarin genotype variants on dose response in CF-LVAD patients. Second, as the genotype data were often not available in time to assist with initial warfarin dose selection, we could not evaluate whether a prospective genotype-guided strategy would enhance anticoagulation management. However, the results of our clinical-based dosing strategy yielded similar findings (e.g., time to therapeutic INR and total warfarin dose) when compared with a previously published genotype-guided report.9 Finally, although our analysis demonstrated that obtaining warfarin genotype information may assist with predicting dose requirements, the cost–effectiveness of such an approach has not been established in CF-LVAD patients.
In conclusion, genotype data in warfarin metabolism/action genes accurately differentiate warfarin dosing and INR kinetics after CF-LVAD implantation. Use of routine genotyping may potentially lead to safer and more effective anticoagulation in these patients. Specifically, knowledge of genotype data before device implantation may allow for avoidance of excessive doses in warfarin sensitive patients with an exaggerated response (prevention strategy 1), and shortening of bridging time in patients who are warfarin resistant with typically prolonged time to therapeutic INR (prevention strategy 2; Figure 4). Given the high prevalence of bleeding and thrombotic events in CF-LVAD patients, warfarin genotype data have the potential to predict these adverse events in genetically susceptible individuals. Our findings emphasize the need for a prospective multicenter study to test the hypothesis that the genotype-directed anticoagulation in CF-LVAD patients is cost-effective and leads to improved clinical outcomes such as reduction in bleeding/thrombotic events, hospital length-of-stay, and readmission rates.
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left ventricular assist device; pharmacogenomics; warfarin; thrombosis
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