In turn, the function of CYP 450 enzymes involved in the hepatic metabolism of exogenous compounds strongly depends on genetic variations, transcriptional regulation, inflammation and other factors [11; 18; 19; 20; 21]. It is worth noting that alterations of CYP 450 enzyme activity might depend on environmental factors, such as drugs and food [22; 23; 24]. Therefore, treatment with DOACs carries the risk of bleeding or a decreased therapeutic effect. Rivaroxaban co-administration with inhibitors of CYP 450 enzymes should be avoided, as they may lead to an increased plasma concentration and risk of bleeding. The opposite effect is observed if strong inducers of CYP 450 enzymes, such as rifampicin, carbamazepine and phenobarbital, are administered [22; 23]. Under fasting conditions, the bioavailability of rivaroxaban was determined to be decreased. Thus, it might increase the risk of thrombosis [25]. Dietary supplements and herbs can interact with anticoagulants. Patients taking rivaroxaban are at particular risk of interaction with dietary supplements, as there are no current governmental regulations or voluntary programs that address dietary supplement interactions with prescription drugs [26]. It was determined that a strong CYP 450 enzyme inducer, St. John's wort, decreases plasma concentrations of all available DOACs [24; 27]. Thus, all of the above-mentioned factors emphasize the need for a clinically significant impact on CYP 450 enzyme identification. The transcriptional regulation of gene expression is one of the major biological processes that coordinate and alter cellular activity after various intra- and extra-cellular signals, such as drug molecules. The expression of CYP 450 is controlled by transcription factors such as CAR (constitutive androstane receptor), PXR (pregnane X receptor), RXR (retinoid X receptor), PPAR\(\alpha\) (peroxisome proliferator-activated receptor) and HNF4\(\alpha\)[19; 20]. During inflammation, the gene transcription of major drug-metabolizing enzymes belonging to P450 is repressed through IL-6 and IL-1\(\beta\) action [21]. Proinflammatory cytokines may cause a phenomenon better known as transient phenoconversion with the downregulation of drug metabolism. During inflammation, extensive metabolizers may become phenotypically poor metabolizers [28]. It is known that enzyme CYP4F2 is activated during processes of inflammation and thrombosis [29]. In humans, CYP4F2 oxidizes naturally occurring arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-HETE). Additionally, CYP4F2 participates in the resolution of inflammation by reducing the level of leukotriene B4 [30]. Previously published studies showed that CYP4F2 has an impact on warfarin dosage during treatment with the anticoagulant warfarin [31]. Our recent works added new data--enzyme CYP4F2 has an impact on the antiplatelet effect in patients with stable and unstable cardiovascular disease and diabetes treated with antiplateletts ticagrelor and clopidogrel [32; 33]. The study, which included 667 patients with acute coronary syndromes and treated with ticagrelor or clopidogrel dual antiplatelet therapy after stent implantation, showed higher CYP4F2 activity in patients without diabetes and in patients receiving ticagrelor [32]. Another study showed that the concentration of CYP4F2 was lower in patients with angina pectoris compared to healthy subjects [33]. These studies carried out on the antiplateletts ticagrelor and clopidogrel and on the anticoagulant warfarin [31] showed a significant impact of CYP4F2 enzyme activity on drug effects. Therefore, there was a possibility that direct oral anticoagulants might have acted on the CYP4F2 enzyme. The endothelial lining of blood vessels is a barrier whose damage is directly involved in thrombosis and inflammation; thus, we decided to evaluate the activity of a novel anticoagulant, rivaroxaban, on the CYP4F2 and transcription factors in HUVEC cells.