PRODUCT LABELING

Please refer to the following sections of the enclosed Full Prescribing Information that are relevant to your inquiry: CLINICAL PHARMACOLOGY.¹

BACKGROUND

Existing and novel anticoagulant medications work by inhibiting specific targets in the coagulation pathway. Therapeutic targets exist at each of the three steps (initiation, propagation, and thrombin activation) of the coagulation pathway, and generally consist of serine protease clotting factors (eg, thrombin, factor VII, etc.). Older anticoagulants include warfarin, which targets factors II (ie, thrombin), VII, IX, and X; heparin and low molecular weight heparin (LMWH), which target both thrombin and FXa (as indirect inhibitors); and direct thrombin inhibitors (eg, bivalirudin, argatroban). Newer anticoagulants target specific steps in anticoagulation and include agents which are direct and indirect inhibitors of FXa.²

FXa catalyzes the conversion of prothrombin (factor II) to thrombin (factor IIa).  Low concentrations of activated thrombin are capable of rapidly amplifying the coagulation cascade via subsequent activation of factors V and VIII, platelets, and platelet-bound factor XI.^2,10,11 FXa serves as a pharmacological target because it is the initial step in the common coagulation pathway, where the intrinsic and extrinsic pathways overlap.¹⁰  Inhibition of this important rate-limiting step of thrombin generation appears to be an effective therapeutic strategy for thromboprophylaxis.¹⁰

FXa exists both in a free, unbound form and in a form that is bound to platelets within the prothrombinase complex or clots.^2,3  Indirect FXa inhibitors inhibit FXa via antithrombin, and are effective only in inhibiting free FXa.²  Inhibition of FXa directly inhibits both free FXa and prothrombinase- or clot-bound FXa.^2,3

Rivaroxaban, a small-molecule FXa inhibitor, selectively blocks the active site of FXa, and does not require a cofactor (such as anti-thrombin III) for activity.^1,5,6

PHARMACOLOGY STUDIES

Platelet Dependent Thrombin Generation Studies (In Vivo)

A randomized, placebo-controlled, open-label, two-way crossover study in 12 healthy adult male subjects evaluated the pharmacologic activity of single doses of rivaroxaban 5 mg or 30 mg.¹² The effect of rivaroxaban on platelet-induced thrombin generation was measured using endogenous thrombin potential (ETP) in platelet-rich plasma (PRP).  Platelet-dependent thrombin generation in plasma was also assessed using the prothrombinase-induced clotting time (PiCT) assay. Both ETP and PiCT results showed that rivaroxaban inhibited platelet-induced thrombin generation in a dose-dependent manner.  Maximum PiCT prolongation occurred at two hours following dose administration. Prolongation of PiCT increased 1.8-fold from baseline for rivaroxaban 5 mg and 2.3-fold for rivaroxaban 30 mg.  At two hours following dose administration, peak ETP (collagen-induced) was reduced by ~80% from baseline for rivaroxaban 5 mg and ~90% for rivaroxaban 30 mg.  The effect of ETP and PiCT was maintained at 24 hours after administration of the 30 mg dose.

Enzyme Assay Studies (In Vivo/In Vitro)

One study assessed the in vitro and in vivo activity of rivaroxaban using various assays (including enzyme, prothrombinase, plasma, and coagulation assays) and animal models (including rat venous stasis model, rat/rabbit arteriovenous-shunt model, rat tail-bleeding model, and rabbit ear-bleeding model).⁴ The results from the in vitro enzyme assay showed that rivaroxaban produced a concentration-dependent inhibition of human factor Xa with a mean ± SEM Ki of 0.4 ± 0.02 nM. In a buffer solution, rivaroxaban had a mean IC₅₀ of 0.7 nM in humans, 0.8 nM in rabbits, and 3.4 nM in rats.  Rivaroxaban displayed high selectivity for factor Xa (>10,000-fold more selective for factor Xa than for other, related serine proteases). A prothrombinase assay showed that rivaroxaban inhibited thrombin generation in a concentration-dependent manner with a mean IC₅₀ of 2.1 nM, indicating that rivaroxaban inhibited prothrombinase-bound factor Xa in addition to free factor Xa.  Rivaroxaban also inhibited endogenous factor Xa generated in human, rabbit, and rat plasma, with mean IC₅₀ values of 21 nM in both humans and rabbits and 290 nM in rats.

Thrombin Generation Studies (In Vivo/In Vitro)

In vivo thrombosis and bleeding models in rats and rabbits showed that rivaroxaban produced a dose-dependent reduction in thrombus formation, inhibition of factor Xa activity, and PT prolongation without significant prolongation of bleeding times at the effective doses. The antithrombotic effect in these animal models was achieved at low to moderate anticoagulant doses.⁴

An in vitro study of clots formed on polystyrene hooks in citrated normal platelet-poor plasma examined the effect of various concentrations of rivaroxaban or fondaparinux (0 to 15,000 nM) on clot-bound factor Xa.³ Results showed that, like fondaparinux, rivaroxaban produced a concentration-dependent inhibition of clot-bound factor Xa. The IC₅₀ for rivaroxaban was 75 nM.  At concentrations of 500 nM or more of rivaroxaban, the inhibition of factor Xa activity was almost complete at 85% to 97%.

Another in vitro study assessed the effects of rivaroxaban on various phases of thrombin generation using whole blood obtained from 16 healthy subjects.¹³ Thrombin generation following activation of the tissue factor (extrinsic) pathway was measured directly in PRP and indirectly using prothrombin fragments 1 + 2 (F₁₊₂), in whole blood.  Results showed that nanomolar concentrations of rivaroxaban significantly inhibited both the initiation and propagation phases of thrombin generation in whole blood as well as in PRP. In PRP, rivaroxaban also produced a decrease in the C_max (peak) of thrombin generated as well as in the ETP. However, in order to produce a 50% decrease in ETP and the C_max of prothrombin F₁₊₂, higher concentrations of rivaroxaban were required compared to the amount required to inhibit the initiation and propagation phases of coagulation.

Kotha et al (2017)¹⁴ conducted a laboratory-based study evaluating the effects of rivaroxaban, a Factor Xa inhibitor, on antiplatelet activity.

Study Design

• Whole blood was collected by venipuncture in 6 healthy volunteers. Light transmission aggregation and thrombin generation assay were performed with aspirin, ticagrelor, and rivaroxaban alone, or a combination of agents.  

Outcomes

• The platelet aggregation endpoint evaluated was change in percent average maximal platelet aggregation.  Thrombin generation endpoints were treatment effects on both average peak thrombin concentration and average thrombin potential.

Results

• Aspirin and ticagrelor monotherapy reduced maximal platelet aggregation 9% and 19%, respectively. Rivaroxaban monotherapy reduced maximal aggregation 18-54% across the concentrations of 15-120 ng/mL (corresponding to 2.5 mg BID to 20 mg BID rivaroxaban dose). Rivaroxaban combination therapy with aspirin and ticagrelor reduced maximal aggregation 53-67% across the concentrations of 15-120 ng/mL.  The rivaroxaban 120 ng/mL combination analysis, corresponding to rivaroxaban 10-20 mg twice daily dosing, had a statistically significant change (P<0.05).
• Thrombin generation assays were conducted in PRP and PPP. Aspirin monotherapy did not affect PRP peak thrombin concentration, and ticagrelor only slightly reduced it.  Increasing concentrations of rivaroxaban, 15-120 ng/mL, reduced PRP peak thrombin concentration by 24-66%, and rivaroxaban combination therapy with aspirin and ticagrelor further reduced peak thrombin concentration by 34-67% across rivaroxaban concentrations.
• Results for the PPP analysis showed inhibition of peak thrombin concentration ranging from 36-87% across rivaroxaban monotherapy 15-120 ng/mL.  Results were statistically significant for inhibition of thrombin generation for rivaroxaban monotherapy (P<0.05).  Monotherapy with aspirin or ticagrelor did not impact peak thrombin concentration, while the addition of aspirin and ticagrelor to rivaroxaban 15-120 ng/mL inhibited peak thrombin by 37-89%.
• Thrombin potential was evaluated, and results were consistent with the trends seen for peak thrombin generation.

Tantry et al (2020)¹⁵ is conducting a prospective, open-label, randomized, phase 4 study to evaluate the effect of dual pathway inhibition with rivaroxaban plus aspirin on platelet activation and aggregation, plasma inflammation and coagulation markers in 30 patients with a history of CAD and PAD. Patients will be randomized to receive rivaroxaban 2.5 mg twice daily plus enteric-coated aspirin 81 mg once daily or enteric-coated aspirin 81 mg once daily alone for 12 weeks. The primary endpoint is the difference in maximal ADP-induced platelet aggregation at 12 weeks. The select secondary endpoints include differences in tissue factor-and α-thrombin-induced platelet aggregation, interleukin-6, fibrinogen, thrombin-antithrombin complexes, and other endpoints. The primary safety endpoint will be the time from randomization to the first occurrence of bleeding as defined by the modified International Society on Thrombosis and Hemostasis major bleeding.

SELECTED ADDITIONAL REFERENCES

The in vitro and in vivo pharmacologic activity of rivaroxaban has also been described in several other publications and poster presentations.^16-28

An additional citation identified during a literature search is included in the REFERENCES section for your review.²⁹

LITERATURE SEARCH

A literature search of MEDLINE^®, EMBASE^®, BIOSIS Previews^®, Derwent^® (and/or other resources, including internal/external databases) was conducted on 18 September 2024.