Laboratory Measurement of Rivaroxaban in Patient with Cancer-Associated Thromboembolism; Focus on Initial Rivaroxaban Therapy.

Background: Data on the levels of rivaroxaban-specific anti-factor Xa activity (AFXaA) within three weeks of starting high-dose rivaroxaban therapy in patients with cancer-associated thromboembolism (CAT) is limited. This study aimed to determine initial levels of rivaroxaban-specific AFXaA in patients with CAT to assist with drug monitoring.

Methods: This study included a total of 33 patients from December 2017 through January 2019. The levels of specific AFXaA, as well as prothrombin time (PT) and activated partial thromboplastin time (aPTT), were measured 3 hours after rivaroxaban administration for peak levels, and immediately before rivaroxaban administration, following 3 days of rivaroxaban therapy, for trough levels. We also reviewed complications such as major bleeding of patients treated with rivaroxaban.

Results: The median levels of specific AFXaA at trough and peak times were 120.7 and 347.2 ng/mL, respectively. The PT showed a positive correlation with AFXaA activity at both peak and trough levels (trough R = 0.92, peak R = 0.8, p < 0.05). However, aPTT was only weakly correlated with rivaroxaban-specific AFXaA (trough R = 0.39, peak R = 0.37). The levels of specific AFXaA were similar in the event group (venous thromboembolism recurrent, major bleeding, or minor bleeding) and the event-free group. The lowest trough level of AFXaA was present in the relapse group, and the highest level was present in the minor bleeding group (p = 0.39).

Conclusions: This study is the first report to determine the trough and peak levels of AFXaA within three weeks of starting high-dose rivaroxaban therapy in patients with CAT in Korea.