Study Design and Oversight

In this phase 2, randomized trial, which had an open-label, parallel group, adaptive design, we compared three FXI-ASO dosing regimens (100 mg, 200 mg, and 300 mg) with 40 mg of enoxaparin. A steering committee in collaboration with the sponsor (Isis Pharmaceuticals) was responsible for the design and oversight of the study. The institutional review board at each participating center approved the protocol. All the patients provided written informed consent. The sponsor was responsible for the collection and maintenance of the data. An independent committee, whose members were unaware of the treatment assignments, adjudicated all venograms for the presence and extent of venous thrombosis, all clinically suspected episodes of venous thromboembolism or bleeding, and all cerebrovascular events. The authors wrote all drafts of the manuscript, verified the data, made the decision to submit the manuscript for publication, and vouch for the completeness of the data, the accuracy of the analyses, and the fidelity of the study to the protocol. The protocol and accompanying documents are available with the full text of this article at NEJM.org.

Patients

Patients 18 to 80 years of age who were undergoing elective primary unilateral total knee arthroplasty and were willing to adhere to the study procedures were eligible for participation in the study. The main exclusion criteria were active bleeding or a high risk of bleeding, a history of brain or spinal surgery within the previous 3 months, anticipated use of intrathecal or epidural catheters, body weight below 50 kg, calculated creatinine clearance below 60 ml per minute, clinically significant liver disease, and venous thromboembolism within the previous year. The full list of exclusion criteria is provided in the protocol.

Randomization and Study Treatment

Figure 2. Figure 2. Enrollment, Randomization, and Populations for Analysis. The initial protocol (Panel A) called for random assignment of patients to one of three dose regimens of FXI-ASO (100 mg, 200 mg, or 300 mg) or enoxaparin. After 14 patients had undergone randomization, the steering committee amended the protocol (Panel B) to discontinue the 100-mg dose regimen. Random assignment to the 300-mg FXI-ASO cohort was discontinued when the target sample of 70 patients was recruited in that group. Enrollment was continued in the 200-mg cohort until the study ended.

The initial protocol called for random assignment of patients to one of three dose regimens of FXI-ASO (100 mg, 200 mg, or 300 mg) or enoxaparin (Figure 2). After 14 patients had undergone randomization, the steering committee amended the protocol to discontinue the 100-mg dose regimen and to add to the 200-mg and 300-mg regimens two additional doses of FXI-ASO before surgery and one additional dose after surgery. This decision was made to ensure a sufficient reduction in factor XI levels during and after surgery. In this article, we report results from the amended protocol (Figure 2), in which patients were randomly assigned to one of two regimens of FXI-ASO (200 mg or 300 mg) and within each regimen to receive treatment with either FXI-ASO or enoxaparin in a 3:1 ratio (for a more detailed description of the randomization procedure, see the Supplementary Appendix, available at NEJM.org).

Treatment with FXI-ASO was initiated 36 days before surgery (day 1 of the study). Patients received three subcutaneous doses of FXI-ASO during the first week of treatment (days 1, 3, and 5) followed by four once-weekly doses on days 8, 15, 22, and 29. On day 36, the day of surgery, patients received a dose 6 hours postoperatively. A final dose was given on day 39 (Fig. S1 in the Supplementary Appendix).

Enoxaparin, at a dose of 40 mg administered subcutaneously once daily, was initiated the evening before or 6 to 8 hours after surgery, according to the investigator's preference, and was to be continued for at least 8 days postoperatively.

Study Outcomes

The primary efficacy outcome was the incidence of adjudicated total venous thromboembolism, which was a composite of asymptomatic deep-vein thrombosis (detected by mandatory bilateral venography), objectively confirmed symptomatic venous thromboembolism, fatal pulmonary embolism, or unexplained death for which pulmonary embolism could not be ruled out. Venography was to be performed 8 to 12 days after the surgery.

Prespecified secondary efficacy outcomes included the individual components of the primary efficacy outcome. An exploratory outcome was the extent of venous thrombosis on venography, as assessed by the adjudication committee according to prespecified categories of bilateral thrombosis, confluent distal-to-proximal thrombosis, isolated proximal deep-vein thrombosis classified as large (≥10 cm) or small (<10 cm), and isolated distal deep-vein thrombosis classified as extensive (≥2 veins) or limited (<2 veins).

The principal safety outcome was the incidence of adjudicated clinically relevant bleeding, which was a composite of major or clinically relevant nonmajor bleeding. Bleeding was defined as major if it was overt and if it was associated with a decrease in hemoglobin of 2 g per deciliter or necessitated transfusion of 2 or more units of blood, occurred in a critical area or organ, or was fatal. Bleeding at the surgical site was defined as major only if it required an intervention or caused hemarthrosis that delayed wound healing or mobilization, prolonged hospitalization, or was associated with deep wound infection.9 Clinically relevant nonmajor bleeding was defined as overt bleeding that did not meet the criteria for major bleeding but required intervention or consultation with a physician or had clinical consequences10 (details are provided in the Supplementary Appendix). All other clinically overt bleeding events were classified as minor. Other safety outcomes included adverse events, abnormal results on liver-function tests, cardiovascular events, and deaths.

Pharmacodynamic outcomes included changes in the activated partial-thromboplastin time and factor XI levels, as measured in a central laboratory. Activated partial-thromboplastin times were determined with the use of standard two-stage assays. Ratios were established by dividing the results of those assays by the mean of the normal control. Factor XI activity was determined with the use of factor XI–depleted plasma. The reference value, which was derived from control plasma, was 1.0 unit per milliliter.

Surveillance and Follow-up

Patients receiving FXI-ASO were evaluated at the time of each preoperative injection, whereas surveillance of patients in the enoxaparin group started with the first dose on the evening before or the day of surgery. All the patients were assessed every day postoperatively until venography was performed and were followed according to a prespecified schedule for 3 months thereafter. Patients were instructed to report symptoms that were suggestive of venous thromboembolism or bleeding.

Statistical Analysis

The study was designed to evaluate the safety and efficacy of FXI-ASO as compared with enoxaparin according to dose regimen and according to the average factor XI levels (≤0.2 vs. >0.2 units per milliliter) between day 36 and day 39 (i.e., the perioperative period). The rationale for this cutoff was based on clinical and experimental observations.3,6,7

The safety population included all patients who received at least one dose of study medication. The modified intention-to-treat population consisted of all patients in the safety population who could be evaluated for the primary efficacy outcome. The per-protocol population comprised all patients in the modified intention-to-treat population, excluding those with major protocol violations (Figure 2).

The primary efficacy analysis tested the hypothesis that FXI-ASO would be noninferior to enoxaparin among patients in the per-protocol population who had average factor XI levels of 0.2 units per milliliter or less between day 36 and day 39 of treatment; the analysis included all events up to 12 days postoperatively. We prespecified that noninferiority would be shown if the upper limit of the 90% confidence interval for the between-group difference in risk was 14% or less. If noninferiority was shown, superiority testing would be performed with the use of the chi-square test or Fisher's exact test, as appropriate. We calculated that with 70 patients in each group, the study would have 80% power to show noninferiority. This calculation assumed rates of 18% for the primary efficacy outcome in all treatment groups. The noninferiority margin was chosen to be the midpoint of the 27-percentage-point difference in risk between the rate with placebo (45%) and the expected treatment rate (18%).11 This margin corresponds to retention of about 50% of the treatment effect of enoxaparin.

Secondary efficacy analyses compared the rates of the components of the primary efficacy outcome among the treatment groups. These analyses were repeated with the inclusion of events up to 4 weeks after venography. We also performed a sensitivity analysis of efficacy using the modified intention-to-treat population and an exploratory analysis of the extent of thrombosis. Analysis of the principal safety outcome and its components was performed in the safety population and included all events during study treatment and follow-up until day 136.